Letter Cite This: J. Phys. Chem. Lett. 2019, 10, 3621−3629
pubs.acs.org/JPCL
Surface Sensitive Photoluminescence of Carbon Nanodots: Coupling between the Carbonyl Group and π‑Electron System Cui Liu,†,§ Lei Bao,† Mengli Yang,† Song Zhang,*,‡,∥ Miaomiao Zhou,‡,∥ Bo Tang,† Baoshan Wang,† Yufei Liu,§ Zhi-Ling Zhang,*,† Bing Zhang,‡,∥ and Dai-Wen Pang†
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†
Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, State Key Laboratory of Virology, The Institute for Advanced Studies, and Wuhan Institute of Biotechnology, Wuhan University, Wuhan 430072, P. R. China ‡ State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China § Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education), Centre for Intelligent Sensing Technology, College of Optoelectronic Engineering, Chongqing University, Chongqing 400044, P. R. China ∥ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: The functional groups and π-electron system of carbon dots (C-dots) were carefully controlled by several innovative chemical methods, without any changes in size, to unravel the relationship between the surface structure and photoluminescence (PL). The results of experiments and theoretical calculations reveal that the PL of C-dots is related to the surface state. The energy gap is determined by the coupling of the πelectron system and carbonyl group, and the quantum yield (QY) is dependent on the carbonyl group. The carbonyl group is the main factor increasing the ratio of nonradiation to radiation recombination, thereby leading to the low QY of C-dots. This work provides a strategy for effectively tuning the structure of C-dots, giving rise to the tunable PL emission wavelength and highly desirable QY, which enables us to further unravel the PL mechanism.
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our previous works,8,31,32,40 we revealed that the energy gap of C-dots is governed by both the size and degree of surface oxidation. Despite that, among several types of oxidation, i.e., C−O, CO, and COO−, the one that contributes to the PL (i.e., spectrum and quantum yield) of C-dots is unclear. The key to addressing this issue is controlling the content of those surface functional groups. In this paper, we have developed several chemical methods for tuning the oxygen functional groups semiquantitatively on the surface of C-dots that are ∼2 nm in size. Such a strategy results in a tunable π-electron system and functional groups of C-dots, which also facilitates the unraveling of the surface-related photoluminescence. The emission wavelength of C-dots is strongly associated with both the extent of delocalization of the π-electron system and the carbonyl-group content, while the QYs of C-dots are more dependent on carbonyl-group content. When different samples prepared under various synthesis conditions were studied together, the inferred PL mechanisms were not convincing enough because too many factors affected the PL. Therefore, in this work, the PL mechanism of C-dots
arbon nanodots (C-dots) have attracted an immense amount of attention1−4 due to their low toxicity, unique optical properties, good water dispersity, facile preparation, and ease of functionalization.5−10 Because of these characteristics, C-dots have great potential in a wide range of applications, including detection,11−14 bioimaging,15−18 lightemitting devices,19−23 therapy,24 drug delivery,25,26 catalysis,27−29 and energy conversion.30 The quantum yield (QY) and emission wavelength largely determine the prospects of Cdots in these applications. However, controlling the photoluminescence (PL) properties of C-dots is very difficult because the PL mechanism remains ambiguous. As many reports declared, the optical properties of C-dots largely rely on synthetic methods, reaction conditions,31,32 and carbon precursors.33,34 The PL of C-dots is considered to be dependent on size,35−37 conjugated structure,38,39 surface state,32,40 molecule state,33,41 etc. A deep understanding of the PL mechanism will allow us to tune the PL of C-dots as that of quantum dots.42−44 Among these proposed PL mechanisms, surface state-related PL emission has attracted much attention. Ding et al.34 obtained full-color light-emitting C-dots through hydrothermal synthesis and separation via chromatography. They confirmed that various degrees of oxidation result in different emission wavelengths of C-dots. In © 2019 American Chemical Society
