Achieving a solar-to-chemical efficiency of 3.6% in ambient conditions by inhibiting interlayer charges transport

Huang, Yuyan; Shen, Minhui; Yan, Huijie; He, Yingge; Xu, Jianqiao; Zhu, Fang; Yang, **n; Ye, Yu-**n; Ouyang, Gangfeng

doi:10.1038/s41467-024-49373-z

Achieving a solar-to-chemical efficiency of 3.6% in ambient conditions by inhibiting interlayer charges transport

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Published: 26 June 2024

Volume 15, article number 5406, (2024)
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Achieving a solar-to-chemical efficiency of 3.6% in ambient conditions by inhibiting interlayer charges transport

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Abstract

Efficiently converting solar energy into chemical energy remains a formidable challenge in artificial photosynthetic systems. To date, rarely has an artificial photosynthetic system operating in the open air surpassed the highest solar-to-biomass conversion efficiency (1%) observed in plants. In this study, we present a three-dimension polymeric photocatalyst achieving a solar-to-H₂O₂ conversion efficiency of 3.6% under ambient conditions, including real water, open air, and room temperature. The impressive performance is attributed to the efficient storage of electrons inside materials via expeditious intramolecular charge transfer, and the fast extraction of the stored electrons by O₂ that can diffuse into the internal pores of the self-supporting three-dimensional material. This construction strategy suppresses the interlayer transfer of excitons, polarizers and carriers, effectively increases the utilization of internal excitons to 82%. This breakthrough provides a perspective to substantially enhance photocatalytic performance and bear substantial implications for sustainable energy generation and environmental remediation.

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Introduction

Solar-to-chemical conversion (SCC) provides a promising avenue for resolving the energy and environmental crises that afflict contemporary society by harnessing the largest renewable energy sources on Earth^{1,2,3,4,5,6,7,8}. Among the diverse artificial photocatalytic systems currently available, the photosynthesis of hydrogen peroxide (H₂O₂) offers a compelling strategy, enabling cost-effective synthesis in an open system and maximizing photon efficiency through simultaneous utilization of oxidation and reduction reactions.

Photocatalytic processes generally involve several steps. Firstly, the catalyst can be excited to a depth of up to 100 nanometers by photons to generate excitons, which then transform into loosely bound polarons. Typically, the internal excitons and polarons need to migrate to the surface and further separate into free charges to react with the substrates⁹. Whereas, the transfer distance of excitons or polarons in organic photocatalysts is usually no more than 20 nanometers, and a massive amount of them recombine before reaching the surface^1,10,11. Despite the successful separation of polarons into free charges before recombination, the internal free charges still require to traverse the interlayer to reach the surface, which recombine inevitably when the electrons and holes from different layers get close under the Coulomb attraction force. It has been reported that >90% of excitons recombine rapidly within sub-microsecond (sub-μs), leading to unpromising photocatalytic efficiency of H₂O₂ which is far from the requirements of practical applications¹².

To overcome the recombination of excitons, polarons or charge carriers during the migration, one of the most studied methods is to reduce the transfer distance by constructing ultrathin two-dimensional (2D) semiconductors (Fig. 1)¹³. However, these low-dimensional materials are always severely stacked owing to their high surface energies, resulting in bulk recombination of free charges during transportation. Another plausible solution to this challenge is to construct electron donor-acceptor (D-A) polymeric photocatalysts possessing ordered donor-on-donor (for hole transport) and acceptor-on-acceptor (for electron transport) bi-continuous π-columnar structures were proposed^{14,15,16,17,18}. Unfortunately, it is challenging to precisely construct such kind of highly ordered microstructures, and donor-on-acceptor alternative stacking is more likely to form owing to favorable electrostatic interactions, which tends to induce the recombination of electrons and holes¹⁹. Additionally, the severely stacked structures also embed the dominant active sites inside the photocatalysts and only leave small portions of the active sites on the outer surfaces. Thus, the optimized utilization of internal excitons and active sites is a feasible means to develop high-performance photocatalysts.

**Fig. 1: Schematic illustration comparing previous works and this work.**

Herein, we propose a strategy to enhance the utilization of internal excitons by suppressing the recombination of metastable excitons, polarons, and photogenerated charge carriers during interlayer transfer (Fig. 1). This is achieved by inhibiting their interlayer transfer through rapid dissociation of polarons into free charges via expeditious intramolecular electron transfer, followed by their storage within the catalyst. Furthermore, by exposing storage sites through the three-dimensional architecture of the photocatalyst, reactants can effectively access and extract free or stored charge carriers from deep within the material, preventing recombination during interlayer transfer. This approach yields a break-through photocatalytic rate of H₂O₂ ranging from 9257 to 9991 μmol·g⁻¹·h⁻¹, accompanied by a solar-to-chemical conversion efficiency reaching 3.6% under ambient conditions, i.e. real water, open air and room temperature. To our best knowledge, this is an impressive SCC achieved among the photosynthetic systems of H₂O₂, and is among the rare instances where SCC in ambient conditions has surpassed the highest solar-to-biomass conversion (SBC) rate of typical plants. Importantly, this study introduces an approach for attributing peaks in femtosecond transient absorption spectroscopy, which is challenging and prone to confusion. By utilizing sacrificial agents and small molecule monomers, we differentiate and identify various photoinduced transient species (excitons, polarons, electrons). Based on this approach, the mechanism of accelerated electron extraction from the three-dimensional catalysts on photophysical processes was further explored by switching atmospheres in situ. These techniques enable the clarification of the transfer pathways (interlayer or intramolecular) and provide insights into the corresponding proportions of each transfer method. Specifically, most of the free electrons (82.2% in air and 89.2% in O₂) were utilized through intramolecular transfer, while the long-distance interlayer electron transport was substantially suppressed, thus circumvented the recombination during interlayer transport. This study offers a perspective to substantially enhance photocatalysis performance by improving photon utilization and bears substantial implications for sustainable energy generation and environmental remediation.

