Tuning the Nonlinear Optical Response of Graphitic Carbon Nitride by

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Tuning the Nonlinear Optical Response of Graphitic Carbon Nitride by Doping Li Atoms Juan Juan Tan, and Feng Long Gu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08242 • Publication Date (Web): 02 Nov 2018 Downloaded from http://pubs.acs.org on November 5, 2018

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The Journal of Physical Chemistry

Tuning the Nonlinear Optical Response of Graphitic Carbon Nitride by Doping Li Atoms Juan Juan Tan,† and Feng Long Gu†* †Key

Laboratory of Theoretical Chemistry of Environment, Ministry of Education;

South China Normal University, Guangzhou 510006.

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ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS: nonlinear optics, first hyperpolarizability, changer transfer, carbon nitride.

ABSTRACT: In this work, seven complexes based on graphitic carbon nitride (g-C3N4) were systematically investigated. Density functional theory (DFT) calculations demonstrated that the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the complexes conspicuously narrowed by doping Li atom. The value of HOMO-LUMO energy gap is decreased from 3.89 eV for C18N27H8 to 0.78 eV for C18N27H8Li, similar regularity is also found in other non-doped and doped Li atom g-C3N4 complexes. It has been shown that the Li doping can remarkably enhance the first hyperpolarizability (β0) of g-C3N4. The Li doped g-C3N4 complex C18N27H8Li exhibits its large β0 up to 1.66×104 a.u., which is much larger than that of the non-doped complex C18N27H8 (1.03×102 a.u.). In this study, the calculated NLO properties show that the β0 values of non-doped and doped Li atom g-C3N4 complexes are in the range of 0 ~ 2.36×103 a.u. and 1.66×104 ~ 1.25×106 a.u., respectively. This theoretical study implies that Li doped g-C3N4 is a novel candidate for high performance NLO materials. The theoretical evidence for the possibility of using g-C3N4 to construct materials with excellent NLO properties has been provided.

1. Introduction Carbon based nanomaterials, such as fullerenes, carbon nanotubes, and graphene, have been widely used to design the opticelectric materials because of their delocalized π-electron.1-5 Carbon nitride complexes as a member of carbon based nanomaterials have a pretty long history. The first example in the carbon nitride family reported by Liebig could be traced back to 1834.6 Up to now, there are various -2-


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methods can be adopted to fabricate g-C3N4, such as sol-gel, chemical vapor deposition, shock-wave compression, solvothermal method, polymer precursor’s condensation, and pyrolysis.6 -10 g-C3N4 has attracted intensive attention in emission devices,11 energy conversion,12 solar cells13 and humidity and gas sensors,14 hydrogen15 and carbon dioxide storage,16 bioimaging,17 and so forth18,19, because of their highly thermal stability, unique electronic structure, and low-cost preparation.

Recent years have witnessed the development of designing and synthesising nonlinear optical (NLO) materials for their extensive potential applications.20-25 Up to now, many researches have been focused on investigating the NLO response of materials, especially on carbon nanostructures which were composed of sp2 hybridized.26,27 Carbon nanostructures as potential candidates with high performance NLO materials, because they composed of abundant delocalized π-electrons. In the last few decades, many efforts have been devoted to design high performance NLO materials.28-31 For example, the effects of the nature of the π-conjugated systems have been monitored by using series of polyphenylenes, polyenes, polyynes, their diphenyl-substituted analogues and molecules with heterocyclic π-systems.32,33, 24 Similar to the π-conjugated systems, carbon nanosystems with their extensive π-conjugation in sp2 hybridized carbon atoms emerged as potential candidates for NLO applications. The graphene with abundant π-electrons can obtian structures with excellent NLO properties by a rational chemistry modification. 3,5,34For example, it has been successfully proved that the donor–graphene nanoribbon–acceptor (D– GNR–A) framework has large NLO response, and NH2–GNR–NO2 exhibits quite -3-



large static first hyperpolarizabilities (β0) up to 2.5×106 a.u.. 3 Besides, it was reported that different shaped graphene quantum dots can be used as a conjugated bridge into the D–π–A framework.

