DOI: 10.1002/chem.201405088
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& Nanoparticles
Intramolecular Hydrogen Bonds Quench Photoluminescence and Enhance Photocatalytic Activity of Carbon Nanodots Pengju Yang,[a, b] Jianghong Zhao,*[a] Lexi Zhang,[c] Li Li,[a] and Zhenping Zhu*[a] Abstract: Understanding the photoluminescence (PL) and photocatalytic properties of carbon nanodots (CNDs) induced by environmental factors such as pH through surface groups is significantly important to rationally tune the emission and photodriven catalysis of CNDs. Through adjusting the pH of an aqueous solution of CNDs, it was found that the PL of CNDs prepared by ultrasonic treatment of glucose is strongly quenched at pH 1 because of the formation of intramolecular hydrogen bonds among the oxygen-containing surface groups. The position of the strongest PL peak and its
Introduction Carbon nanodots (CNDs) have superior optical and electronic properties combined with significant advantages such as high aqueous solubility, robust chemical inertness, easy functionalization, low toxicity, and high resistance to photobleaching. Thus, many studies have been performed on CNDs in a wide range of applications, such as biological labeling, bioimaging, drug delivery, sensors, photocatalysis, and optoelectronic devices.[1–4] Photoluminescence (PL) is the most interesting property of CNDs; most studies have focused on developing controllable synthetic techniques for CNDs and tuning their strong PL emissions.[2, 3] To rationally achieve these objectives, understanding the factors affecting the emission properties of CNDs and investigating their PL mechanism are very important. However, engineering a unique PL emission and understanding the PL mechanism remain difficult because of the highly nonstoichiometric and heterogeneous structure, size, and surface state of CNDs. For example, the mechanism underlying the [a] Dr. P. Yang, Dr. J. Zhao, L. Li, Prof. Dr. Z. Zhu State Key Laboratory of Coal Conversion Institute of Coal Chemistry, Chinese Academy of Sciences Taiyuan, 030001 (P. R. China) E-mail: [email protected] [b] Dr. P. Yang University of Chinese Academy of Sciences Beijing, 100039 (P. R. China) [c] Dr. L. Zhang Department of Material Physics Institute of Material Science and Engineering Tianjin University of Technology Tianjin, 300384 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405088. Chem. Eur. J. 2015, 21, 8561 – 8568
corresponding excitation wavelength strongly depend on the surface groups. The origins of the blue and green emissions of CNDs are closely related to the carboxyl and hydroxyl groups, respectively. The deprotonated COO¢ and CO¢ groups weaken the PL peak of the CNDs and shift it to the red. CNDs alone exhibit photocatalytic activity towards degradation of Rhodamine B at different pH values under UV irradiation. The photocatalytic activity of the CNDs is the highest at pH 1 because of the strong intramolecular hydrogen bonds formed among the oxygen-containing groups.
unique excitation-wavelength-dependent emission feature remains unclear, although several origins of PL in CNDs have been tentatively proposed, including radiative recombination of excitons, emissive traps, quantum-confinement effect, aromatic structures/oxygen-containing groups, free zigzag sites, and/or edge defects.[1a, 5–8] Factors affecting the emission properties include the size (or size distribution),[9] element doping,[10] and surface states[8b] of CNDs. An increasing amount of research has shown that the abundant surface groups are directly related to the emission intensity, band shape,[8b] and band position[11] of CNDs. The abundant chemistry and flexible reactivity of surface groups are important in producing CNDs with very high sensitivity to the working environment, such as solvent[12] and pH.[13] Therefore, elucidating the PL mechanism of the surface chemistry of CNDs and investigating the functional mechanism of the effect of the environment through surface groups on the PL properties of CNDs are important. Most of the transformations in the above-mentioned applications require aqueous environments as the reaction media; thus, controllably preserving and tuning the PL properties of CNDs in aqueous systems are important. The pH value is one of the most important factors that significantly affect numerous chemical and biological processes in water. Moreover, the PL of CNDs is very sensitive to pH.[8, 14, 15] However, few studies have investigated the mechanism underlying the pH-induced phenomenon. Previously, Pan et al. attributed the quenching of PL under acidic conditions to the formation of a reversible non-emissive complex between the emissive free zigzag sites and H + , while free zigzag sites can be restored under alkaline conditions through deprotonation, which leads to the restoration of PL at high pH.[7] Jeon et al. proposed that at high pH the COOH groups of CNDs are deprotonated to COO¢ groups
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Full Paper bearing an additional electron, which increases their electron density. The electron-withdrawing properties of the carboxyl groups towards CNDs are weakened by the extra electron, and this results in a decreased bandgap of the CNDs and a redshift of the maximum PL peak.[15] Galande et al. found that the fluorescence of graphene oxide (GO) originates from the quasimolecular fluorophores formed by electronic coupling of COOH groups with the nearby sp2 carbon atoms; the pH-dependent fluorescence characteristic of GO is attributed to excited-state protonation of the emitting (G–COO¢)* to (G–COOH)* species.[16] Moreover, Konkena et al. corroborated that the pH-dependent nature of the fluorescence spectra of GO results from excited-state proton transfer involving quasimolecular fluorophores.[13] Carboxyl and hydroxyl groups are generally regarded as the main oxygen-containing groups in CNDs[11, 16] and are often located at the edge and defect sites. These oxygen-containing groups can be protonated easily under appropriate acidic conditions to form intramolecular hydrogen bonds. Intramolecular hydrogen bonding is an important noncovalent interaction with a considerable effect on the geometric, electronic, vibrational, and radiationless-transition properties of substituted aromatic compounds[17] and their related conjugated oligomers or polymers.[18] Thus, substituted molecules would be able to control their absorption and emission performance.[19, 20] To the best of our knowledge, no reports have focused on the effects of intramolecular hydrogen bonds on the optical and electronic properties of CNDs. In the present study, the pH value of an aqueous solution of CNDs was adjusted to ensure that their sizes and core carbon structures are similar under different conditions. TEM, AFM, XRD, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), FTIR spectroscopy, and steady-state PL measurements were performed to characterize the morphology, structure, oxygen-containing groups, and PL spectra of CNDs. Strong intramolecular hydrogen bonds were formed among the oxygen-containing groups of the CNDs at pH 1. The intramolecular hydrogen bonds significantly quenched the PL intensities of the CNDs without dependence on the excitation wavelength. Furthermore, photocatalysis, which is a competitive process of PL, was investigated. Photocatalytic studies on the decomposition of Rhodamine B (RB) showed that CNDs alone have the highest photocatalytic activity at pH 1 because of the formed intramolecular hydrogen bonds.
Results and Discussion The TEM image in Figure 1 a shows that the as-prepared CNDs are well-dispersed nanospheres with homogeneous size. The average diameter of the CNDs, obtained by statistical analysis of 300 nanoparticles, is 3.2 nm (for size distribution, see inset of Figure 1 a). The AFM image (Supporting Information, Figure S1) showed that the height of CNDs is about 3 nm, which is close to the size distribution shown by TEM. Figure 1 b shows the high-resolution TEM (HRTEM) image of the CNDs. The lattice distance of 0.21 nm, which is consistent with the (0110) lattice plane of most CNDs, indicates good crystallinity Chem. Eur. J. 2015, 21, 8561 – 8568
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Figure 1. a) TEM and b) HRTEM images of CNDs prepared by ultrasonic treatment of glucose. Inset: size distribution of the as-prepared CNDs.
of the CNDs.[21] The increased degree of graphitic crystallinity resulted in better photostability. This finding is confirmed by the fact that no aggregation or precipitation could be observed after one-year storage without shielding from light. The XRD pattern of the CNDs (Supporting Information, Figure S3) showed a broad peak at about 208, corresponding to the graphitic structure in nanosized carbon materials.[6c] The Raman spectrum provided compelling evidence for the different types of ordered and disordered bonding environments of the sp2- and sp3-hybridized carbon. In Figure 2 the peak at
Figure 2. Raman spectrum of the CNDs.
1588 cm¢1 is the G band corresponding to the E2g mode of the ordered sp2-bonded carbon atoms, whereas the D band located at 1378 cm¢1 is disordered because of the scattering at the sp3-bonded edge carbon atoms. A weak and broad 2D peak observed at 2770 cm¢1 on the strong fluorescence background indicates better graphitization of the CNDs with few layers of graphene.[22] Moreover, the G and D bands overlaid on a strong fluorescence background are indicative of defects and functional groups in the CNDs.[23] The relative intensity ratio of D to G band (ID/IG) is characteristic of the degree of disorder in graphitic carbons; the relative intensity of the CNDs of 0.77 is smaller than those reported previously.[21, 24] These results confirmed the well-crystallized sp2 cores and the partial amorphous nature of the edges of the CNDs, consistent with the HRTEM and XRD characterizations. In addition to the morphological and structural factors, surface chemistry is important in controlling the electronic and optical properties of carbon nanostructures.[25, 26] XPS is a pow-
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Figure 4. FTIR spectra of the CNDs at different pH values.
Figure 3. High-resolution C 1s (a) and O 1s (b) XPS spectra of the CNDs.