Results

Synthesis and characterization of catalysts

The self-supporting three-dimensional (3D) amorphous photocatalyst was synthesized with triptycenes (TPC) as the self-supporting electron donors, the built-in redox anthraquinone (AQ) moieties as the electron acceptors, and alkynyl as connectors (Fig. 2a). This photocatalyst was named as TPC-3D. The three-dimensional structure of triptycenes units exposes more active sites and allows O₂ to diffuse into the interior of the material. Alkynyl bridges have superior electron transfer properties due to the linear conjugated structure. Furthermore, AQ moieties possess a strong electron withdrawal capacity, which promotes the separation of photo-induced carriers. Critically, AQ has a two-electron storage capacity which effectively inhibits the recombination of charge carriers². This 3D photocatalyst can rapidly store electrons on AQ by intramolecular transfer and allow O₂ diffuse into the catalyst interior to extract electrons, which suppressing the interlayer transport of photogenerated excitons, polarons, and charge carriers. For comparison, another two two-dimensional (2D) structural analogs were synthesized by using pyrene (PYR) and triphenylene (TPL) as the planar electron donors, which were termed PYR-2D and TPL-2D, respectively (Fig. 2a). The method for introducing the alkyne group was shown in Supplementary Fig. 1, and the three conjugated polymers (CPs) were conventionally synthesized through a one-step Sonogashira reaction. The determination of electron donor and acceptor relies on the distribution positions of their Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) in their optimized structures. As depicted in Supplementary Fig. 2, HOMO is predominantly located in TPC, while LUMO is primarily distributed in the AQ moieties.

**Fig. 2: Structural characteristics of catalysts.**

The characteristic signals of alkynyl groups were observed in solid-state cross-polarization/magic-angel-spinning (CP/MAS) ¹³C NMR spectra (Supplementary Fig. 3), Raman spectra (Supplementary Fig. 4), and in FT-IR spectra (Supplementary Fig. 5). Besides, the FT-IR spectra showed the stretching bands of C=O at ~1673 cm⁻¹ (Supplementary Fig. 5), combined with the characteristic benzoquinone groups (~181 ppm) in ¹³C NMR spectra (Supplementary Fig. 3) and X-ray photoelectron spectroscopy (XPS) measurements (Supplementary Figs. 6 and 7), manifesting the retaining of AQ in CPs²⁰. More details are reported in the supplementary information to demonstrate the successful polymerization for all the CPs through the Sonogashira reaction (Supplementary Figs. 2–7)²¹. The powder X-ray diffraction (PXRD) profiles revealed that all the CPs exhibited the features of amorphous carbon²² (Supplementary Fig. 8). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the CPs possessed rough surfaces and uniform structures (Supplementary Figs. 9 and 10). Also, as shown in Fig. 2b, all the CPs exhibited strong absorption throughout the visible light region and even in the near-infrared region, which demonstrated that the CPs possessed large conjugation structures. In TPC-3D, the three phenylene rings of the triptycene influence each other electronically through space (which is often called homoconjugation), which also extends its conjugation structure²³. In addition, all CPs exhibited satisfactory thermal stability (Supplementary Fig. 11).

The energy band structures of the CPs were subsequently determined (Fig. 2c). The conduction band (CB) minima of TPC-3D, PYR-2D, and TPL-2D were determined to be −0.43 eV, −0.47 eV, and −0.48 eV versus the normal hydrogen electrode (NHE) via the Mott-Schottky tests (Supplementary Fig. 12)^24,25,37,38. In addition, TPC-3D essentially maintained its high efficiency, morphology, and component for 5 cycles (Supplementary Figs. 15–18 and Supplementary Note 2). To assess the potential for practical application, TPC-3D was coated onto a glass slide and immersed it in real water for photocatalytic reactions (Fig. 3c). Remarkably, the catalyst was without appreciable loss in performance even after 15 cycles (Fig. 3d), and relevant structural characterizations have provided (Supplementary Fig. 19 and Supplementary Note 2).

Besides, the apparent quantum yield (AQY) of TPC-3D reached up to 30.9%, 20.8%, and 10.6% at 420, 470, and 620 nm in pure water, respectively, and even as high as 44.8% at 420 nm and 11.9% at 620 nm in lake water (Fig. 2b). The AQYs at all the wavelengths are among the leading values reported for the photosynthesis of H₂O₂ (Supplementary Table 2). Significantly, the solar-to-chemical energy conversion (SCC) process achieved successful implementation under low concentration of photocatalyst usage (0.4 g·L⁻¹) and in open air conditions (Supplementary Fig. 20, Supplementary Movies 1 and 2), with an impressive efficiency of 2.4% in pure water and 3.6% in lake water (Fig. 3a). These SCC efficiencies were superior to most of those reported for photosynthetic systems of H₂O₂ (Supplementary Table 3) and were even higher than the highest solar-to-biomass conversion efficiency of typical plants (1%)^20,29. It is notable that other artificial photocatalytic reactions, such as water splitting and carbon dioxide reduction, cannot be performed in the open air. The current photocatalytic system provides a more feasible way for SCC.

