Design of Carbon Dots for Metal-free Photoredox Catalysis - ACS

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Design of Carbon Dots for Metal-free Photoredox Catalysis Simone Cailotto, Raffaello Mazzaro, Francesco Enrichi, Alberto Vomiero, Maurizio Selva, Elti Cattaruzza, Davide Cristofori, Emanuele Amadio, and Alvise Perosa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14188 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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ACS Applied Materials & Interfaces

Design of Carbon Dots for Metal-free Photoredox Catalysis Simone Cailotto,1 Raffaello Mazzaro,3 Francesco Enrichi,2,3 Alberto Vomiero,2,3 Maurizio Selva,1 Elti Cattaruzza,1 Davide Cristofori,1,4 Emanuele Amadio,1* Alvise Perosa 1*

1 Department

of Molecular Sciences and Nanosystems, Università Ca’ Foscari Venezia, Via Torino 155, 30172 Venezia

Mestre, Italy 2

Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 1, 00184 Roma, Italy

3

Division of Materials Science, Department of Engineering Sciences and Mathematics, Luleå University of Technology,

971 87 Luleå, Sweden 4

Centro di microscopia elettronica “G. Stevanato”, Via Torino 155b, 30172 Venezia-Mestre, Italy

* Corresponding authors: Tel: (+39) 041 234 8958; Fax: (+39) 041 234 8979; E–mail: [email protected], [email protected]

Abstract The photoreduction potential of a set of four different carbon dots (CDs) was investigated. First, the CDs were synthesized by using two different preparation methods – hydrothermal and pyrolytic – and two sets of reagents – neat citric acid and citric acid doped with diethylenetriamine. The hydrothermal syntheses yielded amorphous CDs, which were either non-doped (a-CDs) or nitrogendoped (a-N-CDs); while the pyrolytic treatment afforded graphitic CDs, either non-doped (g-CDs) or nitrogen-doped (g-N-CDs). The morphology, structure and optical properties of the four different types of CDs revealed significant differences depending on the synthetic pathway. Next, the photocatalytic activities of the CDs were investigated as such, i.e. in the absence of any other redox mediators, on the model photoreduction reaction of methyl viologen (MV). The observed photocatalytic reaction rates: aN-CDs ≥ g-CDs > a-CDs ≥ g-N-CDs were correlated to the presence/absence of fluorophores, to the graphitic core, and to quenching interactions between the two. The results indicate that nitrogen doping reverses photoredox reactivity between amorphous and graphitic CDs and that amorphous N-doped CDs are the most photoredox-active, a yet unknown fact that demonstrates the tunable potential of CDs for ad hoc applications.

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Keywords: carbon dots, carbon nanomaterials, photocatalysis, photosensitizer, photoreduction, methyl viologen, structure-reactivity relationship, citric acid.


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1 Introduction Nanostructured materials have so far emerged as efficient photocatalytic systems due to their properties and functions, which are closely related to their core-shell composition and size.1, 2, 3, 4, 5, 6 Among these nanomaterials, luminescent carbon dots (CDs) 7 have received attention as a new generation of nanophotocatalysts thanks to their good light-harvesting properties, excellent photostability, low-cost, lowtoxicity and high aqueous affinity.8, 9, 10 However, these carbon-based nanomaterials have so far been generally under-explored in photochemical applications and predominantly in conjunction with other photosensitizers, semiconductors and metallic redox mediators.