erful technique for determining the chemical structure and composition of carbon materials, because the binding energy of the core-level peak provides elemental and chemical information. Figure 3 a shows the high-resolution C 1s XPS spectrum of the as-prepared CNDs. The spectrum can be curve-fitted to five component peaks with binding energies of 284.5, 285.5, 286.6, 287.2, and 288.6 eV, which correspond to sp2-hybridized carbon (C=C), sp3-hybridized carbon (C¢C and C¢H), and hydroxyl (COH), carbonyl (C=O), and carboxyl (O=COH) species,[27] respectively. The sp2 and sp3 C peaks are stronger than the other three peaks from various carbon–oxygen binding configurations. This confirms that the CNDs are constructed mainly from graphitic cores that have a large number of amorphous edges with COH, C=O, and O=COH groups. The high-resolution O 1s XPS spectrum (Figure 3 b) exhibited three component peaks at 531.1, 532.5, and 533.8 eV, which are attributed to quinone, O=CO/C=O, and C¢O species,[28] respectively. Thus, the presence of carbonyl, carboxyl, and hydroxyl groups in the CNDs is further confirmed. The carboxyl and hydroxyl groups are sensitive to pH. FTIR spectroscopy is a very useful structure-sensitive technique to determine the conformation, association, dissociation, and bonding state of the oxygen-containing groups in the CNDs at varying pH values (Figure 4). A broad band from 3800 to 2300 cm¢1 in the IR spectrum at pH 1, corresponds to the O¢H stretching vibration of the adsorbed water molecules and the structural carboxyl and hydroxyl groups.[29] However, this continuously extended absorption is clearly different from the O¢ H stretching band of water and free carboxyl and hydroxyl groups and indicates that strong intramolecular hydrogen bonds are formed among the carbonyl, carboxyl, and hydroxyl Chem. Eur. J. 2015, 21, 8561 – 8568
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groups of the CNDs.[29–31] Moreover, intermolecular hydrogen bonds were not formed between water and the oxygen-containing groups in the CNDs, because their absorption bands remained unaffected on dilution (data not shown). Further evidence for the existence of intramolecular hydrogen bonds is the strong and broadened C=O stretching band in the range of 2000 to 1700 cm¢1 at pH 1. Moreover, the peak located at 1030 cm¢1 in the IR spectrum at pH 1 corresponds to the C¢O stretch of the carboxyl and hydroxyl groups.[31] Overlap of this peak with the strong O¢H bending bands of the carboxyl and hydroxyl groups in the range of 1400 to 1200 cm¢1 yields a single broad absorption band.[32] This finding also confirms the formation of intramolecular hydrogen bonds in the CNDs at pH 1. Increasing the pH from 3 to 9 caused the broad O¢H stretching band to split into a flat and wide peak from 3800 to 3000 cm¢1 and a shoulder peak centered at 2945 cm¢1 in the IR spectrum. The peak at 2945 cm¢1 is assigned to the carboxyl O¢H stretching absorption,[31] whereas that in the range of 3800 to 3000 cm¢1 is due to the overlapping O¢H stretching bands of the adsorbed water molecules and various hydroxyl groups. The splitting of the O¢H stretching band indicates that the intramolecular hydrogen bonds in the CNDs were broken. At pH 12, the flat peak in the range of 3800 to 3000 cm¢1 transformed into a sharp peak centered at 3414 cm¢1, whereas the carboxyl O¢H stretching band at 2945 cm¢1 became very weak. This implies that the carboxyl and hydroxyl groups in the CNDs were heavily deprotonated, which was further confirmed by the appearance of a new absorption peak centered at 1595 cm¢1. This peak corresponds to the asymmetric stretching vibration of the COO¢ groups.[31, 33] Moreover, the C= O stretching band centered at 1750 cm¢1 and the strong and unstructured O¢H bending vibrations within the range of 1400 cm¢1 to 1200 cm¢1 disappeared in the IR spectrum at pH 12, whereas the C¢O stretching band was blue-shifted from 1030 to 1085 cm¢1. These results substantiate the abundant COO¢ and CO¢ groups in the CNDs at pH 12. At pH 9 and 7, the C=O stretching band at 1750 cm¢1 observed for the CNDs at pH 3 and 1 disappeared. The carboxyl O¢H stretching band at 2945 cm¢1 and the unstructured O¢H bending bands in the range of 1400 to 1200 cm¢1 of the CNDs at pH 9 became weaker than those of the CNDs at pH 7 and 3.
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Full Paper Moreover, an additional shoulder of the COO¢ asymmetric stretching band at 1595 cm¢1 appeared in the CNDs at pH 9, in contrast to the CNDs at pH 7 and 3. These findings suggest that the carboxyl groups in the CNDs started to be deprotonated at pH 7, whereas the hydroxyl groups maintained their protonated form. At pH 9, most of the carboxyl groups were deprotonated and the hydroxyl groups started to be deprotonated. This result is in accordance with the typical pKa values in the ranges of 3–6 and 7–10 for the aromatic carboxyl and hydroxyl groups of carbon materials, respectively.[34] At pH 3, the C=O stretching band at 1750 cm¢1 still exists but has became narrower and weaker in contrast to that of the CNDs at pH 1. Simultaneously, the carboxyl O¢H stretching band at 2945 cm¢1 and the unstructured O¢H bending bands in the range of 1400–1200 cm¢1 are stronger than those of the CNDs at pH 7 and 9. These results indicate that the carboxyl and hydroxyl groups in CNDs at pH 3 are all protonated, whereas the intramolecular hydrogen bonds among these groups are significantly broken. Figure 5 shows the UV/Vis absorption spectra of the CNDs at pHs 1, 3, 7, 9, and 12. The common feature is the two main ab-
Figure 5. UV/Vis absorption spectra of the CNDs at different pH values.
sorption peaks in each spectrum. The peak below 250 nm (peak 1) is due to the p–p* transitions of the aromatic sp2 domains in the CNDs, whereas the peak above 250 nm (peak 2) is attributed to the n–p* transitions of C=O in the sp3-hybridized regions. Moreover, an evident change in the absorption spectra is the redshift of peaks 1 and 2 with increasing pH value from 3 to 12. This finding is attributed to deprotonation of the carboxyl and hydroxyl groups, which results in the formation of negatively charged species from these oxygen-containing groups. The negative charges increase the electron density of the C=O moieties and the aromatic sp2 domains, and thus lower the bandgap of the CNDs,[35] which results in a strong redshift of the p–p* and n–p* transition absorption bands. Peak 2 in the spectrum at pH 1 shows an evident redshift compared with the absorption spectrum at pH 3. The FTIR data indicate that this is attributable to the intramolecular hydrogen bonds formed among the carbonyl, carboxyl, and hydroxyl groups of the CNDs at pH 1. After the intramolecular hydrogen bonds are formed, all core-carbon backbones are constrained in a near-planar conformation,[19] and thereby the p conjugaChem. Eur. J. 2015, 21, 8561 – 8568
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tion of the system is extended and charge transfer between the electron-donating hydroxyl and electron-withdrawing carboxyl groups is facilitated. This increases the electron density of the C=O moieties and results in the redshift of the n–p* transition band in the absorption spectrum. The CNDs are fluorescent materials, as shown by the fluorescence images taken under visible and UV irradiation (Supporting Information, Figure S2). The UV-excited CNDs exhibit strong, bright blue luminescence. A series of PL spectra (Figure 6) provided more information regarding the pH-tuning effect on the emission properties of the as-prepared CNDs. The common feature is the redshift of the PL peaks measured at different pH values with increasing excitation wavelength, which suggests that the luminescent CNDs possibly have multiple chromophores.[4b, 25, 36] Moreover, an evident difference is the significant decrease of the PL emission intensities at pH 1 at different excitation wavelengths compared with the PL spectra measured at four other pH values (Figure 6 a). The PL spectra obtained at pH 3–12 clearly showed that the three main PL peaks correspond to blue (centered at 450 nm), green (centered at 520 nm), and yellow (centered at 570 nm) emissions. The intensities of these three peaks depend on the excitation wavelength and change with varying pH value. The strongest PL peak with a broad tail extending to 800 nm is located at 450 nm in the blue spectral region for excitation at 360 nm under acidic conditions at pH 3 (Figure 6 b). The green emission peak at 520 nm exhibited the highest PL intensity under neutral conditions (pH 7) on excitation at a higher wavelength of 420 nm (Figure 6 c). At pH 9 (Figure 6 d), the peak positions and sequence of the three main PL emissions remained unchanged, whereas the intensity of the highest green emission peak on 420 nm excitation decreased significantly. At pH 12, the strongest PL peak for 500 nm excitation centered at 570 nm shifted into the yellow spectral range (Figure 6 e). Remarkably, the intensity of the yellow emission increased sharply at pH 12, which is higher than the pKa values in the range of 7–10 for the aromatic hydroxyl groups in carbon materials.[34] Size, physical structure, chemical defects, and surface passivation significantly affect the emission properties of carbon nanomaterials.[3, 37] In the present study, only pH was adjusted for one type of CNDs. This strategy significantly simplifies the complexity of the factors that determine the PL properties of CNDs. Moreover, the effects of size and physical structure on the PL behavior are excluded, because the size and physical structure of CNDs cannot be changed by varying the pH value. As confirmed by XPS and FTIR spectroscopy, carbonyl, carboxyl, and hydroxyl groups are the main oxygen-containing species in the as-prepared CNDs; the chemical structures of these groups are very sensitive to pH. Therefore, the PL behavior shown in Figure 6 should result from the chemical defect emission states involving oxygen-containing groups, which have been proposed as one of the important origins of PL in carbon nanomaterials.[5, 38] The FTIR spectra (Figure 4) corroborated that the intramolecular hydrogen bonds formed among carbonyl, carboxyl, and hydroxyl groups are broken, whereas the carboxyl and hydroxyl groups exist in their protonated forms at pH 3. At a neutral
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Figure 6. Photoluminescence spectra of the CNDs at a) pH 1, b) pH 3, c) pH 7, d) pH 9, and e) pH 12.
pH of 7, some carboxyl groups are deprotonated to ionic COO¢ groups, whereas the hydroxyl groups remain protonated. At pH 9, some hydroxyl groups are also deprotonated to ionic CO¢ groups, besides the largely deprotonated COO¢ moieties. At pH 12, the carboxyl and hydroxyl groups are deprotonated to their ions. In combination with the FTIR results, the PL spectra indicate that the broad blue PL peak under excitation at 360 nm is closely related to the emission centers involving carboxyl groups, whereas the green PL peak on excitation at a higher wavelength of 420 nm is due to the emission centers related to hydroxyl groups. At pH 3, the blue PL curves excited at 360 nm are deconvoluted into two Gaussian peaks centered at 450 (peak 1) and 515 nm (peak 2; Figure 6 f). At pH 7, the component peak 1 significantly decreased and became weaker than peak 2. These results indicate that two Chem. Eur. J. 2015, 21, 8561 – 8568
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types of carboxyl group are involved in the emission centers (EmC), the quasimolecular structures of which correspond to the aromatic carboxyl group or to a hydroxyl group (EmC 1) and the isolated aromatic carboxyl groups (EmC 2).[11a] The components of the PL peaks 1 and 2 are assigned to EmC 1 and EmC 2, respectively, because EmC 1 exhibits lower pKa values than EmC 2.[13] In addition, under extremely basic conditions of pH 12, the strongest yellow PL peak on excitation at 500 nm originates from ionic COO¢- and > CO¢-related moieties; the CO¢ groups play a more prominent role, because the sharp increase of the yellow emission intensity only appears under extremely basic conditions (pH 12). More importantly, the serious reduction of the PL peak intensities at pH 1 implies that intramolecular hydrogen bonds significantly quench the PL emission of CNDs.