In addition, it was not foreseen that the photosynthetic rate of H₂O₂ by TPC-3D was increased by >55% in real water. Generally, the photocatalytic rate would be decreased in real water due to the intricate interferential components, i.e. inorganic ions, and organic matters^{20,Full size image}

On the other hand, the synthesis of H₂O₂ was significantly suppressed under conditions of argon and the addition of AgNO₃ (Supplementary Fig. 28), indicating that the photocatalytic generation of H₂O₂ primarily proceeds via ORR. For ORR, all CPs exhibited effective two-electron oxygen reduction to produce H₂O₂ (Supplementary Fig. 30). DFT calculation revealed that O₂ could spontaneously adsorb on the alkynyl moieties to form the endoperoxide species in TPC-3D (Fig. 4e and Supplementary Fig. 31). To further explore the adsorption behavior of O₂ on alkynyl moieties, in-situ DRIFTS was conducted. As shown in Fig. 4f, the stretching vibration of the C≡C bond (2219 cm^–1) increased gradually with the injection of O₂ in the dark. The results indicated that alkynyl groups served as the active site for O₂ adsorption as the adsorbed intermediate could result in increased force constants due to symmetry breaking⁴⁵. In contrast, under irradiation, the signals of alkynyl moieties decreased, suggesting that alkynyl participates in the photocatalytic ORR. This portion of O₂ adsorbed on the alkynyl moieties was proven capable of directly reacting with e⁻ and being conversed to H₂O₂, since H₂O₂ could still be successfully produced under the condition of in Ar atmosphere to exclude O₂ from the air and adding hole sacrificial agents added to inhibit the generation of O₂ from water oxidation (Supplementary Fig. 32).

Meanwhile, another oxygen reduction pathway that reduced AQ reacted with O₂ was also proposed. The TD-DFT calculations showed that the active sites were mainly located on the carbonyl oxygen (atoms 35 and 36 in Fig. 4b) and carbonyl carbon (atoms 27 and 28 in Fig. 4b) of AQ moieties for ORR. And the superior electrons accepting ability of the AQ moieties was also verified from the LUMO diagrams as well as from the electron paramagnetic resonance (EPR) spectra (Supplementary Figs. 2 and 33). To further confirm the reaction between reduced AQ and O₂, the photosynthesis was conducted in an argon atmosphere for one hour, and then pure O₂ was injected into the photocatalytic system immediately after stop** the illumination. In the absence of light, where electron-hole pairs cannot be generated, the production of H₂O₂ in a dark environment solely relies on stored electron (Fig. 4g). Consequently, the generation of H₂O₂ persisted for duration of 20 min in dark. These phenomena indicated that the electrons were stored in AQ to reduce O₂ to generate H₂O₂ for 20 min²⁹. In addition, significant color change during the photocatalytic process can be regarded as another visual indicator of the accumulation of electrons. Upon irradiation in an air atmosphere, the material transitions from a brown to an orange-yellow shade, whereas in an argon atmosphere, the material further shifts to a vivid yellow color, which is attributed to the accumulation of electrons (Supplementary Fig. 34). And in the UV/Vis spectra (Supplementary Fig. 34), the increase of absorbance indicated the formation of the reduced species⁴⁸. Moreover, in-situ DRIFTS provided a strong evidence for the formation of reduced species. As shown in Fig. 4h, the peaks at 1358, 1482 and 1680 cm⁻¹ were assigned to the one-electron accumulation state of AQ (AQH), two-electron accumulation state of AQ (AQH₂) and AQ, respectively²⁰. With the progression of irradiation, the peak of AQ diminished in argon, along with the emergence of peaks corresponding to the reduced species AQH and AQH₂. Subsequently, upon the removal of the light source and the injection of oxygen, the peaks of AQH and AQH₂ decreased while the peak of AQ increased, and the infrared vibration peak assigned to the 1,4-endoperoxide intermediate species at 910 cm⁻¹ were significantly enhanced, indicating that the electron-storage reduced species reacted with O₂. These phenomena strongly corroborated the electron storage capability of AQ and the occurrence of the ORR on AQ. Consequently, the ORR process on AQ can be summarized as follows (Fig. 4i): under visible-light irradiation, AQ sequentially formed the one-electron accumulation state of AQ (AQH) and two-electron accumulation state of AQ (AQH₂) via an electron-coupled hydrogenation reaction^20,33. Then, AQH₂ reacted with O₂ to generate AQH₂−1,4-endoperoxide, which subsequently coupled with the adjacent hydrogen in the hydroxyl group to release H₂O₂ and regenerated the AQ redox center⁴⁹. Also, the in-situ EPR spectra had revealed the electron signal on AQ during the reaction (Supplementary Fig. 35 and Supplementary Note 4).

