Dynamic aggregation of carbon dots self‐stabilizes symmetry breaking for exceptional hydrogen production with near‐infrared light

Developing new photosystems that integrate broad‐band near‐infrared (NIR) light harvesting and efficient charge separation is a long‐sought goal in the photocatalytic community. In this work, we develop a novel photochemical strategy to prepare light‐active carbon dots (CDs) under room temperature and discover that the aggregation of CDs can broaden the light absorption to the NIR region due to the electronic couplings between neighboring CDs. Importantly, the dynamic non‐covalent interactions within CD aggregates can stabilize symmetry breaking and thus induce large dipole moments for charge separation and transfer. Furthermore, the weak non‐covalent interactions allow for flexible design of the aggregated degrees and the local electronic structures of CD aggregates, further strengthening NIR‐light harvesting and charge separation efficiency. As a result, the CD aggregates achieve a record apparent quantum yield of 13.5% at 800 nm, which is one of the best‐reported values for NIR‐light‐driven hydrogen photosynthesis to date. Moreover, we have prepared a series of different CDs and also observed that these CDs after aggregation all exhibit outstanding NIR‐responsive photocatalytic hydrogen production activity, suggesting the universality of aggregation‐enhanced photocatalysis. This discovery opens a new promising platform for using CD aggregates as efficient light absorbers for solar conversion.


INTRODUCTION
Photocatalytic water splitting into hydrogen is one of the most promising technologies to realize the conversion and storage of solar energy.Over the past 50 years, various semiconductors have been developed as photocatalysts for hydrogen production, however, most of them suffer from narrow visible light absorption. [1,2]In the solar spectrum, visible light and near-infrared (NIR) light account for 45% and 50% of the solar light, respectively. [3,4][16][17] For example, the short-range electronic couplings of aggregates (J-type aggregate) can cause the red shift of the absorption spectrum. [15,18][21] Impressively, the structure of an aggregate is dynamic in view of the lability of the non-covalent interactions connecting the molecular components of an aggregate. [15,22]This dynamic character may endow aggregates with the ability to undergo a continuous change in its structure through reversible dissociation and association into various configurations, [22] which will further stabilize symmetry-breaking states and thus prolong dipole relaxation time. [23,24]The CDs typically consist of a π-conjugated graphitic core with rich oxygen/nitrogen-containing groups.In principle, multifarious intermolecular attractive interactions (e.g., hydrogen bonds, π-π interactions) could facilitate the aggregation of CDs under appropriate conditions. [25,26]Significantly, the noncovalent interactions and the diversity of surface groups of CDs allow for the flexible design of physicochemical properties of CD aggregates, granting CD aggregates with desired light harvesting and charge separation efficiency.Bearing these benefits in mind, it is expected to obtain adjustable and high-efficiency CD aggregates that concurrently integrate strong red/NIR-light harvesting and robust charge separation.However, there have been no reports to date on CD aggregates as red/NIR-light absorbers for solar conversion, and the underlying interplay between aggregation and red/NIR-light photocatalytic properties has not been articulated.
In this work, we develop a novel photochemical strategy to prepare CDs and discover that the aggregated CDs exhibit strong light harvesting from visible to NIR light regions due to the strong electronic couplings between neighboring CDs.Importantly, the dynamic nature of CD aggregates prolongs the dipole relaxation time by stabilizing symmetry breaking, contributing to a strong and uninterrupted built-in electric field for the separation and transfer of charge carriers.Consequently, the CD aggregates achieve excellent hydrogen production under NIR light irradiation.Furthermore, the physicochemical properties of CD aggregates can be further modified by varying the aggregated degrees and local structural configurations, which synergistically consolidates light harvesting and charge separation.The apparent quantum yield (AQY) of optimized CD aggregates for hydrogen production highly reaches 13.5% at 800 nm, which is one of the best-reported values for NIR-light-driven hydrogen photosynthesis to date.More importantly, this aggregationenhanced photocatalysis was found in a series of different CDs, approving its general principle.The results and the unraveled insights represent an important step towards artificial photosynthesis in general where CD aggregates can function as a new, tunable, and high-efficiency platform for solar conversion.