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Full Paper Numerous studies have shown that intramolecular and intermolecular hydrogen bonds play sensitive roles in controlling the optical absorption and emission properties of molecular chromophores[20, 39] through photoinduced proton transfer or electron transfer.[40] Excited-state proton transfer has been recently been found to be of the key factors that determines the spectral migration of the fluorescence in aqueous dispersions of GO.[15] In the present study, the quasimolecular emission centers in the CNDs were reasonably inferred to function as molecular chromophores; thus, intramolecular hydrogen bonds can modulate the PL characteristics of CNDs through photoinduced proton transfer or electron transfer. This hypothesis requires further experimental investigation and theoretical calculations. CNDs function as excellent photoinduced electron donors and acceptors.[41] In many of the composite photocatalysts reported in the literature, the CNDs have multiple functions in promoting a wider absorption spectrum, separating electrons and holes, and stabilizing the photolysis of semiconductors.[41d, 42, 43] However, limited studies reported that CNDs alone can function as an efficient photocatalysts,[44, 45] but research on the detailed photocatalytic features of CNDs is insufficient. The present study explored the photocatalytic behavior of the as-prepared CNDs towards RB at different pH values under UV (300 W high-pressure Hg lamp), visible (300 W Xe lamp with a cutoff filter), and UV/Vis light (300 W Xe lamp). The CNDs have no photocatalytic activity for RB degradation under visible-light irradiation at varying pH values (data not shown). However, the photocatalytic activity of the CNDs for RB degradation significantly increased with decreasing pH value under UV irradiation (Figure 7 a), and only the CNDs at pH 1 showed
evident photocatalytic ability (Figure 7 b). These results (with a deviation of 5 % from the average) indicate that the as-prepared CNDs alone are the UV-responsive photocatalysts. The difference in photocatalytic performance under UV and UV/Vis irradiation is due to the discrepancy in UV light intensity from high-pressure Hg and Xe lamps. This finding is different from the reported visible-light-active carbon-dot photocatalysts,[45, 46] the visible-light responsivity of which is partly attributed to the exceptional upconversion property.[43, 46] The effects of RB adsorption on the photocatalytic activity of CNDs were excluded in the present study by establishing an adsorption/desorption equilibrium in the dark prior to illumination. Moreover, the UV intensity in the spectrum produced by the Xe lamp is lower than that emitted by the Hg lamp. Thus, the different photocatalytic behaviors of similar CNDs at similar pH values under UV and UV/Vis irradiation originate from the light-intensity-induced differentiation of the intrinsic photocatalytic ability of the CNDs with modulated structures caused by the change in pH values (e.g., the formation of intramolecular hydrogen bonds at pH 1). This finding indicates that the highest photocatalytic activity of the CNDs at pH 1 can be attributed to the formation of intramolecular hydrogen bonds in the carbon structure. Long-lived charge-separated states are key factors that determine the photocatalytic activity in solar energy utilization. Hydrogen bonds can cause thermodynamic and kinetic stabilization of the charge-separated states in the photoactive molecular systems.[17, 47, 48] As the key surface states, the oxygen-containing groups can possibly determine the photocatalytic activity of the carbon dots alone because of their upward band-bending effect, which controls the separation and combination of the photogenerated electron–hole pairs.[45] In the present study, the intramolecular hydrogen bonds constrain the core-carbon backbone in a nearly planar conformation, which extends the p conjugation of the system and facilitates charge transfer between the electron-donating hydroxyl and electron-withdrawing carboxyl groups. The enhanced synergistic band-bending effect induced by the hydrogen bonds formed among the carboxyl and hydroxyl groups can significantly benefit the separation and stabilization of the photogenerated electron–hole pairs and result in faster carrier migration and hence impressively high photocatalytic activity.
Conclusion
Figure 7. Photodegradation of RB over CNDs under a) UV light and b) UV/Vis light. Chem. Eur. J. 2015, 21, 8561 – 8568
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We have systematically investigated the pH-dependent PL characteristics of CNDs and determined the mechanisms underlying the PL behavior. The luminescent CNDs exhibit multiple chromophores and the oxygen-containing groups significantly affect the position of the strongest PL peak and its excitation wavelength. The origins of the blue and green emissions in CNDs are closely related to carboxyl and hydroxyl groups, respectively. The deprotonated COO¢ and CO¢ groups weakened and caused redshifts of the PL peak of the CNDs. More importantly, the PL of the as-prepared CNDs was strongly quenched at pH 1 because of the formation of intramolecular hydrogen bonds among the oxygen-containing groups. Furthermore, CNDs alone exhibited impressive and high photoca-
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Full Paper talytic activity for RB degradation at pH 1 under UV irradiation. The hydrogen bonds formed among carboxyl and hydroxyl groups enhanced the synergistic band-bending effect. This enhancement is significantly advantageous for the separation and stabilization of photogenerated electron–hole pairs and therefore results in high photocatalytic activity. These findings provide new insight into the complexity of the optical properties of CNDs and the effects of the environment on their structures and properties to control and modify the applications of carbon dot materials.
Experimental Section Ultrasonic synthesis of CNDs All chemicals were of analytical grade and used without further purification. The CNDs were prepared by a typical ultrasonic method,[47] in which glucose (9 g) was dissolved in deionized water (50 mL) to form a clear solution, after which HCl (50 mL, 36– 38 wt %) was added and the solution was treated ultrasonically for 4 h to obtain a solution of CNDs. The pH of the CND solution was adjusted with HCl or NaOH.
Characterization TEM images were obtained by using a JEOL JEM-2010 microscope with an accelerating voltage of 200 kV. FTIR was performed with a Nicolet Magna-IR 550-II spectrometer by using KBr pellets. Raman spectra were recorded with a LABRAM-HR Raman spectrometer with excitation wavelength of 514 nm. UV/Vis absorption spectroscopy was performed on the CND solution in a 1 cm quartz cuvette by using a Shimadzu UV-3600 UV/Vis/NIR spectrophotometer at room temperature. PL was determined by using a Hitachi F7000 FL spectrophotometer. All UV/Vis and PL measurements were performed for three parallel samples under the same conditions to obtain high reproducibility of the spectra. The height of CNDs was characterized by AFM (Bruker MultiMode V). The photographs under visible and UV light irradiation were obtained under a fluorescence microscope (Leica DM4000M). XRD analysis was conducted on a D8 Advance Bruker X-ray diffractometer with CuKa radiation (l = 0.15406 nm) operating at 40 kV.
Measurement of photocatalytic activity Photocatalytic activity was determined by adding the CND solution (20 mL, about 0.02 g of CNDs) to 150 mL of RB (20 mg L¢1). The solution was irradiated under UV (300 W high-pressure Hg lamp), visible (300 W Xe lamp with a cut-off filter), and UV/Vis light (300 W Xe lamp) at ambient atmosphere. The concentration of the residual target molecule (RB) was determined by means of the absorbance at 554 nm. The solution of RB dye and CND photocatalyst was by stirred in the dark for 12 h prior to illumination to exclude the effect of dye adsorption on the photocatalytic activity. Each photocatalytic measurement was performed three times with good reproducibility.
Acknowledgements We acknowledge the financial support from the Nature Science Foundation of China (No. 21173250), the Knowledge Innovation Project of Chinese Academy of Sciences (No. KGCX2-EWChem. Eur. J. 2015, 21, 8561 – 8568
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311), the Youth Innovation Foundation of Institute of Coal Chemistry, and Chinese Academy of Sciences (2011SQNRC19). Keywords: carbon · hydrogen bonds · luminescence · nanoparticles · photochemistry [1] a) Y. P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. Wang, J. Am. Chem. Soc. 2006, 128, 7756; b) S. Zhu, Q. Meng, L. Wang, J. Zhang, Y. Song, H. Jin, K. Zhang, H. Sun, H. Wang, B. Yang, Angew. Chem. Int. Ed. 2013, 52, 3953; Angew. Chem. 2013, 125, 4045. [2] a) S. N. Baker, G. A. Baker, Angew. Chem. Int. Ed. 2010, 49, 6726; Angew. Chem. 2010, 122, 6876; b) S. Zhu, S. Tang, J. Zhang, B. Yang, Chem. Commun. 2012, 48, 4527. [3] H. T. Li, Z. H. Kang, Y. Liu, S. T. Lee, J. Mater. Chem. 2012, 22, 24230. [4] a) J. Shen, Y. Zhu, X. Yang, C. Li, Chem. Commun. 2012, 48, 3686; b) Z. P. Zhang, J. Zhang, N. Chen, L. T. Qu, Energy Environ. Sci. 2012, 5, 8869. [5] L. Bao, C. Liu, Z.-L. Zhang, D.-W. Pang, Adv. Mater. 2015, 27, 1663. [6] a) K. Lingam, R. Podila, H. Qian, S. Serkiz, A. M. Rao, Adv. Funct. Mater. 2013, 23, 5062; b) L. A. Ponomarenko, F. Schedin, M. I. Katsnelson, R. Yang, E. W. Hill, K. S. Novoselov, A. K. Geim, Science 2008, 320, 356; c) J. Peng, W. Gao, B. K. Gupta, Z. Liu, R. Romero-Aburto, L. Ge, L. Song, L. B. Alemany, X. Zhan, G. Gao, S. A. Vithayathil, B. A. Kaipparettu, A. A. Marti, T. Hayashi, J. J. Zhu, P. M. Ajayan, Nano Lett. 2012, 12, 844; d) L. Wang, S. J. Zhu, H.-Y. Wang, S. N. Qu, Y. L. Zhang, J. H. Zhang, Q. D. Chen, H. L. Xu, W. Han, B. Yang, H. B. Sun, ACS Nano 2014, 8, 2541. [7] D. Pan, J. Zhang, Z. Li, M. Wu, Adv. Mater. 2010, 22, 734. [8] a) S. Zhu, J. Zhang, X. Liu, B. Li, X. Wang, S. Tang, Q. Meng, Y. Li, C. Shi, R. Hu, B. Yang, RSC Adv. 2012, 2, 2717; b) S. Zhu, J. Zhang, S. Tang, C. Qiao, L. Wang, H. Wang, X. Liu, B. Li, Y. Li, W. Yu, X. Wang, H. Sun, B. Yang, Adv. Funct. Mater. 2012, 22, 4732. [9] Y. Dong, H. Pang, H. B. Yang, C. Guo, J. Shao, Y. Chi, C. M. Li, T. Yu, Angew. Chem. Int. Ed. 2013, 52, 7800; Angew. Chem. 2013, 125, 7954. [10] a) D. Kozawa, Y. Miyauchi, S. Mouri, K. Matsuda, J. Phys. Chem. Lett. 2013, 4, 2035; b) K. Hola, A. B. Bourlinos, O. Kozak, K. Berka, K. M. Siskova, M. Havrdva, J. Tucek, K. Safarova, M. Otyepka, E. P. Gianneis, R. Zboril, Carbon 2014, 70, 279. [11] X. Li, H. Wang, Y. Shimizu, A. Pyatenko, K. Kawaguchi, N. Koshizaki, Chem. Commun. 2011, 47, 932. [12] B. Konkena, S. Vasudevan, J. Phys. Chem. Lett. 2014, 5, 1. [13] a) X. Jia, J. Li, E. Wang, Nanoscale 2012, 4, 5572; b) H. Liu, T. Ye, C. Mao, Angew. Chem. Int. Ed. 2007, 46, 6473; Angew. Chem. 2007, 119, 6593; c) R. Ye, C. Xiang, J. Lin, Z. Peng, K. Huang, Z. Yan, N. P. Cook, E. L. G. Samuel, C. C. Hwang, G. Ruan, G. Ceriotti, A. R. O. Raji, A. A. Marti, J. M. Tour, Nat. Commun. 2013, 4, 2943; d) D. Pan, J. Zhang, Z. Li, C. Wu, X. Yan, M. Wu, Chem. Commun. 2010, 46, 3681; e) Y. Q. Dong, C. Q. Chen, X. T. Zheng, L. L. Gao, Z. M. Cui, H. B. Yang, C. X. Guo, Y. W. Chi, C. M. Li, J. Mater. Chem. 2012, 22, 8764. [14] S. H. Jin, D. H. Kim, G. H. Jun, S. H. Hong, S. Jeon, ACS Nano 2013, 7, 1239. [15] C. Galande, A. D. Mohite, A. V. Naumov, W. Gao, L. Ci, A. Ajayan, H. Gao, A. Srivastava, R. B. Weisman, P. M. Ajayan, Sci. Rep. 2011, 48, DOI: 10.1038/srep00085. [16] J. Hankache, M. Niemi, H. Lemmetyinen, O. S. Wenger, J. Phys. Chem. A 2012, 116, 8159. [17] Y. H. Tian, M. Kertesz, Macromolecules 2009, 42, 2309. [18] W. Hu, Q. Yan, D. Zhao, Chem. Eur. J. 2011, 17, 7087. [19] T. Han, Y. Zhang, X. Feng, Z. Lin, B. Tong, J. Shi, J. Zhi, Y. Dong, Chem. Commun. 2013, 49, 7049. [20] D. Pan, L. Guo, J. Zhang, C. Xi, Q. Xue, H. Huang, J. Li, Z. Zhang, W. Yu, Z. Chen, Z. Li, M. Wu, J. Mater. Chem. 2012, 22, 3314. [21] Z. Z. Sun, Z. Yan, J. Yao, E. Beitler, Y. Zhu, J. M. Tour, Nature 2010, 468, 549. [22] W. Wan, Z. Zhao, H. Hu, Y. Gogotsi, J. Qiu, Mater. Res. Bull. 2013, 48, 4797. [23] Y. Shin, J. Lee, J. Yang, J. Park, K. Lee, S. Kim, Y. Park, H. Lee, Small 2014, 10, 866. [24] S. K. Das, Y. Liu, S. Yeom, D. Y. Kim, C. I. Richards, Nano Lett. 2014, 14, 620.