Recent studies demonstrated that doping alkali metal Li atom into a proper complexants is an effective way to enhance static β0.35,20,21, 23, 34For example, the Li doped organic molecules Li+(calix[4]pyrrole)M−(M = alkaliatoms) with large β0 values (up to 35934 a.u. when M = K) have been reported. 21 And the Li doped boron nitride nanocone molecules exhibit large static β0 up to 5.05×103 a.u., much larger than that of the non-doped boron nitride nanocone (1.07×102 a.u.).36 It is noted that g-C3N4 contains “nitrogen pots” with abundant melon moieties. “Nitrogen pots” are considered to be ideal sites for the modification of molecular electronic structures. We can take advantage of it to modify the properties of g-C3N4. The heteroatomic doping with substituent atoms (non-noble metallic: Fe, Co, Mn,37-39 et al. and nonmetallic: S, P, B, O, halogen40-48) has been researched in recent years. It is worth mentioning that some alkali-modified g-C3N4 have been successfully synthesized by the experiment.49-52 For example, Hu and co-workers reported that the conduction band (CB) and valence band (VB) potentials are altered after K doping into g-C3N4, the band energy is decreased after K doped, which indicates that the potassium has a strong influence on the optical properties. 51

Graphitic carbon nitride (g-C3N4) bears strong covalent C-N bonds instead of C-C bonds in graphene and has high thermal stability as well as fascinating electronic

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properties. Paying further attention to the graphite-like sp2 bonded C-N structure of g-C3N4 might open up new perspectives to develop their further applications in the field of nanotechnology. Since graphene with a typical sp2 structure is a good candidate of NLO material, an idea of investigating the NLO properties for g-C3N4 is highly intriguing, because the structure of graphene and g-C3N4 is very similar. It has been reported that the nonlinearity of g-C3N4 can be improved substantially by grafting lightly with nine-atom silver quantum clusters in lab.53 Ruan et al. revealed that Li doping can evidently extend the optical absorption of g-C3N4 sheet.54 However, the NLO properties of g-C3N4 and the effect of Li doped g-C3N4 complexes is less researched. In fact, g-C3N4, which is rich in pyridine-like nitrogen, can capture variety of transitional metals to form potential active sites, thus promote the charge transfer (CT).

Given the general facts that Li atom has the ability to trigger NLO property, it is of great interest to explore if Li atom doping in g-C3N4 is an efficient and viable way of achieving significant NLO response. To reach this goal, we designed seven complexes based on g-C3N4. Our investigation focused on the molecular structure, electronic structure, and NLO response. Significantly, as we expected, the value of β0 is great and quite different after doping Li atom. The results would be helpful for designing materials with high-performance NLO response and they can help us understand the nature of the interaction between the g-C3N4 and the Li atom.

2. Theoretical Methods

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The geometrical structures without imaginary frequency were optimized at the density functional theory (DFT) B3LYP/6-31G(d) level. At the same level, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) gaps were calculated, and the natural population analysis (NPA) charges were evaluated to quantitatively analyze CT, and the vertical ionization energies (VIE) was calculated as VIE  E  M    E  optM 

(1)

where E[opt M] and E[M+] are the energies of an optimized neutral molecule and its cation with the same optimized geometrical structure as the neutral molecule, respectively. In order to understand their stability, the interaction energies (Eint) were computed based on the optimized geometries. To eliminate the basis set superposition error (BSSE) in the interaction energy, Eint, the counterpoise procedure correction55,56 is used as shown in eq. (2),

Eint  E A  X AB   EB  X AB   E AB  X AB 

(2)

where the same basis set of XAB is used for both the subunit energy calculation (EA and EB) and for the compound energy (EAB) calculation. The isotropic average polarizability (α) was calculated as 1

α = 3(𝛼𝑥𝑥 + 𝛼𝑦𝑦 + 𝛼𝑧𝑧)

