Two-step coupled photoelectrochemical chlorination and oxygenation of C(sp³)–H bonds mediated by chlorine radicals over a modified BiVO₄ photoanode

Abstract

Photoelectrochemical (PEC) cells are emerging tools for fine chemical synthesis, but often suffer from low solar-to-product conversion efficiency, especially in energy-demanding reactant activation. Herein, we report chlorination and oxygenation of energy-demanding C(sp³)–H bonds using a two-step coupled PEC cell, avoiding the direct generation of high-energy chlorine radicals (Cl˙). The photoanode consists of a BiVO₄ semiconductor modified with TiO₂ and a CoNi₂O_x chlorine evolution reaction (CER) catalyst. Under 1 sun illumination, the BiVO₄/TiO₂/CoNi₂O_x photoanode showed a photocurrent density of 2.9 mA cm⁻² for CER at 0.8 V vs. the reversible hydrogen electrode (RHE) with the highest applied bias photon-to-current efficiency of 3.20%. Subsequent homolysis of Cl₂ under white light generates Cl˙, activating C(sp³)–H bonds following hydrogen atom transfer. The PEC cell selectively chlorinated hydrocarbons under argon, and enabled oxygenation to afford aldehydes, ketones, and alcohols when the atmosphere was switched to dioxygen, offering a green and efficient synthetic approach. Studies on the reaction mechanism revealed that Cl˙ is the key reactive intermediate responsible for C(sp³)–H bonds activation. This work offers a solar-driven energy-efficient strategy for the generation of Cl˙ from chloride salts and activation of energy-demanding C(sp³)–H bonds, highlighting its great potential in advancing green chemical synthesis.

---

Introduction

The activation and functionalization of inert C(sp³)–H bonds are essential goals in synthetic organic chemistry, with significant implications for fine chemical production.^1–4 Traditional methods for C(sp³)–H activation, such as chlorination and oxygenation, often require external oxidants (e.g., 2-iodoxybenzoic acid,^5,6 S₂O₈,^2–7etc.), hazardous reagents (e.g., Cl₂,⁸N-chlorosuccinimide,^9,10 NaClO,¹¹etc.), expensive metal-based catalysts (e.g., Au,¹² Rh,¹³ Pd,¹⁴etc.), and harsh reaction conditions (e.g., elevated temperature and pressure,¹⁵etc.). These challenges limit their application in green and sustainable chemistry (Scheme 1).

Scheme 1 C(sp³)–H bond chlorination and oxygenation driven by traditional routes and by the two-step coupled PEC approach in this work.

Photoelectrochemical (PEC) cells, which use sunlight to drive chemical reactions, offer a promising alternative to conventional methods, as they enable chemical transformations under milder conditions.^16–22 PEC synthesis has been successfully applied to form C–N,²³ C–O,^24,25 C–P,²⁶ and C–X²⁷ bonds via C–H functionalization.^28–31 However, the direct activation of C(sp³)–H bonds, especially those in alkanes with bond dissociation energies (BDE) of approximately 96–101 kcal mol⁻¹, remains difficult in PEC systems due to their high activation energy.^32,33 One promising approach to overcome these challenges is the use of redox mediators that facilitate hydrogen atom transfer (HAT).^32,34–40 The chlorine radical (Cl˙) is particularly effective as a HAT mediator, because it is capable of abstracting hydrogen atoms from hydrocarbons to form carbon-centered radicals.^27,41–46 While the generation of Cl˙ from one electron oxidation of Cl⁻ is energy-demanding and requires a high reduction potential (E°(Cl˙/Cl⁻) = 2.2–2.4 V vs. normal hydrogen electrode (NHE) in aqueous solutions^47,48), this challenge can in principle be mitigated through a two-step coupled photoelectrochemical process.

