Solar-driven fast photocatalytic hydrogen evolution using size-minimized organic heterojunctions

Domain: physics
Published: 2025-12-30
Editorial outcome: minor_revision

Abstract
Pursuing small catalyst sizes has remained a constant endeavor to maximize reaction yields by exposing more active sites and/or enhancing charge extraction. Organic particle photocatalysts typically remain tens to hundreds of nanometers in size, constrained by the quasi-infinite conjugation of commonly used polymers. Here, we reveal the size-minimized organic heterojunction nanoparticles by using all small molecule photovoltaic materials, and significantly enhance their photocatalytic activities for solar-driven hydrogen evolution. Owing to the intrinsically weak intermolecular forces and the absence of molecular entanglement of small molecules, (sub)nanometer-scale diameters of polymer-free nanoparticles are yielded, representing a size reduction of 1-2 orders of magnitude than traditional polymer-containing nanoparticles. Optimized polymer-free nanoparticles attaching on covalent frameworks achieve photocatalytic mass-united hydrogen evolution rates of up to 527.2~3180.7 mmol h−1 g−1 at varied concentrations under simulated sunlight, and the external quantum efficiency is up to 32.8% at near-infrared light, marking a competitive performance for organic photocatalysts.

Introduction
Hydrogen, a kind of clean and sustainable energy, possesses tremendous advantages, such as high gravimetric energy density and a wide range of use as industrial raw materials1. Global hydrogen consumption was up to 95 million tons in 2022 and has been increasing annually, more than 80% of which was gray hydrogen, produced from fossil fuels with more than 900 million tons of CO2 emissions, bringing enormous pressure on climate change2. The photocatalytic water splitting through the Honda-Fujishima effect can convert solar energy directly into chemical energy in sustainable or green hydrogen without carbon emissions3. The particle suspension reactor is one potentially economical option in photocatalytic hydrogen evolution to meet the target of a hydrogen price of $2–4 kg−1 set by the United States Department of Energy4. However, the narrow absorption spectrum of traditional particle photocatalysts, such as TiO2 and SrTiO3 5, limited the utilization of visible to near-infrared solar light6. In pursuit of efficient photocatalysts, organic semiconductors, such as conjugated polymer/molecules7–15, have shown potential to be a viable option, due to their structural diversity, broadband absorption, easily tunable optoelectronic property, and low-cost processing.

Especially more and more high-performance organic photovoltaic materials, including electron donors (D) and acceptors (A), have been developed for bulk heterojunction (BHJ) organic solar cells, conversing solar energy to electricity with efficiencies of over 20%16–20, which positively drove the research enthusiasm in the heterojunction nanoparticles (HJ-NPs) based on organic photovoltaic materials for solar hydrogen evolution via direct photocatalysis. Like most chemical reactions and chemical engineering processes employing particle reactants/catalysts, it is crucial to manage the size and morphology of the utilization of visible to near-infrared solar light6. In pursuit of efficient photocatalysts, organic semiconductors, such as conjugated polymer/molecules7–15, have shown potential to be a viable option, due to their structural diversity, broadband absorption, easily tunable optoelectronic property, and low-cost processing.

Especially more and more high-performance organic photovoltaic materials, including electron donors (D) and acceptors (A), have been developed for bulk heterojunction (BHJ) organic solar cells, conversing solar energy to electricity with efficiencies of over 20%16–20, which positively drove the research enthusiasm in the heterojunction nanoparticles (HJ-NPs) based on organic photovoltaic materials for solar hydrogen evolution via direct photocatalysis. Like most chemical reactions and chemical engineering processes employing particle reactants/catalysts, it is crucial to manage the size and morphology of McCulloch and coworkers have developed a surfactant modulation approach to manage the morphology of organic HJ-NPs from a common but unfavorable core-shell structure to a BHJ structure with a heterogeneous surface, and greatly improved photocatalytic hydrogen evolution rate (HER)9. However, there are few reports on minimizing the sizes of organic NP catalysts, which typically range from dozens to hundreds of nanometers beyond their charge-carrier/exciton diffusion length, imposing inherent limitations on photocatalytic activity.

Here, we reveal the size-minimized organic NPs with BHJ structure by employing all small molecule D/A pairs, and significantly enhance their photocatalytic activities for solar-driven hydrogen evolution. Compared to widely used polymer-containing D/A pairs, polymer-free systems intrinsically tend to form smaller NPs due to weaker intermolecular interactions and the absence of molecular entanglement. The polymer-free NPs exhibit (sub)nanometer-scale diameters, which are 1–2 orders of magnitude smaller than those of traditional polymer-containing NPs and situated within the exciton/carrier diffusion length of organic semiconductors. Among the candidates screened, the NPs based on BTR:L8-BO D/A pair exhibit the most promising photocatalytic activity under AM1.5G illumination (100 mW cm−2). Then, size-minimized BTR:L8-BO NPs supported by covalent organic framework (NP@COF) show much better stability than the supportless NPs, and yield the optimized average photocatalytic mass-united HER (HER-m) of 527.2 (±34.0)~3180.7 (±234.5) mmol h−1 g−1 and the area-united HER (HER-a) of 137.4 (±10.1)~364.4 (±23.5) mmol h−1 m−2 at varied NP concentrations. The external quantum efficiencies (EQEs) of BTR:L8-BO NP@COF reach 18.4–32.8% under red to near-infrared light, representing a competitive performance among organic photocatalysts (OPCs)9–11,21,22, at which most inorganic catalysts reported in literatures fail to respond23.

Results
Design principle for size-minimized NPs
The microemulsion method stands out as one of the most extensively employed techniques for preparing NPs, in which the dispersed phase is mixed with the continuous phase, forming nanodroplets (NDs) stabilized by the surfactants, and the ND sizes dictate the NP sizes24. The size of steady-state NDs is influenced by its internal/external pressure (Pin/Pex) difference (ΔP=Pin−Pex) on NDs and interfacial tension (γ) between dispersed and continuous phase, which can be described by the Young–Laplace equation25:

ΔP= 2γ / R (1)

where R denotes the ND radius. In a given continuous phase, the ND sizes are primarily determined by interfacial tension and intermolecular forces within the dispersed phase. The intermolecular forces are expected to correlate positively with the inward force and negatively with the magnitude of the outward ΔP at the ND interface26. Intermolecular forces include dipole–dipole interaction, induction force, and dispersion force (the details are seen in Supplementary Note 1). Compared to small molecules, polymers possess more repeating units, which confer more internal rotational degrees of freedom and a higher number of conformations. This increases the likelihood of molecular distortion, thereby enhancing the non-uniformity of electron distribution and increasing dipole–dipole interactions within polymers27. More importantly, small molecule semiconductors with finite conjugated length tend to exhibit weaker dispersion forces and induction forces when interacting with surrounding molecules, including the material itself and/or solvents. In contrast, polymers with quasi-infinite conjugated length have more delocalized and polarized electrons, resulting in stronger intermolecular interactions28. These factors restrict the size-minimization of NDs and then NPs (Fig. 1).

The viscosity is positively correlated with intermolecular forces29, and thus serves as a macroscopic indicator of them. Here, the viscosity of several typical organic photovoltaic materials in solution was investigated by employing the Ubbelohde viscometer30. The kinematic viscosities of typical polymer photovoltaic materials, such as PM6 and PCE10 (Supplementary Fig. 1), in chloroform solution at a concentration of 0.5 mg mL−1, are 0.42 mm2 s−1 and 0.49 mm2 s−1, respectively. These viscosities are lower than those of many common organic solvents, such as chlorobenzene (0.72 mm2 s−1), methylbenzene (0.67 mm2 s−1), and tetrahydrofuran (0.59 mm2 s−1), but remain noticeably higher than that of pure chloroform (0.38 mm2 s−1)31. As a contradistinction, chloroform solutions of small molecule photovoltaic materials (B1, DRCN5T, and BTR, chemical structures shown in Fig. 2a)32–34 at the same concentration as polymer solutions, present viscosities of about 0.38 mm2 s−1, nearly identical to that of pure chloroform (Fig. 2b)31. These results experimentally confirm that typical small molecule photovoltaic semiconductors possess intermolecular interactions comparable to or even weaker than those in common organic solvents, in stark contrast to conjugated polymers with significantly stronger interactions. Such a weak intermolecular interaction is unlikely to hinder the reduction in ND and NP sizes. Furthermore, self-aggregation occurs in polymer solutions due to their larger conjugation length and stronger intermolecular forces, resulting in particle sizes of 10–100 nm at concentrations of 0.25–1.00 mg mL−1, as measured by dynamic light scattering (DLS, Fig. 2c), and the much larger particles are formed in the solution with a higher concentration of 2.00 mg mL−1. In contrast, no scattering signal was detected in small molecule solutions such as BTR and B1, indicating their dispersion diameter is below the DLS detection limit (ca. 0.3 nm). This absence of self-aggregation and weaker intermolecular interactions render small molecules ideal candidates for fabricating size-minimized NPs.