8, 9, 10, 11, 12, 13, 14, 15

In addition, CDs

represent a diverse family of nanomaterials that can be prepared through different methods, starting from different carbon sources, and incorporating different dopants. Therefore, it is difficult to generalize their properties and photoreactivity. An example of the use of CDs as photocatalysts was reported by Reisner and co-workers who highlighted the influence of different amorphous and graphitic-like CDs on photoinduced hydrogen production.16, 17 The different CDs were used as photosensitizers to activate a nickel catalytic redox mediator, via photo-induced electron transfer (PET). Improved hydrogen evolution was observed when using graphitic nitrogen-doped CDs, nonetheless a complete and precise rationalization of the effects of the CD morphology and dopant on the photocatalytic performance and light-harvesting properties cannot be drawn since different carbon sources (citric acid for non-doped CDs and aspartic acid for the N-doped ones) were used, and, furthermore, only graphitic CDs were tested, not amorphous nitrogen-doped ones. Very recently, Prato and co-workers prepared and characterized a library of photoredox N-doped CDs highlighting their potential applicability as photocatalysts thanks to their tunable oxidation/reduction potential. However, they synthetized and tested only amorphous CDs and not graphitic analogues.18 Therefore, significant gaps still exist in the knowledge of how to properly design photoredox-active CDs (in terms of choice of the synthetic method, carbon sources and dopants). CDs can be synthesized easily from a wide variety of cheap and renewable carbon sources (orange juice,19 tea,20 chocolate,21 protein,22 sugars,23 cellulose and waste paper,24 lignin25 or willow bark26) by a choice of hydrothermal or pyrolytic methods,8, 10, 27, 28, 29,30 but citric acid is currently the most common starting material used to obtain highly luminescent CDs, usually in combination with nitrogen-containing dopants. 8, 31, 32, 33, 34, 35, 36, 37, 38,39, 40 In general, the choice of the synthetic method34, 39, 40, 41 allows control of the degree of carbonization (i.e. the extent of formation of a purely carbonaceous material), size and morphology of the CDs: the hydrothermal procedures usually lead to incomplete carbonization and to



the presence of molecular fluorophores and amorphous CDs,

34, 35, 36 ,37

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while the harsher pyrolytic

conditions yield CDs with a predominantly graphitic core structure.8, 16, 32, 38, 39, 40 This synthetic flexibility makes for easy syntheses but for little predictability, control, and reproducibility of morphology, optical properties and photoredox activity, limiting their usefulness in photocatalysis. Recent research has indicated that the emission type and intensity of the CDs are due to cooperative effects between molecular fluorophores embedded in the nanoparticles and the defect states and the graphitic cores.34,

35, 36, 37, 42, 43

The molecular fluorophores are mainly responsible for light

absorption and for the generation of photo-excited states; these then relax back to the ground state either by the classical direct radiative pathways (fluorescence or phosphorescence), or by intramolecular energy transfer to low-lying structural defect states, or even by quenching from the graphitic crystalline cores.37 Overall, the existing studies have started to describe the nature and the structure-luminescence relationships of these carbon-based nanomaterials. However, the fundamental role played by the morphology of the CDs, in particular whether they are mainly molecular-CDs or graphitic-CDs, and the effect of nitrogen dopants in controlling the photo-chemical reactivity are still open issues. Here, we describe the synthesis and complete characterization of four different carbon dots (Figure 1) and we compare them in terms of morphology, optical properties and photoredox catalysis. We used two different preparation methods – hydrothermal and pyrolytic, that yield amorphous and graphitic CDs, respectively – and two sets of reagents – citric acid and citric acid coupled with diethylenetriamine as dopant, to obtain the following materials: 1. amorphous non-doped CDs (a-CDs), 2. graphitic non-doped CDs (g-CDs), 3. amorphous nitrogen-doped CDs (a-N-CDs), 4. graphitic nitrogen-doped CDs (g-N-CDs). Photoredox catalysis with the CDs was then tested towards the photoreduction of methyl viologen (MV) by using the CDs as the sole photocatalysts in the absence of any other photosensitizers, semiconductors or metallic redox mediators. We demonstrate that the a-N-CDs and the g-CDs possess high photocatalytic activity, in the same range previously observed by Reisner16,17 and Prato,18 and that the morphology, structure, and optical properties of the CDs all come into play in determining the photoredox activity of the CDs. This study paves the road towards the rational design of carbonnanoparticles for efficient photocatalytic organic transformations, as alternative to the currently used precious-metal inorganic dyes.