In this work, we propose a novel two-step coupled PEC process for generating Cl˙ from Cl⁻. The principle is straightforward: since the one-electron oxidation of Cl⁻ in aqueous solution is highly energy-demanding, we design the photoanode to intentionally facilitate a two-electron oxidation of Cl⁻ to Cl₂ through photoelectrocatalysis first (E°(Cl₂/Cl⁻) = 1.48 V vs. RHE).⁴⁷ Cl₂ in the aqueous phase diffuses away from the photoanode and to the organic phase where Cl˙ is then photogenerated through the homolysis of Cl₂ in the second step. Importantly, our mechanistic study found that Cl˙ intermediate in the second step, rather than other chlorine species like Cl₂ or dichloride radical anion (Cl₂˙⁻),⁴⁹ plays a crucial role in the PEC cell. This design of merging photoelectrocatalysis and photocatalysis/photolysis is inspired by the Z-scheme of natural photosynthesis, where the production of highly reducing NADPH equivalents and oxygen evolution is spatially separated across two photosystems and the electron transport chain. Therefore, two redox-difficult reactions can proceed at relatively high efficiency under mild conditions.^50,51 This two-step coupled approach allows for the activation of energy-demanding C(sp³)–H bonds with readily available NaCl aqueous solution in a PEC cell, avoiding the need for toxic reagents or organic chloride sources (Scheme 1).

Results and discussion

Design and characterization of the modified BiVO₄ photoanode

Monoclinic BiVO₄ photoanodes were prepared according to prior reports with minor modifications.^52,53 To synthesize the BiVO₄/TiO₂/CoNi₂O_x photoanode, a protective TiO₂ overlayer⁵⁴ was deposited onto the surface of BiVO₄ through atomic layer deposition (ALD). Then, CoNi₂O_x was decorated on the BiVO₄/TiO₂ surface as a CER catalyst⁵⁵ by drop casting (Fig. 1a). The structural and morphological properties of the resulting BiVO₄/TiO₂/CoNi₂O_x photoanode were first analyzed using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD revealed the presence of monoclinic BiVO₄ peaks, confirming the preservation of the crystal structure after modifications (Fig. S1). HRTEM analysis showed that the TiO₂ overlayer had a uniform thickness of ∼4.4 nm on the BiVO₄ surface (Fig. 1b). The CoNi₂O_x catalyst displayed a nanosheet-type morphology with polygonal shapes and widths ranging from 100–400 nm (Fig. S3). X-ray photo-electron spectroscopy (XPS) confirmed the presence of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ species (Fig. S2). Scanning electron microscopy (SEM) further illustrated the successful loading of CoNi₂O_x onto the BiVO₄/TiO₂ surface (Fig. 1c), with close attachment of the nanosheets to the photoanode (Fig. 1d and e). The presence of Bi, V, Ti, O, Co and Ni in the BiVO₄/TiO₂/CoNi₂O_x photoanode (Fig. 1f–k) was confirmed using scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) with a Co/Ni ratio of 1 : 2 (Fig. S4 and Table S1).

Fig. 1 (a) Schematic illustration of the construction of the BiVO₄/TiO₂/CoNi₂O_x photoanode. (b) HRTEM image of BiVO₄/TiO₂. (c) SEM image of BiVO₄/TiO₂/CoNi₂O_x. (d and e) HRTEM images of BiVO₄/TiO₂/CoNi₂O_x. (f–k) STEM-EDS maps showing the distribution of Bi, V, Ti, O, Co, and Ni in BiVO₄/TiO₂/CoNi₂O_x.