(3)

The static first hyperpolarizability (β0) was obtained as

0 =



2 x

  y2   z2 

(4)

where

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i 

3  iii  ijj  ikk  , i, j, k  x, y, z 5

For calculating the polarizabilities and hyperpolarizabilities, traditional DFT methods (such as B3LYP) have deficiencies for such systems.57,58 Noted that the Coulomb-attenuated hybrid exchange-correlation function (CAM-B3LYP59,60) developed to overcome these limitations, and is appropriate for predicting the NLO property of large conjugation systems, for example, in fullerene dimers61 and similar heteronanotubes27,

62.

Therefore, the polarizability and the first hyperpolarizability

were calculated by using an analytical CAM-B3LYP/6-31+G(d) approach, together with the transition energy ∆E, oscillator strength f0, and the difference of the dipole moment ∆μ between the ground state and the crucial excited state with the largest oscillator strength, all calculated with the TD-CAM-B3LYP method at the same basis set. All of the calculations were performed by the Gaussian09 program package.63

3. Results and Discussions 3.1 Geometrical characteristics and stability The optimized structures are shown in Figure 1. All carbon atoms bear 3-fold coordination by the neighboring three nitrogen atoms. The edged N atoms are 2-fold coordination by two C atoms, whereas, the bridged N atoms and the inner N atoms are all 3-fold coordination by three C atoms. The structure of complex C6N10H6 is a planar structure with D3h symmetry, however, the structure of the complex C18N27H8 ~ the complex C36N52H10Li3 is no longer maintaining the planar structure but a corrugated structure. In one unit, the distance between the two N atoms is about 6.9 Å. -7-



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Table 1. VIE (eV), HOMO-LUMO energy gap (Egap, eV), interaction energies (Eint, kcal/mol), average NPA charges of the N atoms in the cavity neighboring Li (QN),and average NPA charges value of Li atoms(QLi). Complexes C6N10H6 C18N27H8 C18N27H8Li C30N44H12 C30N44H12Li2 C42N61H15 C42N61H15Li3

VIE 7.76 7.23 4.35 7.10 3.97 6.99 4.09

Egap 4.96 3.89 0.78 3.67 0.72 3.62 0.44

Eint

78.59 79.66/84.35 78.63/79.61/80.66

QN -0.518 -0.594 -0.525 -0.649 -0.522 -0.626

QLi

+0.894 +0.891 +0.890

To explore their stability, the VIE for these complexes were calculated. The VIE values are ranging from 3.97 eV to 7.76 eV (see Table 1), which indicates all the complexes have high stability. The VIE value (7.23 eV) of complex C18N27H8 is higher than that (4.35 eV) of the complex C18N27H8Li, which exhibits the effect of Li-doping on the VIE values. Similar trends are found in the formations of other non-doped and doped Li atom of g-C3N4 complexes as shown in Table 1.

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C6N10H6(D3h)

C18N27H8

C18N27H8Li

C30N44H12

C30N44H12Li2

C42N61H15(Cs)

C42N61H15Li3

Figure 1. Optimized structures based on g-C3N4.

Furthermore, the interaction energies (Eint) between Li atom and g-C3N4 were also calculated to explore the stabilities of the Li doped g-C3N4 complexes. Table 1 shows that the values of Eint are in the range of 78.59~84.35 kcal/mol, the interaction

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between Li atom and g-C3N4 cavities is quite strong, indicating the remarkable stabilities of the Li doped g-C3N4 complexes under investigation.

3.2 Electronic Structure Features

The frontier molecular orbitals (FMO) can be used to understand the relationship between the optical and electric properties of a molecule.64 Further, the HOMO and LUMO energy gap is used to reveal the charge-transfer (CT) interaction occurring within a molecule. Correspondingly, HOMO and LUMO are depicted in Figure 2. In the case of complex C18N27H8, the specific localization of HOMO and LUMO on the whole molecule can be noted, which means no significant inter-molecule CT in the complex, thus C18N27H8 may be expected with small NLO responses. But when Li is encapsulated inside the complex C18N27H8 to form the complex C18N27H8Li, the obvious differences have been found in the HOMO and LUMO, so the complex C18N27H8Li has strong inter-molecular CT and should be expected to have larger NLO responses. Similar regularity also exists in other non-doped and Li doped g-C3N4 complexes, which indicates that doping Li atom may exhibit larger NLO responses.