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2. Results and Discussion 2.1 Synthesis and characterization of the CDs Gaining control on the synthesis, morphology, structure and photophysical properties of CDs as a function of the carbonaceous precursors and of the reaction conditions is crucial to understand and tune their photocatalytic potential. To this aim a study of the effect of synthetic conditions (hydrothermal vs. pyrolytic) and of the presence/absence of diethylenetriamine (DETA) as dopant was carried out. Citric acid was used as the carbon source. DETA was chosen because it had been previously reported as an Ndoping agent for CDs using a range of different methods such as thermolysis,44 hydrothermal,45 and microwave46 heating. Figure 1 summarizes and schematizes the four categories of CDs described in this paper.

Figure 1. Schematic representations of the four CDs studied in this work: purely carbogenic (a-CDs, g-CDs) or N-doped (a-N-CDs, g-N-CDs) obtained by HTS (180°C, hours, water) or thermolysis (220 °C, days, neat) of citric acid with or without diethylenetriamine as doping agent.

2.1.1 Synthesis of non-doped CDs The amorphous a-CDs were synthesized by heating an aqueous solution of citric acid in a sealed autoclave for 24 h at 180 °C (details in the Supporting Info). The resulting clear suspension of the aCDs, analysed by Gas Chromatography- Mass Spectrometry (GC-MS), 1H and 13C{1H} NMR, revealed the presence of citraconic (1) and itaconic (2) anhydrides, non-fluorescent molecules (see Figures S4 and



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S5 for the NMR and MS fragmentation spectra). Diffusion-ordered NMR spectroscopy (DOSY)23 showed the presence of heavier species with lower diffusion coefficients (green line, Figure 2a), with an estimated Molecular Weight (MW) = 600-700 Da (further details in Supporting Info). ElectroSpray Ionisation- Mass Spectrometry (ESI-MS) analyses were in agreement with the NMR data and confirmed the presence of a distribution of species having MWs in the range 800-900 Da (Figure S6).

Figure 2. 2D DOSY NMR of a-CDs (a) and a-N-CDs (b) in D2O pD =7.

The graphitic-like g-CDs were synthetized by a modified procedure

17, 39, 47

via thermolysis of

neat citric acid in air at 220 °C for 48 h followed by dialysis (See Supporting Info for details). The resulting materials were predominantly carbonaceous solids as confirmed by High Resolution Transmission Electron Microscopy (HR-TEM), Fourier Transform Infrared Spectroscopy (FT-IR), Xray Photoelectron Spectroscopy (XPS), and by the absence of signals in the 1H and

13C{1H}

and 2D

DOSY NMR spectra (Figures S8-10). The NMR experiments excluded the presence of molecular and/or oligomeric soluble species. The morphology and surface properties of the amorphous and graphitic CDs were investigated by HR-TEM, FT-IR and XPS. HR-TEM of the a-CDs, (Figure 3a) revealed the presence of amorphous poorly defined very rarefied structures. Instead, the thermally synthesized g-CDs, (Figure 3b) showed a homogeneous and well-dispersed population of small nanoparticles with a quasi-spherical shape about 2-7 nm in size. Interestingly, the size of the CDs was affected by the synthetic method: harsher conditions lead to smaller nanoparticles with a higher degree of carbonization. Only the g-CDs synthesized by thermolysis show the presence of lattice fringes with an interlayer spacing of 2.4 Å in the HR-TEM, confirming their crystalline graphene structure.27


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Figure 3. HR-TEM images of a-CDs (a), g-CDs (b), a-N-CDs (c) and g-N-CDs (d) at low (left) and high (right) magnification with size distribution histograms.