Screening polymer-free NPs
Typical small molecule photovoltaic donors B1, DRCN5T, and BTR were employed to prepare polymer-free HJ-NPs through the microemulsion method, by blending four typical small molecule acceptors named PC71BM, EH-IDTBR, BTP-eC9, and L8-BO (Fig. 2a) with type II D/A heterojunction energy level offset (Supplementary Fig. 2, Supplementary Table 1), and here the D/A weight ratio is 4:6, and sodium 2-(3-thienyl)-ethyloxybutylsulfonate (TEBS, 0.4 wt.%) was used as the stabilizer9. The DLS volume distribution (Supplementary Fig. 3 and Supplementary Table 2) and transmission electron microscopy (TEM) analysis (Supplementary Figs. 4, 5 and Supplementary Table 3) revealed that the diameters of these polymer-free HJ-NPs ranged from 1 to 10 nm. These values are already 1–2 orders of magnitude smaller than those of traditional polymer-containing NPs (ca. 50–100 nm), which are composed of polymer donor and small molecule acceptors (Fig. 2d, Supplementary Table 4)9–11,14,15,35. For instance, BTR:L8-BO NPs exhibited a mean diameter of approximately 5.5 nm according to the DLS volume distribution (Fig. 2e), consistent with the statistical mean value obtained from the TEM image (Fig. 2f).

The photocatalytic activities of these polymer-free HJ-NPs, consisting of 12 D/A pairs from three donors and four acceptors, were subsequently evaluated using 0.2 M ascorbic acid (AA) as the sacrificial agent and 30 wt.% Pt loading as cocatalyst, under simulated sunlight (AM1.5G, 100 mW cm−2). Despite the unoptimized fabrication of these polymer-free NPs, most of them already exhibited notable HER-m (Supplementary Fig. 6 and Supplementary Table 5), surpassing those reported for typical polymer-containing HJ-NPs in the literature9–11,14. Specifically, when coupled with acceptor molecule L8-BO, the B1, DRCN5T, or BTR-based HJ-NPs exhibited photocatalytic HER-m of 598 mmol h−1 g−1, 393 mmol h−1 g−1, and 754 mmol h−1 g−1 for 4 h, respectively. Meanwhile, B1:EH-IDTBR also exhibited a favorable HER-m, with the value reaching 237 mmol h−1 g−1 under the same conditions (Fig. 2g). These results suggest that the size-minimized NPs do benefit the enhanced photocatalytic activity.

Examination of dimension minimization
The diameters of NDs in ultrasonically formed microemulsions of polymer-free systems, here taking the BTR:L8-BO and B1:EH-IDTBR pair as examples, are primarily concentrated at 1–10 nm for BTR:L8-BO NDs and at 10–30 nm for B1:EH-IDTBR NDs (Supplementary Fig. 7), and the resulting NPs remain small after chloroform removal (Supplementary Fig. 8). In contrast, the ND diameters of polymer-containing systems, such as PM6:Y6 and PCE10:EH-IDTBR pairs, are larger, ranging from 50 to 100 nm. These findings indicate that the diminished size of those polymer-free NPs is contingent upon their precursor NDs, ruling out the possibility of ND reaggregation during the chloroform removal.

As indicated by Eq. (1), the lower interfacial tension γ of the dispersed/continuous phase for NP fabrication promotes the formation of smaller NDs and subsequently generated NPs, alongside the weak intermolecular interaction within the dispersed phase. The surfactant concentration is a critical parameter that governs the interfacial tension, thereby regulating the ND size and consequently the size of the resulting NPs. We measured γ using the Du Noüy ring method36, in which varying aqueous solutions of TEBS surfactant (continuous phase) and chloroform solution (0.5 mg mL−1) of various polymer-containing or polymer-free D/A pairs (dispersed phase) are employed. The chloroform solution of BTR:L8-BO, B1:EH-IDTBR, PM6:Y6, and PCE10:EH-IDTBR (D:A, w:w, 4:6) exhibited similar γ values to TEBS aqueous solution (aq.) at low concentrations, such as 21 mN m−1, 24.5 mN m−1, 23.1 mN m−1, or 23.7 mN m−1 in 0.5 wt.% TEBS aq., respectively, indicating γ should not be the primary determinant of the size reduction in polymer-free NPs relative to polymer-containing NPs. As the TEBS concentration increased from 0.5 to 10 wt.%, the γ between dispersed phase and TEBS aq. phase decreased rapidly and reached almost stable values with further increasing TEBS concentration for both polymer-containing and polymer-free systems. At a concentration of 10 wt.%, γ of PCE10:EH-IDTBR, PM6:Y6, B1:EH-IDTBR or BTR:L8-BO (CHCl3 solution)/TEBS aq. was measured at 3.1 mN m−1, 11.6 mN m−1, 7.1 mN m−1, and 6.2 mN m−1, respectively (Fig. 3a). Although the size of NPs decreased as the TEBS concentration increased for all types of NPs, a significant difference remained in the minimum achievable sizes between polymer-containing and polymer-free NPs. Notably, even at 30 wt.% TEBS, PCE10:EH-IDTBR NPs and PM6:Y6 NPs still showed a diameter of 19.2 nm and 17.8 nm, respectively, and in contrast, the diameters of polymer-free NPs are all below 5 nm (Fig. 3b).

Likewise, the surfactant structure is also a critical factor that governs the interfacial tension. Subsequently, various types of surfactants with gradient concentration, including sodium dodecyl sulphate (SDS), sodium dodecylbenzenesulphonate (SDBS), and sodium cholate (SC), were further used to prepare NPs. Similar to TEBS surfactant, the concentrations of SDS, SDBS, and SC aq. exhibit a negative correlation with their interfacial tensions with polymer-free/polymer-containing chloroform solution (B1:EH-IDTBR, BTR:L8-BO, PM6:Y6, and PCE10:EH-IDTBR, 4:6, w:w, 0.5 mg mL−1), reaching minimum values when the concentrations are up to approximately 1 wt.% (Fig. 3a). At concentrations exceeding 0.01–0.05 wt.% for SDS, SDBS, or SC, the diameters of the produced polymer-free NPs (B1:EH-IDTBR and BTR:L8-BO) are as small as 5 nm, substantially smaller than those (>20 nm) of polymer-containing NPs (PM6:Y6 and PCE10:EH-IDTBR, Fig. 3b). This indicates that, regardless of the surfactant type or dosage, polymer-free NPs consistently exhibited significantly smaller sizes than polymer-containing NPs.

Then, the external factors such as ultrasound power (100–300 W for 1–5 min) did not significantly affect the size of polymer-free NPs in the range of 1–10 nm and polymer-containing NPs (>20 nm) (Supplementary 9). These findings confirm that the shear energy applied in this study (250 W for 5 min) was sufficient to generate well-dispersed NDs under thermodynamic equilibrium and was not a limiting factor for the resulting NP size. By tuning the NP preparation parameters, including interfacial tension and shear energy, we found that small molecules, due to their weaker intermolecular interactions and the absence of molecular entanglement, are inherently more likely to forming smaller NPs than polymers.