Structural characterization of CD aggregates
The CDs are prepared using sucrose and sodium nitrite as precursors via a photochemical method at room temperature.The calculated bond energy values of precursors by density functional theory (DFT) suggest that sucrose and sodium nitrite can be easily splitted into C/N/O fragments by UV photolysis.Thus, the formation of CDs can be mainly attributed to the self-assembly of C/N/O fragments, as displayed in Figure S1. Figure 1A-C displays the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of CDs.The HRTEM image shows an interplanar spacing of 0.21 nm, which can be ascribed to the (1 0 0) diffraction facet of carbon materials. [27]It is seen that the average size of CDs is about 3 nm (Figure 1B).The powder X-ray diffraction pattern of CDs is revealed in Figure S2.The CDs display a broad peak at approximately 20.3 • , corresponding to the (0 0 2) diffraction facet of carbon materials. [28]The atomic force microscopy (AFM) image (Figure 1D,E and Figure S3) suggests that the height of CDs is about 1.2 nm, reflecting the few-layered feature of CDs.Raman spectroscopy was used to further determine the structure of CDs.In Figure 1F the peak at 1590 cm −1 is the G band, whereas the D band is located at 1370 cm −1 . [25,29]The relative ratio of G to D bands (I G /I D ) is 1.44, suggesting the well-crystallized core of CDs.
Fourier-transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra were performed to determine the surface chemical compositions of CDs.A broad band at around 3440 cm −1 corresponds to the OH stretching mode (Figure S4).The peaks at 1604 and 1688 cm −1 can be assigned to the C=C/C=N and C=O stretching vibration, respectively.The XPS survey spectrum confirms the presence of C, N, and O (Figure S5).The C 1s XPS can be curve-fitted to three component peaks with binding energies of 284.6, 286.3, and 288.1 eV (Figure S5), which correspond to sp 2 -hybridized carbon (C=C), hydroxyl (C-OH), and carbonyl (C=O) species, respectively.The binding energies at 399.7, 400.2, and 401.1 eV from the N 1s spectrum can be attributed to pyridinic N, pyrrolic N, and graphitic N species (Figure 1G), respectively.The O 1s spectrum suggests the presence of C=O, C-OH and COOH groups (Figure 1H).Solid-state nuclear magnetic resonance (SSNMR) spectroscopy was employed to further analyze the structure of CDs (Figure 1I).In the SSNMR spectrum, signals in the range of 50-85 ppm are associated with the aliphatic (sp 3 ) carbon atoms (C-O/C-N), and signals from 100 to 120 ppm are indicative of sp 2 carbon atoms (C=C).Signals in the range of 150-185 ppm correspond to C=O and/or C=N. [30]Based on the aforementioned characterizations, a proposed model for the CDs is presented in the inset of Figure 1I.
Figure 2A,B displays the UV-vis absorption spectra of the CDs with different concentrations.The CDs solution (0.007 mg/mL) possesses a main absorption peak at 257 nm, which can be attributed to the π→π* electronic transitions of the aromatic sp 2 domains of CDs. [31]When increasing the concentrations of CDs, the light adsorption intensity of CDs can be obviously increased.When the concentration is 0.1 mg/mL, the π→π* electronic transition absorption is shifted from 257 to 262 nm.According to the molecular exciton coupling theory, the spectral redshift suggests the formation of J-type aggregates.With the further increase of CD concentration (0.2 mg/mL), the absorption peak can be shifted to 270 nm.When the concentration of CDs reaches 0.4 mg/mL, the π→π* electronic transition absorption peak gets increasingly red-shifted and the light absorption intensity from 400 to 1000 nm is noticeably heightened, reflecting the formation of higher J-aggregates.To further check the formation of CD aggregates, a molecular dynamics (MD) simulation was employed.As revealed in Figure 2C, MD simulations confirm that the CDs can assemble into aggregates.The average energy for the interaction pair is calculated to be −19.1 kJ/mol, signifying that the aggregation is a spontaneous process.Reduced density gradient (RDG) analysis was employed to explore the intermolecular interactions within CD aggregates (Figure 2D).RDG analysis approves the presence of multiple weak interactions (hydrogen bonds, π-π interaction), indicating that the non-covalent interactions are the primary driving force for CD aggregation.Furthermore, the CD-CD contact map intuitively approves that the CD can interact with the adjacent CD (Figure 2E).What is more, we have examined the TEM image of CDs under a high concentration (0.43 mg/mL).Obviously, CD aggregates were witnessed (Figure 2F), agreeing with the results of UV-vis absorption spectra and MD simulations.
Figure S6 displays the steady-state photoluminescence (PL) emission of CD solution with different concentrations.With the increase in CD concentrations, the PL emission peaks display a noticeable red shift, which indicates a decreased bandgap of CD aggregates.Furthermore, we have observed the generation of photocurrent of CD powder under low-energy red/NIR light irradiation (Figure S7), also confirming the decreased bandgap of CD aggregates.Previous studies have established that the J-aggregates can narrow the bandgap energy, thus red-shifting the absorption spectrum. [32]To uncover the effects of J-type aggregates on the bandgap energy, we have calculated the highest occupied molecular orbital-least unoccupied molecular orbital (HOMO-LUMO) gaps of CD aggregates with different aggregated degrees by DFT.Expectedly, the HOMO-LUMO gaps of CD aggregates gradually decrease, as the aggregated degree increases (n = 1, 2, 3, and 4) (Figure 2G and Figure S8).
Orbital interaction diagram analysis is used to further study the electronic couplings of CD aggregates and how fundamental molecular orbitals (MOs) of CD monomers are mixed to form CD aggregates. [33,34]The orbital interaction diagram of the CD monomer, dimer, trimer, and tetramer is revealed in Figure 3 and Figures S9 and S10.As highlighted by the pink arrows in Figure 3, the LUMO orbit of the trimer is mainly derived from the 99th orbit of monomer C (77%), and the energy is decreased by 0.34 eV.The HOMO orbit is mainly derived from the 98th orbit of monomer B (81%) and the energy is increased by 0.22 eV.The LUMO+1 and HOMO-1 orbits are mainly from the 100th orbit of monomer C (72%) and the 98th orbit of monomer B (83%), respectively.These results confirm the existence of strong electronic couplings between adjacent CDs and thus make the rearrangement of orbital energies, consistent well with the MOs results of CD dimer and tetramer. [35]o explore the effects of aggregation on the charge carrier separation of CDs, femtosecond transient absorption (fsTA) spectra of CD monomer and CD aggregates were measured.The fsTA spectra of CD monomer and CD aggregates were demonstrated in Figure S11 and Figure 4B.The CD monomer shows a negative ground state bleaching (GSB) peak and a weak positive absorption band (excited state absorption, ESA) (Figure S11). [36,37]The fitting lifetime of ESA at 490 nm is 21.78 ps (Table S1), demonstrating a fast recombination process.Meanwhile, the CD aggregates display a strong ESA signal, suggesting the efficient separation of electrons/holes.The absorption band at 470 nm can be attributed to the formation of Frenkel excitons with a decay lifetime of 482.73 ps (Figure 4D and Table S1). [36,37]Thereafter, the transient species at about 650 nm belong to the chargetransfer (CT) states. [38]The formation of CT states can be ascribed to the intermolecular electronic coupling effects of CD aggregates.The kinetic decay lifetime of CT states was calculated to be as long as 2.11 ns (Figure 4D and Table S1).Compared with the CD monomer, the CD aggregates demonstrate a longer ESA lifetime, reflecting efficient separation of charge carriers. [39,40]Furthermore, the PL lifetimes of CD monomer and CD aggregates were also estimated (Figure S12 and Table S2) and the PL lifetime of CD aggregates is longer than that of the CD monomer, supporting the fsTA results.
To analyze the charge mobility ability of the CD monomer and CD aggregates, we have calculated their electron reorganization energy λe by first-principles calculations. [41,42]We chose the standard four-point scheme to calculate their λe (Figure S13 and Table S3).Geometry optimization is carried out by Gaussian 09 package using B3LYP functional with a basis set of 6-31G (d).It can be found that the λe sharply decreases from 445.45 meV for the CD monomer to 140 meV for the CD trimer.The decreased λe reflects that the CD aggregates have a high migration rate of charge carriers.In principle, the spontaneous symmetry breaking of aggregates can cause a transient dipole state and thus generate a built-in electric field for the separation and transfer of charge carriers.To check this deduction, DFT calculations were carried out.The CD monomer displays a small dipole moment of 8.9 Debye (denoted as D to simplify) (Figure S14).The dipole moments of CD aggregates with different aggregated degrees were carefully analyzed.The dipole moments for dimers (from 10.5 to 14.1 D) are obviously larger than that of CD monomers (Figure S14).Furthermore, we have calculated the dipole moments of trimers with different local structures (Figure S15).As expected, these trimers all exhibit increased dipole moments (from 14.2 to 19.2 D).Furthermore, the randomly selected dimer and trimer based on our MD simulations also exhibit large dipole moments of 14-20 D (Figure S16).
In thermodynamics, the weak non-covalent interactions of aggregates allow for structural fluctuations and thus endow CD aggregates with dynamic features.This dynamic nature of aggregates will substantially prolong dipole relaxation time by maintaining symmetry breaking and hence provide a continuous built-in electric field for the separation and transfer of charge carriers. [43,44]To check this, the struc-tural change of a trimer aggregate was recorded with time by MD simulations.The randomly selected snapshot images of the trimer aggregate recorded at 45, 46, 47, 48, 49, and 50 ns are presented in Figure 4E.Interestingly, the distances of adjacent CDs change with time, suggesting the dynamic feature of CD aggregates (Figure 4C-E).[47] The negative surface potential of CD overlaps the positive surface potential of adjacent CD (Figure S17), signifying the existence of strong electronic couplings.Importantly, the surface potential distribution of CD aggregates is observed to fluctuate with time.This result also reveals the dynamic behavior of CD aggregates, which is further corroborated by the optical microscope measurement (Figure S18).It should be noted that the dynamic feature of CD aggregates can consecutively maintain symmetry-breaking states and generate large dipole moments in the range of 19.7-21.9D. [48][49][50] Therefore, based on the above analysis, the efficient separation and transfer of charge carriers of CD aggregates can be mainly attributed to the formation of a steady and strong built-in electric field stemming from uninterrupted dipole states.Although the dynamic behavior has been observed in some areas such as in supramolecular chemistry, the correlation with photophysical properties (charge separation and transfer) has not been explored.The insight into the linking between dynamic aggregation and charge separation in this work may bring new inspiration and knowledge for photocatalysis.