The PEC performance of different photoanodes for CER was first evaluated using linear sweep voltammetry (LSV). As illustrated in Fig. 2a, BiVO₄ showed a low photocurrent density of 1.2 mA cm⁻² at 1.2 V vs. the reversible hydrogen electrode (RHE) under AM 1.5G illumination (100 mW cm⁻²). Fortunately, the photocurrent density was significantly enhanced by the addition of TiO₂ or CoNi₂O_x, reaching 2.1 mA cm⁻² and 3.0 mA cm⁻² for BiVO₄/TiO₂ and BiVO₄/CoNi₂O_x, respectively. It is worth noting that the thickness of the TiO₂ overlayer can influence the PEC performance of BiVO₄/TiO₂; the optimal thickness for CER was found to be ∼4.4 nm (Fig. S5). The Co/Ni ratio of the CER catalyst also has a significant influence on the overall performance (Fig. S6a and b). The optimized photoanode composition is BiVO4/TiO₂/CoNi₂O_x, exhibiting a remarkable photocurrent density of 2.9 mA cm⁻² at 0.8 V vs. RHE and 5.4 mA cm⁻² at 1.2 V vs. RHE (Fig. 2a). However, when we excluded the BiVO₄ layer from the BiVO₄/TiO₂/CoNi₂O_x architecture, i.e. TiO₂/CoNi₂O_x that had no visible-light absorption, no photocurrent was generated, confirming the critical role of BiVO₄ as the primary light-absorbing unit. Additionally, the incident photon-to-current conversion efficiency (IPCE) of different photoanodes revealed that the BiVO₄/TiO₂/CoNi₂O_x displayed excellent IPCE values above 80% from 400 to 450 nm (Fig. 2b), consistent with its outstanding PEC performance for CER. Other photoanode configurations showed much lower IPCE values within the same wavelength range.

Fig. 2 (a) Chopped LSV curves of different photoanodes in 0.5 M NaCl solution (pH = 2) under AM 1.5G light illumination (100 mW cm⁻²). (b) IPCE spectra of the different photoanodes at 1.2 V vs. RHE. (c) Plot of k_tr, k_rec, and η_ct values calculated from the IMPS measurements for different photoanodes. (d) J–t curves for long-term PEC CER tests of different photoanodes in 0.5 M NaCl (pH = 2) aqueous solution at 1.2 V vs. RHE. (e) FE values of various photoanodes for Cl₂ and O₂ evolution. Reaction time: 1 h. (f) Current density from PEC using modified BiVO₄ photoanode as a function of E_app and comparison with electrochemical (EC) oxidation of Cl⁻ to Cl₂. The vertical dashed lines represent the E_app needed for the two oxidation processes when the current density at each indicated (photo)electrode reaches 2.9 mA cm⁻².

Intensity modulated photocurrent spectroscopy (IMPS) characterizations were carried out to probe the hole transfer kinetics of photoanodes from bulk to surface. The charge transfer rate constant (k_tr) and charge recombination rate constant (k_rec) were determined from the IMPS spectra (Fig. S7a and b).⁵⁶ The k_tr and k_rec values of various photoanodes are shown in Fig. 2c. BiVO₄ displayed a k_rec value of 0.29 s⁻¹ along with a k_tr value of 0.33 s⁻¹. For BiVO₄/TiO₂, the TiO₂ overlayer slowed down the charge recombination and enhanced the hole transfer with k_rec = 0.15 s⁻¹ and k_tr = 0.47 s⁻¹, indicating its role as a hole transport layer. Meanwhile, the BiVO₄/CoNi₂O_x photoanode showed slightly larger k_rec and k_tr than BiVO₄. For the BiVO₄/TiO₂/CoNi₂O_x photoanode, k_tr is about 20-fold higher than k_rec (2.34 s⁻¹vs. 0.12 s⁻¹). The charge transfer efficiency (η_ct) calculated from the k_rec and k_tr values [η_ct = k_tr/(k_rec + k_tr) × 100%] is over 95%, implying almost quantitative charge separation at the photoanode surface (Fig. 2c and Table S2). The charge transport from photoanode surface to electrolyte was investigated by electrochemical impedance spectroscopy (EIS). The EIS spectra collected from different photoanodes fit well with Randle's equivalent circuit (Fig. S8). In this model, lower charge transfer resistance (R_ct) indicates faster hole transfer from photoanode surface to electrolyte.^57,58 BiVO₄/TiO₂/CoNi₂O_x photoanode had the lowest R_ct among all tested configurations, further confirming the enhancement in charge separation and transfer efficiency at the photoanode-electrolyte interface shown in Table S3.