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Figure 2. The frontier molecular orbital diagrams for C18N27H8 and C18N27H8Li.

To gain deeper insight into the characteristics of the studied complexes, the HOMO-LUMO gap, which is the most intrinsic property of a molecule, has been calculated. The results are collected in Table 1. As shown in Table 1, the HOMO-LUMO gap of g-C3N4 can be narrowed by doping Li atom. For example, the value of HOMO-LUMO gap for the complex C18N27H8 and the complex C18N27H8Li is 3.89 eV and 0.78 eV, respectively. Obviously, doping Li atom has a great effect on the occupied/unoccupied FMOs as showed in Figure 2. When Li atom is doped, the interaction between the 2s electron of Li and the lone pair electron of neighboring nitrogen atoms become stronger, and push away the electrons near the cavity in g-C3N4, thus lead to the inter-molecule CT, and thus cause high NLO response. In addition, we can also explain the inter-molecule CT from the perspective of the difference in electronegativity between Li and N atoms. Due to the large difference of electronegativity between Li and N atoms, when Li atom is doped, charge transfer from Li atom to N atom. Similar trends are found in the formations of other - 11 -



non-doped and doped Li atom g-C3N4 complexes. This indicates that the g-C3N4 complex by doping Li atom should exhibit better NLO property than the corresponding g-C3N4 complex. This is a good proof that the lower is the optical gap, the greater will be the nonlinearity.

The NPA has been used to explain the interaction between Li atom and the g-C3N4 moiety.

From Table 1, one can see that the charges on Li atoms are close to +1e,

+0.894e for C18N27H8Li, and the average charge of the Li atoms is +0.891e and +0.890e for C30N44H12Li2 and C42N61H15Li3, respectively. As a result, Li salts with Li+ cation are formed. Further, the NPA charges of the N atoms in the cavity neighboring the Li atom are about -0.518e for C18N27H8, and about -0.594e for complex C18N27H8Li. The fact that the average charges of the N atoms in the cavity neighboring the Li atom become more negative is of great importance, from which we can draw a conclusion that doping Li atom into the g-C3N4 moiety can increase the electron density of the N atoms in the cavity. As a result, the negative g-C3N4 moiety acts as the electron acceptor and leads to the CT from Li atom to g-C3N4 moiety. Such similar regularity is also found in other non-doped and doped Li atom g-C3N4 complexes. Based on the above analysis, the Li atom has a significant effect on its neighboring N atoms, then it's reasonable to expect that the Li doped g-C3N4 salts will show some special properties compared with g-C3N4. Due to the presence of inter-molecule CT, the Li doped g-C3N4 complexes should be expected to have large NLO responses.

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3.3 NLO Properties

The inter-molecule CT found in the Li doped g-C3N4 complexes prompts us to further investigate the NLO properties. To calculate the first hyperpolarizability of the Li doped g-C3N4 complexes, a suitable functional is important. Several studies have pointed out that the CAM-B3LYP functional which adds a long-range correction using the Coulomb-attenuating method is suitable to predict the molecular NLO properties of large systems, because it can provide semiquantitative accuracy (very similar to the desirable coupled cluster methods) with a reasonable computational cost.65-67 Therefore, we only take the CAM-B3LYP functional as an example to evaluate the first hyperpolarizabilities of the Li doped g-C3N4 complexes.