8567
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Full Paper [25] G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, M. Chhowalla, Adv. Mater. 2010, 22, 505. [26] L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, K. S. Teng, C. M. Luk, S. Zeng, J. Hao, S. P. Lau, ACS Nano 2012, 6, 5102. [27] a) D. Fan, C. Zhang, J. He, R. Hua, Y. Zhang, Y. Yang, J. Mater. Chem. 2012, 22, 18564; b) X. Yu, Y. Huo, J. Yang, S. Chang, Y. Ma, W. Huang, Appl. Surf. Sci. 2013, 280, 450. [28] a) B. H. Stuart, Infrared Spectroscopy: Fundamentals and Applications. Wiley, 2004; b) T. Szabû, E. Tombcz, E. Ill¦s, I. D¦kny, Carbon 2006, 44, 537. [29] a) J. J. Max, C. Chapados, J. Phys. Chem. A 2004, 108, 3324; b) N. Mori, Y. Asano, T. Irie, Y. Tsuzuki, Bull. Chem. Soc. Jpn. 1969, 42, 482. [30] M. B. Hay, S. C. B. Myneni, Geochim. Cosmochim. Acta 2007, 71, 3518. [31] S. E. Cabaniss, J. A. Leenheer, I. F. McVey, Spectrochim. Acta Part A 1998, 54, 449. [32] a) W. Dong, Y. Zhou, D. Yan, H. Li, Y. Liu, Phys. Chem. Chem. Phys. 2007, 9, 1255; b) C. H. Specht, F. H. Frimmel, Phys. Chem. Chem. Phys. 2001, 3, 5444. [33] a) B. Konkena, S. Vasudevan, J. Phys. Chem. Lett. 2012, 3, 867; b) J. T. Paci, H. B. Man, B. Saha, D. Ho, G. C. Schatz, J. Phys. Chem. C 2013, 117, 17256. [34] M. M. Dudkina, A. V. Tenkovtsev, H. Komber, L. Haussler, F. Bohme, Macromolecules 2004, 37, 8389. [35] a) J. Hofkens, W. Schroeyers, D. Loos, M. Cotlet, F. Kohn, T. Vosch, M. Maus, A. Herrmann, K. Mullen, T. Gensch, F. C. De Schryver, Spectrochim. Acta Part A 2001, 57, 2093; b) S. K. Sonkar, M. Ghosh, M. Roy, A. Begum, S. Sarkar, Mater. Express 2012, 2, 105. [36] a) P. G. Luo, S. Sahu, S. T. Yang, S. K. Sonkar, J. Wang, H. Wang, G. E. LeCroy, L. Cao, Y. P. Sun, J. Mater. Chem. B 2013, 1, 2116; b) X. T. Zheng, A. Ananthanarayanan, K. Q. Luo, P. Chen, Small 2015, 11, 1620. [37] H. Z. Zheng, Q. L. Wang, Y. J. Long, H. J. Zhang, X. X. Huang, R. Zhu, Chem. Commun. 2011, 47, 10650. [38] T. A. Fayed, M. A. El-Morsi, M. N. El-Nahass, J. Photochem. Photobiol. A 2011, 224, 38. [39] a) S. C. Hung, A. N. Macpherson, S. Lin, P. A. Liddell, G. R. Seely, A. L. Moore, T. A. Moore, D. Gust, J. Am. Chem. Soc. 1995, 117, 1657; b) K. Kawai, Y. Osakada, T. Takada, M. Fujitsuka, T. Majima, J. Am. Chem. Soc. 2004, 126, 12843; c) S. Fukuzumi, K. Okamoto, Y. Yoshida, H. Imahori, Y.
Chem. Eur. J. 2015, 21, 8561 – 8568
www.chemeurj.org
[40] [41]
[42]
[43]
[44] [45] [46] [47] [48]
Araki, O. Ito, J. Am. Chem. Soc. 2003, 125, 1007; d) K. Okamoto, S. Fukuzumi, J. Phys. Chem. B 2005, 109, 7713; e) S. Fukuzumi, Y. Yoshida, K. Okamoto, H. Imahori, Y. Araki, O. Ito, J. Am. Chem. Soc. 2002, 124, 6794; f) J. Hankache, D. Hanss, O. S. Wenger, J. Phys. Chem. A 2012, 116, 3347; g) K. Sakai, S. Takahashi, A. Kobayashi, T. Akutagawa, T. Nakamura, M. Dosen, M. Kato, U. Nagashima, Dalton Trans. 2010, 39, 1989; h) M. Jozefowicz, K. A. Kozyra, J. R. Heldt, Chem. Phys. 2005, 320, 45. X. Wang, L. Cao, F. Lu, M. J. Meziani, H. Li, G. Qi, B. Zhou, B. A. Harruff, F. Kermarrec, Y. P. Sun, Chem. Commun. 2009, 3774. a) B. Y. Yu, S. Y. Kwak, J. Mater. Chem. 2012, 22, 8345; b) H. Zhang, H. Huang, H. Ming, H. Li, L. Zhang, Y. Liu, Z. Kang, J. Mater. Chem. 2012, 22, 10501; c) H. Li, R. Liu, Y. Liu, H. Huang, H. Yu, H. Ming, S. Lian, S. T. Lee, Z. Kang, J. Mater. Chem. 2012, 22, 17470; d) D. Pan, C. Xi, Z. Li, L. Wang, Z. Chen, B. Luc, M. Wu, J. Mater. Chem. A 2013, 1, 3551; e) D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, Z. Xie, Z. Sun, Nanoscale 2013, 5, 12272; f) X. Yu, J. Liu, Y. Yu, S. Zuo, B. Li, Carbon 2014, 68, 718; g) L. Wang, Y. Wang, T. Xu, H. Liao, C. Yao, Y. Liu, Z. Li, Z. Chen, D. Pan, L. Sun, M. Wu, Nat. Commun. 2014, 5, 5357. a) H. Yu, Y. Zhao, C. Zhou, L. Shang, Y. Peng, Y. Cao, L. Z. Wu, C. H. Tung, T. Zhang, J. Mater. Chem. A 2014, 2, 3344; b) K.-A. Tsai, Y.-J. Hsu, Appl. Catal. B 2015, 164, 271; c) H. Li, J. Xing, Z. Xia, J. Chen, Carbon 2015, 81, 474. a) S. Hu, R. Tian, Y. Dong, J. Yang, J. Liu, Q. Chang, Nanoscale 2013, 5, 11665; b) L. Cao, S. Sahu, P. Anilkumar, C. E. Bunker, J. Xu, K. A. S. Fernando, P. Wang, E. A. Guliants, K. N. Tackett, Y. P. Sun, J. Am. Chem. Soc. 2011, 133, 4754. S. Hu, R. Tian, L. Wu, Q. Zhao, J. Yang, J. Liu, S. Cao, Chem. Asian J. 2013, 8, 1035. S. J. Zhuo, M. W. Shao, S. T. Lee, ACS Nano 2012, 6, 1059. O. S. Wenger, Acc. Chem. Res. 2013, 46, 1517. H. Li, X. He, Y. Liu, H. Huang, S. Lian, S. T. Lee, Z. Kang, Carbon 2011, 49, 605. L. Liu, J. Hao, Y. Shi, J. Qiu, C. Hao, RSC Adv. 2015, 5, 3045.
