Graphitic Nitrogen Doping in Carbon Dots Causes Red-Shifted

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Graphitic Nitrogen Doping in Carbon Dots Causes Red-Shifted Absorption Sunandan Sarkar, Mária Sudolská, Matúš Dubecký, Claas J. Reckmeier, Andrey L. Rogach, Radek Zboril, and Michal Otyepka J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10186 • Publication Date (Web): 21 Dec 2015 Downloaded from http://pubs.acs.org on December 22, 2015

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The Journal of Physical Chemistry

Graphitic Nitrogen Doping in Carbon Dots Causes Red-Shifted Absorption Sunandan Sarkar,†,¶ Mária Sudolská,†,¶ Matúš Dubecký,† Claas J. Reckmeier,‡ Andrey L. Rogach,‡ Radek Zboˇril,† and Michal Otyepka∗,† Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký University Olomouc, Tˇr. 17. listopadu 12, 771 46 Olomouc, Czech Republic, and Department of Physics and Materials Science and Centre for Functional Photonics, City University of Hong Kong, 83 Tat Chee Avenue, Hong Kong SAR, China E-mail: [email protected]

Abstract

applications in bioimaging, drug delivery, photocatalysis, sensors, and optoelectronic devices. 10,11 Doping with heteroatoms (e.g., oxygen, nitrogen, or sulfur) is one way of tuning their electronic and optical properties. 12–19 Nitrogen-doped CDs (N-CDs) have several properties that can be advantageously exploited in multicolor cell imaging, metal-free catalytic reactions and light-emitting diodes. 18,20–26 However, N-doping is often accompanied by the introduction of various oxygencontaining functionalities arising from the synthetic process or starting materials. 23,27–29 In N-CDs, some of the carbon atoms at core/edges of the CD honeycomb matrix are replaced by nitrogen, or N-containing groups reside at edges. 26,30,31 The most common types of N centers found in N-CDs are graphitic, pyridinic, pyrrolic, and amino centers (Figure 1). 24–26 In recently reported N-doped CDs, pyridinic, pyrrolic and amino N atoms were mainly located at edge sites, with pyridinic and pyrrolic N centers being more abundant. 23–25 However, the relative and absolute abundance of different N dopant types depends on several factors including the starting materials used to prepare the N-CDs, the method of preparation, and the experimental conditions. 15,21 Recent experiments have shown that the absorption and emission peaks of N-CDs shift to longer wavelengths as the nitrogen content increases 24,25,32 and that the electronic properties of N-CDs can differ significantly from those of the

The knowledge gap on how different types of nitrogen centers affect the optical properties of Ndoped carbon dots (CDs) hinders the rational design and synthesis of these nanostructures. We present a systematic theoretical study of 1 nm small CD models containing nitrogen and oxygen functional groups designed to explore the effects of various nitrogen centers on the absorption characteristics of CDs. Graphitic nitrogen centers are shown to have an electron doping effect that alters the systems’ electronic energy levels and causes pronounced red-shift of their absorption spectra. Other kinds of nitrogen centers including pyridinic, pyrrolic, and amino centers had no appreciable effects on the CDs’ absorption properties.

Introduction Carbon dots (CDs) have attracted considerable interest because of their outstanding biocompatibility, low cytotoxicity, tunable electronic and optical properties, and straightforward synthesis from various biomaterials. 1–9 These physical and chemical properties mean that CDs have many potential ∗ To

whom correspondence should be addressed University Olomouc ‡ City University of Hong Kong ¶ Authors contributed equally. † Palacký

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corresponding non-doped CDs. 28,29,33,34 Using experimental methods alone, it is very difficult to determine how the optical properties of N-CDs are affected by introducing specific types of nitrogen center because no existing experimental techniques can readily separate the contributions of individual N centers. An example of such spectrum is reported in the results. Consequently, much of the information required to rationally design and synthesize N-CDs with desired optical properties is currently unavailable. 35 Computational modeling can complement experiments in this respect because it can be used to study systems with known contents of individual dopant types. Here we present a comparative computational study on the absorption spectra of N-doped and Nfunctionalized CDs. The studied two-layer models of N-CDs (Figure 2, bottom) containing graphitic, pyridinic, pyrrolic and amino N centers were assembled from pyrene-based (Figure 2, top) and coronene-based (Figure 2, middle) fixed-size subunits 36 to rule out size-effects. The calculations based on time-dependent density functional theory (TD-DFT) revealed that graphitic N-doping of CDs causes considerable red-shift of their absorption spectra but other nitrogen configurations have no such effects. The red-shift associated with graphitic N-doping is attributed to the formation of deep levels in the original gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied MO (LUMO), which reduces the energies of transitions with a strong frontier/nearfrontier orbital component.

