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Metal-doped graphitic carbon nitride (g-C3N4) as selective NO2 sensors: A first-principles study
Applied Surface Science 455 (2018) 1116–1122
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Metal-doped graphitic carbon nitride (g-C3N4) as selective NO2 sensors: A first-principles study
T
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Hong-ping Zhanga, , Aijun Dub, Neha S. Gandhic, Yan Jiaod, Yaping Zhanga, Xiaoyan Lina, Xiong Lue, Youhong Tangf a
State Key Laboratory of Environmental Friendly Energy Materials, Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Sichuan 621010, China b School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Queensland 4001, Australia c School of Mathematical Sciences, Queensland University of Technology, Queensland 4000, Australia d School of Chemical Engineering, The University of Adelaide, South Australia 5005, Australia e Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China f Institute for NanoScale Science and Technology, College of Science and Engineering, Flinders University, South Australia 5042, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 Metal-doped NO2 Sensitive Density functional theory
In this research, the potential application of metal-doped g-C3N4 as highly sensitive molecule sensors for NO2 detection was studied using density function theory (DFT) calculations. Various metal-doped (Ag-, Au-, Co-, Cr-, Cu-, Fe-, K-, Li-, Na-, Mn-, Pt-, Pd-, Ti-, V-) g-C3N4 sheets were considered. CO, CO2, NH3, N2 and NO2 molecules were found to adsorb on metal-doped g-C3N4 via strong chemical bonds. Chemisorbed gas molecules and metaldoped g-C3N4 formed charge transfer complexes with different charges transferring from metal-doped g-C3N4 to gas molecules. Pristine and metal-doped g-C3N4 sheets were demonstrated as potential capturers for certain gas molecules according to the adsorption energy, isosurface of electron density difference, and density of states analysis. Among the diverse metal-doped g-C3N4 sheets, Ag-, K-, Na-, and Li-doped g-C3N4 were found to be clearly sensitive to the NO2 molecule. The adsorption energies between NO2 and Ag-, K-, Na-, and Li-doped gC3N4 were significantly greater than those of the other gas molecules (CO, CO2, N2, and NH3). The density of states indicates that the NO2 adsorption on Ag-, K-, Na-, and Li-doped g-C3N4 induced the shift of the total density of state in the positive energy direction. Charge transfer results also demonstrate that chemical interactions existed between NO2 and Ag-, K-, Na-, and Li-doped g-C3N4. All these results suggest the strong potential of Ag-, K-, Na-, and Li-doped g-C3N4 for application as highly sensitive molecule sensors.
1. Introduction Two-dimensional (2D) layer-structured nanomaterials have attracted much research attention because of their ultrahigh specific surface area, adjustable band structure, and excellent mechanical and thermal properties [1]. Due to their quantum size effects, these materials, including graphene-like materials (phosphorene, graphitic carbon nitrides, silicene, etc.) and 2D transition-metal dichalcogenides have shown promise in nanoelectronics, catalysis, energy storage, gas sensors, and other fields in materials science [2–5]. Among the various graphene-like 2D nanomaterials, carbon nitride (C3N4) has spurred interest because of its potential applications as photocatalyst and optoelectronic conversion devices [6]. Graphitic carbon nitride (g-C3N4) has a stacked 2D structure without metals and is the most stable allotrope of
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Corresponding author. E-mail address: [email protected] (H.-p. Zhang).
https://doi.org/10.1016/j.apsusc.2018.06.034 Received 26 February 2018; Received in revised form 24 May 2018; Accepted 6 June 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
C3N4. g-C3N4 possesses strong covalent CeN bonds rather than CeC bonds in each layer and the layers are bound by van der Waals forces only. Coupling g-C3N4 with other substances (graphene, semiconductors, metals, etc) and/or introducing defects as found in any other 2D nanomaterial can give rise to the structuring of g-C3N4 into different morphologies such nanosheets, mesoporous structures, nanorods, hierarchical structures, and films. These structures and morphologies are strongly related to the physical, electronic, and chemical properties and applications of g-C3N4. C3N4 exhibits extensive advantages in hydrogen evolution, pollution degradation and photocatalytics, its low utilization of visible light and other weakness in photocatalytic activities attract the research attentions. Combining gC3N4 with other materials was demonstrated to be an effective way to improve the photocatalytic activity of g-C3N4 [7].
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program Dmol3 in Materials Studio (Accelrys, San Diego, CA), in which the physical wave functions were expanded in terms of numerical basis sets. The double numerical basis set with polarisation function (DNP) [26,27], that is comparable to the 6-31G** basis set, was utilised during the calculations [28]. The core electrons were treated with DFT semicore pseudopotentials. The exchange-correlation energy was calculated using the PBE and GGA methods [29]. Special point sampling integration over the Brillouin zone was employed using Monkhorst-Pack schemes with a 5 × 5 × 1 k-point mesh [30]. A Fermi smearing of 0.005 Ha and a global orbital cutoff of 7 Å were employed. The convergence criteria for the geometric optimization and energy calculation were set as follows: (1) self-consistent field tolerance of 1.0 × 10−6 Ha/ atom, (2) energy tolerance of 1.0 × 10−5 Ha/atom, (3) maximum force tolerance of 0.002 Ha/Å, and (4) maximum displacement tolerance of 0.005 Å. A vacuum slab with 30 Å was added onto the g-C3N4 or metal doped g-C3N4 surface to avoid the interaction influence of the periodic boundary conditions.