Received: September 1, 2014 Published online on April 29, 2015
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& Nanoparticles
Intramolecular Hydrogen Bonds Quench Photoluminescence and Enhance Photocatalytic Activity of Carbon Nanodots Pengju Yang,[a, b] Jianghong Zhao,*[a] Lexi Zhang,[c] Li Li,[a] and Zhenping Zhu*[a] Abstract: Understanding the photoluminescence (PL) and photocatalytic properties of carbon nanodots (CNDs) induced by environmental factors such as pH through surface groups is significantly important to rationally tune the emission and photodriven catalysis of CNDs. Through adjusting the pH of an aqueous solution of CNDs, it was found that the PL of CNDs prepared by ultrasonic treatment of glucose is strongly quenched at pH 1 because of the formation of intramolecular hydrogen bonds among the oxygen-containing surface groups. The position of the strongest PL peak and its
Introduction Carbon nanodots (CNDs) have superior optical and electronic properties combined with significant advantages such as high aqueous solubility, robust chemical inertness, easy functionalization, low toxicity, and high resistance to photobleaching. Thus, many studies have been performed on CNDs in a wide range of applications, such as biological labeling, bioimaging, drug delivery, sensors, photocatalysis, and optoelectronic devices.[1–4] Photoluminescence (PL) is the most interesting property of CNDs; most studies have focused on developing controllable synthetic techniques for CNDs and tuning their strong PL emissions.[2, 3] To rationally achieve these objectives, understanding the factors affecting the emission properties of CNDs and investigating their PL mechanism are very important. However, engineering a unique PL emission and understanding the PL mechanism remain difficult because of the highly nonstoichiometric and heterogeneous structure, size, and surface state of CNDs. For example, the mechanism underlying the [a] Dr. P. Yang, Dr. J. Zhao, L. Li, Prof. Dr. Z. Zhu State Key Laboratory of Coal Conversion Institute of Coal Chemistry, Chinese Academy of Sciences Taiyuan, 030001 (P. R. China) E-mail: [email protected] [b] Dr. P. Yang University of Chinese Academy of Sciences Beijing, 100039 (P. R. China) [c] Dr. L. Zhang Department of Material Physics Institute of Material Science and Engineering Tianjin University of Technology Tianjin, 300384 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405088. Chem. Eur. J. 2015, 21, 8561 – 8568
corresponding excitation wavelength strongly depend on the surface groups. The origins of the blue and green emissions of CNDs are closely related to the carboxyl and hydroxyl groups, respectively. The deprotonated COO¢ and CO¢ groups weaken the PL peak of the CNDs and shift it to the red. CNDs alone exhibit photocatalytic activity towards degradation of Rhodamine B at different pH values under UV irradiation. The photocatalytic activity of the CNDs is the highest at pH 1 because of the strong intramolecular hydrogen bonds formed among the oxygen-containing groups.
unique excitation-wavelength-dependent emission feature remains unclear, although several origins of PL in CNDs have been tentatively proposed, including radiative recombination of excitons, emissive traps, quantum-confinement effect, aromatic structures/oxygen-containing groups, free zigzag sites, and/or edge defects.[1a, 5–8] Factors affecting the emission properties include the size (or size distribution),[9] element doping,[10] and surface states[8b] of CNDs. An increasing amount of research has shown that the abundant surface groups are directly related to the emission intensity, band shape,[8b] and band position[11] of CNDs. The abundant chemistry and flexible reactivity of surface groups are important in producing CNDs with very high sensitivity to the working environment, such as solvent[12] and pH.[13] Therefore, elucidating the PL mechanism of the surface chemistry of CNDs and investigating the functional mechanism of the effect of the environment through surface groups on the PL properties of CNDs are important. Most of the transformations in the above-mentioned applications require aqueous environments as the reaction media; thus, controllably preserving and tuning the PL properties of CNDs in aqueous systems are important. The pH value is one of the most important factors that significantly affect numerous chemical and biological processes in water. Moreover, the PL of CNDs is very sensitive to pH.[8, 14, 15] However, few studies have investigated the mechanism underlying the pH-induced phenomenon. Previously, Pan et al. attributed the quenching of PL under acidic conditions to the formation of a reversible non-emissive complex between the emissive free zigzag sites and H + , while free zigzag sites can be restored under alkaline conditions through deprotonation, which leads to the restoration of PL at high pH.[7] Jeon et al. proposed that at high pH the COOH groups of CNDs are deprotonated to COO¢ groups
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Full Paper bearing an additional electron, which increases their electron density. The electron-withdrawing properties of the carboxyl groups towards CNDs are weakened by the extra electron, and this results in a decreased bandgap of the CNDs and a redshift of the maximum PL peak.[15] Galande et al. found that the fluorescence of graphene oxide (GO) originates from the quasimolecular fluorophores formed by electronic coupling of COOH groups with the nearby sp2 carbon atoms; the pH-dependent fluorescence characteristic of GO is attributed to excited-state protonation of the emitting (G–COO¢)* to (G–COOH)* species.[16] Moreover, Konkena et al. corroborated that the pH-dependent nature of the fluorescence spectra of GO results from excited-state proton transfer involving quasimolecular fluorophores.[13] Carboxyl and hydroxyl groups are generally regarded as the main oxygen-containing groups in CNDs[11, 16] and are often located at the edge and defect sites. These oxygen-containing groups can be protonated easily under appropriate acidic conditions to form intramolecular hydrogen bonds. Intramolecular hydrogen bonding is an important noncovalent interaction with a considerable effect on the geometric, electronic, vibrational, and radiationless-transition properties of substituted aromatic compounds[17] and their related conjugated oligomers or polymers.[18] Thus, substituted molecules would be able to control their absorption and emission performance.[19, 20] To the best of our knowledge, no reports have focused on the effects of intramolecular hydrogen bonds on the optical and electronic properties of CNDs. In the present study, the pH value of an aqueous solution of CNDs was adjusted to ensure that their sizes and core carbon structures are similar under different conditions. TEM, AFM, XRD, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), FTIR spectroscopy, and steady-state PL measurements were performed to characterize the morphology, structure, oxygen-containing groups, and PL spectra of CNDs. Strong intramolecular hydrogen bonds were formed among the oxygen-containing groups of the CNDs at pH 1. The intramolecular hydrogen bonds significantly quenched the PL intensities of the CNDs without dependence on the excitation wavelength. Furthermore, photocatalysis, which is a competitive process of PL, was investigated. Photocatalytic studies on the decomposition of Rhodamine B (RB) showed that CNDs alone have the highest photocatalytic activity at pH 1 because of the formed intramolecular hydrogen bonds.
Results and Discussion The TEM image in Figure 1 a shows that the as-prepared CNDs are well-dispersed nanospheres with homogeneous size. The average diameter of the CNDs, obtained by statistical analysis of 300 nanoparticles, is 3.2 nm (for size distribution, see inset of Figure 1 a). The AFM image (Supporting Information, Figure S1) showed that the height of CNDs is about 3 nm, which is close to the size distribution shown by TEM. Figure 1 b shows the high-resolution TEM (HRTEM) image of the CNDs. The lattice distance of 0.21 nm, which is consistent with the (0110) lattice plane of most CNDs, indicates good crystallinity Chem. Eur. J. 2015, 21, 8561 – 8568
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Figure 1. a) TEM and b) HRTEM images of CNDs prepared by ultrasonic treatment of glucose. Inset: size distribution of the as-prepared CNDs.
of the CNDs.[21] The increased degree of graphitic crystallinity resulted in better photostability. This finding is confirmed by the fact that no aggregation or precipitation could be observed after one-year storage without shielding from light. The XRD pattern of the CNDs (Supporting Information, Figure S3) showed a broad peak at about 208, corresponding to the graphitic structure in nanosized carbon materials.[6c] The Raman spectrum provided compelling evidence for the different types of ordered and disordered bonding environments of the sp2- and sp3-hybridized carbon. In Figure 2 the peak at
Figure 2. Raman spectrum of the CNDs.
1588 cm¢1 is the G band corresponding to the E2g mode of the ordered sp2-bonded carbon atoms, whereas the D band located at 1378 cm¢1 is disordered because of the scattering at the sp3-bonded edge carbon atoms. A weak and broad 2D peak observed at 2770 cm¢1 on the strong fluorescence background indicates better graphitization of the CNDs with few layers of graphene.[22] Moreover, the G and D bands overlaid on a strong fluorescence background are indicative of defects and functional groups in the CNDs.[23] The relative intensity ratio of D to G band (ID/IG) is characteristic of the degree of disorder in graphitic carbons; the relative intensity of the CNDs of 0.77 is smaller than those reported previously.[21, 24] These results confirmed the well-crystallized sp2 cores and the partial amorphous nature of the edges of the CNDs, consistent with the HRTEM and XRD characterizations. In addition to the morphological and structural factors, surface chemistry is important in controlling the electronic and optical properties of carbon nanostructures.[25, 26] XPS is a pow-
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Figure 4. FTIR spectra of the CNDs at different pH values.
Figure 3. High-resolution C 1s (a) and O 1s (b) XPS spectra of the CNDs.