As summarized above, photo-induced electrons can be directly captured by absorbed O₂ in TPC-3D or stored in AQ for subsequent ORR. To elucidate the ORR pathway, EPR spectra were conducted to detect ·O₂⁻ which involved in 2e⁻ ORR with 5,5-dimethyl−1-pyrroline N-oxide (DMPO) as the spin-trap agent. As depicted in Supplementary Fig. 36a, TPC-3D containing both alkynyl and AQ exhibited the typical six-line characteristic peaks of DMPO-·O₂⁻, indicative of the presence of superoxide. For comparison, we synthesized a polymer devoid of alkynyl groups using triptycenes as the electron donor and anthraquinone as the electron acceptor, designated as TPC-AQ (Supplementary Fig. 19a). It is noteworthy that TPC-AQ, which contains AQ but not alkynyl, did not exhibit the peak of DMPO-·O₂⁻ (Supplementary Fig. 36b). The comparison suggested that ORR pathway occurred on the alkynyl via a two-step 2e− process involving the formation of the ·O₂⁻ intermediates, while the ORR pathway on AQ reduced O₂ to produce H₂O₂ via a one-step 2e− process bypassing ·O₂⁻. To investigate the percentage of ORR occurring on different sites, we conducted a photocatalysis experiment with the addition of superoxide dismutase (SOD), which acts as a scavenger of ·O₂⁻. The presence of SOD did not significantly affect the H₂O₂ yield of TPC-AQ, while it reduced the yield in TPC-3D by about 19% (Supplementary Fig. 36c), indicating the percentage of ORR pathway on alkynyl involving ·O₂⁻ intermediates less than one-fifth. And this differential response to SOD treatment confirms our hypothesis regarding the distinct ORR pathways on the alkynyl and AQ moieties. These two pathways coexist, and which pathway occurs depends on whether oxygen is pre-adsorbed or not (See Supplementary Note 5 for details).The investigation revealed that the production pathways of H₂O₂ by 2D CPs were analogous (Supplementary Figs. 30, 33, 37, 38). In 2D CPs, it also included the pathway that the AQ stored the photo-generated electron to reduce the charge carrier recombination through back electron transfer effectively²⁹. However, the recombination caused by interlayer electron transfer remained unavoidable. The electrons may become trapped inside the catalyst through AQ, preventing them from coming into contact with the oxygen on the surface of catalyst. Alternatively, the electrons may still need to migrate a long distance from the interior of the catalyst to the interface. Both of these processes resulted in a decrease in the electron utilization efficiency. This limitation was successfully addressed in TPC-3D, which boasts a 3D structure that facilitates O₂ diffusion into the pores and exposes a multitude of active sites, promoting efficient electron utilization. As shown in Supplementary Fig. 39, TPC-3D possessed a specific surface area 40%–50% larger than that of PYR-2D and TPL-2D due to its 3D structure, allowing better exposure of the active sites. Besides, as seen from the O₂ physical adsorption experiments (Supplementary Fig. 40a), O₂ temperature programmed desorption (O₂-TPD) experiments (Supplementary Fig. 40b), EPR measurements (Supplementary Fig. 33) and H₂O adsorption experiments (Supplementary Fig. 41), TPC-3D possessed more active sites for O₂ adsorption, electron storage and WOR than PYR-2D and TPL-2D (see Supplementary Note 6 for details). Interestingly, although the 2D catalysts owned higher ratios of O₂ adsorption sites, specifically alkynyl moieties (electron donors:alkynyl = 1:3 in TPC-3D, while it was 1:4 in PYR-2D and 1:6 in TPL-2D), their overall oxygen adsorption capacity was lower. This was owing to the stacking structures of PYR-2D and TPL-2D embedded substantial portions of active sites inside the 2D CPs. In addition, a significant delay in O₂ desorption for TPC-3D also indicated that O₂ was likely to be adsorbed inside TPC-3D (Supplementary Fig. 40b). The more accessible active sites in TPC-3D were convinced to circumvent the long-distance electron transfer to the surface of 2D materials. Moreover, the photocurrent densities in pure O₂ were significantly decreased to only half of that in air in TPC-3D (Fig. 5). It was inferred that the photoinduced electrons were effectively captured by the internal O₂ rather than transported to the electrode. On the contrary, the trends of photocurrent densities for PYR-2D and TPL-2D were 3- and 10-fold higher in O₂ than that in air. These results demonstrated the photoinduced electrons in the 2D CPs preferred to transport to the electrode. The electrochemical impedance spectra (EIS) also showed that the electrical resistance of TPC-3D was significantly higher than PYR-2D and TPL-2D, indicating that the charges transportation in the two-dimensional catalysts were more fluent (Supplementary Fig. 42).

**Fig. 5: Electron transportation investigation of the photocatalysts.**

Electron utilization

After identifying the photochemical pathways, it is known that there are three pathways for photoinduced electrons to be utilized (Fig. 6). In order to further investigate the underlying mechanism of affecting the three pathways resulting from the increased accessibility of oxygen inside the catalyst due to the 3D structure, we characterized the rates and proportions of electron transfer in air and O₂ atmosphere using transient absorption (TA) spectroscopy (Fig. 7 and Supplementary Figs. 43–45)^50,51. A 400 nm laser was selected to excite the electron donors and the broad absorption ranging from 800 nm to 1200 nm were observed in the TA spectroscopy of CPs. This broad adsorption was assigned to electronic absorption because they decreased in the presence of AgNO₃ as electron scavengers⁵⁰.

**Fig. 6: Schematic illustration of the behaviors of excitons, polarons and charge carriers in 2D and 3D photocatalysts.**

**Fig. 7: Femtosecond time-resolved transient absorption (fs-TA) measurements in the near-infrared region of TPC-3D.**

To clarify the reaction times corresponding to these three pathways for photoinduced electrons to be utilized, the TA decay kinetics under O₂ and air conditions were fitted by the tri-exponential function, resulting in τ₁, τ₂, τ₃ and their corresponding ratios. In O₂, the τ₁ component was accelerated and the proportion was increased, indicating τ₁ could be attributed to the direct reaction between electrons and oxygen (Fig. 6 Pathway I). In comparison, the proportion of τ₂ component was nearly the same in O₂ and air conditions, which was likely to be affiliated to the storage of electrons in reduced AQ (Fig. 6 Pathway II). In addition, the τ₃ component was much longer than the other two, which was assigned to electrons transferring across stacked layers to the surfaces and reacting with O₂ (Fig. 6 Pathway III). Notably, the contribution of the τ₃ component was found to decrease significantly from 17.8% in the presence of air to 10.8% in the presence of O₂ for TPC-3D (Fig. 7d). In contrast, no significant changes were observed in the τ₃ components for PYR-2D and TPL-2D when exposed to O₂ or air. This can be attributed to the fact that, in TPC-3D, O₂ can penetrate into the interior of the catalyst and reduce the interlayer electron transfer, which ultimately decreases the lifetime of the electrons. Conversely, in two-dimensional materials, O₂ cannot penetrate the material’s interior, and thus, it does not affect the electron transfer pathway within the catalyst, resulting in no decrease in electron lifetime.