Photocatalytic performances of CD aggregates
Inspired by the broad light absorption and high-efficiency charge separation, we have demonstrated the photocatalytic hydrogen production of CD aggregates under visible and NIR light irradiation (formic acid as electron donor, Pt as a cocatalyst).As displayed in Figure 5A,B, the CDs with low concentration (monodisperse state) are only active under UV and narrow visible light irradiation, and no hydrogen was detected under red/NIR light.Impressively, the CDs with high concentrations (CD aggregates) can trigger hydrogen production with low-energy red/NIR photons.The hydrogen production rates of CD aggregates (0.43 mg/mL) at 630 and 800 nm highly reach 4.86 and 3.60 μmol/h (1619.8 and 1200 μmol g −1 h −1 ), respectively (Figure 5C).Moreover, we have also analyzed the hydrogen production of the Pt/formic acid system without CD aggregates.Under 630 nm light irradiation, only trace hydrogen was detected (Figure S19).Then, we tested the photocatalytic activity of CD aggregates with higher CD concentrations (0.7 and 1 mg/mL).The hydrogen production of CD aggregates can be further improved by increasing the concentrations of CDs (Figure S20).However, the increased amplitude for hydrogen production of CD aggregates with higher CD concentrations is relatively small, which probably relates to the reduced light transmittance caused by high CD concentrations.Furthermore, the energy band alignment of CD aggregates was analyzed (Figures S21  and S22), which satisfies the thermodynamic potential for hydrogen production.The HRTEM image of Pt/CD aggregates was revealed in Figure S23.The particle size of the Pt co-catalyst is about 2-3 nm and the lattice spacing of 0.22 nm corresponds to the Pt (1 1 1) crystal plane. [51]These results suggest that the CD aggregates can act as a new and promising light-harvester for red/NIR light hydrogen production and also reflect that the aggregated degree was the essential factor that controls red/NIR-light-responsive photocatalysis.To the best of our knowledge, this is the first observation that CD aggregates can drive hydrogen production reaction under low-energy red and NIR light irradiation.
To further check the effects of aggregated degrees on the photocatalytic performance of CD aggregates, the CDs with different concentrations are confined in water-in-oil droplets.The CD concentrations in droplet-A, droplet-B, and droplet-C are 0.25, 0.75, and 1.0 mg/mL, respectively.The overall CD amounts within these three droplets are controlled to be 0.5 mg. Figure 5D-F exhibits the microscope images of droplet-A, droplet-B, and droplet-C.The photocatalytic activities of droplet-confined CDs follow an order of droplet-C > droplet-B > droplet-A (Figure 5G).Apparently, the photocatalytic hydrogen evolution rates are positively correlated with the aggregated degrees of CDs.This result undoubtedly confirms that the aggregated degree is the central factor that determines the red/NIR-light photocatalysis of CD aggregates.
Furthermore, the electronic transition type and the upconversion properties of CD aggregates were carefully investigated.Orbital distribution simulation approves that the CD monomer and CD aggregates exhibit typical π→π* electronic transition (Figure S24).Besides, it is found that the CD monomer and CD aggregates both show weak up-conversion emission (Figure S25), proposing that the aggregation has negligible impact on the up-conversion behaviors of CDs.Consequently, the influence of electronic transition type and up-conversion properties on the red/NIRlight-driven hydrogen production of CD aggregates can be safely excluded.