We also observed that modifying BiVO₄ with TiO₂ could improve the photoanode stability. BiVO₄ showed poor stability at 1.2 V vs. RHE after 1 h of PEC Cl₂ production, with the photocurrent density decreasing from 1.3 mA cm⁻² to 0.7 mA cm⁻². In contrast, BiVO₄/TiO₂ and BiVO₄/TiO₂/CoNi₂O_x displayed much better stability, demonstrated the other role of TiO₂ as a protective overlayer (Fig. 2d). Next, we quantified the faradaic efficiency for the CER (FE_CER) of the different photoanodes using the N,N-diethyl-p-phenylenediamine (DPD) colorimetric method. The amount of dioxygen byproduct from water oxidation was determined by gas chromatography (GC). The FE_CER values for bare BiVO₄ (43%) and BiVO₄/TiO₂ (54%) were enhanced by the deposition of CoNi₂O_x on BiVO₄/TiO₂ (94% for BiVO₄/TiO₂/CoNi₂O_x, Fig. 2e). The incorporation of CoNi₂O_x significantly enhanced the FE_CER while suppressing oxygen evolution, indicating its role as a CER catalyst. When we deposited CoNi₂O_x or TiO₂ on the FTO substrates and tested their LSV curves (Fig. S9), we found that FTO/CoNi₂O_x exhibited a significant catalytic current compared to FTO and FTO/TiO₂, showing increased current density and reduced onset potential. This indicates the role of CoNi₂O_x as a CER catalyst as well. For the long-term CER test (24 h), an acceptable FE_CER above 80% was obtained and an outstanding photocurrent density above 3.0 mA cm⁻² was maintained for 24 h at 0.8 V vs. RHE using BiVO₄/TiO₂/CoNi₂O_x photoanode (Fig. S10). With the systematic testing for each component of the modified photoelectrode, we summarize that BiVO₄ acts as the light-absorbing unit; the TiO₂ overlayer serves as the hole transport and protective layer, enhancing charge transfer efficiency and protecting electrode materials; and CoNi₂O_x primarily functions as the CER catalyst, enhancing FE_CER. The three components synergistically operate together to achieve the optimized performance.

Fig. 2f illustrates the advantages of the PEC method for CER by plotting the CER current density as a function of the applied potential (E_app). The onset potential for Cl₂ evolution at the BiVO₄/TiO₂/CoNi₂O_x photoanode was around 1 V less than that at a carbon electrode. The significantly lower E_app for the PEC method (0.8 V vs. RHE) compared to electrochemical oxidation (∼1.5 V vs. RHE) is crucial. This reduced E_app critically lowers the overall cell voltage (E_cell) requirement. Significantly, E_cell below 1.48 V, corresponding to the thermodynamic potential difference of E°(Cl₂/Cl⁻) and E°(H⁺/H₂) under our experimental conditions, enables solar energy harvesting for subsequent organic synthesis as quantified by the applied bias photon-to-current efficiency (ABPE) being a positive value (vide infra). This enables C(sp³)–H bond activation initiated by Cl˙ under a low E_app using the PEC method.

Finally, the efficiency of using the incident light in photoelectrochemical conversion by the BiVO₄/TiO₂/CoNi₂O_x-based PEC cell was evaluated by ABPE (eqn (1)),⁵⁹

where P_{AM 1.5G} is the incident light intensity (100 mW cm⁻² for AM 1.5G illumination), J_p is the photocurrent density, E_cell is the cell voltage between the working electrode and counter electrode (Pt), and E° represents the thermodynamic potential of the cell reaction (E°(Cl₂/Cl⁻) – E°(H⁺/H₂)), which is 1.48 V under our experimental conditions.⁴⁷ An outstanding ABPE value of 3.20% was obtained with the BiVO₄/TiO₂/CoNi₂O_x photoanode at 0.8 V, which is 10 times higher than that with bare BiVO₄ (Fig. S11). Through this new strategy of Cl₂ production by PEC, we have greatly reduced the E_app from values up to 1.5 V vs. RHE in electrochemical Cl₂ generation to 0.8 V vs. RHE while ensuring efficient Cl₂ evolution.