FT-IR and XPS were used to detect the surface functional groups present on the CDs. FT-IR of the aCDs and g-CDs (Figure S22) showed strong broad adsorptions in the region 3500 - 3000 cm-1 related to the O-H stretching and weak signals for the C-H stretching at 2900-2800 cm-1. The peaks at 1722 or 1738 cm−1 were assigned to the C=O stretching of carboxylic acids. Additionally, peaks at 1626 or 1633 cm-1, assigned to aromatic C=C stretching, were also observed. XPS of the a-CDs and g-CDs showed C and O in the ratios 45/55 and 65/35, respectively, as well as the presence in both samples of C=C, C-O and C=O functional groups48 assigned to the three strong peaks in the C1s band (Figure S23) and centered around 284.6, 286.2 and 288.6 eV, respectively. The lower oxygen content of the g-CDs was coherent with the higher degree of carbonization reached by thermolysis.39 The optical properties of the a-CDs and g-CDs were investigated by Ultraviolet–Visible spectroscopy (UV-Vis) and fluorescence spectroscopy. Interestingly, while the UV absorption spectra

(Figure S21) of both CDs showed weak absorption bands at 350 nm assigned to the n-* transition related to the defects states, some differences were instead observed analyzing their photoluminescence. As shown in Figure S27a and S27b and in the contour plots in Figure 4a and 4b, both the nanomaterials showed an excitation-dependent emission band whose maximum ranged between 420 and 500 nm. While no other contribution was visible for the a-CDs, a second sharp band was noticed for the g-CDs, whose intensity maximum at 385 nm seemed to be only slightly influenced by the excitation wavelength. The relatively simple emission spectrum of the a-CDs was tentatively attributed to the presence of multiple defects on the nanoparticle surface. The additional high energy transition of the emission spectra of the g-CDs could be instead ascribed to the crystalline core.49



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Figure 4. Contour plot of the emission map at different excitation wavelengths of a-CDs (a), g-CDs (b), a-N-CDs (c) and g-N-CDs (d). The Intensity scale is going from dark violet to red passing through blue, green and yellow.

2.1.2 Synthesis of nitrogen-doped CDs The nitrogen-doped a-N-CDs were synthetized by a procedure similar to the one described for the non-doped a-CDs. Hydrothermal treatment of citric acid in the presence of DETA at 180 °C for 6 h yielded the a-N-CDs as a brown solid in 72%wt yield (See Supporting Info for experimental details). Interestingly, the 1H and 13C{1H} NMR spectra (Figure S11-12) of the as-synthesized a-N-CDs showed a series of singlets and triplets between 6 - 5 ppm and 4.8 - 2.8 ppm together with carbonyl (170-160 ppm), sp2 (150-80 ppm) and aliphatic sp3 (50-30 ppm) carbon environments that were assigned to molecular fluorophores with structures analogous to that of IPCA,34,35,36 , e.g. 3 (Figure S15) that is also responsible for the most abundant peak observed in the ESI-(+/-)MS (Figure S13; m/z = 224.1 [3 + H+]). The comparison of the experimental and the simulated isotopic patterns of the cationic species [3·H]+ are reported in Figure S14. In addition to the identified IPCA-like molecular compounds, the a-N-CDs samples contained also a series of un-defined oligomers with MW ranging from the signal at 270 Da assigned to 3, up to 1000-1200 Da as revealed by both DOSY (Figure 2b, Table S2) and ESI-MS (Figure S13). The graphitic g-N-CDs were prepared by thermolysis followed by dialysis to selectively recover the doped carbogenic structures as a dark-brown solid in 18%wt yield (See Supporting Info for details). The 1H and 13C{1H} NMR spectra (Figure S18 and S19) did not show significant resonances, while ESIMS (Figure S20) of the purified g-N-CDs led to detect the fluorophore 3 and a series of low-intensity signals spanning 200 - 2000 Da (upper instrumental detection limit). These data were consistent with the presence of complex mixtures formed by the coexistence of small fluorophores (probably non-covalently bonded to the dot), oligomers and/or extended carbonaceous cores. HR-TEM of a-N-CDs yielded amorphous and poorly defined structures, while HR-TEM of the g-N-CDs, (Figure 3d) revealed the presence of a homogeneous and well-dispersed population of small nanoparticles with a quasi-spherical shape. Furthermore, as expected, only the g-N-CDs sample showed ACS Paragon Plus Environment