Internal morphology
In addition to NP size, the internal morphology of HJ-NPs is another major intrinsic factor influencing their photocatalytic activity, and the BHJ-like morphology is favored for the extraction of photogenerated hole/electron, according to the previous study by McCulloch and coworkers9. Small-angle X-ray scattering (SAXS) is used to analyze the NP morphology statistically, and X-ray scattering profiles provide insights into nanostructural inhomogeneities of electron density within bulk materials. Then, the analysis of SAXS data from coherent scattering events provides information about the distributions of material components. Figure 3c, d display the two-dimensional SAXS patterns of BTR:L8-BO NPs stabilized by 0.05 wt.% SDS and 0.4 wt.% TEBS, respectively, and their corresponding one-dimensional profiles are shown in Fig. 3e, f. The X-ray scattering profiles of single-component NPs are shown in Supplementary Fig. 10. Here, we use Debye–Anderson–Brumberger and the fractal-like network model37–39 and core-shell model40 (the details of the model seen in Supplementary Note 2) for the fitting analysis of the measured scattering profile, respectively. The SAXS profiles of BTR:L8-BO NPs stabilized by SDS or TEBS, fit well with the Debye–Anderson–Brumberger (DAB) and the fractal-like network model (fitting coefficients close to 1) (Fig. 3e, the parameters of fitting are listed in Supplementary Tables 6, 7), which indicates that the BTR:L8-BO NPs based on either SDS or TEBS have the BHJ-like continuous interpenetrating network structure. On the contrary, the model describing core-shell particles fails to fit well the SAXS profile of BTR:L8-BO NPs, which have coefficients of determination of 0.98 at most for BTR:L8-BO NPs stabilized by SDS and only 0.06 for BTR:L8-BO NPs stabilized by TEBS (Fig. 3f, the parameters of fitting seen in Supplementary Tables 8, 9), when setting the radius as the corresponding value and applicable to a Log-normal distribution (Supplementary Table 10) with polydisperse shell thickness (Gaussian distribution). Similarly, SAXS profiles of B1:EH-IDTBR NPs stabilized by SDS or TEBS can also be fitted well with the model describing the continuous interpenetrating network structure (fitting coefficient of almost 1 while failing to be fitted with the core-shell model (Supplementary Fig. 11 and Supplementary Table 11–14). The above results suggest that size-minimized polymer-free HJ-NPs tend to form a continuous interpenetrating network structure rather than a core-shell structure, independent of molecular structure or surfactant types. This is different from the typical polymer-containing HJ NPs, of which SDS-stabilized NPs and TEBS-stabilized NPs tend to exhibit core-shell or BHJ structures, respectively9,41.

To further investigate internal morphology, we examined TEBS-stabilized BTR:L8-BO NPs as a representative example using cryo-TEM. In contrast to the core-shell structure typically observed in polymer-containing HJ-NPs, the BTR:L8-BO polymer-free NPs exhibit continuous amorphous morphology with only a few crystalline domains dispersed inside. These crystalline domains exhibit two primary lattice spacings of 1.23–1.24 nm and 1.50 nm (Supplementary Fig. 12) that is corresponding to L8-BO lamellar stacking and BTR unit crystal cell, respectively33,42. This predominance of amorphous regions further supports the formation of a BHJ-like structure in the polymer-free HJ-NPs, consistent with the SAXS results. This BHJ-preferred morphology is likely strongly associated with the size minimization of polymer-free HJ-NPs. From a geometric standpoint, such small (<5 nm) polymer-free NPs can be treated as molecular clusters only composed of several molecules, and thereby it is difficult to form a core-shell structure with a continuous shell to fully cover up another paired material. Such cover is usually found in SDS-stabilized polymer-containing HJ-NPs41, and the smaller particles prefer tighter polymer shell encapsulation, thereby further hindering the electron transfer to Pt, which may contribute to the decreased HER recently reported in SDS-stabilized polymer-containing NPs below 60 nm43. Then the SDS-stabilized size-minimized BTR:L8-BO NPs here with similar BHJ morphology also show lower photocatalytic activities than their TEBS-stabilized counterparts, despite the SDS concentration used is much lower than the value in the literature (Supplementary Fig. 13), which can be attributed to the decreased Pt deposition caused by the long alkyl chain of SDS44.

Photoinduced charge-carrier dynamics
Preliminary screening indicates that polymer-free NPs based on BTR:L8-BO exhibit the most promising photocatalytic activity. The BTR:L8-BO HJ-NPs display broad absorption from the visible to near-infrared region (Supplementary Fig. 14a, b) and exhibit efficient charge separation, as evidenced by photoluminescence (PL) quenching compared to pristine donor or acceptor NPs (Supplementary Fig. 14c, d), which meets the prerequisite for efficient solar-driven photocatalysis. To further investigate the exciton dissociation dynamics in BTR:L8-BO HJ NPs, transient absorption (TA) measurements were first carried out with pump fluence dependence under selective excitation of the acceptor at 780 nm (Supplementary Fig. 15)45–47. As shown in Supplementary Fig. 16, the ground state bleaching signal of L8-BO, peaking at 784 nm and consistent with the absorption profile of L8-BO, shows no significant change in decay lifetime across different excitation intensities (0.82–3.27 μJ cm−2), indicating the absence of exciton-exciton annihilation at these weak excitation fluences11. Subsequently, TA spectra were collected for BTR:L8-BO (w:w, 4:6) NPs along with polymer-containing PM6:L8-BO (w:w, 4:6) NPs as a reference under 780 nm pump excitation at a fluence of 1.63 μJ cm−2. As illustrated in Fig. 4a–d, the three emerged excited-state absorption bands with peaks at ~550, ~900 and ~1350 nm can be assigned to the reduced form of L8-BO, the localized excitation (LE) state of L8-BO, and the intra-moiety excited (i-EX) state of L8-BO10,48,49, respectively, which are also present in the TA spectra of L8-BO NPs (Supplementary Fig. 17). Kinetic analysis of the reduced L8-BO signal reveals that the small-molecule system BTR:L8-BO exhibits a longer decay lifetime (t1/t2 = 1.2/350.8 ps) compared to the polymer-containing PM6:L8-BO system (t1/t2 = 0.8/22.4 ps), indicating slower recombination of the reduced acceptor in the small-molecule system (Fig. 4e). Meanwhile, the lifetimes of LE state (t1/t2 = 0.6/12.5 ps) and the i-Ex state (t1/t2 = 5.2/206.1 ps) in BTR:L8-BO are shorter than those in PM6:L8-BO (LE: t1/t2 = 0.6/13.4 ps; i-Ex: t1/t2 = 8.5/286.3 ps), suggesting that the continuous interpenetrating network structure in the size-minimized polymer-free HJ-NPs facilitates shorter diffusion paths and more efficient exciton dissociation compared to the polymer-containing system, which is expected to contribute to enhanced photocatalytic hydrogen evolution (Fig. 4f, g)9.

Solar-driven photocatalytic hydrogen evolution
Then, BTR:L8-BO HJ NPs were optimized for photocatalytic sacrificial hydrogen evolution under simulated sunlight, in which AA (0.2 M) acted as the sacrificial agent and Pt was photo-deposited in situ on the HJ-NP surface as a cocatalyst (Fig. 5a). The highest HER-m achieved at D/A ratio of 4:6 (w:w) (Supplementary Fig. 18), could be attributed to more efficient charge separation relative to other D/A ratios which was supported by the shortest decay lifetime of the L8-BO i-Ex signal observed in TA experiments (Supplementary Figs. 19 and 20). The optimized TEBS/H2O weight ratio and Pt-loading ratio were determined to be 0.4 wt.%, and 70 wt.% respectively, with detailed optimization results provided in Supplementary Figs. 21–24. Determined by ICP-MS according to the reported method44,50, the actual Pt loading (mass of Pt on OPC/mass of OPC) was 25.5 wt.% (Supplementary Table 15). Furthermore, TA spectra of NPs in the presence of AA and AA + Pt were measured. Upon the addition of AA, the hole in the donor BTR domain can be effectively extracted, while charge recombination of reduced L8-BO in the acceptor domain is not significantly affected, leading to a similar reduced acceptor lifetime (t1/t2 = 1.2/365.3 ps). In contrast, the lifetime of reduced L8-BO decreases markedly to t1/t2 = 0.2/1.1 ps after in-situ Pt deposition, indicating rapid electron transfer from the NPs to Pt (Supplementary Figs. 25 and 26).