erful technique for determining the chemical structure and composition of carbon materials, because the binding energy of the core-level peak provides elemental and chemical information. Figure 3 a shows the high-resolution C 1s XPS spectrum of the as-prepared CNDs. The spectrum can be curve-fitted to five component peaks with binding energies of 284.5, 285.5, 286.6, 287.2, and 288.6 eV, which correspond to sp2-hybridized carbon (C=C), sp3-hybridized carbon (C¢C and C¢H), and hydroxyl (COH), carbonyl (C=O), and carboxyl (O=COH) species,[27] respectively. The sp2 and sp3 C peaks are stronger than the other three peaks from various carbon–oxygen binding configurations. This confirms that the CNDs are constructed mainly from graphitic cores that have a large number of amorphous edges with COH, C=O, and O=COH groups. The high-resolution O 1s XPS spectrum (Figure 3 b) exhibited three component peaks at 531.1, 532.5, and 533.8 eV, which are attributed to quinone, O=CO/C=O, and C¢O species,[28] respectively. Thus, the presence of carbonyl, carboxyl, and hydroxyl groups in the CNDs is further confirmed. The carboxyl and hydroxyl groups are sensitive to pH. FTIR spectroscopy is a very useful structure-sensitive technique to determine the conformation, association, dissociation, and bonding state of the oxygen-containing groups in the CNDs at varying pH values (Figure 4). A broad band from 3800 to 2300 cm¢1 in the IR spectrum at pH 1, corresponds to the O¢H stretching vibration of the adsorbed water molecules and the structural carboxyl and hydroxyl groups.[29] However, this continuously extended absorption is clearly different from the O¢ H stretching band of water and free carboxyl and hydroxyl groups and indicates that strong intramolecular hydrogen bonds are formed among the carbonyl, carboxyl, and hydroxyl Chem. Eur. J. 2015, 21, 8561 – 8568
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groups of the CNDs.[29–31] Moreover, intermolecular hydrogen bonds were not formed between water and the oxygen-containing groups in the CNDs, because their absorption bands remained unaffected on dilution (data not shown). Further evidence for the existence of intramolecular hydrogen bonds is the strong and broadened C=O stretching band in the range of 2000 to 1700 cm¢1 at pH 1. Moreover, the peak located at 1030 cm¢1 in the IR spectrum at pH 1 corresponds to the C¢O stretch of the carboxyl and hydroxyl groups.[31] Overlap of this peak with the strong O¢H bending bands of the carboxyl and hydroxyl groups in the range of 1400 to 1200 cm¢1 yields a single broad absorption band.[32] This finding also confirms the formation of intramolecular hydrogen bonds in the CNDs at pH 1. Increasing the pH from 3 to 9 caused the broad O¢H stretching band to split into a flat and wide peak from 3800 to 3000 cm¢1 and a shoulder peak centered at 2945 cm¢1 in the IR spectrum. The peak at 2945 cm¢1 is assigned to the carboxyl O¢H stretching absorption,[31] whereas that in the range of 3800 to 3000 cm¢1 is due to the overlapping O¢H stretching bands of the adsorbed water molecules and various hydroxyl groups. The splitting of the O¢H stretching band indicates that the intramolecular hydrogen bonds in the CNDs were broken. At pH 12, the flat peak in the range of 3800 to 3000 cm¢1 transformed into a sharp peak centered at 3414 cm¢1, whereas the carboxyl O¢H stretching band at 2945 cm¢1 became very weak. This implies that the carboxyl and hydroxyl groups in the CNDs were heavily deprotonated, which was further confirmed by the appearance of a new absorption peak centered at 1595 cm¢1. This peak corresponds to the asymmetric stretching vibration of the COO¢ groups.[31, 33] Moreover, the C= O stretching band centered at 1750 cm¢1 and the strong and unstructured O¢H bending vibrations within the range of 1400 cm¢1 to 1200 cm¢1 disappeared in the IR spectrum at pH 12, whereas the C¢O stretching band was blue-shifted from 1030 to 1085 cm¢1. These results substantiate the abundant COO¢ and CO¢ groups in the CNDs at pH 12. At pH 9 and 7, the C=O stretching band at 1750 cm¢1 observed for the CNDs at pH 3 and 1 disappeared. The carboxyl O¢H stretching band at 2945 cm¢1 and the unstructured O¢H bending bands in the range of 1400 to 1200 cm¢1 of the CNDs at pH 9 became weaker than those of the CNDs at pH 7 and 3.
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Full Paper Moreover, an additional shoulder of the COO¢ asymmetric stretching band at 1595 cm¢1 appeared in the CNDs at pH 9, in contrast to the CNDs at pH 7 and 3. These findings suggest that the carboxyl groups in the CNDs started to be deprotonated at pH 7, whereas the hydroxyl groups maintained their protonated form. At pH 9, most of the carboxyl groups were deprotonated and the hydroxyl groups started to be deprotonated. This result is in accordance with the typical pKa values in the ranges of 3–6 and 7–10 for the aromatic carboxyl and hydroxyl groups of carbon materials, respectively.[34] At pH 3, the C=O stretching band at 1750 cm¢1 still exists but has became narrower and weaker in contrast to that of the CNDs at pH 1. Simultaneously, the carboxyl O¢H stretching band at 2945 cm¢1 and the unstructured O¢H bending bands in the range of 1400–1200 cm¢1 are stronger than those of the CNDs at pH 7 and 9. These results indicate that the carboxyl and hydroxyl groups in CNDs at pH 3 are all protonated, whereas the intramolecular hydrogen bonds among these groups are significantly broken. Figure 5 shows the UV/Vis absorption spectra of the CNDs at pHs 1, 3, 7, 9, and 12. The common feature is the two main ab-
Figure 5. UV/Vis absorption spectra of the CNDs at different pH values.
sorption peaks in each spectrum. The peak below 250 nm (peak 1) is due to the p–p* transitions of the aromatic sp2 domains in the CNDs, whereas the peak above 250 nm (peak 2) is attributed to the n–p* transitions of C=O in the sp3-hybridized regions. Moreover, an evident change in the absorption spectra is the redshift of peaks 1 and 2 with increasing pH value from 3 to 12. This finding is attributed to deprotonation of the carboxyl and hydroxyl groups, which results in the formation of negatively charged species from these oxygen-containing groups. The negative charges increase the electron density of the C=O moieties and the aromatic sp2 domains, and thus lower the bandgap of the CNDs,[35] which results in a strong redshift of the p–p* and n–p* transition absorption bands. Peak 2 in the spectrum at pH 1 shows an evident redshift compared with the absorption spectrum at pH 3. The FTIR data indicate that this is attributable to the intramolecular hydrogen bonds formed among the carbonyl, carboxyl, and hydroxyl groups of the CNDs at pH 1. After the intramolecular hydrogen bonds are formed, all core-carbon backbones are constrained in a near-planar conformation,[19] and thereby the p conjugaChem. Eur. J. 2015, 21, 8561 – 8568
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tion of the system is extended and charge transfer between the electron-donating hydroxyl and electron-withdrawing carboxyl groups is facilitated. This increases the electron density of the C=O moieties and results in the redshift of the n–p* transition band in the absorption spectrum. The CNDs are fluorescent materials, as shown by the fluorescence images taken under visible and UV irradiation (Supporting Information, Figure S2). The UV-excited CNDs exhibit strong, bright blue luminescence. A series of PL spectra (Figure 6) provided more information regarding the pH-tuning effect on the emission properties of the as-prepared CNDs. The common feature is the redshift of the PL peaks measured at different pH values with increasing excitation wavelength, which suggests that the luminescent CNDs possibly have multiple chromophores.[4b, 25, 36] Moreover, an evident difference is the significant decrease of the PL emission intensities at pH 1 at different excitation wavelengths compared with the PL spectra measured at four other pH values (Figure 6 a). The PL spectra obtained at pH 3–12 clearly showed that the three main PL peaks correspond to blue (centered at 450 nm), green (centered at 520 nm), and yellow (centered at 570 nm) emissions. The intensities of these three peaks depend on the excitation wavelength and change with varying pH value. The strongest PL peak with a broad tail extending to 800 nm is located at 450 nm in the blue spectral region for excitation at 360 nm under acidic conditions at pH 3 (Figure 6 b). The green emission peak at 520 nm exhibited the highest PL intensity under neutral conditions (pH 7) on excitation at a higher wavelength of 420 nm (Figure 6 c). At pH 9 (Figure 6 d), the peak positions and sequence of the three main PL emissions remained unchanged, whereas the intensity of the highest green emission peak on 420 nm excitation decreased significantly. At pH 12, the strongest PL peak for 500 nm excitation centered at 570 nm shifted into the yellow spectral range (Figure 6 e). Remarkably, the intensity of the yellow emission increased sharply at pH 12, which is higher than the pKa values in the range of 7–10 for the aromatic hydroxyl groups in carbon materials.[34] Size, physical structure, chemical defects, and surface passivation significantly affect the emission properties of carbon nanomaterials.[3, 37] In the present study, only pH was adjusted for one type of CNDs. This strategy significantly simplifies the complexity of the factors that determine the PL properties of CNDs. Moreover, the effects of size and physical structure on the PL behavior are excluded, because the size and physical structure of CNDs cannot be changed by varying the pH value. As confirmed by XPS and FTIR spectroscopy, carbonyl, carboxyl, and hydroxyl groups are the main oxygen-containing species in the as-prepared CNDs; the chemical structures of these groups are very sensitive to pH. Therefore, the PL behavior shown in Figure 6 should result from the chemical defect emission states involving oxygen-containing groups, which have been proposed as one of the important origins of PL in carbon nanomaterials.[5, 38] The FTIR spectra (Figure 4) corroborated that the intramolecular hydrogen bonds formed among carbonyl, carboxyl, and hydroxyl groups are broken, whereas the carboxyl and hydroxyl groups exist in their protonated forms at pH 3. At a neutral
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Figure 6. Photoluminescence spectra of the CNDs at a) pH 1, b) pH 3, c) pH 7, d) pH 9, and e) pH 12.
pH of 7, some carboxyl groups are deprotonated to ionic COO¢ groups, whereas the hydroxyl groups remain protonated. At pH 9, some hydroxyl groups are also deprotonated to ionic CO¢ groups, besides the largely deprotonated COO¢ moieties. At pH 12, the carboxyl and hydroxyl groups are deprotonated to their ions. In combination with the FTIR results, the PL spectra indicate that the broad blue PL peak under excitation at 360 nm is closely related to the emission centers involving carboxyl groups, whereas the green PL peak on excitation at a higher wavelength of 420 nm is due to the emission centers related to hydroxyl groups. At pH 3, the blue PL curves excited at 360 nm are deconvoluted into two Gaussian peaks centered at 450 (peak 1) and 515 nm (peak 2; Figure 6 f). At pH 7, the component peak 1 significantly decreased and became weaker than peak 2. These results indicate that two Chem. Eur. J. 2015, 21, 8561 – 8568
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types of carboxyl group are involved in the emission centers (EmC), the quasimolecular structures of which correspond to the aromatic carboxyl group or to a hydroxyl group (EmC 1) and the isolated aromatic carboxyl groups (EmC 2).[11a] The components of the PL peaks 1 and 2 are assigned to EmC 1 and EmC 2, respectively, because EmC 1 exhibits lower pKa values than EmC 2.[13] In addition, under extremely basic conditions of pH 12, the strongest yellow PL peak on excitation at 500 nm originates from ionic COO¢- and > CO¢-related moieties; the CO¢ groups play a more prominent role, because the sharp increase of the yellow emission intensity only appears under extremely basic conditions (pH 12). More importantly, the serious reduction of the PL peak intensities at pH 1 implies that intramolecular hydrogen bonds significantly quench the PL emission of CNDs.