Electronic and textural properties regulation of CD aggregates
To examine the adjustability of aggregated degrees of CDs, a charge screening strategy was employed.The CDs possess negative charges due to the presence of abundant carboxylic and hydroxyl functional groups (Figure 6A).Polyaluminium chloride (PAC), which mainly consists of Keggin-Al 13 (AlO 4 Al 12 (OH) 24 (H 2 O) 12 7+ ), has the characteristics of high positive charge density (Figure 6B). [52]In principle, the CDs can interact with PAC by a positive-negative interaction, which should facilitate the aggregation of CDs by a charge screening effect.Figure 6A gives the zeta potentials of CDs, PAC, and CDs/PAC.Compared with the CDs, the CDs/PAC exhibit a decreased zeta potential, suggesting the occurrence of electrostatic attraction between the CDs and the PAC.This strong electrostatic attraction certainly will induce the aggregation of CDs because the repulsive Coulombic forces are reduced between nearby CDs. [53]We have analyzed the TEM image of CDs after the introduction of PAC.As revealed in Figure 6C, big CD aggregates were witnessed, signifying the formation of higher aggregates.Besides, the optical absorption of CDs/PAC aggregates in the red/NIR light region is obviously enhanced (Figure S26), also reflecting the formation of higher CD aggregates.
Moreover, the CDs/PAC aggregates have a decreased PL emission intensity compared with the CD aggregates (Figure S27), suggesting a fast separation of charge carriers.The carrier dynamics of CDs/PAC aggregates are explored by fsTA spectra.Two-dimensional fsTA demonstrates the absorption spectra versus time and wavelength (Figure 6D).Specifically, the GSB, ESA, and CT states are observed in the fsTA spectra of CDs/PAC (Figure 6E). [44,54]Kinetic fitting (Figure 6F) shows that the decay times of the ESA (510 nm) and CT states (650 nm) are 1.08 and 3.30 ns (Table S4), respectively.Compared with the CD aggregates (Figure 4B), the CDs/PAC aggregates have a longer lifetime of excited states.The results reveal that the high aggregation of CDs is beneficial to the separation and transfer of charge carriers.Impressively, the CDs/PAC aggregates display enhanced hydrogen production activity (Figure 6G) and good stability (Figure S28).The hydrogen production rates at 630 and 800 nm are 10.1 and 6.05 μmol/h (3366.3 and 2016.5 μmol g −1 h −1 ), respectively.The AQYs for hydrogen production at 630 and 800 nm correspondingly reach as high as 14.75% and 8.97% at 30 • C.
To check the generality of charge screening for promoting CDs aggregation, a series of salts (sodium alginate, ZnCl 2 , K 2 CO 3 , InCl 3 , and NaH 2 PO 2 ) was employed to modulate the aggregation of CDs (Salts consisting of equal numbers of cations and anions.The positively charged cations of salts can enable the aggregation of CDs by a charge screening effect.).Interestingly, the light absorption of CD aggregates can be apparently heightened after the introduction of these salts, reflecting the formation of higher CD aggregates (Figure S29). Figure 6H,I demonstrates the photocatalytic hydrogen production performances of these CD aggregates.These systems all exhibit enhanced hydrogen production activities, ratifying the universality of chargescreening-induced aggregation and the important role of aggregation degree in promoting photocatalytic activity.
Due to the presence of abundant O/N-containing groups on the surface of CDs, urea molecules can potentially adjust the electronic structures of CD aggregates by hydrogenbond interactions.Expectedly, urea molecules can implant the matrix of CD aggregates by hydrogen-bond interactions based on MD simulations (Figure 7A and Figure S30).The CD-CD contact map suggests that urea molecules have little effect on the aggregation degree of CDs (Figure S31).DFT calculations propose that the urea insertion narrows the bandgap energy of CD aggregates (Figure S32), consistent with the optical analysis (Figure S33).Furthermore, we have studied the orbital interactions between the CD aggregates (trimer) and urea.As presented in Figure S34, the LUMO orbit is mainly derived from the 295th orbit of CD aggregates (−2.97 eV) (68%), and the HOMO orbit is derived from the 186th orbit of the inserted urea (−6.27 eV) (96%).These results suggest that the local hydrogen-bond modulation can greatly rearrange the orbits of CD aggregates, thus regulating their electronic properties.
The hydrogen-bond modulation can also promote symmetry breaking through a local increase of distortion, inducing larger dipole moments compared with the CD aggregates (Figure S35).The randomly selected snapshot images of urea-functionalized CD aggregates recorded at 45, 46, 47, 48, 49, and 50 ns authorize the dynamic feature of aggregates (Figure S36) and also reflect this dynamic behavior is beneficial to maintaining large dipole moments of 25-34 D (Figure S37).The increased dipole moments definitely will accelerate the separation and transfer of charge carriers, reinforced by the observations of the HOMO and LUMO electron distribution of these aggregates (Figure S38) and the weak PL emission (Figure S39).Besides, the fsTA spectra confirm the CDs/urea aggregates have longer decay kinetics compared with the CD aggregates and the CDs/PAC aggregates (Figure 7C-E).The lifetime of the ESA and CT states reach 4.09 ns and 10.10 ns, respectively (Figure 7E and Table S4), further approving a better separation and transfer of electrons/holes.
As expected, the CD aggregates after urea modification have excellent hydrogen production activity under visible and red/NIR light (Figure 7B).The hydrogen production rate of the CDs/urea aggregates is much higher than that of the CD aggregates.The AQYs for hydrogen production at 630 and 800 nm were estimated to be as high as 17.4% and 10.37% at 30 • C, respectively.Furthermore, the AQY for hydrogen production at 800 nm can be increased to 13.5% at 50 • C. The AQYs for hydrogen production of reported red/NIRlight-active systems are summarized in Table S5.Apparently, the AQY for hydrogen production of CD aggregates is much higher than most of the reported values, suggesting the huge potential of CD aggregates for solar conversion.The reducibility and stability of photocatalysts are very important.In most cases, a few hours or at most one day of photocatalyst stability are reported.Herein, the stability of CDs/urea aggregates was studied by 10 runs of 200 h.As shown in Figure 7F, the hydrogen production activity of CDs/urea aggregates has no obvious decrease in 200 h reaction time, indicating admirable durability.
Considering the availability of electronic structures regulation for promoting photocatalytic activity, boracic acid, 2-methylimidazole, ascorbic acid, and thymine were also employed to modify the local arrangements of CD aggregates.DFT calculations suggest that these small molecules can interact with CDs by hydrogen-bond interactions and modulate the electronic properties of CD aggregates (Figure 7G and Figure S40).The CD aggregates after these molecules modification all display increased dipole moments for the separation of charge carriers, further reinforced by the electron distribution results of HOMO and LUMO orbits of these aggregates and the steady-state PL emission (Figure S41).Impressively, these systems all demonstrate superior photocatalytic activities (Figure 7H and Figure S42).These above results validate the tunable characters of electronic properties of CD aggregates, which offers a huge opportunity for further improvement in photocatalytic activity.
Wood, as a sustainable natural product, possesses a great of low-tortuosity channels providing a natural confined space for CD self-aggregation.To eliminate the influence of lignin on the photocatalytic activity of CD aggregates, a delignified wood (DW) was prepared. [55]Due to the removal of chromophoric groups from the wood, the DW turned completely white (Figure S43).The CDs/DW features a brown color under natural light, proposing that the CDs were successfully incorporated with the DW.Furthermore, we have analyzed the scanning electron microscopy images of a longitudinal section of DW and CDs/DW (Figure S44).Compared with the smooth pore surface of DW, the pore surface of the CDs/DW sample was relatively rough and some CD aggregates were observed, demonstrating the formation of CDs/DW.Besides, the CDs/DW demonstrated a strong absorption in visible/NIR light region compared with the DW (Figure S45).The enhanced light harvesting of CDs/DW can be attributed to the incorporation of CDs into the DW.These above results all reflect that the CDs were successfully confined into DW.DFT calculations suggest that the CDs can interact with DW by hydrogen bonds (Figure S46).Excitingly, the CDs/DW system can trigger hydrogen production with red/NIR light (Figure S47).Due to the advantages of large-scale, self-floating, and low-cost wood, the wood-confined CD aggregates can act as a large-scale and floating light-harvesting platform for hydrogen production.
A proposed photocatalytic hydrogen evolution mechanism of CD aggregates is presented in Figure S48.The CD aggregates are formed via π-π interactions and hydrogen-bonds interactions in an aqueous solution.The aggregation of CDs can broaden the light absorption from UV to NIR light range due to the electronic couplings between neighboring CDs.Importantly, the dynamic behaviors of CD aggregates can induce large dipolar states by stabilizing symmetry breaking, thus generating a robust built-in electric field for the separation and transfer of photo-generated charge carriers.Accordingly, the photogenerated holes are transported to the surface of CD aggregates and oxidize formic acid to release protons, while the photogenerated electrons reduce protons into hydrogen.Thanks to the unique aggregation endows the CD aggregates with exceptional red/NIR light harvesting and efficient charge separation, hence achieving high-performance hydrogen production even under red/NIR light irradiation.