The optimized BTR:L8-BO NPs exhibited the highest average HER-m of up to 1786 mmol h−1 g−1 at a low concentration of 3.33 μg mL−1 for 4 h under AM 1.5G, 100 mW cm−2. Then the concentration of the photocatalyst was increased for almost saturated absorption (26.67 μg mL−1), BTR:L8-BO NPs showed average photocatalytic HER-a of 161 mmol h−1 m−2 and HER-m of 468 mmol h−1 g−1 for 4 h. Further increasing the concentration to 53.33 μg mL−1, a HER-a of 194 mmol h−1 m−2 and a HER-m of 281 mmol h−1 g−1 are yielded (Supplementary Table 16), and under this concentration, the external quantum efficiencies (EQEs) of BTR:L8-BO NPs were 9.2%, 7.2%, 12.7% and 19.2% at 500, 600, 700, and 800 nm, respectively (Supplementary Fig. 27 and Supplementary Table 17).

Furthermore, a surfactant named 2-phenylethoxybutylsulfonate sodium (PEBS, the structure shown in Fig. 5b) with a phenyl instead of thiophenyl in TEBS was synthesized (the details of synthesis are shown in Methods) and used to stabilize polymer-free HJ-NPs for further improving photocatalytic hydrogen evolution. The optimized D/A ratio, PEBS/H2O weight ratio, and Pt-loading ratio are 5:5 (w:w), 0.4 wt.%, and 20 wt.%, respectively, and the detailed results are shown in Supplementary Figs. 28–31. Then the actual Pt load in this condition is 11.6 wt.% from ICP-MS measurement (Supplementary Table 15). The optimized BTR:L8-BO NPs stabilized by PEBS exhibited the average HER-m of up to 2314.1±306.6 mmol h−1 g−1 at a low photocatalyst concentration of 3.33 μg mL−1, and as the concentration increased to 26.67 μg mL−1~53.33 μg mL−1, the average HER-m fell to 656.9 (±64.8)~368.8 (±29.0) mmol h−1 g−1 and the average HER-a raised to 227.1 (±22.4)~255.0 (±20.0) mmol h−1 m−2 for 4 h under AM 1.5G, 100 mW cm−2 (Fig. 5c, Supplementary Table 18). The improvement in hydrogen generation activity can be attributed to the further reduced size of NPs stabilized by PEBS (3.13 nm) compared to TEBS (5.47 nm, Supplementary Fig. 32). These notable HER-m results support that minimizing HJ-NPs by employing polymer-free pairs to the nanometer size is efficient for photocatalytic activity enhancement. Recently, Li, Zhu, Cooper, and coworkers have developed a high-throughput automated platform for screening molecular heterojunction photocatalysts, which also suggests the advantage of small molecules in flow synthesis13.

The nanoscale particle size of polymer-free HJ NPs presents a challenge in terms of stability during the photocatalytic hydrogen evolution, as they are susceptible to aggregation and precipitation, thereby limiting their practical applications. The aggregation of these NPs was prohibited by introducing COF (named as Py-TAPD-COF, Supplementary Fig. 33) which, under the mildly acidic conditions generated by AA, becomes partially protonated at the imine or amine sites51, thereby acquiring a positive surface charge that electrostatically attracts and anchors the negatively charged NPs, preventing their aggregation and sedimentation and thus enhancing stability and sustaining photocatalytic activity over time. Consequently, numerous NPs adhered to COF were observed by using scanning electron microscopy (SEM, Fig. 5d and Supplementary Fig. 34). The NPs supported by COF (NP@COF) show much better photocatalytic stability than that of the NPs without COF (Supplementary Fig. 35). Subsequently, we have carried out ultraviolet-visible (UV-vis) absorption spectra and mass spectrometry to characterize the structural stability. UV-vis absorption spectra and mass spectrometry verify that the chemical structure of NP@COF remains essentially unchanged (Supplementary Fig. 36). These results demonstrate the structural robustness of the NP@COF composite during repeated photocatalytic operation. The optimized BTR:L8-BO NP@COF, stabilized with TEBS, exhibited the highest average HER-m of up to 1735 mmol h−1 g−1 at a low concentration of 3.33 μg mL−1 and the highest average HER-a of 183 mmol h−1 m−2 at a concentration of 53.33 μg mL−1 for 4 h under AM 1.5G, 100 mW cm−2 illumination (Supplementary Figs. 37, 38 and Supplementary Table 19). The corresponding maximum EQEs are 8.8%, 7.5%, 12.1% and 19.8% at 500, 600, 700, and 808 nm, respectively (Supplementary Fig. 39, Supplementary Table 17). The close EQEs between these NPs and NP@COF indicate that their similar photocatalytic activity in the initial stage is mainly attributed to the enhanced stability rather than the catalytic activity of Py-TAPD-COF itself, while pure Py-TAPD-COF does not produce hydrogen under the same conditions.

To more directly investigate whether the Py-TAPD-COF participates in charge transfer, we first measure the absorption of the Py-TAPD-COF with a relatively wide band gap (Supplementary Fig. 40a), which exhibits limited intrinsic absorption with a peak at 500 nm (longer wavelength signal attributed to scattering). Subsequently, PL spectra show that the Py-TAPD-COF itself exhibits almost no fluorescence emission (Supplementary Fig. 40b). When single-component L8-BO NPs are loaded onto the Py-TAPD-COF and excited at 780 nm, the PL intensity decreases by 32% compared to those NPs without the Py-TAPD-COF (Supplementary Fig. 40c). Similarly, when BTR-only NPs are loaded onto the Py-TAPD-COF, the PL intensity also decreases by 18% (Supplementary Fig. 40b). The electron transfer should be limited by a type I contact between Py-TAPD-COF and L8-BO within HJ-NPs (Supplementary Fig. 40d). Meanwhile, although the LUMO energy of BTR is a little shallower than Py-TAPD-COF, which may allow the electron transfer from BTR to Py-TAPD-COF, the spatial separation between the Py-TAPD-COF and BTR by the surfactant TEBS likely restricts the possibility of charge transfer. Therefore, the observed fluorescence reduction maybe attributed to decreased excitation light absorption by L8-BO/BTR due to the light scattering by Py-TAPD-COF. Both mechanism insights and initial hydrogen evolution performance indicate that negligible charge transfer occurs between the Py-TAPD-COF and NPs, with the Py-TAPD-COF primarily functioning as a support. At the same time, when stabilized with PEBS instead of TEBS, the optimized BTR:L8-BO NP@COF exhibited further enhanced average HER-m of up to 3180.7±234.5 mmol h−1 g−1 at a low concentration of 3.33 μg mL−1, and average HER-a of 364.4±23.5 mmol h−1 m−2 at a concentration of 53.33 μg mL−1 for 4 h under simulated sunlight (Fig. 5c, Supplementary Fig. 41 and Supplementary Table 20), demonstrating competitive performance among reported OPCs (Supplementary Table 21). The corresponding maximum EQEs are 19.1, 23.2, 22.3, 18.4, and 32.8% at 400, 500, 600, 700, and 808 nm (Fig. 5e, Supplementary Table 17), and the notable EQE values verify the high photocatalytic activity, especially in the near-infrared region (Supplementary Table 21)21, at which most existing inorganic photocatalysts do not respond. Although the EQE test is conducted under conditions of light absorption saturation, the EQE at different wavelengths still varies due to differences in charge transfer pathways. When the acceptor is excited under near-infrared illumination, electrons transfer directly to the Pt cocatalyst, whereas donor excitation requires an additional step from donor to acceptor before reaching Pt11. The shorter transfer pathway and lower recombination probability in the acceptor-excited process account for the higher quantum efficiency observed under near-infrared illumination.