Received: May 10, 2019 Accepted: June 14, 2019 Published: June 14, 2019 3621
DOI: 10.1021/acs.jpclett.9b01339 J. Phys. Chem. Lett. 2019, 10, 3621−3629
Letter
The Journal of Physical Chemistry Letters
the Fourier transform infrared (FT-IR) spectrum of C-dots (Figure 1C), the broad bands at ∼3415, 1724, and 1231 cm−1 assigned to the O−H, CO, and C−O stretching vibrations, respectively, were very strong. The broad absorption band at ∼2567 cm−1 was a hydrogen bonded O−H stretching vibration of carboxyl groups.46 After being combined with NH4OH, the carboxylic acid (−COOH) of C-dots was converted to ammonium carboxylate (−COONH4). The peaks at 1724, 1231, and ∼2567 cm−1 disappeared. Instead, two strong bands at 1604 and 1400 cm−1 appeared, which corresponded to the asymmetric and symmetric stretching vibrations of −COO−, respectively. Simultaneously, the intensity of the band at 3415 cm−1 attributed to the absorption of the stretching vibration of O−H of −COOH decreased, and the band at 3100 cm−1 assigned to the N−H of the −COONH4 stretching vibration appeared. These results demonstrate that the surface of C-dots contains abundant carboxyl groups. The C 1s X-ray photoelectron spectroscopy (XPS) spectrum of the initial C-dots (Figure 1D) can be fitted by four peaks: 284.5 eV (graphitic carbons, sp2), 286.0 eV (alcoholic carbons, C−O), 287.8 eV (carbonyl carbons, C O), and 289.0 eV (carboxyl carbons, COOH).31 According to the integrating fitting curve area of the four peaks, the contents of CC, CO, C−OH, and COOH were calculated to be 63.6%, 19.3%, 11.0%, and 6.2% (Table S1), respectively. A large fraction of functional groups on C-dots were generated under vigorous oxidation, which is convenient for surface engineering. To unravel which functional group contributes to the PL (i.e., spectrum and quantum yield) of C-dots, the surface structure was carefully controlled. NaBH4 is commonly chosen as the reductant for reduction of aldehydes and ketones to the corresponding alcohols.47 Therefore, it was used to react with the initial C-dots (room temperature, 48 h) for reducing carbonyl groups, and the resulting sample was denoted as Cdots-NaBH4. The optimal emission wavelength of C-dotsNaBH4 was blue-shifted from 534 to 475 nm (Figure 2A), and the QY doubled, compared with that of the initial C-dots. The transmission electron microscopy image (Figure S2) and XRD pattern (Figure S1) of C-dots-NaBH4 indicated that the
was thoroughly studied via structure tuning based on only one kind of C-dots. The initial C-dots with a size of 1.8 ± 0.3 nm (Figure 1A) were prepared via the oxidation of carbon fiber
Figure 1. (A) Transmission electron microscopy image and corresponding size distribution and (B) Raman spectrum of the initial C-dots. (C) FT-IR spectra of the initial C-dots before and after being alkalified by ammonia. (D) C 1s high-resolution XPS spectrum of the initial C-dots with identification of peaks by curve fitting.
powder in a nitric acid solution as in our previous report45 (details in the Supporting Information). The structure of Cdots was characterized. As shown in the X-ray diffraction (XRD) pattern (Figure S1), the diffraction peaks with 2θ values of approximately 22−25° and 42° were attributed to the 002 and 100 facets, respectively, of graphite [powder diffraction file (PDF Card No. 75-1621)], indicating the initial C-dots with a graphite crystal structure. The Raman spectrum of C-dots (Figure 1B) exhibited two peaks at 1590 and 1350 cm−1 that can be attributed to the G and D bands, respectively, and the intensity ratio of the D band to the G band of C-dots was 0.88, implying C-dots with a large portion of defects. In
Figure 2. (A) PL spectra of the initial C-dots, C-dots-NaBH4, C-dots-NaBH4-NaOH, and C-dots-NaOH. C 1s high-resolution XPS spectra of (B) C-dots-NaBH4, (C) C-dots-NaOH, and (D) C-dots-NaBH4−NaOH with identification of peaks by curve fitting. (E) FT-IR and (F) 1H nuclear magnetic resonance spectra of the initial C-dots, C-dots-NaBH4, C-dots-NaOH, and C-dots-NaBH4-NaOH as indicated. 3622
DOI: 10.1021/acs.jpclett.9b01339 J. Phys. Chem. Lett. 2019, 10, 3621−3629
Letter
The Journal of Physical Chemistry Letters