Generality of aggregation-enhanced photocatalysis
To demonstrate the universality of aggregation-enhanced photocatalytic hydrogen production, a series of different CDs was prepared.The CDs prepared from glucose, sucrose, cotton, and methyl cellulose precursors by a hydrothermal method are denoted as CD-G, CD-S, CD-C, and CD-M, respectively.The CDs from sucrose precursor by a photochemical method are denoted as CD-ph.The CDs from glucose precursor by a pyrolysis routine are denoted as CDpy.As revealed in Figure 8B-G, these CDs have different morphologies and sizes (six examples).The XPS, Raman, Xray diffraction, and FTIR results of these CDs are revealed in Figures S49-S53, proposing their different compositions and structures.The absorption spectra of all samples obviously red-shift with increasing concentrations (Figure S54), suggesting the formation of aggregates.What's more, we have observed obvious photocurrent of these CDs powder irradiated by red/NIR light (Figure S55).These results reflect that these CD aggregates are operative under red/NIR light.
Furthermore, we have investigated the photocatalytic hydrogen production performances of these CD aggregates under visible and red/NIR light irradiation, as revealed in Figure 8H and Figures S56 and S57.Although these CDs have different structures, compositions, sizes, and morphologies, aggregation-enhanced photocatalysis was observed in all samples, representing the generality of this principle.Besides, PAC and urea were also used to modulate the aggregation degrees and electronic structures of CD-py aggregates, respectively.Unsurprisingly, the CD-py aggregates after introducing PAC and urea both display enhanced photocatalytic activity (Figure 8I,J).In a word, all these results support the fact that the CDs after aggregation are capable of capturing the red/NIR light and simultaneously enabling the charge separation, which can serve as a fresh and promising platform for efficient solar conversion.