Hydrogen evolution in varied water sources
The enhanced stability of NP@COF catalysts made them promising for the direct utilization of some high-salinity water sources other than scarce freshwater resources, like some wastewater and even seawater (96.5% of the Earth’s water), in which most supportless NPs suffer severe condensation and catalytic deactivation, because the heightened concentrations of free ions disrupt the double layer surrounding the NPs, prompting their aggregation and subsequent sedimentation52. Benefiting from the support of COF, NP@COF can be stable in water as a colloid showing the obvious Tyndall phenomenon under 650 nm laser illumination, even if the electric double layer structure of NPs is disrupted in high-salinity water (Fig. 5f). Here we utilize a 0.6 M sodium chloride aqueous solution to emulate the ionic and high-salinity environment of seawater (global average salinity of 3.5%) for photocatalytic hydrogen evolution, and in such highly ionic water NP@COF still exhibit obviously photocatalytic activity with an average AA-sacrificial HER-m of 10.9 mmol h−1 g−1 under AM 1.5G, 100 mW cm−2 for 10 h (Fig. 5g). In contrast, NPs without COF are susceptible to rapid aggregation and sedimentation when stirred under dark conditions in the sodium chloride aqueous solution. Although the HER of NP@COF in simulated seawater is markedly lower than that observed in pure water due to much more sodium ions than hydrogen ions accumulated on the particle surfaces, this development represents a significant improvement over the severe instability of NPs in saline environments53.

The photocatalytic sacrificial hydrogen evolution usually suffers additional operating costs due to the consumption of chemical sacrificial agents like AA used here. Benefiting from the enhanced stability of NP@COF in high-salinity water, reducing agents widely present in high-salinity wastewater show considerable potential to replace the commercial sacrificial agents for hydrogen evolution. Here, we take several antibiotic wastewaters as examples, in which antibiotics are widely used in both human medicine and animal husbandry, posing substantial environmental risks due to their persistence and potential for adverse effects on non-target organisms. The total annual consumption of antibiotics gradually increases around the world, reaching over 106 tons, and a considerable portion is discharged into wastewater, where antibiotic concentrations detected in surface water spans from ng L−1 to mg L−1, and these biologically active reductive compounds can have detrimental impacts on the environmental matrix, necessitating thorough evaluation of their ecological ramifications54,55. The NP@COF maintains stability (Supplementary Fig. 42) and exhibits photocatalytic activity in 0.2 M antibiotic solutions without AA sacrificial agents, where lincomycin hydrochloride (LM), gentamicin sulfate (GM, a mixture of three compounds), and levofloxacin hydrochloride (LX) serve as sacrificial agents with HER-m of 6.6 mmol h−1 g−1, 7.9 mmol h−1 g−1, and 3.8 mmol h−1 g−1, respectively, under AM 1.5G illumination at 100 mW cm−2 for 4 h at a NP concentration of 6.67 μg mL−1 (Fig. 5g). In contrast, NPs without COFs aggregate after 5 min (Supplementary Fig. 43), resulting in no hydrogen evolution. At a concentration of 0.2 M, AA has a pH of 2.4 and ion concentration of 10−3 M, and in comparison, antibiotics at 0.2 M have ion concentrations of 10−1 M, two orders of magnitude higher than AA. At a NP concentration of 53.33 μg mL−1, NP@COF remains stable in antibiotic solutions, with HER-m of 1.9 mmol h−1 g−1 in GM and 1.6 mmol h−1 g−1 in LM (Supplementary Fig. 44). This result supports that the COF-stabilized size-minimized NPs have the capability for generating hydrogen in wastewater with antibiotics and other reducing agents, and then reducing the operational cost of energy supply by using the wastewater partially instead of the commercial sacrificial agents.

In summary, we reveal and demonstrate that the NP size can be minimized in polymer-free HJ NPs employed with all small molecule D/A pairs, overcoming the limit of widely-used polymer NPs and bringing enhanced performance in photocatalytic activity; and then we stabilize size-minimized NPs by supporting them on COFs, even in high-salinity water. These polymer-free NPs exhibit diameters ranging from 1 to 10 nm at 0.4 wt.% TEBS/H2O, which have already been 1–2 orders smaller than those (ca. 50–100 nm) reported for traditional polymer-containing NPs, and the weak intermolecular forces and the absence of entanglement between small molecules mainly contribute to the smaller size of polymer-free NDs and NPs than polymer-containing counterparts in the same fabrication conditions. Profiting from size-minimization, the preferred BHJ morphology is formed within polymer-free HJ-NPs, independent of the surfactant type. As a result, both polymer-free HJ-NPs without and with COFs exhibit enhanced performance for photocatalytic hydrogen evolution under AM 1.5G, 100 mW cm−2; the optimized polymer-free NP@COF achieves a HER-a of up to 364.4±23.5 mmol h−1 m−2 with the near-infrared EQE of up to 32.8%, and HER-m values of 527.2 (±34.0)~3180.7 (±234.5) mmol h−1 g−1 at varied OPC concentration. Furthermore, the NP@COF with enhanced stability exhibits promising hydrogen evolution in seawater and wastewater containing antibiotics. Our work confirms the advantage of small molecules for heterogeneous reactions as nano-catalysts, which boosts the development of organic NP photocatalysts for solar hydrogen evolution. This strategy to minimize the size of NPs and then stabilize them on frameworks should be extended to other chemical and chemical engineering processes, such as pollutant photodegradation and industrial catalysis, as well as other applications where stable nanosized particles are required or preferred.

Methods
Materials and characterization
The chemical reagents and materials were purchased from commercial companies and were used without further purification. Except for L8-BO and Py-TAPD-COF, organic semiconductor materials were purchased from Solarmer Energy, Co., Ltd. L8-BO was purchased from Nanjing Zhiyan Technology Co., Ltd. Py-TAPD-COF, LX, LM, and GM were purchased from Beijing Innochem Technology Co., Ltd. Organic solvents were purchased from Tianjin Concord Technology Co., Ltd. UV-vis absorption spectra were measured by a UH4150 spectrophotometer. PL spectra were measured by a FLS980 fluorescence spectrometer (Edinburgh Instruments). Hydrodynamic diameters of NPs and NDs were measured by DLS (Zetasizer Nano-ZS, Malvern Instruments) and batch-to-batch variations under identical conditions were within experimental uncertainty. Interfacial tension were measured employing the du Noüy ring method. The liquid under scrutiny was ensconced within a 10 mL vitreous vessel. An organic phase concentration of 0.5 mg mL−1 was meticulously selected with a corresponding concentration for the surfactant aqueous counterpart. The interfacial tension were recorded by an automatic tensiometer (FST300C, AFES). SEM measurement was performed using a Hitachi S-4800. Silicon wafers underwent ultraviolet ozone treatment for 20 min, followed by direct deposition of 3 μL NPs onto their surfaces. After vacuum drying at 40 °C for 2 h, before SEM measurement. TEM measurement was performed using the Hitachi HT7700 TEM. Ultra-thin microgrids were subjected to ultraviolet ozone treatment for 20 min. Subsequently, 3 μL of the NP suspension was directly deposited onto the pretreated microgrids, followed by vacuum drying at 40 °C for 2 h before TEM measurement.

Synthesis of PEBS
To a solution of 2-Phenethanol (5 mL; 41.7 mmol) in freshly distilled toluene (200 mL), 1.18 g of sodium hydride (48 mmol) was added slowly, which led to a white precipitate. After the mixture was stirred at 25 °C for 0.5 h, 1,4-butane sultone (48 mmol) was added dropwise. The mixture was then refluxed for 2 h under the protection of nitrogen. By filtering and washing with toluene and ethanol, and vacuum drying at 40 °C, the final pale-yellow precipitate was obtained with a yield of 46%. 1H NMR (400 MHz, D2O): δ 7.23 (m, 5H), 3.66 (t, 2H), 3.43 (t, 2H), 2.78 (m, 4H), 1.57 (m, 4H). 13C NMR (100 MHz, D2O): δ 139.12, 128.89, 128.64, 126.44, 70.99, 69.88, 50.63, 35.02, 27.37, 20.86. ESI-MS (TOF): [M-Na]− calcd. for C12H17O4S− 257.08, found 257.14.