Photoelectrochemical chlorination of C(sp³)–H bonds

The PEC chlorination of the C(sp³)–H bonds was conducted in 30 mL 0.5 M NaCl (pH = 2) solution using cyclohexane as the model substrate. E_app of 0.8 V vs. RHE was applied to different photoanodes for PEC chlorination tests with 2 mL cyclohexane. After 2 h, the chlorinated products were quantified using GC (Fig. S13a–c) and thus the FE toward the chlorinated products was obtained. Among the different photoanodes, BiVO₄/TiO₂/CoNi₂O_x displayed the highest production rate of 51.5 μmol cm⁻² h⁻¹ toward chlorocyclohexane with a FE of 93% for chlorinated products (Fig. 3a and Table 1, entry 1). For the chlorination of 0.15 mmol (16.2 μL) cyclohexane over BiVO₄/TiO₂/CoNi₂O_x, a high yield of 93% for chlorocyclohexane was obtained with white light irradiation (100 mW cm⁻²) after 10 h (Fig. 3b). However, the decreased initial substrate loading significantly compromised FE. To maintain practical FE, we employed milliliter-scale substrate quantities (>10 mmol) for other compounds, though complete conversion would require impractically long reaction times (>200 h). This trade-off led us to prioritize reporting faradaic efficiencies and product selectivity data.

Fig. 3 (a) Production rate of chlorocyclohexane from the C–H chlorination of cyclohexane. Reaction time: 2 h (30 mL NaCl electrolyte, 2 mL cyclohexane, 0.8 V vs. RHE, Ar atmosphere). (b) The J–t curve for the chlorination of 0.15 mmol cyclohexane and the yield of chlorocyclohexane at 0.8 V vs. RHE under an Ar atmosphere. Reaction time: 10 h.

Table 1 PEC chlorination of hydrocarbons over BiVO₄/TiO₂/CoNi₂O_x photoanode in Ar^a

Entry	Substrate	Chlorinated product	FEmain^b
a Reaction conditions: 30 mL 0.5 M NaCl (pH = 2) solution, 1–2 mL hydrocarbons, white LED (100 mW cm^−2), 1 atm Ar. b FE of main chlorinated products was determined by NMR, GC and GC-MS.
1			93%
2			96%
3			82%
4			84%
5			83%
6			67%
7			90%
8			33%

---

A wider scope of substrates was then tested to further establish the general applicability of PEC chlorination of C(sp³)–H bonds over the BiVO₄/TiO₂/CoNi₂O_x photoanode (Table 1). For the chlorination of cycloheptane, 4-chloro-1-methylcyclohexane was the primary product, likely due to a carbon radical rearrangement (Table 1, entry 2). We then investigated the site selectivity of the chlorination using methylcyclohexane as the substrate (Table 1, entry 3). Chlorination predominantly occurred at the secondary C–H bond, with minimal primary chlorination and no tertiary chlorination detected via gas chromatography-mass spectrometry (GC-MS) analysis. This high selectivity for secondary C–H chlorination is consistent with previously reported site-selective aliphatic C–H chlorination using N-chloroamides and visible light.⁶⁰