(Table S1), as predicted. The QYs of C-dots-NaOH and Cdots-NaBH4-NaOH increased 5- and 6-fold, respectively, compared with that of the initial C-dots. However, the shifts in emission wavelength were opposite, indicating that other factor has a dramatic effect on the energy gap of C-dots except for carbonyl groups. The C−OH content decreased for both C-dots-NaOH and C-dots-NaBH4-NaOH, whereas their CC content increased from 63.6% to 68.8% and from 64.7% to 72.6% (Table S1), respectively. This increase was due to the elimination of hydroxyl groups at high temperatures.55 The 1H NMR spectra of C-dots-NaOH and C-dots-NaBH4-NaOH (Figure 2F) showed that the magnitudes of the peaks in the range of 7.0−8.5 and 5.5 ppm attributed to the 1H nuclei in aromatic rings and alkene, respectively, increased obviously, while the magnitudes of the peaks in the range of 3.3−4.3 ppm ascribed to the α-H of alcohol and keto decreased. Simultaneously, the magnitudes of the peaks in the range of 1.8−2.8 ppm attributed to the α-H of the carboxyl group dramatically increased. These results suggested that the C-dots contained some α-diketone and/or β-keto acid/alkone, which underwent α-diketone rearrangement and/or acid form decomposition, respectively. These results were in line with those of XPS. The ID/IG of C-dots-NaOH (1.06) was larger than that of C-dots, while that of C-dots-NaBH4-NaOH (0.96) was consistent with the ratio of C-dots-NaBH4 (Figure S3). The result was consistent with previous work.56 After hydrothermal treatment in a concentrated NaOH solution, the elimination of hydroxyl groups led to the increase in the CC content (repairing the aromatic structure, boosting the G band), and α-diketone rearrangement and/or acid form decomposition of β-keto acid/alkone in C-dots resulted in more defect (enhancing the D band). Because C-dots-NaBH4 had a hydroxyl-group content higher than that of C-dots, C-dots-NaBH4-NaOH exhibited a lower ID/IG. Moreover, as shown in the FT-IR spectra (Figure 2E), the bands in the range of 3000−3200 cm−1, which contributed to the unsaturated carbon−hydrogen bond of C-dots-NaOH and C-dots-NaBH4-NaOH, were much stronger, further indicating the increase in the CC content. These results indicated that the factor affecting the emission wavelength of C-dots was the CC content. It is known that the PL emission wavelength is determined by the energy gap between the ground and excited states.57 The increase in CC content implies that the extent of delocalization of the π-electron system becomes higher, which decreases the energy gap and further leads to the red-shift of the emission wavelength of C-dots.31,39 However, the decrease in carbonyl-group content has the opposite effect. The PL spectra of C-dots-NaBH4-NaOH and C-dots-NaOH did not split into two peaks, which thereby indicated that the effects of carbonyl and CC on the band gap of C-dots were not irrelevant. Accordingly, we propose that the carbonyl groups and the π-electron system can strongly couple with each other, which alters the electronic states of C-dots and further affects the emission wavelength. Consequently, the red-shift in Cdots-NaBH4-NaOH is due to the more substantial increase in CC content, which plays a dominant role in the change in PL emission. To confirm the effect of the coupling between carbonyl groups and the π-electron system on the PL of C-dots, we chose HI in acetic acid as the reductant to selectively reduce the carbonyl group of the initial C-dots, and the resulting sample was donated as C-dots-HI. If the carbonyl groups are
reduction reaction did not change the physical size or the crystalline structure, being in line with previous reports.48 Hence, these PL changes in C-dots should be attributed to structural changes rather than size or crystallinity. NaBH4 is often used to reduce C-dots and graphene quantum dots (GQDs), but the resulting change in structure is ambiguous. Li et al.49 reported that the CC/C−C content increased and the oxygenated C and nitrous C content decreased after reduction by NaBH4. The oxygen-containing carbon included C−OH, CO, C−O, O−CO, etc. Meanwhile, Zhu et al.50 used NaBH4 to reduce GQDs, and they showed that the contents of carbonyl and epoxide decreased, the nitrous carbon disappeared, and the percentage of graphitic carbon increased. Here, to confirm which structure changed, the XPS and 1H nuclear magnetic resonance (NMR) spectra were recorded. The relative contents of different functional groups were calculated by integrating the fitting curve area of C 1s XPS (Figure 2B). After reduction by NaBH4, the decrement of carbonyl groups (3.0%) was consistent with the increment of hydroxyl groups (2.7%), thereby elucidating the