Various doped C3N4 combinations have been reportedly fabricated successfully in recent years to enhance the performance of g-C3N4 in various fields [3,8,9], among which the element doping strategy plays an essential role. Sulfur-doped C3N4 nanosheets (S-g-C3N4) has shown promising photocatalytic activity for H2 evolution under visible light with good stability compared to that of pristine g-C3N4 bulk, with sulfur substitute carbon in g-C3N4 [10]. Similarly, to achieve high photocatayltic performance, environmentally friendly and scalable methods to synthesize novel porous P-doped g-C3N4 nanosheets (PCN-S) by a combination of P doping and its precursors and thermal exfoliation have been reported [11]. Sulfur could be introduced as a dopant into the lattice of carbon nitride within a sulfur-doped graphitic carbon nitride (CNS) nanosheet [12]. The C3N4/S-C3N4 heterojunction obtained by growing CN on preformed sulfur-doped CN nanosheet has shown enhanced photoelectrochemical performance compared with various control counterparts including CN, CNS and physically mixed CN and CNS (CN + CNS) [13]. Besides these nonmetallic dopants, a few metal-doped C3N4 materials have also been successfully synthesized. Fe-doped C3N4 has the capability of oxidizing benzene to phenol using hydrogen peroxide without the use of strong acids or alkaline substances [14]. Co-doped C3N4 (Co-g-C3N4) materials on graphene substrates was synthesized to form a novel nitrogen-metal macrocyclic catalyst for the oxygen reduction reaction (ORR) in alkaline fuel cells [15]. The improvement in ORR activity was attributed to the abundant Co-Nx active-sites and fast charge transfer at the interfaces of Co-g-C3N4. Palladium-doped g-C3N4 (Pd-g-C3N4) was fabricated as a catalyst for the photodegradation of bisphenol A. Meanwhile, Pd-g-C3N4 composites showed good stability across a wide range of pH 3.08–11.00 and resistance to photocorrosion after reuse several times [16]. A Ni and NiO co-doped g-C3N4 electrochemical sensor was developed for sensing octylphenol, based on the sensing mechanism of its catalytic oxidation ability to octylphenol [17]. A novel electrocatalyst based on Cu-doped g-C3N4 with supermolecular structure was prepared, that possessed excellent anti-acid ability [18]. Mg-doped g-C3N4 composites prepared by the thermal polymerization method exhibited high benzaldehyde conversion of 97.4% at 70 °C and good cycling stability [19]. Theoretical studies are normally used to demonstrate the electrocatalytic properties of doped C3N4 [20], but no systematic theoretical study of doping C3N4 has been reported. The selective detection of certain gas species is one of the most critical issues in different fields like security, health care, and industrial process control. 2D layered nanomaterials such as graphene, MoS2, and silicane have been considered promising gas sensors because of their tunable electronic structures [21–23]. For g-C3N4, studies related to sensing are very limited. Peyghan et al. reported an ion sensor based on C3N4 nanotubes for detecting alkali and alkaline earth cations [24]. Although several literature reviews have focused on the catalytic activities of doped g-C3N4, limited data exists on the sensing capabilities of C3N4 or doped C3N4 materials, especially selectivity of the gases. From that perspective, we aim to systematically investigate the doping behaviors of g-C3N4 materials with different metal elements and the sensing behaviors of metal-doped g-C3N4 materials on gases. In detail, DFT calculations using the Perdew-Burke-Ernzerhof (PBE) and generalized gradient approximation (GGA) methods, including long-range dispersion corrections, [25] were conducted to predict the structural and promising applications of metal-doped g-C3N4 nanocomposites. The interactions between various metal-doped g-C3N4 nanocomposites and gas molecules (CO, CO2, N2, NH3, NO2) were carefully studied. We demonstrate, for the first time, that metallic doping could tune the interactions between gas molecules and g-C3N4 nanocomposites.
2.2. Analysis methods The adsorption energy (), indicatingtheintensityofinteractionbetweenmetallicdopantsand ), indicating the intensity of interaction between metallic dopants and g-C3N4 surface and also the intensity of interactions between gas molecules and doped g-C3N4 surfaces was derived according to Eqs. (1) and (2):
Eads = Eg − C3N4 + metal atom −
(Eg−C N
3 4
+ Emetal atom
)
(1)
where , Eg − C3 N4 , and Emetal atom represent the total energy of the metallic doping system, the energy of the g-C3N4 surface, and the energy of the metallic atom, respectively.
Eads = Edoped g − C3N4 + gas −
(Edoped g−C N
3 4
+ Egas
)
(2)
where Edoped g − C3N4 + gas , Edoped g − C3N4 , and Egas represent the total energy of the adsorption system, the energy of the g-C3N4 surface, and the energy of the gas molecule, respectively. A negative correspondstostableadsorptionandmorenegativeresultscorrespondtoamore
stableadsorptionsystem. Theelectrondensitydifferencerevealsthechangei nelectrondensityduringtheinteractionprocessandiscalculatedbysubtractingthe electrondensityoftheisolatedgasmolecule ( corresponds to stable adsorption and more negative results correspond to a more stable adsorption system. The electron density difference reveals the change in electron density during the interaction process and is calculated by subtracting the electron density of the isolated gas molecule (ρgas ) and the gC3N4 surface (ρg − C3 N4 ) from the total electron density of the system (ρg − C3 N4 + gas ), as shown in Equation (3):
Δρ = ρg − C3 N4 +
gas −ρ gas −ρg − C3 N4
(3)
The DOS analysis was used to study the interactions between gas molecules and the g-C3N4 surface at the electronic level. The Hirshfeld population analysis was used to analyze the details of charge transfer during the gas adsorption process. 3. Results and discussion 3.1. Metal-doped g-C3N4 The g-C3N4 model used in this study was based on a well-characterized experimental structure [31]. The optimized lattice constant of g-C3N4 was 2.46 Å, which was very close to the reported theoretical and experimental value [32]. To further evaluate the g-C3N4 model, its band gap was calculated by the state-of-the-art hybrid functional (HSE06). The calculated band gap of g-C3N4 was 2.67 eV, very similar to the reported value of 2.73 eV. For the metal-doped g-C3N4 model, five different doping sites were investigated in this study to obtain the most realistic results, as shown in Fig. S1. The binding energy between various metallic dopants and g-C3N4 is listed in Table S1. As indicated by
2. Computational methods 2.1. Simulation parameters All the calculations in the study were performed using the DFT 1117
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Fig. 1. Isosurface of electron density difference of metal-doped g-C3N4 sheet with isovalue of 0.002. The charge accumulation and depletion are represented in blue and yellow, respectively. (a) Ag-doped, (b) Au-doped, (c) Co-doped, (d) Cr-doped, (e) Cu-doped, (f) K-doped, (g) Li-doped, (h) Na-doped, (i) Ni-doped, (g) Pd-doped, (k) Pt-doped, (l) Ti-doped. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
adsorption energy of around −0.8 eV indicating stability. Fig. S2 shows the isosurface of electron density difference between g-C3N4 and distinct gas molecules, indicating an apparent charge transfer between gC3N4 and distinct gas molecules. Among CO, CO2, N2, NH3, and NO2, during the adsorption process, only NH3 lost electrons, whereas CO, CO2, N2, and NO2 accepted electrons, suggesting that the latter four gas molecules could be stably adsorbed on pristine g-C3N4. To further explore the interactions between these gas molecules and g-C3N4, the DOS analysis was performed and is reported in Fig. S3. It was established that for the five different gas molecules, the total DOS of the g-C3N4/gas molecule systems were very similar. Together, the results demonstrated that although CO, CO2, N2, NH3, and NO2 could be chemisorbed onto the pristine g-C3N4, they evinced weak selectivity for the pristine gC3N4.