To gain insight into the active species driving chlorination, we analyzed the chlorinated products of toluene and its derivatives, because toluene contains both active benzyl C(sp³)–H bonds and aryl C(sp²)–H bonds. Benzyl chloride was the predominant product, with only trace amounts of phenyl chloride, indicating that chlorination was driven by Cl˙ radical-mediated HAT rather than electrophilic aromatic substitution by Cl₂ (Table 1, entry 4). Substrates prone to oxidation, such as toluene, were not directly oxidized at the electrode interface in the aqueous solution, but instead mediated by Cl˙ in the organic phase due to the insolubility of organic substrates in NaCl aqueous solution in the PEC cell. This approach demonstrated an excellent product selectivity. Electron-donating (Table 1, entry 5) and electron-withdrawing groups (Table 1, entry 6) had minimal influence on selectivity. However, the chlorination of 4-chlorotoluene yielded a lower FE, likely due to its higher BDE. Similarly, for ethylbenzene, the reaction selectively produced (1-chloroethyl)benzene, because secondary C–H bonds are weaker than primary bonds (Table 1, entry 7). Chlorination of tert-butylbenzene yielded neophyl chloride as the main product, despite the increased inertness of alkyl C(sp³)–H bonds compared to toluene (Table 1, entry 8).

Interestingly, alicyclic hydrocarbons such as cyclohexane demonstrate higher FE compared to aromatic hydrocarbons, despite their higher BDE. We tentatively attribute this to the variable absorption properties of Cl₂ in different solvents. As shown in Fig. S21, there is a red-shift of Cl₂ absorption spectra in cyclohexane compared to 0.5 M NaCl (pH = 2) aqueous solution, while a blue-shift is observed in toluene and similar aromatic hydrocarbons. This enables Cl₂ to be more easily excited by visible light in alicyclic hydrocarbons, whereas in aromatic hydrocarbons such as toluene, there is less absorption of visible light and therefore less efficient generation of Cl˙, resulting in lower FE. However, it should be noted that FE does not equate to product selectivity. The selectivity for chlorinated products remained consistently above 90%, as evidenced by GC and NMR analyses which revealed only negligible amounts of byproducts. Overall, the PEC chlorination results suggest that C–H chlorination over BiVO₄/TiO₂/CoNi₂O_x photoanode was primarily driven by Cl˙.²⁷

Mechanistic investigation of C(sp³)–H activation

To determine whether Cl˙ serves as the HAT agent driving the C–H chlorination, we first investigated the possible reaction of Cl₂ and cyclohexane in the dark. As shown in Fig. 4a, only trace amounts of chlorocyclohexane were detected. However, when Cl₂ was exposed to AM 1.5G light or AM 1.5G light coupled with a long-pass filter (λ > 405 nm), the production of chlorocyclohexane increased significantly (Fig. 4a). These results suggest that the CER product, Cl₂, is insufficient to activate C–H bonds directly and that light excitation is needed to generate reactive intermediates, such as Cl˙, to drive C–H chlorination.⁶¹

Fig. 4 (a) Chlorocyclohexane yield for cyclohexane chlorination at indicated illumination conditions. (b) Schematic illustration for the control trials of PEC chlorination. Left: gap between the photoanode and the inner wall of the cell, allowing Cl₂ to absorb blue photons; right: no gap, Cl₂ could not absorb blue photons. (c) Selectivity of chlorinated products of diene 1a with different setups: with a gap or no gap.

Next, we conducted PEC C–H chlorination in the presence of various scavengers. The addition of the radical scavenger 2,2,6,6-tetramethylpiperidinyl-1-oxide (TEMPO) and the hole scavenger ammonium oxalate (AO) completely suppressed the formation of chlorocyclohexane (Fig. S27). This result indicates that both radicals and holes are involved in the reaction, further confirming the role of Cl˙ as the active species. Based on these findings, two possible pathways for C–H chlorination with the BiVO₄/TiO₂/CoNi₂O_x photoanode were proposed: (1) Cl˙ is directly generated on the photoanode surface and quickly reacts with cyclohexane before Cl₂ evolves, or (2) Cl₂ is first evolved from the photoanode, and then Cl˙ is produced via light excitation.