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graphitic-N (core site)

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Figure 1: Common types of Ndoping/functionalization in N-CDs. Green, blue and grey represent carbon, nitrogen and hydrogen atoms, respectively.

Figure 2: The single-layer (1L) pyrene-based (top row) and coronene-based (middle row) building blocks used to construct the studied double layer (2L) O/N-doped CD models (bottom row): nitrogen free models, and graphitic (g1, g2), pyridinic (pd), pyrrolic (pl) and amino (am) N-doped models (left to right). Green, red, blue and white vertices represent carbon, oxygen, nitrogen and hydrogen atoms, respectively. correlation functional and the 6-31+G(d) basis set. This combination is reportedly suitable for studying excited states with potential charge transfer. 47–49 We assumed a vacuum environment because trial calculations on one-layer models revealed that the results obtained using the conductor-like polarizable continuum solvent model (C-PCM) 50,51 did not show qualitative differences (see Figure S1 and related description in SI); see also Ref. 36. All calculations were conducted using the Gaussian 09.D01 software package. 52

Computational Methods Each model (Figure 2 and Figure S4) was subjected to geometry optimization and frequency analyses to confirm that it corresponded to local minimum. The optimizations were conducted using density functional theory (DFT) with the Becke three-parameter hybrid density functional (B3LYP), 44 an empirical dispersion correction D3, 45 and the 6-31++G(d,p) Pople basis set. The absorption spectra (in the ultra violet and visible range) were obtained using TD-DFT with the range-separated hybrid ω B97xD 46 exchange-


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Experimental Methods

layer systems exhibit a broad absorption at 230300 nm with a shoulder that extends to about 400 nm. This can be assumed to be a sum of the absorption spectra of the corresponding single-layer subunits. It should be noted that excitonic coupling effects due to the proximity 38 of the two monolayers are weak 36 (see below). The absorption bands of the two-layer N-CDs with pyridinic, pyrrolic and amino nitrogen motifs (pd-2L, pl-2L, and am-2L) are qualitatively similar to those of the N-free 2L model, i.e., N-doping does not appear to cause any significant red-shift or alteration of the spectra between 200 and 350 nm in these cases. Above 350 nm, the pyridinic doping in the pd-2L model causes a slight blue-shift (of approximately 30 nm) of the transition with highest wavelength relative to 2L. Effective blue-shift with respect to the undoped model 2L was also observed in the low energy regions of the spectra of the pyrrolic (pl-2L) and amino (am-2L) two-layer models. From the comparison of the spectra of the oxygen-containing models (2L, g1-2L, g2-2L and pd-2L) with the spectra of models without oxygen functional groups (i.e., purely N-doped and functionalized models pl-2L and am-2L, respectively) it follows that the absorption at 400-430 nm originates from the presence of OH/COOH electron donating/withdrawing groups bound to the conjugated system, which tunes the absorption by extension of the aromatic system. Furthermore, in 2L and pd-2L, the highest wavelength transitions (429 nm and 399 nm, respectively) are dominated by the contributions from HOMO to LUMO/LUMO+1 with the latter orbitals spanning over the COOH groups (Table 1, Figure 4). In pl-2L, no such effects are present, and the absorption is given by the size of aromatic system itself. Since amino groups donate electron density to the aromatic systems (like -OH groups), the absorption above 400 nm can be connected with the presence of COOH groups. The spectra of the N-doped and N-free singlelayer components (see the insets in Figure 3) show the same trends as were observed for the two-layer models (for the sake of completeness, the spectra of pyrene and coronene are included in the SI (Figure S2). While graphitic-like N-doping (g1-1Lp, g1-1Lc, g2-1Lp, and g2-1Lc) causes significant red-shift, the positions of the absorption bands in

The graphitic CDs containing nitrogen and oxygen with mean size of 3.5 nm were synthesized hydrotermally from citric acid and ethylenediamine following the procedure described elsewhere. 27,37 The absorption spectrum was measured in aqueous solution. 37