In order to choose a suitable basis set, we calculated the β0 value of complex C18N27H8 at the CAM-B3LYP level of theory using the 6-31g basis set and its extensions, such as, 6-31g(d), 6-31+g(d), 6-31+g(d,p), 6-31++g(d,p), and 6-311++g(d,p) basis sets. As shown in Table 2, it is noticed that the 6-31+g(d) basis set raises up to 0.20% as compared to the more flexible and polarized 6-311++g(d,p) basis set at the CAM-B3LYP level of theory. Thus, adding diffuse functions and polarization has a very slight influence on the NLO properties of the studied complexes. Considering the computation costs, the CAM-B3LYP/6-31+g(d) was considered as a proper means to calculate the first hyperpolarizabilities the Li doped g-C3N4 complexes.

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Table 2. The first hyperpolarizabilities β0 (a.u.) of complex C18N27H8Li calculated at the CAM-B3LYP level by using the 6-31g basis set and its extensions. basis set 6-31g 6-31g(d) 6-31+g(d) 6-31+g(d,p) 6-31++g(d,p) 6-311++g(d,p)

β0 19011.25 15375.50 16622.64 16636.26 16652.26 16282.61

We calculated β0 for the case of g-C3N4 doped with Li atom, the results are collected in Table 3. Once the Li atom is interacted with the C18N27H8, the CT occurred between the two moieties, may lead to large NLO response. As listed in Table 3, compared C18N27H8 with C18N27H8Li, one can see that the β0 value is greatly increased from 1.03×102 a.u. to 1.66×104 a.u., as well as for the other non-doped and doped Li atom g-C3N4 complexes, which demonstrates the Li doped effect indeed can greatly increase the static first hyperpolarizabilities.

The two level model, which decipher the mechanism on how the first hyperpolarizability of doped systems may are triggered, can be expressed as68-70:

0 

    f 0  E  3

where ∆E, f0, and ∆μ are the transition energy, oscillator strength, and the difference of dipole moments between the ground state and the crucial excited state with the largest oscillator strength, respectively.

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Table 3. The polarizability α (a.u.), the first hyperpolarizability β0 (a.u.), transition energy ΔE (eV), the largest oscillator strength f0, and differences of dipole moment between ground state and crucial excited state Δμ (D). Complexes C6N10H6 C18N27H8 C18N27H8Li C30N44H12 C30N44H12Li2 C42N61H15 C42N61H15Li3

α 154.12 469.20 668.32 791.91 1298.18 1122.06 2465.60

β0 0 1.03×102 1.66×104 1.13×103 6.05×104 2.36×103 1.25×106

f0Δμ/(∆E)3

ΔE

f0

Δμ

0.005 0.094 0.008 0.589 0.031 1.767

5.1039 1.5584 5.0806 1.1146 4.7419 1.0263

1.3361 0.2933 1.4835 0.3729 2.8757 0.4461

0.500 1.217 0.750 2.188 1.143 4.281

As β0 is inversely proportional to the third power of ∆E, even a slight change in ∆E may affect the magnitude of β0. As can be seen in Table 3, the ∆E value of C18N27H8 is 5.1039 eV, when it is doped with Li atom to form C18N27H8Li, the ∆E sharply decreases to 1.5584 eV. The ∆E value can be further decreased as C18N27H8 > C30N44H12 > C42N61H15 > C18N27H8Li > C30N44H12Li2 > C42N61H15Li3. Obviously, the Li doping is an ideal way to make g-C3N4 based complexes own a large β0 value.

Another predominant factor of β0 is Δμ, as shown in the two level model, the β0 value is directly proportional to the Δμ. Date in Table 3 show that the Δμ value varying from 0.500 to 4.281 Debye, and increased as C18N27H8 < C30N44H12 < C42N61H15 < C18N27H8Li < C30N44H12Li2 < C42N61H15 Li3.

We put all the factors together to interpret the variation of β0 value using the two level formula f0Δμ/(∆E)3. It can be seen in Table 3 that the change tendencies of β0 and f0Δμ/(∆E)3, and increased as C18N27H8 < C30N44H12 < C42N61H15 < C18N27H8Li

Tuning the Nonlinear Optical Response of Graphitic Carbon Nitride by

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