conversion of CO into C−OH. Nevertheless, the CC and carboxyl-group content barely changed (Table S1). In the proton NMR (1H NMR) spectrum of C-dots-NaBH4 (Figure 2F), the new absorption peaks in the range of 0.8−1.5 ppm corresponding to 1H nuclei of aliphatic hydrocarbons appeared. This phenomenon was due to the conversion from the α-H of the carbonyl group to the β-H of the newly formed hydroxyl group. As shown in Figure S3, the intensity ratios of the D to G bands (ID/IG) for the initial Cdots (0.88) and C-dots-NaBH4 (0.90) were not obviously different. These results clearly indicated that the structural change was the carbonyl being converted to a hydroxyl group, which was different from the results of previous works.49,50 This could be due to the higher degree of oxidation of C-dots in this work. Fan’s group51 synthesized graphene quantum dots (GQDs) by electrochemical exfoliation of a graphite rod followed by refluxing in a mixed solution of concentrated nitric and sulfuric acid. From the data of FT-IR and XPS spectra, they observed the existence of the epoxy group, which can be converted to C−OH by NaBH4 reduction. Nevertheless, we did not observe the C−O−C vibration band (1062 cm−1) in the FT-IR spectrum of C-dots. In the C 1s high-resolution XPS spectrum of the initial C-dots, the peak at 286.7 eV attributed to C−O− C is weak enough to be ignored. Therefore, we did not consider the influence of epoxy groups on the PL of C-dots. Hence, the blue-shift of the emission wavelength (from 534 to 475 nm) was clearly ascribed to the only structural change, i.e., the decrease in the carbonyl-group content. The carbonyl group can change the electronic transitions to the n → π* type, which has an energy lower than that of the π → π* type.52,53 The high carbonyl-group content (16.3%) of C-dots-NaBH4 implied that most of the carbonyl groups were hardly reduced by NaBH4. Hence, to further reduce the carbonyl-group content, the initial C-dots and C-dots-NaBH4 underwent a hydrothermal route in a NaOH solution, and the resulting sample was donated as C-dots-NaOH and C-dots-NaBH4NaOH, respectively. Through the hydrothermal route, the physical size and the crystalline structure still did not change (Figures S1 and S2), coinciding with the previous report.54 According to the result of XPS (Figure 2C,D) the carbonylgroup content decreased from 19.3% to 13.6% for C-dotsNaOH and from 16.3% to 10.6% for C-dots-NaBH4-NaOH 3623
DOI: 10.1021/acs.jpclett.9b01339 J. Phys. Chem. Lett. 2019, 10, 3621−3629
Letter
The Journal of Physical Chemistry Letters
Figure 3. PM6 Hamiltonian-optimized structures of the (A) initial C-dots, (B) C-dots-NaBH4, and (C) C-dots-NaOH and (D) calculated electronic transitions of the three kinds of C-dots as indicated. In the structures of the three kinds of C-dots, the red and white spheres represent O and H atoms, respectively.
cm−1 attributed to the hydrogen bonded O−H stretching vibration of carboxyl groups disappeared. The peak at 1724 cm−1 assigned to the CO stretching vibrations of carboxyl groups shifted to 1740 cm−1 for C-dots modified by alcohols, while that peak shifted to 1655 cm−1 for C-dots modified by amines. These changes indicated that the carboxyl groups of Cdots were sufficiently converted to esters and amides. In addition, C-dots modified by alcohols or amines changed from hydrophilic to lipophilic (Figure S5C), indicating that C-dots can be transferred between aqueous and oil phases through the reaction of their carboxyl groups. However, the PL spectra (Figure S5D) did not change, and the QYs of the C-dots after their modification by n-hexanol, benzyl alcohol, n-butylamine, and tert-butylamine are 2.5%, 2.8%, 1.9%, and 1.9%, respectively, which are not significantly different from that of the initial C-dots. As mentioned above, the absorption peak of CO in the carboxyl group was at 1724 cm−1, which provided strong evidence that the carboxyl groups in our C-dots are not conjugated with the π-electron system, which is different from the observations of Zboril,60 who stated that the carboxyl groups are conjugated with aromatic rings, leading to the C O absorption peak below 1700 cm−1. Thus, the carboxyl groups in our C-dots have no obvious effect on the PL due to the absence of a conjugate effect between carboxyl groups and the π-electron system. To verify the PL mechanism of C-dots, theoretical calculations were performed. Considering the average height of C-dots of