Results and Discussion The compatibility of peak positions in the calculated spectra of the two-layer systems (Figure 3b,c) and range of experimental absorption (Figure 3g) presumably indicates that the mean size of π -conjugated regions in our models and illustrative experimental N-CDs is similar. 37 As can be seen in Figure 3, the absorption spectra of the two-layer N-CDs containing graphitic-like N centers (g1-2L and g2-2L) were strongly redshifted relative to the corresponding N-free CD model (2L). Unlike g1-2L, whose spectrum contains low intensity peaks at wavelengths in the range 450-600 nm, the spectrum of g2-2L features two intense characteristic transitions at 475 nm and 576 nm (f >0.1 a.u.). The significant contributions from the HOMO and HOMO-1 orbitals to low energy virtuals near LUMO (Table 1) are linked to the intense low-energy transitions of the one-layer subsystems (which occur at 462 nm and 567 nm in g2-1Lp and g2-1Lc, respectively). The intensity of these low energy peaks may be due to the fact that whereas both nitrogen centers in the coronene layer of g1-2L are located at edge sites, one of the nitrogen centers in the coronene layer of g2-2L is located in the middle of the sp2 carbon lattice. 53 We note that g2-2L exhibits very lowintensity transitions in the low energy region of the visible spectrum (>600 nm), and the absorption spectrum of g1-2L also contains additional transitions at 610 nm and 732 nm with f ∼ 0.1 a.u. (Table 1). All of the low-energy transitions mentioned above are dominated by the excitations from the set of the highest occupied MOs (HOMO, HOMO-1, HOMO-2) to the lowest unoccupied orbitals (LUMO, LUMO+1, LUMO+2, LUMO+3 and LUMO+4) (Table 1, Figure 4). In addition, the absorption spectra of the both graphitic-N double


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parable to those for the N-free counterparts 1Lp and 1Lc (Figure 3a inset). Moreover, pyridinic Ndoping induces a slight blue-shift of the lowest energy band (from 420 nm in N-free 1Lp to 390 nm in its doped counterpart), and the absence of oxygen groups in pyrrolic and amino N-doped subsystems eliminates all absorption above 400 nm. A comparison of the calculated spectra for the two-layer models and the spectral shapes obtained by summation of the corresponding one-layer subunits (SI, Figure S3) indicates that the excitonic coupling between the excited states originating from the subsystems is rather weak within the twolayer structures. Although this conclusion may not hold in general, 38 it is consistent with the reported behavior of oxygen-containing CDs. 36 Based on this observation, we propose that the basic absorption properties of composite CDs can be predicted by summing the separately calculated spectra of their single-layer components. The relative molecular orbital energy levels of the two-layer models plotted in Figure 5 explain some of the variation observed in the absorption spectra of the model systems. While the HOMOLUMO gaps of the pyridinic, pyrrolic and amino N systems (6.38, 5.98 and 6.45 eV, respectively) are quite similar to that of the parent system (6.13 eV), graphitic doping reduces the gap by more than 2.2 eV (3.61 eV in g1-2L and 3.89 eV in g2-2L). As shown in Figure 5, graphitic N-doping (g12L, g2-2L) creates deep gap-states in the original HOMO-LUMO gaps, reducing the energy of lowlying optical transitions that have a strong frontier-

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Figure 3: Calculated absorption spectra of the N-free 2L model (a) and N-doped models with graphitic (b, c), pyridinic (d), pyrrolic (e) and amino (f) nitrogen centers, and (g) a measured absorption spectrum of oxygen- and nitrogencontaining CDs (average size 3.5 nm). The insets show the absorption spectra of the corresponding monolayer subunits. The structures of the model systems are shown in Figure 2. The envelope function assumes Gaussian broadening of peaks with σ = 30 nm. the spectra of subunits doped with other types of N centers (insets of Figure 3d-f) are often com-


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Table 1: Summary of the significant vertical excitations of the two-layer models (Figure 2) indicating the wavelength (λabs in nm), oscillator strength ( f ) and CI contributions (major MO involvement in %). For brevity, we denote here HOMO orbital as H and LUMO as L.