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the presence of lattice fringes with an interlayer spacing of 2.4 Å, confirming their graphitic nature. In analogy to the above reported non-doped CDs, the size of the a-N-CDs and g-N-CDs was affected by the synthetic method: the harsher conditions led to the formation of smaller nanoparticles. However, Ndoping increased their average dimensions from 3 to 17 nm g-CDs/g-N-CDs. When analyzing the FTIR spectra (Figure S25) of the a-N-CDs and g-N-CDs, along with the broad absorption bands around 3400 cm−1 corresponding to the O−H vibrations, new peaks due to the N-H stretching of amines/amide were detected around 3300-3000 cm-1. The presence of the doping reagent is also responsible for the additional very broad signals revealed around 3000-2800 cm-1 reasonably due to ammonium (RNH3+; R2NH2+) groups. The strong absorption bands around 1773-1600 cm−1 reflected the presence of acids, ketones and amide groups. Furthermore, the multiple signals at 1600-1400 cm-1 were assigned to both C-C stretching of aromatic or conjugated double bonds, and N-H bending of the above discussed N containing groups. Interestingly, only for the a-N-CDs sample uncommon signals were revealed at 2025 and 2015 cm-1 which were tentatively assigned to the presence of molecules containing -C≡N or -C=Ngroups. The XPS analyses of the N-doped nanomaterials revealed a dependence of the C, O, N compositions on the synthetic protocols, with the total amount of N decreasing for pyrolysis (C,O,N ratio of 70/20/10 for a-N-CDs while 65/30/5 for g-N-CDs). The high resolution XPS spectra (Figure S26) also in this case confirmed the FTIR assignments, with a C1s region exhibiting C=C (around 284.7 eV), C-O and/or C-N (around 286.3 eV), C=O and/or C=N (around 288.6 eV) signals. The N1s peak shows two bands, related to both pyridinic environments or -NH2 groups (around 399.1 eV),16 and to the C-NC groups48 (around 400.4 eV); this last band was absent in the g-N-CDs, consistent with the formation of an N-doped graphitic core, due to the harsh synthetic protocol. The optical properties of the N-doped CDs were investigated by UV-Vis and fluorescence spectroscopy. The UV spectra (Figure S24) showed similar patterns with two main absorption bands centered at 350 nm and 240 nm reasonably assigned to the presence of the fluorophore 3 which has been detected in both the a-N-CDs and g-N-CDs.34 The molecular fluorophore affected also the PL spectrum of the a-N-CDs and g-N-CDs which resulted in a similar excitation independent wavelength behavior with an emission peak centered at 450 nm (Figure 4c,d and Figure S27c, d). In summary, the data of the four a-CDs, g-CDs, a-N-CDs and g-N-CDs confirmed different morphology and chemical composition: the hydrothermal treatment produced amorphous nanoaggregates containing low molecular weight compounds (e.g. 1 and 2) while pyrolysis formed small graphene-like structures. Addition of a dopant amine results in the formation of luminescent fluorophores (such as 3) and of larger aggregates/oligomers. ACS Paragon Plus Environment


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2.2 Photocatalytic studies We envisaged that the photocatalytic activity of the four different types of CDs could be rationalized by correlating their morphology, structure and optical properties with the photoinduced electron transfer process whereby an electron is transferred from the photoexcited state of the CDs to an acceptor molecule. To this end, the CDs-catalyzed photo-reduction of methyl viologen (MV2+) to its radical cation MV+. was studied. This simple photoreaction is an established reactivity test to prove the PET efficiency of carbon dots.16, 17, 18 Preliminarily, we verified the occurrence of PET between the CDs and MV2+ by photoluminescence quenching experiments of the CDs in the presence of increasing concentrations of MV2+. The results (Figure S28 in the SI section) show that the fluorescence emission of all our CDs (aCDs, a-N-CDs, g-CDs, g-N-CDs) was effectively quenched upon increasing the concentration of MV, at constant CDs concentration. More interestingly, the quenching resulted to be highly dependent on the nature of the nanodots. To establish the kinetics of these photo-physical intermolecular interactions and to determine the quenching rate constants (kq), the Stern–Volmer relationship was applied: 𝐼0 𝜏0 , = 1 + 𝑘𝑞𝜏0[𝑀𝑉2 + ] 𝐼 𝜏