Viscosity measurement
Viscosities of organic semiconductor material solutions were measured by an Ubbelohde viscometer. Initially, the efflux time for a series of solvents with different viscosities, such as chloroform (0.38 mm2 s−1), dichloromethane (0.32 mm2 s−1), toluene (0.67 mm2 s−1), ethyl acetate (0.50 mm2 s−1), tetrahydrofuran (0.59 mm2 s−1), n-hexane (0.50 mm2 s−1), chlorobenzene (0.72 mm2 s−1), and dichlorobenzene (1.01 mm2 s−1)31, were determined by a Ubbelohde viscometer for three times. The efflux time is 132±0.63 s, 115±0.4 s, 230±0.8 s, 170±0.49 s, 188±0.4 s, 182±0.63 s, 246±0.4 s, and 355±0.4 s for chloroform, dichloromethane, toluene, ethyl acetate, tetrahydrofuran, n-hexane, chlorobenzene, and dichlorobenzene, respectively. The working curve of efflux time to viscosity was calculated from the above data:

V=0:00288×t*+0:00536 (2)

where V and t* represented viscosity and efflux time. Subsequently, the efflux time for the chloroform solutions of the organic semiconductor materials were measured by a Ubbelohde viscometer. The efflux time values are 145±0.63 s, 167±0.49 s, 129±0.49 s, 131±0.4 s, and 129±0.63 s for 0.5 mg mL−1 chloroform solution of PM6, PCE10, BTR, DRCN5T, and B1, respectively. Then, the corresponding viscosity can be calculated according to formula (2).

Fabrication of NPs
The semiconductor materials were dissolved in chloroform for 2 h with a concentration of 0.5 mg mL−1. Chloroform solutions with corresponding D/A weight ratios were added to pre-configured TEBS or PEBS aqueous solutions with a chloroform/H2O ratio of 1/4 (v/v). The mixed solutions were sonicated for 5 min using the ultrasonic processor (SCIENTZ-IID, Scientz Biotechnology) to obtain the mini-emulsion. The chloroform in mini-emulsion was removed using rotary evaporation for 2–3 min to obtain a surfactant-stabilized NPs dispersion in water. Finally, the NPs dispersion was filtered (0.45 μm mixed cellulose) to remove large aggregates. The NPs stabilized by SDS, SDBS, and SC were fabricated using the same method. The final concentration was determined by the following: 2 mL of methanol was added to 1 mL of NPs for de-emulsification. Subsequently, 2 mL chloroform was introduced to extract the semiconductor material from the aforementioned mixture. Following the extraction, the organic solvents were removed using rotary evaporation. Then, 1 mL of chloroform was added to the residue. Finally, the absorbance of the solution was measured by UV–vis absorption spectra to ascertain the final mass of NPs in the solution. Notably, we did not have a long time of mechanical stirring during the formation of microemulsion or heating during the removal of organic solvents. In contrast, literature reports typically use a long-term heating or stand method to remove solvents, which usually requires 1–20 h9,11,41,44,56,57.

Fabrication of NP@COF
The optimized polymer-free NPs (BTR:L8-BO NPs stabilized by 0.4 wt.% TEBS or PEBS) were dispersed in an aqueous solution containing 0.2 M AA (7.5 mL), and then the platinum precursor was added to the NPs dispersion. The specific platinum precursor is 1 mg mL−1 K2PtCl6 aqueous solution (the amount added is calculated based on the required platinum mass ratio). Unless otherwise specified, the Pt loading ratio refers to the added Pt amount/OPC amount. The system was continuously irradiated for 4 h under LED white light source (100 mW cm−2). The 2 mg Py-TAPD-COF was added to 8 mL purified water, and was subjected to sonication using an ultrasonic power of 250 W for a duration of 5 min, resulting in an aqueous dispersion of COF with a concentration of 250 μg mL−1. Various proportions of Py-TAPD-COF were introduced into the Pt-loaded NPs dispersion. Subsequently, distillation was conducted under reduced pressure at 60 °C for a period of 1 h, with water added intermittently during the distillation process to ensure that the system does not undergo complete evaporation. Finally, the concentration of the NPs was adjusted to 50 μg mL−1 to obtain NP@COF. Following this processing, the BTR NP@COF and L8-BO NP@COF samples for photoluminescence measurement were prepared at NP:COF weight ratio of 10:8, without Pt loading.

Hydrogen evolution
Various volumes of photocatalyst NP solutions or NP@COF solutions were added to a pre-prepared 0.2 M aqueous solution of sacrificial agents (such as AA, LM, GM, or LX), either in the presence or absence of 0.6 M NaCl, to achieve different NP concentrations. The total volume of the reaction system was maintained at 7.5 mL. Depending on the specific reaction conditions, an appropriate quantity of K2PtCl6 aqueous solution (1 mg mL−1) was added to the corresponding NP solutions. For conducting hydrogen evolution under AM1.5G light, the combined reaction mixture was transferred to a temperature-controlled recirculating batch reactor (cross-sectional area: 5.786 cm2; light penetration depth: 1.3 cm) and the temperature was maintained at 5 °C. This reactor was evacuated to eliminate oxygen, and the pressure was adjusted to 1.2 kPa. The NP suspensions were exposed to a 300 W xenon lamp fitted with an AM1.5G filter, and the light intensity onto samples is 100 mW cm−2. The light intensity was calibrated using an OHSP-350S radiometer (HOPOO light & color technology Co., Ltd.). No additional side-products were detected during the test. To enhance the efficiency of screening for the optimal hydrogen evolution condition in terms of the NPs:COF ratio and PEBS NP screening, multiple-channel reactors (PCX-50C, Perfect Light) were employed. The NP@COF composites formed under various conditions were individually introduced into separate offline reactors, each featuring a cross-sectional area of 3.1 cm2 and a light penetration depth of 2.4 cm. These reactors were filled with 7.5 mL of an aqueous solution containing 0.2 M AA and subsequently purged repeatedly with nitrogen to eliminate atmospheric oxygen. The suspensions containing the photocatalyst NPs were agitated at room temperature and illuminated using a light source that spanned the spectrum of 400–800 nm (100 mW cm−2). Hydrogen evolution was comprehensively assessed using an all-glass automatic online trace gas analysis system (Labsolar 6A, PLS-SXE300+, Perfect Light), equipped with an online gas chromatograph featuring a thermal conductivity detector and N2 as the carrier gas. Notably, the multiple-channel photochemical reaction system incorporated an LED lamp (400–800 nm, 100 mW cm−2) instead of the reference spectrum (AM1.5G, 100 mW cm−2) typically used for standard tests of solar energy conversion systems. Additionally, the reactor exhibited a light-concentrating effect, resulting in an actual irradiation intensity exceeding 100 mW cm−2. Consequently, to ascertain the optimal conditions, all data obtained from the multiple-channel reactors were normalized to the maximum value recorded within each set of parallel experiments.

SAXS
SAXS measurements were performed on a Xeuss 2.0 SAXS/WAXS System (Xenocs) utilizing a X-ray of λ = 0.152 nm. Sample preparation was conducted as follows: excess surfactants in the NP dispersion were removed via ultrafiltration centrifugation, and the dispersion was concentrated to a final concentration of 1 mg mL−1. The concentrated sample was dropped onto a polyimide film substrate, followed by drying at 40 °C for 2 h. The intensity data were subjected to analysis predicated upon the scattering vector (q), which is meticulously centered around the beam’s coordinates.

Cryo-TEM
Cryo-TEM measurements were performed on a Themis 300 TEM (Thermo Fisher Scientific). Samples were prepared using an automated plunge freezer: 3.5 μL of the NP dispersion was deposited onto the grids, followed by a 1 s blotting duration and immediate plunging into liquid ethane. Subsequently, Cryo-TEM analysis was conducted with the microscope operating at an acceleration voltage of 300 kV.