λabs 429 306 303 302 296 g1-2L 732 610 414 380 g2-2L 576 475 334 pd-2L 399 328 316 307 302 pl-2L 313 307 298 am-2L 315 298 297 Model 2L

f CI Contributions (%) 0.075 H→L (74) 0.249 H-2→L(20) 0.312 H-1→L+2(30) 0.358 H-1→L(55) 0.207 H-2→L+2(49), H-2→L(26) 0.102 H→L+1 (41), H-1→L+2 (33) 0.078 H→L+3 (49), H→L+2 (32) 0.056 H-1→L+5 (44), H→L+6 (36) 0.128 H-2→L (74) 0.134 H→L+3(59) 0.314 H→L+4(38),H-1→L+4(26) 0.106 H-2→L+2(36), H-2→L+1(33) 0.093 H→L+1(52), H→L(35) 0.052 H→L+2(46) 0.07 H→L+3(54) 0.065 H-1→L+1(44),H-1→L(32) 0.501 H-1→L+2(32) 0.349 H→L+1(45), H-1→L(27) 0.366 H-1→L+1(22), H→L+2(22) 0.175 H-2→L+1(48) 0.088 H-1→L+1(47) 0.356 H-3→L+1(53), H-2→L(28) 0.397 H-3→L(46), H-2→L+1(21) e.g., HOMO-1 and LUMO+1 in g1-2L, may contribute, cf. Table 1). The consequences of this can be seen by comparing Figure 3b and Figure 3c to Figure 3a. Graphitic N centers are known to alter the optical properties of N-CDs because of their ability to inject up to one excess electron into the unoccupied π ∗ orbitals of the carbon sp2 lattice. Doping of this sort in graphene strongly affects its electronic structure, converting it into an ntype semiconductor. 39,40 On the other hand, a pyridinic N atom forms two σ -bonds and one π -bond with neighboring C atoms. Its remaining localized electron lone pair lies in the plane perpendicular to the conjugated π -system so it cannot participate in aromatic conjugation and has little effect on the electronic gap. 39,41 Pyrrolic N atoms form three σ bonds with neighboring atoms (two carbon atoms and one hydrogen atom), which account for three of their five valence electrons. The remain-

Figure 5: Relative energy levels of the occupied (red lines) and unoccupied (blue lines) molecular orbitals of the two-layer models. The ∆E (in eV) values indicate the energy difference between the LUMO and HOMO. orbital character (not strictly HOMO-LUMO, but possibly also related near-degenerate levels like,


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ing pair of electrons satisfies the aromatic sextet rule, so no ‘extra’ electrons are available to occupy higher-lying electronic states. Therefore, no charge doping effect occurs. 42 The amino groups have an effect similar to that previously reported for hydroxy groups 43 in oxygen-CDs (which was also observed in our models): they cause a small gap opening of ∼0.32 eV. Overall, these findings indicate that pyridinic, pyrrolic and amino nitrogens do not significantly alter the original HOMOLUMO gap by introducing new states. 17,35 Consequently, doping with such nitrogen centers will have little impact on the optical properties of CDs via gap-alternation effects. Conversely, graphitic N centers cause strong red-shift because they can significantly reduce the HOMO-LUMO gap and thus the energies of the corresponding optical transitions. Two additional mixed two-layer models with graphitic N doping (Figure S4) were analyzed to better understand how the spectral shape is affected by varying the dopant concentration (Figure S5) and the introduction of new dopant levels in the original HOMO-LUMO gap (see the Mixed Models section in the SI, Figure S6 and Figure S7). In the case of the two-layer model g1-2LNc, which consists of the undoped pyrene-based unit 1Lp and the N-doped coronene-based g1-1Lc, graphitic N-doping induced one additional electronic transition at 699 nm (f ∼ 0.15 a.u.), but the spectrum envelope in the range 200-600 nm was not altered. However, significant red-shift was observed for the g1-2L-Np system (Figure S5c) in which the smaller pyrene-based subunit is Ndoped but the larger coronene unit is not. These findings suggest that the optical properties of NCDs depend partly on the dopants’ local environment and concentration.

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an electron doping effect that reduces the magnitude of the electronic gap. However, doping with pyridinic, pyrrolic, or amino N centers did not cause significant red-shifts, which was attributed to the absence of electron doping. This work represents a step towards a more systematic understanding of the relationship between the absorption properties of N-doped CDs and their structures, and provides information that may facilitate the design and synthesis of CDs with improved optical properties. Acknowledgement Support from the Ministry of Education, Youth and Sports of the Czech Republic (project No. LO1305) is gratefully acknowledged. MS and MD were funded by institutional support from Palacký University. ALR acknowledges the NPRP grant No 8-878-1-172 from the Qatar National Research Fund (A Member of the Qatar Foundation). Supporting Information Available: Additional results: comparison of absorption spectra in water and vacuum (single-layer models), spectra of pyrene and coronene in vacuum, comparison of summed spectra of single-layer building blocks to the original absorption spectra of two-layer models, additional analysis of models with reduced N content. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphitic Nitrogen Doping in Carbon Dots Causes Red-Shifted

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