Where I0 and τ0 are the initial photoluminescence intensity and lifetime, respectively. The initial lifetime τ0 and the one measured at each MV2+ concentration were determined by averaging the multiple contributions to the multi-exponential photoluminescence decay as described in the SI section, table S3, while the intensity values are obtained by integrating the emission spectrum over the 400-650 nm wavelength range. Figure S29 reports the Stern-Volmer plots for all samples and the relative fitting curves for both photoluminescence intensity and average lifetime variation. All the values can be successfully fitted with a linear function, except for the g-CDs sample, where two distinct linear functions are necessary to fit the photoluminescence intensity quenching trend. The kq values (Table 1) reveal high CDs-MV2+ interaction for both doped and non-doped CDs. The behavior of the N-doped a-N-Cd and gN-CD was ascribed to a diffusion-limited dynamic quenching process,50,51 since the kq values obtained from the intensity and lifetime decay fittings are close to each other and to the diffusion limit.53 On the contrary, for the non-doped samples the large discrepancy between the lifetime quenching rate and the photoluminescence quenching rate highlights a more complex situation. Indeed, the photoluminescence intensity quenching constant is far larger than the one obtained from the lifetime decay interpolation, ACS Paragon Plus Environment

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whose value is close to 0 in the a-CDs sample. In addition, the kq (I) for these samples is larger than the expected diffusion limit, suggesting the occurrence of a static quenching process deriving from the ground-state interaction between CDs and MV2+. Table 1. Quenching constants of the CDs with MV, calculated on the fluorescence intensity and lifetime

Entry

kq (I)

kq () a

(x 1010 Ms-1)

(x 1010 Ms-1)

a-CDs

19

0

g-CDs

59

1.6

a-N-CDs

1.6

0.78

g-N-CDs

4.1

1.8

Interestingly, these trends correlate nicely with the -potential of the graphitic nanoparticles. The potential of the g-CD is approximately -50 mV, more negative than the -potential of g-N-CD (ca. -15 mV, figure S33) ascribable to the presence of carboxylate moieties in the former and to positively charged ammonium groups in the latter.52 The strong negative -potential registered for the non-doped g-CDs favors the ground state CDs-MV physical encounter, through strong coulombic interactions, while the less negative value of the N-doped g-N-CDs results in a more dynamic interaction between the g-NCDs* excited state and the MV quencher. These features might explain both the strong difference between kq (I) and kq (), the fact that kq (I) is above the diffusion limit, and the lack of a simple linear fitting for the Stern-Volmer plot. Along with the rationalization of the nature of the MV-CDs supramolecular interactions, the luminescence quenching experiments pointed out a surface-dependent photoexcitation behavior of the CDs which is bound to affect also their photocatalytic behavior. To investigate the relationship between the photoredox activity and the structure of each of our CDs, we measured the photoreduction rates of MV (-0.45V vs NHE) in aqueous solution in the presence of EDTA as a sacrificial electron donor under LED light irradiation (365 nm) using a concentration of each CDs normalized for absorption (See SI for further details). As shown in Figure 5 and Table 2, for each of the CDs the PET reactivity is different, and it is clearly influenced by morphology and composition of the CDs. In detail, the a-N-CDs (red points) exhibited high initial photoreduction rate (v0 = 7.83 x 10-8 Ms-1), high TOF (195 g*CDs-1*s-1) and the highest MV2+ conversion (27%), comparable to the initial rate ca. 8 x 10-8 Ms-1 measured by Reisner and Prato.17,18 The second-best performance was shown by the g-CDs (v0 = 5.06 x 10-8 Ms-1; TOF = 72 g*CDs-1*s-1), while poorer initial rates were instead ACS Paragon Plus Environment