To distinguish between these two pathways, we measured the FE of chlorocyclohexane production with different PEC cell configurations (Fig. 4b). In the first setup, a gap between the photoanode and the inner wall of the PEC cell allowed Cl₂ to diffuse and absorb blue photons from the light source, and the BiVO₄/TiO₂/CoNi₂O_x photoanode exhibited a high FE of 88% for chlorocyclohexane production (Fig. 4b, left). In the second setup, the photoanode was placed directly against the inner wall, preventing Cl₂ from being excited by the incident light (which was mostly absorbed by BiVO₄), and the FE dropped significantly to 2.3% (Fig. 4b, right). These results confirm that the excitation of Cl₂ to form Cl˙ is crucial for driving C–H bond activation (the second pathway). This mechanism was further supported by a trapping experiment using a diene substrate (1a), which produced distinct chlorination products that depend on whether Cl₂ (1b & 1c) or Cl˙ (1d) was the active species.⁶² The amount of 1d was found to decrease greatly when the photoanode was in the second setup (Fig. S28 and 4c). In addition, when there was no E_app or when inert SO₄²⁻ was present instead of Cl⁻, no chlorocyclohexane was detected, indicating that the PEC process, which oxidizes Cl⁻ to produce Cl₂, is essential for the overall chlorination reaction. Overall, control experiments were conducted to separate the photocatalysis (PC) step and the PEC step to illustrate their individual roles in our two-step coupled system (Table S4). We found that the PEC process efficiently generates Cl₂ at a lower E_app, while the PC process excites Cl₂ to produce Cl˙, initiating the C–H chlorination reaction. Both of the two steps are essential, with the PEC step first and the PC step second in a coupled manner.

We also investigated the effect on the FE of chlorocyclohexane production of varying the irradiation wavelengths with appropriate combinations of long-pass and short-pass filters (see Experimental section). Under the same conditions, the FE for chlorinated products as a function of wavelength directly correlates with the absorptance of Cl₂ (Fig. S12a). The external quantum efficiency (EQE), which represents the number of product molecules produced per incident photon,^63,64 also aligns with the FE trend (Fig. S12b). This also confirms that the excitation of Cl₂ is the key step in generating Cl˙, which then initiates the C–H chlorination reaction.

Because of the existence of an association equilibrium for the formation of Cl₂˙⁻ when Cl˙ is photogenerated (eqn (2), K_eq ∼ 1.4 ×10⁵ M⁻¹),^47,65,66 Cl₂˙⁻ is a possible alternative reactive intermediate for activation of C(sp³)–H bonds, as was observed in a prior report.⁶⁷

Cl˙ + Cl⁻ ⇌ Cl₂˙⁻ (2)

To elucidate the distinct roles of Cl˙ and Cl₂˙⁻ species in the reaction system, TA measurements were conducted in 0.5 M NaCl aqueous solution containing Cl₂. Upon photoexcitation of Cl₂, a positive absorption signal spanning 300–450 nm was observed (Fig. 5a). The characteristic absorption peak at 340 nm corresponds to Cl₂˙⁻,⁶⁷ while the weaker Cl˙ signal at 320 nm arises from two factors: (1) partial spectral overlap between these species, and (2) competing ground-state bleaching of Cl₂ at 320 nm, which superimposes with and obscures the Cl˙ transient signal. The Cl₂˙⁻ species exhibits a characteristic lifetime of 4.5 μs at 340 nm. However, the addition of cyclohexane to this solution did not alter the kinetics of Cl₂˙⁻ (Fig. 5b), suggesting that Cl₂˙⁻ is not a sufficiently potent oxidant to activate the cyclohexane C–H bond. This observation aligns with previous estimates of the redox potential difference between Cl₂˙⁻/Cl⁻ and Cl˙/Cl⁻, which indicates that Cl₂˙⁻ is a weaker oxidant than Cl˙.⁶⁸ Although this result is contrary to a previous work by Wu, et al.,⁴⁹ the observed differences between our study and this earlier work likely arise from the differing reaction conditions, including pH values, aqueous/organic phase ratios (10 : 1 vs. 1 : 4), and Cl˙ generation methods, which collectively influence the concentration of Cl₂˙⁻ species and product selectivity.