the adsorption energy results, the first doping site was the most stable of the five. The metallic dopants (Ag, Au, Co, Cr, Cu, K, Li, Na, Pt, Pd, Ni, V, Mn, Fe, Ti) formed chemical bonding with g-C3N4. Among these metallic dopants, Pt, Pd, Au, and Ag had relatively low binding energy with g-C3N4. Fig. 1 shows the isosurface of electron density difference of metal-doped g-C3N4 sheets, indicating that the charge was transferred from metallic dopants and around carbon or nitrogen atoms of g-C3N4. The charge depletion occurred on the metallic dopants. Thus, both energy and electron density difference analysis demonstrated stable chemical interactions between various metallic dopants and g-C3N4. Due to the strongest binding ability to metals, doping site 1 was chosen to carry out the subsequent gas adsorption studies. 3.2. Gas adsorption on metal-doped g-C3N4 After the DFT calculations, the adsorption energy between five different gas molecules and 15 diverse metal-doped g-C3N4 including the pristine g-C3N4, electron density difference and DOS analyses were performed on each separate system, investigating the effect of metallic dopants on the interactions between gas molecules and g-C3N4.
3.2.2. Metal-doped g-C3N4/gas To improve the selectivity of gas molecules adsorption on g-C3N4, we designed a metal-doped g-C3N4 gas sensor based on the DFT calculation and others’ experimental work [14-17]. Through systematic study of the adsorption behavior of CO, CO2, N2, NH3, and NO2 molecules on 15 distinct metallic elements doped g-C3N4, it was found that Ag-, Li-, Na-, and K-doped g-C3N4 exhibited significant selectivity for NO2. Fig. S4 shows the top and side views of the optimized configurations for gas/Ag-doped g-C3N4 interaction systems. The other 14 metal-doped systems had similar configurations and are not shown here. Table 1 shows the adsorption energy values of different gas molecules and different metal-doped g-C3N4. It can be seen that among the
3.2.1. Pristine g-C3N4/gas To explore the effect of the metallic dopants on the interactions between g-C3N4 and distinct gas molecules, the adsorption behaviors of CO, CO2, N2, NH3, and NO2 on pristine g-C3N4 were systematically studied. According to the adsorption energy analysis, all these gas molecules could be adsorbed on the pristine g-C3N4, with the 1118
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Table 1 Adsorption energy between gas molecules (CO, CO2, N2, NH3, NO2) and various doped C3N4 (Unit: eV).
CO CO2 N2 NH3 NO2
Ag
Au
Co
Cr
Cu
Fe
K
Li
Mn
Na
Ni
Pd
Pt
Ti
V
Pure
−0.28 −0.24 −0.39 −0.30 −1.63
−1.56 −0.43 −0.44 −0.77 −2.13
−3.19 −2.10 −2.40 −2.04 −3.81
−1.70 −0.48 −1.07 −0.39 −2.45
−1.62 −0.34 −0.92 −0.29 −2.29
−1.65 −0.37 −0.96 −1.39 −2.62
−0.36 −0.16 −0.36 −0.66 −1.72
−0.64 −0.43 −0.56 −0.30 −1.79
−1.65 −0.40 −0.95 −0.29 −2.31
−0.57 −0.47 −0.53 −0.39 −1.84
−2.14 −0.55 −1.32 −0.57 −2.66
−1.46 −0.47 −0.52 −0.47 −1.86
−2.14 −0.51 −0.52 −0.46 −1.81
−1.58 −0.60 −1.12 −0.40 −2.90
−1.66 −0.53 −0.63 −0.38 −3.52
−0.75 −0.84 −0.79 −1.26 −1.02
Fig. 2. Isosurface of electron density difference of diverse gas molecules adsorbed onto Ag-doped C3N4 sheet. The charge accumulation and depletion are represented in yellow and teal, respectively. (a) Ag-C3N4/NO2, (b) Ag-C3N4/CO, (c) Ag-C3N4/NH3, (d) Ag-C3N4/CO2, (e) Ag-C3N4/N2. Isosurface value is 0.002.
energy. The NH3/Li-C3N4 system has the lowest adsorption energy (about −0.3 eV) among the five gas/Li-C3N4 systems. According to the result of the electron density difference (Fig. S5), Li-C3N4 exhibits no significant selectivity for the NO2 molecule; nevertheless, the adsorption energy between NO2 and Li-C3N4 is the highest among the five gas/ Li-C3N4 systems. DOS analysis was carried out to investigate the interactions between NO2 and Li-C3N4 (Fig. S6), revealing a positive shift in the total DOS and a new electron state around the Femi level for the NO2/Li-C3N4 system compared to the other four gas/Li-C3N4 systems. Fig. S7 shows the isosurface of electron density difference of the five different kinds of gas/Na-C3N4 interaction systems. The situations for the gas/Na-C3N4 systems are quite similar to those of the gas/Li-C3N4 systems. No apparent selectivity can be found for NO2, except in the result of the electron density difference. However, according to the results of the adsorption energy and DOS analysis, it is found that LiC3N4 also exhibits excellent selectivity on NO2. The adsorption energy of the NO2/Li-C3N4 system is the highest among the five gas/Li-C3N4 systems. The positive shift in DOS and a new electron density state were also found in the NO2/Na-C3N4 system (Fig. S8), which did not appear in the other four gas/Na-C3N4 systems. Fig. S9 shows the isosurface of electron density difference of the five different gas/K-C3N4 interaction systems. The charge transfers between gas molecules and K-C3N4 systems was found except for the CO2/K-C3N4 system. That phenomenon also corresponds well with the results of the adsorption energy. CO2 has
15 different metal-doped g-C3N4, the Ag-, K-, Na-, and Li-doped g-C3N4 are reasonably different from the other 11 metal-doped g-C3N4. The adsorption energies between NO2 and these particular four kinds of metal-doped g-C3N4 are the highest among the five gas molecules. With the other 11 kinds of metal-doped g-C3N4, no similar phenomenon can be observed. Fig. 2 shows the isosurface of electron density difference of the five different kinds of gas/Ag-C3N4 interaction systems. Among them, NO2 exhibits the most obvious charge transfer with the Ag-C3N4 sheet. According to Table 1, the adsorption energy between NO2 and AgC3N4 sheet is the highest. Thus, the results of the electron density difference align with the adsorption energy results. Due to the apparent differences in adsorption energy, it can be considered that the Ag-C3N4 sheet has excellent selectivity for NO2 but not for CO, CO2, N2 and NH3. As well as the above results, the DOS analysis was performed, and the results are shown in Fig. 3. It is interesting that for the NO2/Ag-C3N4 sheet system, the DOS curve is relatively different from that of the other four systems. The total DOS for the NO2/Ag-C3N4 sheet shifts towards the positive energy direction. A peak that refers to a new energy state appears in the range of −1 eV to 0 eV. For Li-C3N4, the charge transferring between itself and the gas molecules is quite different from that for the Ag-C3N4. There is an apparent charge transfer between gas molecules and the Li-C3N4 sheet, except for the NH3/Li-C3N4 system. This phenomenon corresponds well with the results for adsorption 1119
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Fig. 3. Total density of states (DOS) of gas molecules/Ag-doped C3N4, Ag-doped C3N4, and gas molecules, respectively.