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observed for the a-CDs and g-N-CDs, the latter exhibiting also an induction time of 6 min. Such induction period was confirmed by carrying out 3 repeated runs with the same CDs and resulted to be an intrinsic property of g-N-CDs not associated to an “irreversible” structural/morphological/chemical transformations (See SI for further details). The results clearly demonstrate opposite structure/reactivity trends for the two families of CDs, specifically: the graphitic domains proved to be essential when using non-N doped systems (g-CDs > a-CDs); instead, for the N-doped nanostructures (a-N-CDs > g-N-CDs) the presence of molecular-like fluorophores, rather than the carbogenic core, is of crucial importance. As a control, all the experiments performed without CDs and without light gave no conversion of MV (Figure S30, Table S4), confirming that both a light source and the sensitizers were needed for the evolution of the reaction. These results contribute to a deeper understanding of the structure-activity relationship of CDs for photocatalysis, in particular by demonstrating for the first time that the amorphous a-N-CDs are more active than both the non-doped a-CDs and g-CDs as well as the N-doped graphitic-like homologues (gN-CDs), the latter considered the most promising class of nanoparticles for solar hydrogen production.16

Figure 5. Reaction kinetics of formation of MV.+ using a,g-CDs (a) and a,g-N-CDs (b).

A comparison between the Stern-Volmer analysis and the results of the photocatalytic reactions pointed to some interesting conclusions. First, the photocatalytic activity for the non-doped carbon dots followed the order g-CDs > a-CDs. However, the higher ground state interaction and the higher quenching constant of the g-CDs (5.9·1011 vs. 1.9·1011 Ms-1, respectively, Table 1) were not sufficient to explain the photocatalysis results. In fact, the doped analogues a-N-CDs and g-N-CDs showed higher or similar performance than the g-CDs notwithstanding that their quenching constants kq were more than one order of magnitude lower (1.6·1010 vs. 4.1·1010 Ms-1, respectively, table 1).


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To fully explain the photocatalytic reactions, time-resolved photoluminescence (PL) was applied to evaluate the PL-lifetimes. A single exponential PL decay was observed only for the a-N-CDs (red trace, Figure 6), while for all the other CDs the PL-decays were characterized by a multi-exponential behavior, whose parameters are reported in Table S3. The average lifetime () of the a-N-CDs exhibited the longest  = 13 ns, followed by the others in the order g-N-CDs > g-CDs > a-CDs with values of8.1, 4.3 and 3.6 ns, respectively. The more linear character of the a-N-CDs was consistent with a molecularlike emission,53 explained by the presence in its structure of fluorophores such as 3 and other IPCA-like derivatives as discussed above.

Figure 6. Time-resolved photoluminescent (PL) measurements of a,g-CDs and the a,g-N-CDs.

Quantum yields (QYs, based on quinine sulfate) were also measured for the four sets of CDs: the QYs for a-CDs, g-CDs and the a-N-CDs, g-N-CDs were respectively 1.0, 1.2% and 17.3, 2.4%. The large QY observed for the a-N-CDs was attributed to the presence of molecular fluorophores such as 3 and the IPCA analogs, known to possess high QY values.34, 35, 36, 37 Another parameter that was taken into account was ε, which resulted to be mainly dependent on the presence of the nitrogen-dopant:  was larger for the a-N-CDs and g-N-CDs with values of 13.93, 11.62 L·g-1·cm-1, respectively; while it was lower for the non-doped materials a-CD and g-CDs with values of 1.16, 4.68 L·g-1·cm-1. As for the PLlifetimes and QYs, also in this case the a-N-CDs showed the highest value. For easier comparison, Table 2 summarizes the average lifetime () of the excited state, the quantum yield (QY) and the mass absorption coefficients (ε, see SI and Figure S32) for the four synthetized CDs, together with the photocatalytic activities (initial rate and TOF). Table 2. Photocatalytic performance, average lifetime (), quantum yield (QY) and mass absorption coefficients (ε) of the synthesized CDs