CONCLUSION
In this work, we report a discovery that CD after aggregation can widen the light absorption up to the NIR light region.Concurrently, the dynamic noncovalent interactions endow CD aggregates with large and stable dipole moments and thus accelerate the separation of charge carriers.As a result, the CD aggregates exhibit excellent photocatalytic hydrogen production under red/NIR light.Furthermore, the non-covalent interactions and the diversity of surface groups of CDs allow for the rational design of physicochemical properties of CD aggregates by various strategies (charge screening, local hydrogen-bond rearrangements, and space confinement), significantly improving photocatalytic efficiency.Moreover, this aggregation-enhanced photocatalysis was observed in a series of CDs with different sizes, structures, and compositions, proposing the generality of this concept.These results highlight the achievement of using CD aggregates as NIR-light absorbers to trigger photocatalytic hydrogen production, which opens a new and exciting avenue to manipulating light absorption by aggregation.What is more, the idea of dynamic aggregation towards the reg-ulation of charge separation offers new perspectives and knowledge in photocatalysis science.It is anticipated that this work, combined with the rational use of molecular recognition, nano-scale assembly, and dynamic chemistry, may enable the wide development of non-covalent aggregates as red/NIR-light harvesters for artificial photosynthesis and other photochemistry-related applications.

Preparation of CDs
First, 10 g sucrose, 5 g NaNO 2 , and 1 g NaOH were dissolved in water (240 mL deionized water).Then, the solution was irradiated by a high-pressure Hg lamp (300 W) at 30 • C for 11 h.After the reaction, the solution was dialyzed with deionized water for 3 days, then via rotary evaporation and freeze to get a yellow powder sample (denoted as CDs).The CD-ph was synthesized as described above except that 5 g of NaNO 2 was not added.

Preparation of other CDs
First, 7 g of glucose was added to 420 mL of deionized water and stirred magnetically to dissolve it.The solution was then dispensed into 50 mL hydrothermal reactors and reacted in an oven at 160 • C for 6 h.After the reaction, a brown solution was obtained, which was then dialyzed with deionized water for 3 days and further filtered using a microfiltration membrane to obtain the target solution.Finally, the black powder obtained by freeze drying was denoted as CD-G.The other CDs prepared from sucrose, cotton, and methyl cellulose precursors by the above hydrothermal method were denoted as CD-S, CD-C, and CD-M, respectively.The CDs from glucose precursor by a pyrolysis routine were denoted as CD-py.Typically, 5 g glucose was placed in a tube furnace and calcined at 280 • C for 2 h.After natural cooling, the black product was ultrasonically treated in deionized water to form a brown dispersion.Then, the dispersion was dialyzed with deionized water for 3 days and the black powder was obtained by freeze-drying.