the lowest adsorption energy among the five gas/K-C3N4 systems. But the adsorption energy between NO2 and the K-C3N4 sheet is also different from that of the other four systems. The DOS results also exhibit a similar phenomenon, demonstrating the excellent selectivity of K-C3N4 sheet for NO2, as shown in Fig. S10. To further investigate the selectivity of the specific metal-doped C3N4 on NO2, comparisons of the adsorption energy ratio of NO2 vs NH3, NO2 vs N2, NO2 vs CO2, and NO2 vs CO on different types of doped
C3N4 were performed and are shown in Fig. 4. It was found that dopants on C3N4 could regulate the interactions between gas molecules and C3N4. The pristine C3N4 did not recognize the different gas molecules because of the similarity of the adsorption energy. From Fig. 4, it can be found that g-C3N4 has stronger interactions with NO2 than with other gas molecules. However, according to the adsorption energy ratio of NO2 vs NH3, NO2 vs N2, and NO2 vs CO2, scant selectivity of Ag-, K-, Li-, and Na-doped C3N4 on NO2 can be found, but the other dopants seem
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Fig. 4. Comparison of the adsorption energy of NO2 versus that of NH3, N2, CO2, and CO on different metal-doped C3N4, respectively.
have similar ability. As indicated by the adsorption energy ratio of NO2 vs CO, the selectivity of Ag-, K-, Li-, and Na-doped C3N4 on NO2 can be clearly found. Thus, according to the results presented here, it can be
concluded that among the 15 kinds of metal-doped C3N4 sheets, Ag-, Li-, Na-, and K-doped C3N4 sheets exhibited excellent selectivity for NO2 in comparison with CO, CO2, N2, and NH3. This phenomenon can be 1121
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illustrated by the physiochemical properties difference between NO2 and other four gas molecules, among NO2, N2, NH3, CO2 and CO, only NO2 is a radical molecule.
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4. Conclusions The DFT method was used to study adsorption behaviors of CO, CO2, N2, NH3, and NO2 gas molecules on pristine and 15 kinds of metaldoped (Ag-, Li-, Na-, Co-, Cu-, Au-, Cr-, Ti-, V-, Fe-, Mn-, Ni-, Pt-, Pd-, K-) C3N4. The adsorption energy, charge transfer, and DOS analysis indicated that the pristine C3N4 exhibited good capture capacity for all five gas molecules. However, no apparent selectivity for any particular gas molecules was observed for pristine C3N4. Among the metal-doped C3N4 in this study, Ag-, Li-, Na-, and K-doped C3N4 exhibited the highest adsorption energy for NO2. The positive shift of DOS and the new electron states around the Femi level for NO2/Ag-, Li-, Na-, and K-doped C3N4 confirmed the excellent selectivity of Ag-, Li-, Na-, and K-doped C3N4 for NO2. These findings indicate that Ag-, Li-, Na-, and K-doped C3N4 are potential candidates for gas molecule sensors with unique and high sensitivity for NO2. Acknowledgements H. Zhang is grateful to the National Natural Science Foundation of China (NSFC; Grant No. 31300793) and Longshan Academic Talent Research Supporting Program of SWUST (Grant No. 17LZX411) for supporting this research. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.06.034. References [1] S. Siahrostami, C. Tsai, M. Karamad, R. Koitz, M. García-Melchor, M. Bajdich, A. Vojvodic, F. Abild-Pedersen, J.K. Nørskov, F. Studt, Two-dimensional materials as catalysts for energy conversion, Catal. Lett. 146 (10) (2016) 1917–1921. [2] A. Carvalho, M. Wang, X. Zhu, A.S. Rodin, H. Su, H. Antonio, C. Neto, Phosphorene: from theory to applications, Nat. Rev. Mater. 1 (2016) 16061–16077. [3] J. Zhang, Y. Chen, X. Wang, Two-dimensional covalent carbon nitride nanosheets: synthesis, functionalization, and applications, Energy Environ. Sci. 8 (11) (2015) 3092–3108. [4] W. Hu, N. Xia, X. Wu, Z. Li, J. Yang, Silicene as a highly sensitive molecule sensor for NH3, NO and NO2, PCCP 16 (15) (2014) 6957–6962. [5] S. Manzeli, D. Ovchinnikov, D. Pasquier, O.V. Yazyev, A. Kis, 2D transition metal dichalcogenides, Nat. Rev. Mater. 2 (2017) 17033–17048. [6] J. Liu, H. Wang, M. Antonietti, Graphitic carbon nitride “reloaded”: Emerging applications beyond (photo) catalysis, Chem. Soc. Rev. 45 (2016) 2308–2326. [7] M.Q. Wen, T. Xiong, Z.G. Zang, W. Wei, X.T. Tang, F. Dong, Synthesis of MoS2/gC3N4 nanocomposites with enhanced visible-light photocatalytic activity for the removal of nitric oxide (NO), Opt. Exp. 24 (2016) 10205–10212. [8] Z. Zhao, Y.J. Sun, F. Dong, Graphitic carbon nitride based nanocomposites: a review, Nanoscale 7 (1) (2015) 15–37.