In addition, considering the catalytic reaction occurred under air conditions in an open system, the differences in the electron transfer pathways of the three CPs in air condition may warrant further exploration. As shown in Fig. 7d, τ₃ of TPC-3D was 55% and 40% shorter than those of PYR-2D and TPL-2D in air, respectively. These results verified that electrons in the bulk of 2D CPs inevitably required long-length transport to the surface for being utilized, whereas free electrons in TPC-3D, they could be rapidly extracted by O₂ even in the air condition, thanks to its 3D structure. Thus most of the free electrons of TPC-3D (82.2% in air and 89.2% in O₂) were utilized through intramolecular transfer (Pathway I and Pathway II). Remarkably, compared to interlayer transfer (Pathway III, τ₃), intramolecular transfer (Pathway I, τ₁ and Pathway II, τ₂) has a rate that is more than one order of magnitude faster. The estimated lifetimes of the electrons in TPC-3D were 0.90 ps for τ₁, 15.92 ps for τ₂ and 206.60 for τ₃, respectively. Due to the high proportion and faster rate of intramolecular electron transfer in TPC-3D, its estimated electron lifetime was 191.61 ps, which was 50% and 40% lower than PYR-2D and TPL-2D, respectively. Based on the above research, it has been demonstrated that increasing the proportions of intramolecular electron transfer can significantly enhance the total electron transfer rate. Moving forward, the expeditious electron transfer rate effectively improved the electron utilization efficiency. The photocurrent densities in the presence of O₂ strongly support the high-efficiency utilization of electrons in TPC-3D, as they were significantly lower than those observed under argon conditions (Fig. 5). This finding demonstrated that the photoinduced electrons were more likely to be extracted by the O₂ within the catalyst, rather than being transported to the surface. The improved performance of photocatalytic H₂O₂ synthesis in the presence of O₂, compared to air, indicated that increasing the accessibility could improve the utilization efficiency of electrons (Supplementary Fig. 46).

In summary, TPC-3D achieved expeditious intramolecular transfer to store electrons in the catalyst and exposed the storage sites through the 3D structure, allowing electrons deep within the material to be efficiently extracted by O₂, preventing recombination during interlayer transfer. More importantly, the intramolecular electron transfer ratio increased in TPC-3D, leading to an accelerated total electron transfer rate and improved electronic utilization efficiency (Fig. 8). This provides a promising approach for enhancing the efficiency of photocatalytic systems.

**Fig. 8: Schematic illustration depicting the interaction among the behavior of excitons, polarons, and electrons in TPC-3D.**

From polarons dissociation to electron generation

After confirming the faster and more efficient utilization of electrons in TPC-3D, we proceeded to further investigate the impact of this accelerated electron utilization on the polarons dissociation. Firstly, it was necessary to identify the peaks of excitons and polarons in the TA spectra initially and subsequently measured their respective lifetimes. All the electron donor and CPs exhibited positive peaks immediately and decay rapidly, which were attributed to the absorption of the excitons (Supplementary Figs. 47a, 48a and 49a). The decay of the peaks was attributed to the transformation of the excitons to polarons, pairs of more loosely bound charges by Coulomb attraction forces^52,53. Subsequently, signals formed after the dissociation of the excitons appeared as the highest positive photoinduced absorption (PIA) peaks at 645 nm in TPC-3D, 720 nm in PYR-2D, and 580 nm in TPL-2D (Supplementary Figs. 47b, 48b, 49b). These signals were drastically reduced regardless of the addition of hole sacrificial agents (sodium oxalate) or electron sacrificial agents (AgNO₃) (Fig. 9a and Supplementary Figs. 50–52). Considering that polarons involve a bound state of electrons and holes, it can be inferred that the acceleration of electron or hole extraction through the use of sacrificial agents may potentially expedite the separation of polarons. Thus, these signals could be attributed to polarons. The time delay between the appearance of excitons and polarons signals was the time at which the excitons started to dissociate into polarons, i.e. 630 fs for TPC-3D, 1820 fs for PYR-2D, and 755 fs for TPL-2D (Fig. 9c and Supplementary Figs. 48c, 49c). Owing to the high dissociation efficiencies of the excitons in all the CPs (discussed in detail below), the decay of the PIA peaks was assigned to the transformation of polarons into free charges, other than the geminate recombination. The average lifetimes of polarons for TPC-3D, PYR-2D, and TPL-2D were 389.20 ps, 729.54 ps, and 550.26 ps, respectively (Fig. 9d and Supplementary Figs. 51e, 52e). It was notable that the dissociation speed of the polarons for TPC-3D was accelerated by 47% and 29% compared to PYR-2D and TPL-2D.

**Fig. 9: Femtosecond time-resolved transient absorption (TA) measurements of TPC-3D in the visible region.**

To further explore the mechanism leading to the accelerated dissociation of the polarons in TPC-3D, the dissociation kinetics of the polarons were fitted with the tri-exponential decay function as shown in Fig. 9b, d, e. The fast components τ₁ and τ₂ were assigned to the dissociation of the polarons formed at the intramolecular D-A interfaces^54,55. The long-lived component τ₃ was commonly assigned to the dissociation of polarons after interlayer diffusion (Fig. 6 Pathway 2). As shown in Fig. 9e, the ratios of each lifetime component were nearly the same in all the CPs in pure O₂ compared to those in air, which indicated the decay pathways of polarons were not affected by a higher concentration of O₂. However, the average lifetimes (τ_avg) and the lifetimes of τ₁, τ₂, τ₃ in O₂ were reduced by 40%, 81%, 46%, and 41% in TPC-3D, respectively, compared with those in air. These results demonstrated that pure O₂ could accelerate the dissociation of polarons. In comparison, each component was changed by <20% for PYR-2D and TPL-2D and all the dissociation pathways of polarons in TPC-3D were faster than that in the 2D CPs (Fig. 9e).