TA
Data was collected using a femtosecond TA spectroscopy system (Vitara T-Legend Elite-TOPAS-Helios-EOS-Omni). The method of fabrication of samples was illustrated in NP Fabrication. The optical path length of the aqueous NP suspension was 1 mm. The relative polarizations of pump and probe pulses were parallel. The instrument response function was approximately 120–140 fs. The pump beam was modulated at 500 Hz with a pulse duration of 100 fs and focused to a spot diameter of 2–3 mm. With a pump power ranging from 20 to 80 μW, the pump fluence was calculated to be 0.82–3.27 μJ cm−2, based on the average spot diameter of 2.5 mm.

ICP-MS
ICP-MS (5800 VDV ICP-OES, Agilent) was employed for quantifying the concentration of residual Pt cations after the photodeposition process. NPs (stabilized by TEBS or PEBS) were subjected to platinization for 4 h under the same experimental conditions as the photocatalytic tests including AM1.5G simulated sunlight (100 mW cm⁻²), 0.05 mg NPs, 7.5 mL reaction system volume, 0.2 M AA as the sacrificial agent, and a corresponding quantity of Pt derived from K2PtCl6 solution. Then, the NPs were isolated by ultrafiltration centrifugation, and the filtrate was used for ICP-MS analysis.

EQE
EQE measurements were conducted in a reactor with a cross-sectional area of 5.786 cm² and a light penetration depth of 1.3 cm, where the total volume of the reaction solution was maintained at 7.5 mL and assessed using an all-glass automatic online trace gas analysis system (Labsolar 6A, PLS-SXE300+, Perfect Light), equipped with an online gas chromatograph featuring a thermal conductivity detector and N2 as the carrier gas. Monochromatic lights at wavelengths of 400, 500, 600, 700, and 800/808 nm were sourced from a 300 W xenon lamp equipped with corresponding band-pass filters respectively. Prior to the EQE test, the catalyst sample was irradiated with AM1.5G simulated solar light (100 mW cm−2) for 2 h to realize in-situ photodeposition of Pt. Subsequently, the reactor was evacuated to eliminate all hydrogen gas produced during this process. The EQE was calculated using equations 10:

EQE = 2 × moles of hydrogen / moles of incident photons (3)

Ip = Np / t = N A h c / A λ t (4)