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Full Length Article
Metal-doped graphitic carbon nitride (g-C3N4) as selective NO2 sensors: A first-principles study
T
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Hong-ping Zhanga, , Aijun Dub, Neha S. Gandhic, Yan Jiaod, Yaping Zhanga, Xiaoyan Lina, Xiong Lue, Youhong Tangf a
State Key Laboratory of Environmental Friendly Energy Materials, Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Sichuan 621010, China b School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Queensland 4001, Australia c School of Mathematical Sciences, Queensland University of Technology, Queensland 4000, Australia d School of Chemical Engineering, The University of Adelaide, South Australia 5005, Australia e Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, China f Institute for NanoScale Science and Technology, College of Science and Engineering, Flinders University, South Australia 5042, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 Metal-doped NO2 Sensitive Density functional theory
In this research, the potential application of metal-doped g-C3N4 as highly sensitive molecule sensors for NO2 detection was studied using density function theory (DFT) calculations. Various metal-doped (Ag-, Au-, Co-, Cr-, Cu-, Fe-, K-, Li-, Na-, Mn-, Pt-, Pd-, Ti-, V-) g-C3N4 sheets were considered. CO, CO2, NH3, N2 and NO2 molecules were found to adsorb on metal-doped g-C3N4 via strong chemical bonds. Chemisorbed gas molecules and metaldoped g-C3N4 formed charge transfer complexes with different charges transferring from metal-doped g-C3N4 to gas molecules. Pristine and metal-doped g-C3N4 sheets were demonstrated as potential capturers for certain gas molecules according to the adsorption energy, isosurface of electron density difference, and density of states analysis. Among the diverse metal-doped g-C3N4 sheets, Ag-, K-, Na-, and Li-doped g-C3N4 were found to be clearly sensitive to the NO2 molecule. The adsorption energies between NO2 and Ag-, K-, Na-, and Li-doped gC3N4 were significantly greater than those of the other gas molecules (CO, CO2, N2, and NH3). The density of states indicates that the NO2 adsorption on Ag-, K-, Na-, and Li-doped g-C3N4 induced the shift of the total density of state in the positive energy direction. Charge transfer results also demonstrate that chemical interactions existed between NO2 and Ag-, K-, Na-, and Li-doped g-C3N4. All these results suggest the strong potential of Ag-, K-, Na-, and Li-doped g-C3N4 for application as highly sensitive molecule sensors.
1. Introduction Two-dimensional (2D) layer-structured nanomaterials have attracted much research attention because of their ultrahigh specific surface area, adjustable band structure, and excellent mechanical and thermal properties [1]. Due to their quantum size effects, these materials, including graphene-like materials (phosphorene, graphitic carbon nitrides, silicene, etc.) and 2D transition-metal dichalcogenides have shown promise in nanoelectronics, catalysis, energy storage, gas sensors, and other fields in materials science [2–5]. Among the various graphene-like 2D nanomaterials, carbon nitride (C3N4) has spurred interest because of its potential applications as photocatalyst and optoelectronic conversion devices [6]. Graphitic carbon nitride (g-C3N4) has a stacked 2D structure without metals and is the most stable allotrope of
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Corresponding author. E-mail address: [email protected] (H.-p. Zhang).
https://doi.org/10.1016/j.apsusc.2018.06.034 Received 26 February 2018; Received in revised form 24 May 2018; Accepted 6 June 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
C3N4. g-C3N4 possesses strong covalent CeN bonds rather than CeC bonds in each layer and the layers are bound by van der Waals forces only. Coupling g-C3N4 with other substances (graphene, semiconductors, metals, etc) and/or introducing defects as found in any other 2D nanomaterial can give rise to the structuring of g-C3N4 into different morphologies such nanosheets, mesoporous structures, nanorods, hierarchical structures, and films. These structures and morphologies are strongly related to the physical, electronic, and chemical properties and applications of g-C3N4. C3N4 exhibits extensive advantages in hydrogen evolution, pollution degradation and photocatalytics, its low utilization of visible light and other weakness in photocatalytic activities attract the research attentions. Combining gC3N4 with other materials was demonstrated to be an effective way to improve the photocatalytic activity of g-C3N4 [7].
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program Dmol3 in Materials Studio (Accelrys, San Diego, CA), in which the physical wave functions were expanded in terms of numerical basis sets. The double numerical basis set with polarisation function (DNP) [26,27], that is comparable to the 6-31G** basis set, was utilised during the calculations [28]. The core electrons were treated with DFT semicore pseudopotentials. The exchange-correlation energy was calculated using the PBE and GGA methods [29]. Special point sampling integration over the Brillouin zone was employed using Monkhorst-Pack schemes with a 5 × 5 × 1 k-point mesh [30]. A Fermi smearing of 0.005 Ha and a global orbital cutoff of 7 Å were employed. The convergence criteria for the geometric optimization and energy calculation were set as follows: (1) self-consistent field tolerance of 1.0 × 10−6 Ha/ atom, (2) energy tolerance of 1.0 × 10−5 Ha/atom, (3) maximum force tolerance of 0.002 Ha/Å, and (4) maximum displacement tolerance of 0.005 Å. A vacuum slab with 30 Å was added onto the g-C3N4 or metal doped g-C3N4 surface to avoid the interaction influence of the periodic boundary conditions.