Entry

v0 · 10-8

TOF

(M · s-1) mol MV · g CDs-1 s-1

QY (%)

Page 14 of 21



ε

(L·g-1·cm-1) (ns)

a-CDs

3.45

9

1.0

1.16

3.6

g-CDs

5.06

72

1.2

4.68

4.3

a-N-CDs

7.83

195

17.3

13.93

13

g-N-CDs

1.95

65

2.4

11.62

6

Based on the data reported in Table 2, the a-N-CDs show the highest photocatalytic activity (7.83 x·108

Ms-1) that matches the highest values of QY, ε and . Interestingly, the resulting TOF is also 3 times

higher than that of the graphitic doped counterpart, proving that the presence of free molecular-like fluorophores and the absence of N-doped graphitic domains are among the key morphological features needed to ensure a high PET efficiency of the carbon dots in a fully dynamic MV-CDs interaction system. The trend observed for the other CDs indicates that higher QY of g-N-CDs (2.4%) respect to the a-CDs (1.0%) and g-CDs (1.2%) does not correlate with the kinetics (1.95, 3.45, 5.06 Ms-1, respectively). This apparent inconsistency was explained since QY,  and  are helpful in understanding PET but not strictly correlated to catalytic activity.

3. Conclusions In this work we show how the structure, morphology and properties of CDs can be tuned based on the synthetic procedure, and how in turn these affect the photocatalytic properties of the materials towards the single-electron reduction of methyl viologen. The first significant finding here described is that two structural features act independently in promoting photoredox activity of the CDs: purely carbonaceous – i.e. non-nitrogen-doped CDs – work best in the photoreduction of methyl viologen when their structure is graphitic rather than amorphous; while for nitrogen-doped CDs it is clear that graphitic particles are far less active compared to their amorphous counterpart, where nitrogen incorporation occurs in the form of photoactive molecules that are responsible for the photocatalytic activity. Conversely, the non-doped amorphous particles are less photoredox-active due to the lack of any photoactive molecules as well as to the absence of graphitic defect-states able to promote the excitation of an electron; likewise, the doped graphitic particles are poorly photoredox-active due to the simultaneous presence of molecular fluorophores and of graphitic defect-states that quench each other. ACS Paragon Plus Environment

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It is also shown that the electrostatic interactions between the surface of the CDs and the reactant methyl viologen determine whether the supramolecular attraction is static or dynamic: the strong negative potential of the non-doped CDs determines static quenching favored by strong ground state interactions; while the behavior of the nitrogen-doped CDs was ascribed to a diffusion-limited fully dynamic quenching process between the excited state of the g-N-CDs* and methyl viologen. Overall, the results herein reported highlight the complex nature of the photochemistry of CDs and demonstrate that full understanding of the role of the precursors and of the degree of carbonization are key parameters to achieve the desired photocatalytic properties.

Associated Content Detailed CDs synthesis and procedure optimization, UV-Vis, PL, FT- IR and XPS spectra, 2D-DOSY NMR interpretations, steady state and time resolved photoluminescence, CDs-methyl viologen quenching experiments, photocatalytic blank reaction and study on the induction time, mass absorption coefficient and ζ-potential.

Acknowledgements E.A. acknowledges the European Social Fund for a post-doc scholarship. F.E. acknowledges support by Centro Fermi under the MiFo project and the PLESC project (MIUR) between South Africa and Italy, and VINNOVA (Swedish Innovation Agency), under the Vinnmer Marie Curie Incoming - Mobility for Growth Programme (project “Nano2solar” Ref. N. 2016-02011). A.V. and R.M. acknowledge the Knut & Alice Wallenberg Foundation and the Kempe Foundation for financial support.

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