Preparation of droplet-confined CDs
CDs with different amounts were respectively dissolved in deionized water (0.25 mg/mL for droplet-A, 0.75 mg/mL for droplet-B, and 1 mg/mL for droplet-C).Then, the above CDs aqueous solution was injected into 15 mL ethyl acetate containing 30 mg cetyl trimethyl ammonium bromide, while ultrasonic treatment was carried out for 5 min under continuous stirring to form uniform droplets (the overall CDs amount within the above three droplets are controlled to be 0.5 mg).

Preparation of DW
DW was obtained through delignification in a solution containing ac. 2 wt% NaClO 2 in a buffer solution with an approximate pH value of 4.6.The Balsa wood was immersed into the NaClO 2 solution and boiled for 4 h until the wood turned completely white.Then, the wood was rinsed in pure water thrice to remove residual chemicals and bleached in 5 wt% H 2 O 2 solution.The bleached wood was rinsed with pure water and freeze-dried for 36 h to obtain the DW sample.

Preparation of CDs/DW
A uniform and stable CD solution was obtained via redispersing 3 mg CDs in 3 mL deionized water by ultra-sonication.The solution was then dropped onto the DW.The DW-confined CDs were further dried at room temperature and denoted as CDs/DW.

Characterization
UV-vis absorption spectroscopy was analyzed using a HITACHI UH5700 UV-vis-NIR spectrophotometer.Photoluminescence spectra and time-resolved photoluminescence of all samples were measured by a FL spectrophotometer (FLSP920).Confocal laser microscopy images were obtained with a Carl Zeiss LSM880 instrument.Fourier transform infrared of all sample were analyzed using a Bruker VER-TEX 80v spectrometer.Transmission electron microscopy was measured by a JEOL JEM-F200.AFM was carried out on a Bruker Dimension Icon.Raman spectroscopy was analyzed by Zeta potentials determined by a Malvern Zetasizer Nano ZS90.A powdered XRD pattern was performed using a D8 Advance Bruker X-ray diffractometer.X-ray photoelectron spectroscopy was carried out on a Thermo ESCALAB 250XI.The ultrafast fsTA measurement was conducted on a Helios spectrometer using a regeneratively amplified femtosecond Ti: sapphire laser system (Spitfire Pro-F1KXP; Spectra-Physics).Electrochemical and photocurrent measurements were carried out by using a BAS Epsilon Electrochemical System.

Details of the calculations
Atomistic MD simulations were performed using the Amber 18 and GROMACS 4.6.7 packages [56] with an amber gaff force field for CD molecules [57,58] and an amber14SB force field for the explicit TIP3P model water. [59]During system modeling, the solute molecules were added into a cubic box of 5 × 5 × 5 nm, [58] and then the different solvent molecules were used to fill the box.The particle mesh Ewald summation method was used to describe long-range electrostatics. [60]he cutoff distance is 1.0 nm for nonbonded interactions.The Langevin thermostat and Berendsen barostat were used for temperature and pressure control.In the current study, a 200 ns MD trajectory was collected in each system.After the equilibration checking, only the final equilibrated 50 ns trajectory was used for further analysis.All calculations were completed by Gaussian09 software.DFT was used to optimize the geometric structures of CDs, and time-dependent DFT (TD-DFT) was used to calculate the excited states of CDs and other related CD samples.Geometry optimizations in the ground, excited state, the intermolecular binding energy, were carried out with the B3LYP functional method in combination with the 6-31G* basis set. [61]The convergence accuracy of SCF has reached the default convergence standard.At the same time, the geometric configuration, HOMO-LUMO energy gap, excited state orbital distribution, molecular surface electrostatic potential, and dipole moment of CDs were analyzed and studied. [62]The electrostatic potential and isosurface maps of various orbitals are exported and visualized with Multiwfn 3.8 [62] and VMD 1.9.3 software. [63].9 Photocatalytic reaction Photocatalytic hydrogen evolution reaction was performed using a closed reaction system.In brief, CDs were added into 7 mL deionized water with 0.5 mL formic acid as an electron donor.Then, the reaction bottle was evacuated to eliminate air before the reaction and then the reaction system was full of Ar gas to reach a pressure of 1 atm.The produced hydrogen was analyzed by a gas chromatography (TCD detector).A series of 50 W monochromatic LED lamps were employed as light sources.Cocatalyst Pt was employed for catalyzing hydrogen production by an in-situ photo-deposition approach using H 2 PtCl 6 as a precursor (10 wt% Pt).The reaction temperature was controlled at room temperature (30 ± 2 • C).
The AQY for hydrogen evolution was measured by using a monochromatic LED lamp as excitation light.The irradiation area was controlled at 0.196 cm 2 .The reaction temperature was controlled at room temperature (30 ± 2 • C).The AQY was calculated as follows: where M is the amount of produced H 2 (mol), NA is Avogadro constant (6.02 × 10 23 mol −1 ), h is Planck constant (6.626 × 10 −34 J⋅s), c is light speed (3 × 10 8 m/s), S is irradiation area (cm 2 ), P is light intensity (W/cm 2 ), t is reaction time (s), λ is the wavelength of light irradiation (m).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