Various doped C3N4 combinations have been reportedly fabricated successfully in recent years to enhance the performance of g-C3N4 in various fields [3,8,9], among which the element doping strategy plays an essential role. Sulfur-doped C3N4 nanosheets (S-g-C3N4) has shown promising photocatalytic activity for H2 evolution under visible light with good stability compared to that of pristine g-C3N4 bulk, with sulfur substitute carbon in g-C3N4 [10]. Similarly, to achieve high photocatayltic performance, environmentally friendly and scalable methods to synthesize novel porous P-doped g-C3N4 nanosheets (PCN-S) by a combination of P doping and its precursors and thermal exfoliation have been reported [11]. Sulfur could be introduced as a dopant into the lattice of carbon nitride within a sulfur-doped graphitic carbon nitride (CNS) nanosheet [12]. The C3N4/S-C3N4 heterojunction obtained by growing CN on preformed sulfur-doped CN nanosheet has shown enhanced photoelectrochemical performance compared with various control counterparts including CN, CNS and physically mixed CN and CNS (CN + CNS) [13]. Besides these nonmetallic dopants, a few metal-doped C3N4 materials have also been successfully synthesized. Fe-doped C3N4 has the capability of oxidizing benzene to phenol using hydrogen peroxide without the use of strong acids or alkaline substances [14]. Co-doped C3N4 (Co-g-C3N4) materials on graphene substrates was synthesized to form a novel nitrogen-metal macrocyclic catalyst for the oxygen reduction reaction (ORR) in alkaline fuel cells [15]. The improvement in ORR activity was attributed to the abundant Co-Nx active-sites and fast charge transfer at the interfaces of Co-g-C3N4. Palladium-doped g-C3N4 (Pd-g-C3N4) was fabricated as a catalyst for the photodegradation of bisphenol A. Meanwhile, Pd-g-C3N4 composites showed good stability across a wide range of pH 3.08–11.00 and resistance to photocorrosion after reuse several times [16]. A Ni and NiO co-doped g-C3N4 electrochemical sensor was developed for sensing octylphenol, based on the sensing mechanism of its catalytic oxidation ability to octylphenol [17]. A novel electrocatalyst based on Cu-doped g-C3N4 with supermolecular structure was prepared, that possessed excellent anti-acid ability [18]. Mg-doped g-C3N4 composites prepared by the thermal polymerization method exhibited high benzaldehyde conversion of 97.4% at 70 °C and good cycling stability [19]. Theoretical studies are normally used to demonstrate the electrocatalytic properties of doped C3N4 [20], but no systematic theoretical study of doping C3N4 has been reported. The selective detection of certain gas species is one of the most critical issues in different fields like security, health care, and industrial process control. 2D layered nanomaterials such as graphene, MoS2, and silicane have been considered promising gas sensors because of their tunable electronic structures [21–23]. For g-C3N4, studies related to sensing are very limited. Peyghan et al. reported an ion sensor based on C3N4 nanotubes for detecting alkali and alkaline earth cations [24]. Although several literature reviews have focused on the catalytic activities of doped g-C3N4, limited data exists on the sensing capabilities of C3N4 or doped C3N4 materials, especially selectivity of the gases. From that perspective, we aim to systematically investigate the doping behaviors of g-C3N4 materials with different metal elements and the sensing behaviors of metal-doped g-C3N4 materials on gases. In detail, DFT calculations using the Perdew-Burke-Ernzerhof (PBE) and generalized gradient approximation (GGA) methods, including long-range dispersion corrections, [25] were conducted to predict the structural and promising applications of metal-doped g-C3N4 nanocomposites. The interactions between various metal-doped g-C3N4 nanocomposites and gas molecules (CO, CO2, N2, NH3, NO2) were carefully studied. We demonstrate, for the first time, that metallic doping could tune the interactions between gas molecules and g-C3N4 nanocomposites.
2.2. Analysis methods The adsorption energy (), indicatingtheintensityofinteractionbetweenmetallicdopantsand ), indicating the intensity of interaction between metallic dopants and g-C3N4 surface and also the intensity of interactions between gas molecules and doped g-C3N4 surfaces was derived according to Eqs. (1) and (2):
Eads = Eg − C3N4 + metal atom −
(Eg−C N
3 4
+ Emetal atom
)
(1)
where , Eg − C3 N4 , and Emetal atom represent the total energy of the metallic doping system, the energy of the g-C3N4 surface, and the energy of the metallic atom, respectively.
Eads = Edoped g − C3N4 + gas −
(Edoped g−C N
3 4
+ Egas
)
(2)
where Edoped g − C3N4 + gas , Edoped g − C3N4 , and Egas represent the total energy of the adsorption system, the energy of the g-C3N4 surface, and the energy of the gas molecule, respectively. A negative correspondstostableadsorptionandmorenegativeresultscorrespondtoamore
stableadsorptionsystem. Theelectrondensitydifferencerevealsthechangei nelectrondensityduringtheinteractionprocessandiscalculatedbysubtractingthe electrondensityoftheisolatedgasmolecule ( corresponds to stable adsorption and more negative results correspond to a more stable adsorption system. The electron density difference reveals the change in electron density during the interaction process and is calculated by subtracting the electron density of the isolated gas molecule (ρgas ) and the gC3N4 surface (ρg − C3 N4 ) from the total electron density of the system (ρg − C3 N4 + gas ), as shown in Equation (3):
Δρ = ρg − C3 N4 +
gas −ρ gas −ρg − C3 N4
(3)
The DOS analysis was used to study the interactions between gas molecules and the g-C3N4 surface at the electronic level. The Hirshfeld population analysis was used to analyze the details of charge transfer during the gas adsorption process. 3. Results and discussion 3.1. Metal-doped g-C3N4 The g-C3N4 model used in this study was based on a well-characterized experimental structure [31]. The optimized lattice constant of g-C3N4 was 2.46 Å, which was very close to the reported theoretical and experimental value [32]. To further evaluate the g-C3N4 model, its band gap was calculated by the state-of-the-art hybrid functional (HSE06). The calculated band gap of g-C3N4 was 2.67 eV, very similar to the reported value of 2.73 eV. For the metal-doped g-C3N4 model, five different doping sites were investigated in this study to obtain the most realistic results, as shown in Fig. S1. The binding energy between various metallic dopants and g-C3N4 is listed in Table S1. As indicated by
2. Computational methods 2.1. Simulation parameters All the calculations in the study were performed using the DFT 1117
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Fig. 1. Isosurface of electron density difference of metal-doped g-C3N4 sheet with isovalue of 0.002. The charge accumulation and depletion are represented in blue and yellow, respectively. (a) Ag-doped, (b) Au-doped, (c) Co-doped, (d) Cr-doped, (e) Cu-doped, (f) K-doped, (g) Li-doped, (h) Na-doped, (i) Ni-doped, (g) Pd-doped, (k) Pt-doped, (l) Ti-doped. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
adsorption energy of around −0.8 eV indicating stability. Fig. S2 shows the isosurface of electron density difference between g-C3N4 and distinct gas molecules, indicating an apparent charge transfer between gC3N4 and distinct gas molecules. Among CO, CO2, N2, NH3, and NO2, during the adsorption process, only NH3 lost electrons, whereas CO, CO2, N2, and NO2 accepted electrons, suggesting that the latter four gas molecules could be stably adsorbed on pristine g-C3N4. To further explore the interactions between these gas molecules and g-C3N4, the DOS analysis was performed and is reported in Fig. S3. It was established that for the five different gas molecules, the total DOS of the g-C3N4/gas molecule systems were very similar. Together, the results demonstrated that although CO, CO2, N2, NH3, and NO2 could be chemisorbed onto the pristine g-C3N4, they evinced weak selectivity for the pristine gC3N4.