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Full Paper Numerous studies have shown that intramolecular and intermolecular hydrogen bonds play sensitive roles in controlling the optical absorption and emission properties of molecular chromophores[20, 39] through photoinduced proton transfer or electron transfer.[40] Excited-state proton transfer has been recently been found to be of the key factors that determines the spectral migration of the fluorescence in aqueous dispersions of GO.[15] In the present study, the quasimolecular emission centers in the CNDs were reasonably inferred to function as molecular chromophores; thus, intramolecular hydrogen bonds can modulate the PL characteristics of CNDs through photoinduced proton transfer or electron transfer. This hypothesis requires further experimental investigation and theoretical calculations. CNDs function as excellent photoinduced electron donors and acceptors.[41] In many of the composite photocatalysts reported in the literature, the CNDs have multiple functions in promoting a wider absorption spectrum, separating electrons and holes, and stabilizing the photolysis of semiconductors.[41d, 42, 43] However, limited studies reported that CNDs alone can function as an efficient photocatalysts,[44, 45] but research on the detailed photocatalytic features of CNDs is insufficient. The present study explored the photocatalytic behavior of the as-prepared CNDs towards RB at different pH values under UV (300 W high-pressure Hg lamp), visible (300 W Xe lamp with a cutoff filter), and UV/Vis light (300 W Xe lamp). The CNDs have no photocatalytic activity for RB degradation under visible-light irradiation at varying pH values (data not shown). However, the photocatalytic activity of the CNDs for RB degradation significantly increased with decreasing pH value under UV irradiation (Figure 7 a), and only the CNDs at pH 1 showed
evident photocatalytic ability (Figure 7 b). These results (with a deviation of 5 % from the average) indicate that the as-prepared CNDs alone are the UV-responsive photocatalysts. The difference in photocatalytic performance under UV and UV/Vis irradiation is due to the discrepancy in UV light intensity from high-pressure Hg and Xe lamps. This finding is different from the reported visible-light-active carbon-dot photocatalysts,[45, 46] the visible-light responsivity of which is partly attributed to the exceptional upconversion property.[43, 46] The effects of RB adsorption on the photocatalytic activity of CNDs were excluded in the present study by establishing an adsorption/desorption equilibrium in the dark prior to illumination. Moreover, the UV intensity in the spectrum produced by the Xe lamp is lower than that emitted by the Hg lamp. Thus, the different photocatalytic behaviors of similar CNDs at similar pH values under UV and UV/Vis irradiation originate from the light-intensity-induced differentiation of the intrinsic photocatalytic ability of the CNDs with modulated structures caused by the change in pH values (e.g., the formation of intramolecular hydrogen bonds at pH 1). This finding indicates that the highest photocatalytic activity of the CNDs at pH 1 can be attributed to the formation of intramolecular hydrogen bonds in the carbon structure. Long-lived charge-separated states are key factors that determine the photocatalytic activity in solar energy utilization. Hydrogen bonds can cause thermodynamic and kinetic stabilization of the charge-separated states in the photoactive molecular systems.[17, 47, 48] As the key surface states, the oxygen-containing groups can possibly determine the photocatalytic activity of the carbon dots alone because of their upward band-bending effect, which controls the separation and combination of the photogenerated electron–hole pairs.[45] In the present study, the intramolecular hydrogen bonds constrain the core-carbon backbone in a nearly planar conformation, which extends the p conjugation of the system and facilitates charge transfer between the electron-donating hydroxyl and electron-withdrawing carboxyl groups. The enhanced synergistic band-bending effect induced by the hydrogen bonds formed among the carboxyl and hydroxyl groups can significantly benefit the separation and stabilization of the photogenerated electron–hole pairs and result in faster carrier migration and hence impressively high photocatalytic activity.
Conclusion
Figure 7. Photodegradation of RB over CNDs under a) UV light and b) UV/Vis light. Chem. Eur. J. 2015, 21, 8561 – 8568
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We have systematically investigated the pH-dependent PL characteristics of CNDs and determined the mechanisms underlying the PL behavior. The luminescent CNDs exhibit multiple chromophores and the oxygen-containing groups significantly affect the position of the strongest PL peak and its excitation wavelength. The origins of the blue and green emissions in CNDs are closely related to carboxyl and hydroxyl groups, respectively. The deprotonated COO¢ and CO¢ groups weakened and caused redshifts of the PL peak of the CNDs. More importantly, the PL of the as-prepared CNDs was strongly quenched at pH 1 because of the formation of intramolecular hydrogen bonds among the oxygen-containing groups. Furthermore, CNDs alone exhibited impressive and high photoca-
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Full Paper talytic activity for RB degradation at pH 1 under UV irradiation. The hydrogen bonds formed among carboxyl and hydroxyl groups enhanced the synergistic band-bending effect. This enhancement is significantly advantageous for the separation and stabilization of photogenerated electron–hole pairs and therefore results in high photocatalytic activity. These findings provide new insight into the complexity of the optical properties of CNDs and the effects of the environment on their structures and properties to control and modify the applications of carbon dot materials.
Experimental Section Ultrasonic synthesis of CNDs All chemicals were of analytical grade and used without further purification. The CNDs were prepared by a typical ultrasonic method,[47] in which glucose (9 g) was dissolved in deionized water (50 mL) to form a clear solution, after which HCl (50 mL, 36– 38 wt %) was added and the solution was treated ultrasonically for 4 h to obtain a solution of CNDs. The pH of the CND solution was adjusted with HCl or NaOH.
Characterization TEM images were obtained by using a JEOL JEM-2010 microscope with an accelerating voltage of 200 kV. FTIR was performed with a Nicolet Magna-IR 550-II spectrometer by using KBr pellets. Raman spectra were recorded with a LABRAM-HR Raman spectrometer with excitation wavelength of 514 nm. UV/Vis absorption spectroscopy was performed on the CND solution in a 1 cm quartz cuvette by using a Shimadzu UV-3600 UV/Vis/NIR spectrophotometer at room temperature. PL was determined by using a Hitachi F7000 FL spectrophotometer. All UV/Vis and PL measurements were performed for three parallel samples under the same conditions to obtain high reproducibility of the spectra. The height of CNDs was characterized by AFM (Bruker MultiMode V). The photographs under visible and UV light irradiation were obtained under a fluorescence microscope (Leica DM4000M). XRD analysis was conducted on a D8 Advance Bruker X-ray diffractometer with CuKa radiation (l = 0.15406 nm) operating at 40 kV.
Measurement of photocatalytic activity Photocatalytic activity was determined by adding the CND solution (20 mL, about 0.02 g of CNDs) to 150 mL of RB (20 mg L¢1). The solution was irradiated under UV (300 W high-pressure Hg lamp), visible (300 W Xe lamp with a cut-off filter), and UV/Vis light (300 W Xe lamp) at ambient atmosphere. The concentration of the residual target molecule (RB) was determined by means of the absorbance at 554 nm. The solution of RB dye and CND photocatalyst was by stirred in the dark for 12 h prior to illumination to exclude the effect of dye adsorption on the photocatalytic activity. Each photocatalytic measurement was performed three times with good reproducibility.
Acknowledgements We acknowledge the financial support from the Nature Science Foundation of China (No. 21173250), the Knowledge Innovation Project of Chinese Academy of Sciences (No. KGCX2-EWChem. Eur. J. 2015, 21, 8561 – 8568
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311), the Youth Innovation Foundation of Institute of Coal Chemistry, and Chinese Academy of Sciences (2011SQNRC19). Keywords: carbon · hydrogen bonds · luminescence · nanoparticles · photochemistry [1] a) Y. P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. Wang, J. Am. Chem. Soc. 2006, 128, 7756; b) S. Zhu, Q. Meng, L. Wang, J. Zhang, Y. Song, H. Jin, K. Zhang, H. Sun, H. Wang, B. Yang, Angew. Chem. Int. Ed. 2013, 52, 3953; Angew. Chem. 2013, 125, 4045. [2] a) S. N. Baker, G. A. Baker, Angew. Chem. Int. Ed. 2010, 49, 6726; Angew. Chem. 2010, 122, 6876; b) S. Zhu, S. Tang, J. Zhang, B. Yang, Chem. Commun. 2012, 48, 4527. [3] H. T. Li, Z. H. Kang, Y. Liu, S. T. Lee, J. Mater. Chem. 2012, 22, 24230. [4] a) J. Shen, Y. Zhu, X. Yang, C. Li, Chem. Commun. 2012, 48, 3686; b) Z. P. Zhang, J. Zhang, N. Chen, L. T. Qu, Energy Environ. Sci. 2012, 5, 8869. [5] L. Bao, C. Liu, Z.-L. Zhang, D.-W. Pang, Adv. Mater. 2015, 27, 1663. [6] a) K. Lingam, R. Podila, H. Qian, S. Serkiz, A. M. Rao, Adv. Funct. Mater. 2013, 23, 5062; b) L. A. Ponomarenko, F. Schedin, M. I. Katsnelson, R. Yang, E. W. Hill, K. S. Novoselov, A. K. Geim, Science 2008, 320, 356; c) J. Peng, W. Gao, B. K. Gupta, Z. Liu, R. Romero-Aburto, L. Ge, L. Song, L. B. Alemany, X. Zhan, G. Gao, S. A. Vithayathil, B. A. Kaipparettu, A. A. Marti, T. Hayashi, J. J. Zhu, P. M. Ajayan, Nano Lett. 2012, 12, 844; d) L. Wang, S. J. Zhu, H.-Y. Wang, S. N. Qu, Y. L. Zhang, J. H. Zhang, Q. D. Chen, H. L. Xu, W. Han, B. Yang, H. B. Sun, ACS Nano 2014, 8, 2541. [7] D. Pan, J. Zhang, Z. Li, M. Wu, Adv. Mater. 2010, 22, 734. [8] a) S. Zhu, J. Zhang, X. Liu, B. Li, X. Wang, S. Tang, Q. Meng, Y. Li, C. Shi, R. Hu, B. Yang, RSC Adv. 2012, 2, 2717; b) S. Zhu, J. Zhang, S. Tang, C. Qiao, L. Wang, H. Wang, X. Liu, B. Li, Y. Li, W. Yu, X. Wang, H. Sun, B. Yang, Adv. Funct. Mater. 2012, 22, 4732. [9] Y. Dong, H. Pang, H. B. Yang, C. Guo, J. Shao, Y. Chi, C. M. Li, T. Yu, Angew. Chem. Int. Ed. 2013, 52, 7800; Angew. Chem. 2013, 125, 7954. [10] a) D. Kozawa, Y. Miyauchi, S. Mouri, K. Matsuda, J. Phys. Chem. Lett. 2013, 4, 2035; b) K. Hola, A. B. Bourlinos, O. Kozak, K. Berka, K. M. Siskova, M. Havrdva, J. Tucek, K. Safarova, M. Otyepka, E. P. Gianneis, R. Zboril, Carbon 2014, 70, 279. [11] X. Li, H. Wang, Y. Shimizu, A. Pyatenko, K. Kawaguchi, N. Koshizaki, Chem. Commun. 2011, 47, 932. [12] B. Konkena, S. Vasudevan, J. Phys. Chem. Lett. 2014, 5, 1. [13] a) X. Jia, J. Li, E. Wang, Nanoscale 2012, 4, 5572; b) H. Liu, T. Ye, C. Mao, Angew. Chem. Int. Ed. 2007, 46, 6473; Angew. Chem. 2007, 119, 6593; c) R. Ye, C. Xiang, J. Lin, Z. Peng, K. Huang, Z. Yan, N. P. Cook, E. L. G. Samuel, C. C. Hwang, G. Ruan, G. Ceriotti, A. R. O. Raji, A. A. Marti, J. M. Tour, Nat. Commun. 2013, 4, 2943; d) D. Pan, J. Zhang, Z. Li, C. Wu, X. Yan, M. Wu, Chem. Commun. 2010, 46, 3681; e) Y. Q. Dong, C. Q. Chen, X. T. Zheng, L. L. Gao, Z. M. Cui, H. B. Yang, C. X. Guo, Y. W. Chi, C. M. Li, J. Mater. Chem. 2012, 22, 8764. [14] S. H. Jin, D. H. Kim, G. H. Jun, S. H. Hong, S. Jeon, ACS Nano 2013, 7, 1239. [15] C. Galande, A. D. Mohite, A. V. Naumov, W. Gao, L. Ci, A. Ajayan, H. Gao, A. Srivastava, R. B. Weisman, P. M. Ajayan, Sci. Rep. 2011, 48, DOI: 10.1038/srep00085. [16] J. Hankache, M. Niemi, H. Lemmetyinen, O. S. Wenger, J. Phys. Chem. A 2012, 116, 8159. [17] Y. H. Tian, M. Kertesz, Macromolecules 2009, 42, 2309. [18] W. Hu, Q. Yan, D. Zhao, Chem. Eur. J. 2011, 17, 7087. [19] T. Han, Y. Zhang, X. Feng, Z. Lin, B. Tong, J. Shi, J. Zhi, Y. Dong, Chem. Commun. 2013, 49, 7049. [20] D. Pan, L. Guo, J. Zhang, C. Xi, Q. Xue, H. Huang, J. Li, Z. Zhang, W. Yu, Z. Chen, Z. Li, M. Wu, J. Mater. Chem. 2012, 22, 3314. [21] Z. Z. Sun, Z. Yan, J. Yao, E. Beitler, Y. Zhu, J. M. Tour, Nature 2010, 468, 549. [22] W. Wan, Z. Zhao, H. Hu, Y. Gogotsi, J. Qiu, Mater. Res. Bull. 2013, 48, 4797. [23] Y. Shin, J. Lee, J. Yang, J. Park, K. Lee, S. Kim, Y. Park, H. Lee, Small 2014, 10, 866. [24] S. K. Das, Y. Liu, S. Yeom, D. Y. Kim, C. I. Richards, Nano Lett. 2014, 14, 620.