F I G U R E 1
Characterization of carbon dots (CDs).(A) Transmission electron microscopy (TEM) image of CDs.(B) Size distribution of CDs.(C) Highresolution TEM (HRTEM) image of CDs.(D) Atomic force microscopy (AFM) image of CDs.(E) The height of CDs based on AFM results.(F) Raman spectrum of CDs.(G) N 1s spectrum of CDs.(H) O 1s spectrum of CDs.(I) 13 C NMR spectrum of CDs.F I G U R E 2 Characterization of carbon dot (CD) aggregates.(A) Ultraviolet-visible (UV-vis) absorption spectra of CDs with different concentrations.(B) Enlarged UV-vis absorption spectra of CDs with different concentrations.(C) Structures of CD aggregates by an molecular dynamics (MD) simulation.(D) The function of reduced density gradient and sign(λ 2 )ρ scatter spectrum for CD aggregates is based on our MD simulations.(E) CD-CD contact map based on our MD simulations.(F) Transmission electron microscopy (TEM) image of CD aggregates.(G) Calculated highest occupied molecular orbital-least unoccupied molecular orbital (HOMO-LUMO) gap of CD aggregates.

F I G U R E 3
Orbital interaction diagram for carbon dots (CDs).The vertical axis shows molecular orbital (MO) energies in eV.Blue solid and dashed bars correspond to occupied and unoccupied MOs, respectively.The MOs are plotted as isosurface graphs with an isovalue of 0.02 a.u.Blank texts mark orbital indices.Red texts indicate a major contribution of MOs from the monomer to the MOs of CDs.The orbital compositions were evaluated by the Mulliken method.

F I G U R E 4
Characterization of carbon dot (CD) aggregates.(A) 2D transient absorption surface plots of CDs.(B) fsTA spectra of the CD aggregates (0.43 mg/mL CDs solution).(C) The distances of adjacent CDs change with time based on the result of Figure 4E.(D) Decay kinetics were probed at 470 and 650 nm.(E) Dynamic structure properties of a trimer aggregate in molecular dynamics (MD) snapshot and related dipole moments.

F
I G U R E 5 Photocatalytic performances of carbon dot (CD) aggregates.(A) Photocatalytic performances of CDs under 0.007 mg/mL.(B) Photocatalytic performances of CDs under 0.07 mg/mL.(C) Photocatalytic performances of CDs under 0.43 mg/mL.(D) Optical microscope image of droplet-A.(E) Optical microscope image of droplet-B.(F) Optical microscope image of droplet-C.(G) Photocatalytic hydrogen production of droplet-confined CDs under 630 nm light irradiation.

F I G U R E 6
Structural modulation of carbon dot (CD) aggregates.(A) Zeta potentials of CDs, polyaluminium chloride (PAC), and CDs/PAC.(B) Structure of PAC.(C) Transmission electron microscopy (TEM) image of CD aggregates after introducing PAC.(D) 2D transient absorption surface plots of CDs/PAC.(E) fsTA spectra of the CDs/PAC.(F) Decay kinetics probed at 515 and 650 nm.(G) Photocatalytic performance of CDs/PAC.(H) Photocatalytic performances of CD aggregates after adding different salts under 450 nm light irradiation.(I) Photocatalytic performances of CD aggregates after adding different salts under 630 nm light irradiation.

F I G U R E 7
Structural modulation of carbon dot (CD) aggregates.(A) Structures of CD aggregates after urea modification by an molecular dynamics (MD) simulation.(B) Photocatalytic hydrogen production of CD aggregates after introducing urea.(C) 2D transient absorption surface plots of CDs/Urea.(D) fsTA spectra of the CDs/Urea.(E) Decay kinetics probed at 515 and 650 nm.(F) Stability of CDs/urea aggregates under 630 nm light irradiation.(G) Hydrogenbond interactions between CD and boracic acid, 2-methylimidazole, ascorbic acid, and thymine based on density functional theory (DFT) calculations.(H) Photocatalytic performances of CD aggregates after introducing boracic acid, 2-methylimidazole, ascorbic acid, and thymine under 630 nm light irradiation.

F I G U R E 8
Generality of aggregation-enhanced photocatalysis.(A) Schematic illustration of aggregation-enhanced photocatalysis of carbon dot (CD) aggregates.(B) Transmission electron microscopy (TEM) image of CD-G.(C) TEM image of CD-M.(D) TEM image of CD-C.(E) TEM image of CD-S.(F) TEM image of CD-py.(G) TEM image of CD-ph (inset is the size distribution of CDs).(H) Photocatalytic hydrogen production activity of CD-py aggregates (0.43 mg/mL).(I) Photocatalytic performances of CD-py aggregates after introducing urea under different light-irradiation wavelengths.(J) Photocatalytic performances of CD-py aggregates after introducing polyaluminium chloride (PAC) under different light-irradiation wavelengths.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos.22372094, 21703039, and 21776168), the Natural Science Foundation of Shanxi Province (Grant No.20210302123461), the Central Guidance Local Science and Technology Development in Shanxi Province Project (YDZJSX2021A001), the Science and Technology Major Project of the Shanxi Science and Technology Department (Grant Nos.201903D121003 and 20181102019) and the Foundation of State Key Laboratory of Coal Conversion (Grant No. J22-23-605).