the adsorption energy results, the first doping site was the most stable of the five. The metallic dopants (Ag, Au, Co, Cr, Cu, K, Li, Na, Pt, Pd, Ni, V, Mn, Fe, Ti) formed chemical bonding with g-C3N4. Among these metallic dopants, Pt, Pd, Au, and Ag had relatively low binding energy with g-C3N4. Fig. 1 shows the isosurface of electron density difference of metal-doped g-C3N4 sheets, indicating that the charge was transferred from metallic dopants and around carbon or nitrogen atoms of g-C3N4. The charge depletion occurred on the metallic dopants. Thus, both energy and electron density difference analysis demonstrated stable chemical interactions between various metallic dopants and g-C3N4. Due to the strongest binding ability to metals, doping site 1 was chosen to carry out the subsequent gas adsorption studies. 3.2. Gas adsorption on metal-doped g-C3N4 After the DFT calculations, the adsorption energy between five different gas molecules and 15 diverse metal-doped g-C3N4 including the pristine g-C3N4, electron density difference and DOS analyses were performed on each separate system, investigating the effect of metallic dopants on the interactions between gas molecules and g-C3N4.
3.2.2. Metal-doped g-C3N4/gas To improve the selectivity of gas molecules adsorption on g-C3N4, we designed a metal-doped g-C3N4 gas sensor based on the DFT calculation and others’ experimental work [14-17]. Through systematic study of the adsorption behavior of CO, CO2, N2, NH3, and NO2 molecules on 15 distinct metallic elements doped g-C3N4, it was found that Ag-, Li-, Na-, and K-doped g-C3N4 exhibited significant selectivity for NO2. Fig. S4 shows the top and side views of the optimized configurations for gas/Ag-doped g-C3N4 interaction systems. The other 14 metal-doped systems had similar configurations and are not shown here. Table 1 shows the adsorption energy values of different gas molecules and different metal-doped g-C3N4. It can be seen that among the
3.2.1. Pristine g-C3N4/gas To explore the effect of the metallic dopants on the interactions between g-C3N4 and distinct gas molecules, the adsorption behaviors of CO, CO2, N2, NH3, and NO2 on pristine g-C3N4 were systematically studied. According to the adsorption energy analysis, all these gas molecules could be adsorbed on the pristine g-C3N4, with the 1118
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Table 1 Adsorption energy between gas molecules (CO, CO2, N2, NH3, NO2) and various doped C3N4 (Unit: eV).
CO CO2 N2 NH3 NO2
Ag
Au
Co
Cr
Cu
Fe
K
Li
Mn
Na
Ni
Pd
Pt
Ti
V
Pure
−0.28 −0.24 −0.39 −0.30 −1.63
−1.56 −0.43 −0.44 −0.77 −2.13
−3.19 −2.10 −2.40 −2.04 −3.81
−1.70 −0.48 −1.07 −0.39 −2.45
−1.62 −0.34 −0.92 −0.29 −2.29
−1.65 −0.37 −0.96 −1.39 −2.62
−0.36 −0.16 −0.36 −0.66 −1.72
−0.64 −0.43 −0.56 −0.30 −1.79
−1.65 −0.40 −0.95 −0.29 −2.31
−0.57 −0.47 −0.53 −0.39 −1.84
−2.14 −0.55 −1.32 −0.57 −2.66
−1.46 −0.47 −0.52 −0.47 −1.86
−2.14 −0.51 −0.52 −0.46 −1.81
−1.58 −0.60 −1.12 −0.40 −2.90
−1.66 −0.53 −0.63 −0.38 −3.52
−0.75 −0.84 −0.79 −1.26 −1.02
Fig. 2. Isosurface of electron density difference of diverse gas molecules adsorbed onto Ag-doped C3N4 sheet. The charge accumulation and depletion are represented in yellow and teal, respectively. (a) Ag-C3N4/NO2, (b) Ag-C3N4/CO, (c) Ag-C3N4/NH3, (d) Ag-C3N4/CO2, (e) Ag-C3N4/N2. Isosurface value is 0.002.
energy. The NH3/Li-C3N4 system has the lowest adsorption energy (about −0.3 eV) among the five gas/Li-C3N4 systems. According to the result of the electron density difference (Fig. S5), Li-C3N4 exhibits no significant selectivity for the NO2 molecule; nevertheless, the adsorption energy between NO2 and Li-C3N4 is the highest among the five gas/ Li-C3N4 systems. DOS analysis was carried out to investigate the interactions between NO2 and Li-C3N4 (Fig. S6), revealing a positive shift in the total DOS and a new electron state around the Femi level for the NO2/Li-C3N4 system compared to the other four gas/Li-C3N4 systems. Fig. S7 shows the isosurface of electron density difference of the five different kinds of gas/Na-C3N4 interaction systems. The situations for the gas/Na-C3N4 systems are quite similar to those of the gas/Li-C3N4 systems. No apparent selectivity can be found for NO2, except in the result of the electron density difference. However, according to the results of the adsorption energy and DOS analysis, it is found that LiC3N4 also exhibits excellent selectivity on NO2. The adsorption energy of the NO2/Li-C3N4 system is the highest among the five gas/Li-C3N4 systems. The positive shift in DOS and a new electron density state were also found in the NO2/Na-C3N4 system (Fig. S8), which did not appear in the other four gas/Na-C3N4 systems. Fig. S9 shows the isosurface of electron density difference of the five different gas/K-C3N4 interaction systems. The charge transfers between gas molecules and K-C3N4 systems was found except for the CO2/K-C3N4 system. That phenomenon also corresponds well with the results of the adsorption energy. CO2 has
15 different metal-doped g-C3N4, the Ag-, K-, Na-, and Li-doped g-C3N4 are reasonably different from the other 11 metal-doped g-C3N4. The adsorption energies between NO2 and these particular four kinds of metal-doped g-C3N4 are the highest among the five gas molecules. With the other 11 kinds of metal-doped g-C3N4, no similar phenomenon can be observed. Fig. 2 shows the isosurface of electron density difference of the five different kinds of gas/Ag-C3N4 interaction systems. Among them, NO2 exhibits the most obvious charge transfer with the Ag-C3N4 sheet. According to Table 1, the adsorption energy between NO2 and AgC3N4 sheet is the highest. Thus, the results of the electron density difference align with the adsorption energy results. Due to the apparent differences in adsorption energy, it can be considered that the Ag-C3N4 sheet has excellent selectivity for NO2 but not for CO, CO2, N2 and NH3. As well as the above results, the DOS analysis was performed, and the results are shown in Fig. 3. It is interesting that for the NO2/Ag-C3N4 sheet system, the DOS curve is relatively different from that of the other four systems. The total DOS for the NO2/Ag-C3N4 sheet shifts towards the positive energy direction. A peak that refers to a new energy state appears in the range of −1 eV to 0 eV. For Li-C3N4, the charge transferring between itself and the gas molecules is quite different from that for the Ag-C3N4. There is an apparent charge transfer between gas molecules and the Li-C3N4 sheet, except for the NH3/Li-C3N4 system. This phenomenon corresponds well with the results for adsorption 1119
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Fig. 3. Total density of states (DOS) of gas molecules/Ag-doped C3N4, Ag-doped C3N4, and gas molecules, respectively.