Based on the discussions above, the 3D structure did not alter the pathways of polaron transfer. However, it significantly accelerated polaron dissociation in a revers manner, due to the enhance utilization of electrons (Fig. 8). This phenomenon was not observer in 2D structures.

From excitons dissociation to polarons generation

Building upon the accelerated electron utilization and polaron dissociation discussed above, it was worth exploring whether this precursor effect also contributed to an expedited dissociation of excitons. Additionally, it was important to consider other factors that may influence the rate of excitons dissociation.

As investigated above, both TPC-3D and TPL-2D exhibited significant enhancement in polarons generation rate (630 fs for TPC-3D, 1820 fs for PYR-2D, and 755 fs for TPL-2D) under investigation compared to other reported photocatalysts (Fig. 9c and Supplementary Figs. 48c, 49c)⁵⁶. To elucidate the underlying reasons for this observation, we initially investigated whether the rapid exciton dissociation was due to an acceleration of polarons separation. By comparing the dissociation rates of excitons in air and oxygen, we verified that the dissociation rates of excitons did not increase in the presence of oxygen, thereby eliminating the possibility of accelerated polarons separation contributing to the acceleration of excitons dissociation (Supplementary Figs. 53, 54).

In order to substantiate the efficient dissociation of excitons, steady-state photoluminescence (PL) emission spectra were recorded, and the photoluminescence quenching yields (PLQYs) were determined to evaluate the overall excitonic recombination efficiencies⁵⁷. The photoluminescence of TPC-3D seemed weaker than that of PYR-2D and TPL-2D (Supplementary Fig. 55). The PLQYs of PYR-2D was 0.11 %, and the PLQYs of TPC-3D and TPL-2D were even below the detectable limit (Supplementary Fig. 56), which indicated that the radiative recombination of excitons was almost completely suppressed in the CPs. In other words, the CPs exhibits ultra-high dissociation efficiency.

In order to further evaluate the mechanism behind the lower PLQYs of TPC-3D and TPL-2D, the activation energy of exciton dissociation (E_a) or activation energy of charge recombination (E_ar) was determined through the temperature-dependent photoluminescence (TD-PL) spectra (Fig. 10a–d and Supplementary Fig. 57)^58,59. Notably, the TD-PL intensity increased with elevating temperature for TPC-3D and TPL-2D, indicating that E_a was smaller than E_ar. Thus, the excitons in TPC-3D and TPL-2D could be spontaneously dissociated. The E_ar for TPC-3D and TPL-2D could be estimated to be 23 meV and 47 meV, respectively. In contrast, the E_a for PYR-2D was 120 meV. Moreover, the much more production of ¹O₂ in PYR-2D also confirmed that TPC-3D and TPL-2D exhibited higher excitonic dissociation efficiency (Supplementary Fig. 58a, b). This is because when the excitons could not dissociate efficiently, they would go through spin-flip and transform to triplet excitons, which would react with O₂ to generate ¹O₂ through energy transfer (Supplementary Fig. 58c)³⁰. On the other hand, when the excitons dissociated into electrons, O₂ tended to generate •O₂⁻ through electron transfer (Supplementary Fig. 58d).

**Fig. 10: The mechanism of excitonic dissociation.**

To further investigate the embedded mechanism behind the spontaneously exciton dissociation, exciton binding energy (E_b) calculations calculation was adopted. E_b has been regarded as a crucial parameter for mediating charge separation in polymeric photocatalysts⁶⁰. The adequate dissociation of exciton in TPC-3D was caused by the remarkable low excitonic binding energies (E_b), which indicated that upon photoexcitation, excitons were spontaneously separated into free hole and electron charge carriers after overcoming the Coulomb force (Supplementary Fig. 59)⁶¹. Moreover, TD-DFT calculation confirmed that exciton dissociation in TPC-3D was more adequate than PYR-2D and TPL-2D, resulting in the sufficient charge aggregation (Supplementary Table 5)^62,63. In general, D index is used to measure the distance between the hole center and electron center, and t index is used to measure the separation degree between the hole and the electron from the perspective of charge-separation. Both D index and t index showed that TPC-3D possessed wider distribution and higher separation degree than that of the excited PYR-2D and TPL-2D, which indicated the better charge-separation ability.

In order to find out the reasons for the lower E_ar and E_b of TPC-3D, the electron-giving ability of the three electron-giving unit monomers in the three materials was compared (Fig. 10e), which found that the electron donor of TPC-3D was significantly higher than that of the other two materials, leading to a higher degree of delocalization in TPC-3D. In summary, with the same monomers of the electron acceptor, the stronger electron-giving ability of the electron donor is beneficial to promote the delocalization of the material and the effective separation of excitons, which can be proved by the electrostatic potential the dipole moment (Fig. 10f and Supplementary Figs. 60, 61).

In conclusion, the sufficient excitonic dissociation in excited TPC-3D was attributed to the stronger delocalization in material itself causing by the stronger electron-giving capacity of TPC moieties as electron donors. All the evidence above demonstrates that the stronger delocalization of TPC-3D itself is the fundamental reason for the expeditious generation of polarons but not the acceleration of polarons separation (Fig. 8).

Discussion

Our study has established a highly efficient open system that converts solar energy into in-demand chemicals, while elucidating the regulatory mechanisms on accelerated photophysical process through photochemical process. Specifically, we present a self-supporting 3D amorphous organic photocatalyst that achieves a breakthrough efficiency in the photosynthesis of H₂O₂ in real water, open air, and at room temperature. The photocatalyst exhibits a remarkable production rate of H₂O₂ as high as 9257–9991 μmol·g⁻¹·h⁻¹, and a SCC efficiency of 3.6%, surpassing the highest efficiency of typical plants (1%) and representing the exceptional efficiency in artificial photosynthetic systems in the open air.