where n represented the moles of incident photons and Ip was the radiant power. λ and t were the wavelength and their radiation time (excluding the induction time). h and NA represented Planck constant and Avogadro constant, which are 6.626×10−34 J·s and 6.022×1023 mol−1, respectively. c was the speed of light, which is 3×108 m s−1. The uncertainty of the EQE measurements was estimated to be 5–10%.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This work introduces polymer-free NPs as organic photocatalyst for hydrogen evolution. Moreover, they present the model that the produced hydrogen could be directly used for distributed building heating, which is encouraging. Several suggestions are given as follows:\n1. As it is experimentally confirmed in the work, COF could improve the stability and activity of NPs for photocatalytic hydrogen evolution, but COF itself is inactive. The author also claims that electron transfer is limited between COF and NP due to the Type I structure. What is the mechanism behind the improved stability and activity?\n2. Though cyclic stability test of NP@COF has been present in SI, structure stability of it should be presented. Since molecules like antibiotics could serve as the sacrificial donor in the system, which has been confirmed in the work, why could NPs with similar organic structure maintain stable?\n3. Control experiment for photocatalytic hydrogen evolution that without addition of AA or antibiotics should be provided.\n4. For the experimental section, why the pressure for hydrogen evolution system has been decreased to 1.2 kPa, much lower than atmosphere pressure.\n5. In figure 5e, heating schemes with various methos has been compared. However, for a more convincing comparison, techno-economic evaluation is suggested.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "Si et al, reported photocatalytic hydrogen production from COF supported organic heterojunction nanoparticles and used the hydrogen generated to realize the sustainable heating distribution. The work is interesting and can be reconsidered after addressing the following comments:\n1. Organic molecule heterojunction nanoparticles have been reported for hydrogen production, see Chem. Commun., 2023,59, 5611-5614 and Nature Synthesis,2024, 3, 595–605. These paper are not referred in the manuscript\n2. The authors claimed that the organic molecule heterojunction nanoparticles can have smaller size than the polymer heterojunction nanoparticles. The reviewer does not think it is a right statement which could give misleading information to the community. The size really depends the preparation method, concentration, surfactant, et al.\n3. The function of the COF material is unclear in this paper. It is only a support or it participates in the charge transfer as well with the organic nanoparticles.\n4. For the transient spectra, the reviewer suggests the authors plot the data of amplitude vs. wavelength at different time scale. The current plot with time vs. wavelength at different amplitude is hard to read. The kinetics should be provided to compare the decay of reduced acceptor. Also, it is good to have data with Pt or sacrificial donor to see how the spectra develop. Also, how the author assigned the different excited absorption peaks to different components is not clear.\n5. The heating distribution part is not necessary in this work; the system is still based on sacrificial reagent. Even the authors have used the antibiotics to drive the reaction to realize a “sustainable” counter reaction, the overall concentration antibiotics can not be used to substantially drive heating for a whole community. I would suggest the author should focus on more scientific development and understanding of the organic nanophotocatalysts.", "reviewer_4": "In this article the authors demonstrate novel use for photocatalytic hydrogen evolution by organic semiconductors, where the application, to my knowledge has not been shown before. The work presents in-depth size optimization of the photocatalytic nanoparticles finding those with diameters less than 10 nm perform the greatest. Along with the size optimization, the authors present all small molecule photocatalytic nanoparticles with some of the highest efficiencies reported for hydrogen evolution outcompeting even the well-studied inorganic hydrogen evolution photocatalysts. While the work presents an exceptional use for organic semiconducting nanoparticles, there are areas within the manuscript that are both insufficient in detail, reproducibility and scientific rigor.\nIn the manuscripts current form is not suitable for publication in Nature communications. Specific areas in which the authors can improve the manuscript are detailed below.\nGeneral comments:\n- All hydrogen evolution rates are presented without any error in the measurement. Replicates of the catalysis experiments then presenting the average with twice the standard error is strongly recommended to enhance reproducibility of the work.\n- Figure 2b should have the Y axis minimum set to 0, the selection of making it 0.3 exaggerates the difference in the viscosity between the donors dissolved in organic solvent.\n1) A large amount of the explanation for reduction in nanoparticles size is the removal of the polymeric donor present, with lines 208–211 directly stating the small size is attributed to the weaker molecular interaction of the small molecules compared to the conjugated polymers. Many previous works have investigated both neat small molecule acceptor nanoparticles, (DOI: 10.1021/ac-sanm.2c02350", "reviewer_doi": "10.1038/s41560-022-00990-2) with all these studies showing no consistent\nchange in the nanoparticle size as a function of polymer content or absence of polymer at all. I\nsuggest removing the statement on lines 208–211 relating the size change to polymer content\nand attribute the greater size control to surfactant concentration as per lines 187–189. Where it\nis stated that increasing TEBS concentration always lead to a decrease in nanoparticle size,\ncould this not be the more rational explanation as opposed to the polymer content?", "reviewer_author_response": "Response to Reviewers\nReviewer #1 (Remarks to the Author):\nThis work introduces polymer-free NPs as organic photocatalyst for hydrogen evolution.\nMoreover, they present the model that the produced hydrogen could be directly used for\ndistributed building heating, which is encouraging. Several suggestions are given as follows:", "reviewer_maintain\nstable?\nresponse": "We thank the reviewer for this valuable comment. To check the structural\nstability of NP@COF, we have carried out additional characterizations. Ultraviolet-\nvisible absorption spectroscopy and mass spectrometry confirm that the chemical\nstructures of organic photocatalysts remain essentially unchanged before and after the\ncyclic stability tests (Figures R2a and R2b). These results demonstrate the chemical\nstructural stability of the NP@COF composite during repeated photocatalytic operation.\nThen no obvious aggregation or precipitation of the NP@COF composite was observed\n1\nafter the reaction, which indirectly supports the maintenance of its dispersion state and\nthus implies good stability during the catalytic process.\nThis better stability of NPs than those antibiotics molecules, can be attributed to\nthe fact that the oxidation potentials of the antibiotics or sacrificial agents used in this\nstudy, including ascorbic acid (−4.32 eV), lincomycin (−5.01 eV), gentamicin (−4.95\neV), and levofloxacin (−5.00 eV), are shallower than that of the conjugated donor BTR\n(−5.34 eV) or COF (−5.96 eV). As a result, photogenerated holes can oxidize these\nsacrificial molecules rather than the conjugated molecule itself. On the other hand, in\nconjugated molecules, the extended π-conjugation and strong intermolecular π-π\nstacking facilitate extensive electron delocalization, thereby significantly enhancing the\nelectronic stabilization of the system. Under oxygen-free conditions, self-oxidation of\nthese conjugated species is thus unfavorable (Ref: Chem. Soc. Rev., 2025, 54, 2054-\n2090), ensuring the chemical integrity of the NP@COF composite during\nphotocatalysis.\nFigure R2. a,b, Absorption spectrum (a) and mass spectrometry (b) of NP@COF\ncomposite (BTR:L8-BO 4:6 NPs, 0.4 wt. % TEBS/H O, loading 70 wt. % Pt", "reviewer_absorption": "nsignal in 900-1000 nm is attributed to scattering from the COF, and he peak intensity\nin MALDI-TOF is not quantitatively meaningful.\n3. Control experiment for photocatalytic hydrogen evolution that without addition of AA or\nantibiotics should be provided.", "reviewer_pressure\nresponse": "The hydrogen evolution experiments were performed in a sealed system\ninitially evacuated to remove air and dissolved oxygen, resulting in a system pressure\nof ~1.2 kPa. This low-pressure setup ensures an oxygen-free environment, minimizing\ninterference from other gases in hydrogen detection, while maintaining reliable\nphotocatalytic measurements. The experimental section has been modified to clarify\nthis point.\n5. In figure 5e, heating schemes with various methos has been compared. However, for a more\nconvincing comparison, techno-economic evaluation is suggested.", "reviewer_btr": "L8-BO 4:6 NPs (excited by 568 nm\nmonochromatic light) (d). The NPs stabilized by 0.4 wt. % TEBS.\n7) The authors should verify the Pt loading through ICP-MS as described in the following works:\n- DOI:10.1021/acsanm.2c02350\n- DOI:10.1002/aenm.202403372", "reviewer_6": "L8-BO (4:6) NPs with pump at 550 nm (c) and BTR:L8-\nBO 4:6 NPs with pump at 568 nm (d). e, TA spectra probed at 10 ps of PM6, BTR,\nPM6:L8-BO (4:6) and BTR:L8-BO (4:6) NPs.\n- Additionally, to the comments on spectral assignment, the assigned PM6 excited state shows\n20\nin this work shows overlap with the L8-BO i-EX state while the BTR excited state absorption\ndoes not. When performing the exponential fitting to extract time constants for the i-EX state\nmentioned in 271. The authors should deconvolute the two overlapping features of PM6 and\nL8-BO in order to compare the time constants for the i-EX state between the two different donor\nblends otherwise the comparison is invalid and should not be made in the manuscript.", "reviewer_solvent\nresponse": "Following this suggestion, we have revised Figure 2b in main text by\n11\nresetting the Y-axis minimum to 0 to provide a clear comparison of the viscosity\nbetween different donors (Figure R10).\n0.6\nChloroform Solution Viscosity\n0.49\n)\n1\n-s 0.42\n2 m 0.4 0.38 0.38 0.38 0.38\nm\n(\ny\nt\nsi\no 0.2\nc\ns\nVi\n0.0\nPM6 PCE10 BTR DRCN5T B1 CF\nFigure R10. The viscosity of polymer and small molecule solution with a concentration\nof 0.5 mg mL−1 (the average values ± standard deviations for 3 samples).\n1) A large amount of the explanation for reduction in nanoparticles size is the removal of the\npolymeric donor present, with lines 208–211 directly stating the small size is attributed to the\nweaker molecular interaction of the small molecules compared to the conjugated polymers.\nMany previous works have investigated both neat small molecule acceptor nanoparticles, (DOI:\n10.1021/ac-sanm.2c02350", "reviewer_t": "in ps) were 3.2/114.0 for 0.82 μJ cm−2, 2.1/127.5 for 1.63 μJ cm−2, 2.2/108.8 for\n1 2\n2.45 μJ cm−2, and 1.9/114.6 for 3.27 μJ cm−2, respectively, suggesting the GSB signal\ndecay lifetimes exhibit no significant variation with excitation intensity of 0.82-3.27 μJ\ncm−2. These results indicate that the selected pump fluence (1.63 μJ cm−2) for our main\nTA measurements operates in the low-excitation regime, where no exciton\nconcentration saturation or exciton-exciton annihilation effects occur (Nat. Energy,\n2022, 7, 340-351). These fluence dependence data have been supplemented in the\nsupporting information of the revised manuscript to comply with standard ultrafast\nspectroscopy reporting requirements.\n1.2\nPump: 780nm\n) Probe: 784nm\nu.\nBTR:L8-BO\n(\na.0.9\n0.82 μJ cm−2\nA 1.63 μJ cm−2\nΔ\nd 0.6\n2.45 μJ cm−2\ne 3.27 μJ cm−2\nz\nali\nm\n0.3\nr\no\nN\n0.0\n0.1 1 10 100 1000\nTime (ps)\nFigure R14. Normalized TA traces probed at 784 nm of BTR:L8-BO 5:5 NPs pumped\nat 780 nm with different pulse fluences.\n- The experimental present in the supporting information lacks experimental details such as\npulse duration of the laser, laser pulse spot size for pump and probe, relative polarization of the\npump and probe pulse, as well as if the materials displayed any signs of photo induced\ndegradation. The authors must include these, as in their absence results such as the time\nconstants extracted of species lifetimes could be within the instrument response, and hence not\nreliable.", "reviewer_ref": "Nat. Commun, 2015, 6, 6013). To\nclarify this correspondence, we have revised the manuscript (Page 10) to include\nexplicit references to the crystal structure data of both L8-BO and BTR, thereby\nstrengthening the structural interpretation of our results.\n23"}, "author_response": ""}, {"round": 2, "reviewer_comments": {"reviewer_1": "I think the authors have well addressed the reviewers’ concern; however, there are still some errors.\n1. There is typo in Line 414.\n2. The hydrogen evolution rate in the Abstract and manuscript is extremely high, 527.2~ 3180.7 mmol g-1 h-1, which obviously deviates from the data in Figure 5.", "reviewer_2": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts.", "reviewer_3": "The authors have well addressed the comments. The paper can be considered for publication as is.", "reviewer_4": "All of my comments made in the initial review were adequately addressed by the authors, and I believe the manuscript is now fit for publication in Nature Communications."}, "author_response": "Response to Reviewers\nReviewer #1 (Remarks to the Author):\nI think the authors have well addressed the reviewers’ concern; however, there are still some\nerrors.\n1. There is typo in Line 414.\nResponse: We thank the reviewer for pointing out this typo. The incorrect term\n“polymer-HJ NPs” has now been corrected to “polymer-free HJ-NPs” in the revised\nversion.\n2. The hydrogen evolution rate in the Abstract and manuscript is extremely high, 527.2~ 3180.7\nmmol g-1 h-1, which obviously deviates from the data in Figure 5.\nResponse: The values in the abstract and manuscript are reported in mmol h-1 g-1, while\nthe data in Figure 5 were originally shown in the unit of mol h-1 g-1. We have updated\nFigures 5 by clearly marking the Y-axis as “×103 mmol h-1 g-1” to avoid any ambiguity.\nReviewer #2 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This\nis part of the Nature Communications initiative to facilitate training in peer review and to\nprovide appropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: We appreciate the careful evaluation provided by both reviewers.\nReviewer #3 (Remarks to the Author):\nThe authors have well addressed the comments. The paper can be considered for publication as\nis.\nResponse: We are glad to know the reviewer recommends accepting our manuscript.\nReviewer #4 (Remarks to the Author):\nAll of my comments made in the initial review were adequately addressed by the authors, and\nI believe the manuscript is now fit for publication in Nature Communications.\nResponse: We are glad to know the reviewer recommends accepting our manuscript.\n1"}]

Decision letter