the lowest adsorption energy among the five gas/K-C3N4 systems. But the adsorption energy between NO2 and the K-C3N4 sheet is also different from that of the other four systems. The DOS results also exhibit a similar phenomenon, demonstrating the excellent selectivity of K-C3N4 sheet for NO2, as shown in Fig. S10. To further investigate the selectivity of the specific metal-doped C3N4 on NO2, comparisons of the adsorption energy ratio of NO2 vs NH3, NO2 vs N2, NO2 vs CO2, and NO2 vs CO on different types of doped
C3N4 were performed and are shown in Fig. 4. It was found that dopants on C3N4 could regulate the interactions between gas molecules and C3N4. The pristine C3N4 did not recognize the different gas molecules because of the similarity of the adsorption energy. From Fig. 4, it can be found that g-C3N4 has stronger interactions with NO2 than with other gas molecules. However, according to the adsorption energy ratio of NO2 vs NH3, NO2 vs N2, and NO2 vs CO2, scant selectivity of Ag-, K-, Li-, and Na-doped C3N4 on NO2 can be found, but the other dopants seem
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Fig. 4. Comparison of the adsorption energy of NO2 versus that of NH3, N2, CO2, and CO on different metal-doped C3N4, respectively.
have similar ability. As indicated by the adsorption energy ratio of NO2 vs CO, the selectivity of Ag-, K-, Li-, and Na-doped C3N4 on NO2 can be clearly found. Thus, according to the results presented here, it can be
concluded that among the 15 kinds of metal-doped C3N4 sheets, Ag-, Li-, Na-, and K-doped C3N4 sheets exhibited excellent selectivity for NO2 in comparison with CO, CO2, N2, and NH3. This phenomenon can be 1121
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illustrated by the physiochemical properties difference between NO2 and other four gas molecules, among NO2, N2, NH3, CO2 and CO, only NO2 is a radical molecule.
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4. Conclusions The DFT method was used to study adsorption behaviors of CO, CO2, N2, NH3, and NO2 gas molecules on pristine and 15 kinds of metaldoped (Ag-, Li-, Na-, Co-, Cu-, Au-, Cr-, Ti-, V-, Fe-, Mn-, Ni-, Pt-, Pd-, K-) C3N4. The adsorption energy, charge transfer, and DOS analysis indicated that the pristine C3N4 exhibited good capture capacity for all five gas molecules. However, no apparent selectivity for any particular gas molecules was observed for pristine C3N4. Among the metal-doped C3N4 in this study, Ag-, Li-, Na-, and K-doped C3N4 exhibited the highest adsorption energy for NO2. The positive shift of DOS and the new electron states around the Femi level for NO2/Ag-, Li-, Na-, and K-doped C3N4 confirmed the excellent selectivity of Ag-, Li-, Na-, and K-doped C3N4 for NO2. These findings indicate that Ag-, Li-, Na-, and K-doped C3N4 are potential candidates for gas molecule sensors with unique and high sensitivity for NO2. Acknowledgements H. Zhang is grateful to the National Natural Science Foundation of China (NSFC; Grant No. 31300793) and Longshan Academic Talent Research Supporting Program of SWUST (Grant No. 17LZX411) for supporting this research. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.06.034. References [1] S. Siahrostami, C. Tsai, M. Karamad, R. Koitz, M. García-Melchor, M. Bajdich, A. Vojvodic, F. Abild-Pedersen, J.K. Nørskov, F. Studt, Two-dimensional materials as catalysts for energy conversion, Catal. Lett. 146 (10) (2016) 1917–1921. [2] A. Carvalho, M. Wang, X. Zhu, A.S. Rodin, H. Su, H. Antonio, C. Neto, Phosphorene: from theory to applications, Nat. Rev. Mater. 1 (2016) 16061–16077. [3] J. Zhang, Y. Chen, X. Wang, Two-dimensional covalent carbon nitride nanosheets: synthesis, functionalization, and applications, Energy Environ. Sci. 8 (11) (2015) 3092–3108. [4] W. Hu, N. Xia, X. Wu, Z. Li, J. Yang, Silicene as a highly sensitive molecule sensor for NH3, NO and NO2, PCCP 16 (15) (2014) 6957–6962. [5] S. Manzeli, D. Ovchinnikov, D. Pasquier, O.V. Yazyev, A. Kis, 2D transition metal dichalcogenides, Nat. Rev. Mater. 2 (2017) 17033–17048. [6] J. Liu, H. Wang, M. Antonietti, Graphitic carbon nitride “reloaded”: Emerging applications beyond (photo) catalysis, Chem. Soc. Rev. 45 (2016) 2308–2326. [7] M.Q. Wen, T. Xiong, Z.G. Zang, W. Wei, X.T. Tang, F. Dong, Synthesis of MoS2/gC3N4 nanocomposites with enhanced visible-light photocatalytic activity for the removal of nitric oxide (NO), Opt. Exp. 24 (2016) 10205–10212. [8] Z. Zhao, Y.J. Sun, F. Dong, Graphitic carbon nitride based nanocomposites: a review, Nanoscale 7 (1) (2015) 15–37.
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