This photocatalyst’s superior performance can be attributed to the effective utilization of internal excitons through intramolecular transfer instead of interlayer transfer. We achieved this by storing electrons within the photocatalyst and exposing the electron storage sites to O₂, which facilitates O₂ diffusion into the catalyst to extract electrons. As a result, 82.2% of electrons can be utilized through intramolecular electron transfer, which is an order of magnitude faster than interlayer transfer. This approach effectively circumvents electron recombination during interlayer transfer, thereby enabling faster and more efficient utilization of the electrons.

Moreover, of paramount importance is our ability to attain high-performance through deliberate design, coupled with a comprehensive exploration of the kinetics of each step and the corresponding regulatory mechanisms in the photophysical processes. Under ambient conditions, we observed the process from exciton dissociation to polarons generation within 630 fs. Then the polarons rapidly converted to free electrons within 389 ps. The free electrons were ultimately utilized within another 192 ps. Furthermore, the rapid utilization of electrons accelerated the dissociation of polarons without expediting the dissociation of excitons. The expeditious dissociation of excitons in TPC-3D was attributed to the spontaneous dissociation of excitons owning to the strong delocalization of the material (Fig. 8).

Overall, our findings provide important insights into the mechanisms underlying the superior performance of intramolecular charge transfer-based photocatalysts. This study lays a foundation for future construction of higher-performance solar-to-chemical conversion systems and the development of efficient and sustainable energy conversion technologies that utilize solar energy.

Methods

Photocatalytic experiments

Photocatalytic H₂O₂ production: The suspension contained 1 mg catalysts and 50 mL H₂O was well dispersed by ultrasonication for 30 min. Afterward, the catalyst was irradiated by using a Xe lamp (CEL-HXF300) as the light source. A cutoff filter was used to achieve visible-light irradiation (λ > 400 nm, average intensity: 100 mW·cm⁻²). The experiment was conducted at room temperature. The sample was filtrated with a 0.22 μm filter to further remove the photocatalysts.

Determination of H₂O₂ concentration: the TMB-H₂O₂-HRP enzymatic assay was used to quantify the concentration of H₂O₂, where the reaction between H₂O₂ and TMB could be instantaneously catalyzed by HRP:

$${{{{{{\rm{H}}}}}}}_{2}{{{{{{\rm{O}}}}}}}_{2}\,+{{{{{\rm{TMB}}}}}} {\to }^{{{{{{\rm{HRP}}}}}}} \,{{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}+{{{{{\rm{oxTMB}}}}}}$$

(1)

The 3,3’,5,5’-tetramethylbenzidine (TMB) solution was prepared as follows: 15 mg TMB solid was dissolved in 0.3 mL dimethyl sulfoxide (DMSO), subsequently, 5 mL glycerol solution and 45 mL deionized water containing 20 mg ethylene diamine tetraacetic acid (EDTA) and 95 mg citric acid was added.

The horseradish peroxidase (HRP) solution was prepared as follows: dissolve 2 mg peroxidase (from horseradish) in 10 mL deionized water.

To obtain the calibration curve, a known concentration of H₂O₂ (as the hydrogen donor) was added to the TMB solution. It should be noted that the concentration of HRP was maintained at 12.5 μg·mL⁻¹. After 3 min, added 10 μL concentrated hydrochloric acid and measured by a UV-visible spectrophotometer at 450 nm to obtain a standard calibration curve. Then the H₂O₂ concentration of the samples could be quantified according to the linear relationship between signal intensity and H₂O₂ concentration.

SCC efficiency measurements

The solar-to-chemical energy conversion (SCC) efficiency was measured by photocatalytic experiments employing an AM 1.5 G solar simulator as the light source (100 mW·cm⁻²). The concentration of catalyst is 0.4 g·L⁻¹. Calculated the SCC efficiency (η) via the following equation:

$${{{{{\rm{\eta }}}}}}\left(\%\right)=\frac{\Delta {{{{{\rm{G}}}}}}\left({{{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}}_{2}\right)\times {{{{{\rm{n}}}}}}\left({{{{{{{\rm{H}}}}}}}_{2}{{{{{\rm{O}}}}}}}_{2}\right)}{{{{{{{\rm{t}}}}}}}_{{{{{{\rm{ir}}}}}}}\times {{{{{{\rm{S}}}}}}}_{{{{{{\rm{ir}}}}}}}\times {{{{{{\rm{I}}}}}}}_{{{{{{\rm{AM}}}}}}}}\times 100\%$$

(2)

In the equation, ∆G(H₂O₂) = 117 kJ·mol⁻¹, is the free energy for H₂O₂ generation. n(H₂O₂) is the amount of H₂O₂ generated during the photocatalytic reaction. The irradiation time t_ir is 3600 s, and the irradiated sample area (S_ir) is 1 × 10⁻⁴ m². I_AM, the total irradiation intensity of the AM 1.5 global spectra (300 nm–2500 nm), is 100 mW·cm⁻².

Transient absorption spectroscopy measurements

TA spectra of TPC-3D, PYR-2D, and TPL-2D w ere measured on a Helios femtosecond transient absorption spectrometer (Helios Fire, Ultrafast Systems, LLC). A 400 nm pump pulse was generated via an optical parametric amplifier (OPerA Solo, Coherent). The 400 nm laser intensity was 50 μW. The preparation of samples was as follows: 10 mg of photocatalyst was dispersed in 50 mL deionized water. Then the suspension was subjected to ultrasonication by an ultrasonic cell pulverizer (SCIENTZ-II D) for 48 h. Afterward, the suspension was left overnight to allow the large particles to settle down while the fine particles were stably suspended in water.