8567
Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper [25] G. Eda, Y. Y. Lin, C. Mattevi, H. Yamaguchi, H. A. Chen, I. S. Chen, C. W. Chen, M. Chhowalla, Adv. Mater. 2010, 22, 505. [26] L. Tang, R. Ji, X. Cao, J. Lin, H. Jiang, X. Li, K. S. Teng, C. M. Luk, S. Zeng, J. Hao, S. P. Lau, ACS Nano 2012, 6, 5102. [27] a) D. Fan, C. Zhang, J. He, R. Hua, Y. Zhang, Y. Yang, J. Mater. Chem. 2012, 22, 18564; b) X. Yu, Y. Huo, J. Yang, S. Chang, Y. Ma, W. Huang, Appl. Surf. Sci. 2013, 280, 450. [28] a) B. H. Stuart, Infrared Spectroscopy: Fundamentals and Applications. Wiley, 2004; b) T. Szabû, E. Tombcz, E. Ill¦s, I. D¦kny, Carbon 2006, 44, 537. [29] a) J. J. Max, C. Chapados, J. Phys. Chem. A 2004, 108, 3324; b) N. Mori, Y. Asano, T. Irie, Y. Tsuzuki, Bull. Chem. Soc. Jpn. 1969, 42, 482. [30] M. B. Hay, S. C. B. Myneni, Geochim. Cosmochim. Acta 2007, 71, 3518. [31] S. E. Cabaniss, J. A. Leenheer, I. F. McVey, Spectrochim. Acta Part A 1998, 54, 449. [32] a) W. Dong, Y. Zhou, D. Yan, H. Li, Y. Liu, Phys. Chem. Chem. Phys. 2007, 9, 1255; b) C. H. Specht, F. H. Frimmel, Phys. Chem. Chem. Phys. 2001, 3, 5444. [33] a) B. Konkena, S. Vasudevan, J. Phys. Chem. Lett. 2012, 3, 867; b) J. T. Paci, H. B. Man, B. Saha, D. Ho, G. C. Schatz, J. Phys. Chem. C 2013, 117, 17256. [34] M. M. Dudkina, A. V. Tenkovtsev, H. Komber, L. Haussler, F. Bohme, Macromolecules 2004, 37, 8389. [35] a) J. Hofkens, W. Schroeyers, D. Loos, M. Cotlet, F. Kohn, T. Vosch, M. Maus, A. Herrmann, K. Mullen, T. Gensch, F. C. De Schryver, Spectrochim. Acta Part A 2001, 57, 2093; b) S. K. Sonkar, M. Ghosh, M. Roy, A. Begum, S. Sarkar, Mater. Express 2012, 2, 105. [36] a) P. G. Luo, S. Sahu, S. T. Yang, S. K. Sonkar, J. Wang, H. Wang, G. E. LeCroy, L. Cao, Y. P. Sun, J. Mater. Chem. B 2013, 1, 2116; b) X. T. Zheng, A. Ananthanarayanan, K. Q. Luo, P. Chen, Small 2015, 11, 1620. [37] H. Z. Zheng, Q. L. Wang, Y. J. Long, H. J. Zhang, X. X. Huang, R. Zhu, Chem. Commun. 2011, 47, 10650. [38] T. A. Fayed, M. A. El-Morsi, M. N. El-Nahass, J. Photochem. Photobiol. A 2011, 224, 38. [39] a) S. C. Hung, A. N. Macpherson, S. Lin, P. A. Liddell, G. R. Seely, A. L. Moore, T. A. Moore, D. Gust, J. Am. Chem. Soc. 1995, 117, 1657; b) K. Kawai, Y. Osakada, T. Takada, M. Fujitsuka, T. Majima, J. Am. Chem. Soc. 2004, 126, 12843; c) S. Fukuzumi, K. Okamoto, Y. Yoshida, H. Imahori, Y.
Chem. Eur. J. 2015, 21, 8561 – 8568
www.chemeurj.org
[40] [41]
[42]
[43]
[44] [45] [46] [47] [48]
Araki, O. Ito, J. Am. Chem. Soc. 2003, 125, 1007; d) K. Okamoto, S. Fukuzumi, J. Phys. Chem. B 2005, 109, 7713; e) S. Fukuzumi, Y. Yoshida, K. Okamoto, H. Imahori, Y. Araki, O. Ito, J. Am. Chem. Soc. 2002, 124, 6794; f) J. Hankache, D. Hanss, O. S. Wenger, J. Phys. Chem. A 2012, 116, 3347; g) K. Sakai, S. Takahashi, A. Kobayashi, T. Akutagawa, T. Nakamura, M. Dosen, M. Kato, U. Nagashima, Dalton Trans. 2010, 39, 1989; h) M. Jozefowicz, K. A. Kozyra, J. R. Heldt, Chem. Phys. 2005, 320, 45. X. Wang, L. Cao, F. Lu, M. J. Meziani, H. Li, G. Qi, B. Zhou, B. A. Harruff, F. Kermarrec, Y. P. Sun, Chem. Commun. 2009, 3774. a) B. Y. Yu, S. Y. Kwak, J. Mater. Chem. 2012, 22, 8345; b) H. Zhang, H. Huang, H. Ming, H. Li, L. Zhang, Y. Liu, Z. Kang, J. Mater. Chem. 2012, 22, 10501; c) H. Li, R. Liu, Y. Liu, H. Huang, H. Yu, H. Ming, S. Lian, S. T. Lee, Z. Kang, J. Mater. Chem. 2012, 22, 17470; d) D. Pan, C. Xi, Z. Li, L. Wang, Z. Chen, B. Luc, M. Wu, J. Mater. Chem. A 2013, 1, 3551; e) D. Qu, M. Zheng, P. Du, Y. Zhou, L. Zhang, D. Li, H. Tan, Z. Zhao, Z. Xie, Z. Sun, Nanoscale 2013, 5, 12272; f) X. Yu, J. Liu, Y. Yu, S. Zuo, B. Li, Carbon 2014, 68, 718; g) L. Wang, Y. Wang, T. Xu, H. Liao, C. Yao, Y. Liu, Z. Li, Z. Chen, D. Pan, L. Sun, M. Wu, Nat. Commun. 2014, 5, 5357. a) H. Yu, Y. Zhao, C. Zhou, L. Shang, Y. Peng, Y. Cao, L. Z. Wu, C. H. Tung, T. Zhang, J. Mater. Chem. A 2014, 2, 3344; b) K.-A. Tsai, Y.-J. Hsu, Appl. Catal. B 2015, 164, 271; c) H. Li, J. Xing, Z. Xia, J. Chen, Carbon 2015, 81, 474. a) S. Hu, R. Tian, Y. Dong, J. Yang, J. Liu, Q. Chang, Nanoscale 2013, 5, 11665; b) L. Cao, S. Sahu, P. Anilkumar, C. E. Bunker, J. Xu, K. A. S. Fernando, P. Wang, E. A. Guliants, K. N. Tackett, Y. P. Sun, J. Am. Chem. Soc. 2011, 133, 4754. S. Hu, R. Tian, L. Wu, Q. Zhao, J. Yang, J. Liu, S. Cao, Chem. Asian J. 2013, 8, 1035. S. J. Zhuo, M. W. Shao, S. T. Lee, ACS Nano 2012, 6, 1059. O. S. Wenger, Acc. Chem. Res. 2013, 46, 1517. H. Li, X. He, Y. Liu, H. Huang, S. Lian, S. T. Lee, Z. Kang, Carbon 2011, 49, 605. L. Liu, J. Hao, Y. Shi, J. Qiu, C. Hao, RSC Adv. 2015, 5, 3045.
Received: September 1, 2014 Published online on April 29, 2015
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