Measurements were performed using a cuvette with stopper. For TA spectra in air atmosphere, the suspension was added directly to the cuvette and tested under open conditions. For TA spectra in oxygen atmosphere, oxygen was injected into the suspension and bubbled for 20 min to saturate it, and then the test was performed with the stopper closed. When measuring TA spectra with the addition of sacrificial agents, the concentration of sacrificial agents was maintained at 10 mM.

Computational methods

The Density functional theory (DFT) calculation was conducted as implemented in the Gaussian 09 D.01 program package⁶⁴ and Grimme-D3 dispersion correction was used⁶⁵. GaussView6 was employed for visualization. The geometry optimization and frequency analysis were performed at the B3LYP/6-31 G(g,d) level of theory⁶⁶. The atomic coordinates of the optimized computational models were shown in Supplementary Table 6–8. The optimized structures were utilized to calculate single-point energy with PBE0/ma-TZVP^66,67,68. The convergence criteria for energy, maximum force, root mean square force, maximum displacement, and root mean square displacement was 10⁻⁶ Hartree, 4.5 × 10⁻⁴ Hartrees/Bohr-Radians, 3 × 10⁻⁴ Hartrees/Bohr-Radians, 1.8 × 10⁻³ Bohr-Radians, 1.2 × 10⁻³ Bohr-Radians respectively. Time-dependent density functional theory (TD-DFT) was carried out at the PBE0/6-31 G(d,p) level of theory and applied to the investigation of the transfer direction of electrons^69,70,71. Analysis and visualization of hole and electron distribution were performed by Multiwfn⁶². Exciton binding energy was calculated by the def2-SVP basis set⁵⁸. The electrostatic potential and Mulliken electronegativity involved in the analysis were evaluated by Multiwfn based on an efficient algorithm and filled with colors using VMD1.9.3^58,62,63,72.

The adsorption energy (${{{{{{\rm{E}}}}}}}_{{{{{{\rm{adsorb}}}}}}}$) of O₂ molecule on the surface is calculated as follow:

$${{{{{{\rm{E}}}}}}}_{{{{{{\rm{adsorb}}}}}}}={{{{{{\rm{E}}}}}}}_{{{{{{\rm{total}}}}}}}-{{{{{{\rm{E}}}}}}}_{{{{{{\rm{surface}}}}}}}-{{{{{{\rm{E}}}}}}}_{{{{{{{\rm{O}}}}}}}_{2}}$$

(3)

where ${{{{{{\rm{E}}}}}}}_{{{{{{\rm{total}}}}}}}$ represents the energy of surface with adsorbed O₂ molecule, ${{{{{{\rm{E}}}}}}}_{{{{{{\rm{surface}}}}}}}$ and ${{{{{{\rm{E}}}}}}}_{{{{{{{\rm{O}}}}}}}_{2}}$ represent the energies of isolated surface and O₂, respectively.

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China 22206209 (Y.-X.Ye), 22336007 (G.F.O.Y.), the Natural Science Foundation of Guangdong Province 2022A1515011953 (Y.-X.Ye), the Guangdong Basic Research Center of Excellence for Functional Molecular Engineering Project 31000-42080002 (G.F.O.Y.), and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai) via project No. SML2023SP220 (Y.-X.Ye). The authors thank the PL group of Instrumental Analysis & Research Center, Sun Yat-sen University for their contributions to temperature-dependent PL and PLQYs measurements.

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Authors and Affiliations

Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, LIFM, School of Chemistry, IGCME, Sun Yat-sen University, Guangzhou, 510275, China
Yuyan Huang, Minhui Shen, Jianqiao Xu, Fang Zhu & Gangfeng Ouyang
School of Chemical Engineering and Technology, IGCME, Sun Yat-sen University, Zhuhai, 519082, China
Huijie Yan, Yu-**n Ye & Gangfeng Ouyang
School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou, 510275, China
Yingge He & **n Yang
Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai, Guangdong, 519082, China
Yu-**n Ye & Gangfeng Ouyang

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Yuyan Huang
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Minhui Shen
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Huijie Yan
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Yingge He
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Fang Zhu
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Contributions

Y.Y.H., Y.-X.Ye and G.F.O.Y. co-proposed the idea and designed the experiments. Y.Y.H. contributed to perform the experiments. Y.Y.H. and M.H.S. performed the DFT calculations. Y.Y.H., Y.-X. Ye, M.H.S., H.J.Y., Y.G.H., J.Q.X., F.Z. and X.Y. contributed to performing and analyzing the experiments. Y.Y.H., Y.-X.Ye and G.F.O.Y. participated in writing the manuscript.

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Correspondence to Yu-**n Ye or Gangfeng Ouyang.

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Huang, Y., Shen, M., Yan, H. et al. Achieving a solar-to-chemical efficiency of 3.6% in ambient conditions by inhibiting interlayer charges transport. Nat Commun 15, 5406 (2024). https://doi.org/10.1038/s41467-024-49373-z

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Received: 03 January 2024
Accepted: 04 June 2024
Published: 26 June 2024
DOI: https://doi.org/10.1038/s41467-024-49373-z
Springer Nature Limited

Achieving a solar-to-chemical efficiency of 3.6% in ambient conditions by inhibiting interlayer charges transport

Abstract

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Introduction

Results

Synthesis and characterization of catalysts

Electron utilization

From polarons dissociation to electron generation

From excitons dissociation to polarons generation

Discussion

Methods

Photocatalytic experiments

SCC efficiency measurements

Transient absorption spectroscopy measurements

Computational methods

Data availability

References

Acknowledgements

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Competing interests

Peer review

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Additional information

Supplementary information

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