Fig. 5 (a) Transient absorption spectra of Cl˙ and Cl₂˙⁻ measured at indicated time delays in 0.5 M NaCl (pH = 2) aqueous solution (λ_ex = 355 nm, 4 mJ per pulse). (b) Normalized transient kinetic TA spectral data with 0.5 M NaCl (pH = 2) containing Cl₂ at 340 nm, with or without cyclohexane (λ_ex = 355 nm, 10 mJ per pulse). (c) Transient absorption spectra of the Cl˙-arene adduct measured at indicated time delays in 0.5 M NaCl (pH = 2) aqueous solution containing 1 mL toluene and photoelectrochemically generated Cl₂ under the operando condition (λ_ex = 355 nm, 4 mJ per pulse). Insert: transient kinetic data for the Cl˙-arene adduct monitored at a probe wavelength of 490 nm. (d) ESR detection of Cl˙ formation in the two-step coupled PEC system using DMPO as spin-trapping agent under the indicated light or dark conditions.

The earlier work achieved exceptional product selectivity through a biphasic system where Cl˙ and Cl₂˙⁻ are generated in the aqueous phase.⁴⁹ By removing the possibility of direct contact between the short-lived Cl˙ and the organic substrates, only Cl₂˙⁻ (with a microsecond lifetime) is sufficiently long-lived to diffuse into the organic phase and drive reactions. In our present system, stable Cl₂—rather than transiently lived intermediates—is initially generated by the PEC method, and then diffuses into the organic phase, where it undergoes photolysis to produce Cl˙ that reacts immediately with the substrates. While minor contributions from Cl₂˙⁻-mediated pathways cannot be entirely ruled out for substrates with lower oxidation potential (such as toluene), two factors strongly favor Cl˙-dominated reactivity. Firstly, the extremely low solubility of NaCl in the organic phase severely limits Cl₂˙⁻ formation. Secondly, the rapid generation of Cl˙ in the organic phase, coupled with its significantly higher reaction rate with toluene (Cl˙, k = 10¹⁰ M⁻¹ s⁻¹)⁶⁹ compared to Cl₂˙⁻ (Cl₂˙⁻, k ≤ 10⁶ M⁻¹ s⁻¹),⁷⁰ ensures that even if trace amounts of Cl₂˙⁻ diffuse into the organic phase from the aqueous phase, the Cl₂˙⁻ reactivity remains orders of magnitude slower. We therefore conclude that Cl˙ is the dominant reactive species in this reaction.

Due to the difficulty in observing the TA signals of Cl˙, we turned to observing the transient signals of the complex between Cl˙ and arene. The TA experiment was conducted under operando conditions where Cl₂ is generated from 0.5 M NaCl (pH = 2) aqueous solution in an operating PEC cell. The solution also contained 1 mL of toluene for the detection of transient Cl˙. The characteristic transient absorption signal of the Cl˙-arene adducts (which subsequently form stable chlorinated products^70–72) was observed between 400 and 700 nm upon 355 nm pulsed laser excitation, with a lifetime of about 0.24 μs (Fig. 5c).^73,74 This result confirms the photogeneration of Cl˙ from Cl₂ excitation, and demonstrates the intrinsic reactivity potential of toluene and Cl˙. To further confirm this, ESR measurements were carried out to detect possible radicals. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was used as the spin-trapping reagent to capture the possible radicals generated after the secondary excitation in the two-step coupled system under the experimental conditions. As shown in Fig. 5d, a seven-line ESR spectrum corresponding to DMPOX^75–78 was observed after irradiation, directly indicating the photogeneration of Cl˙ under the secondary light excitation. Recently, Qiu's group⁷⁹ published a study on photoelectrocatalytic Cl˙-mediated C(sp³)–H aminomethylation of hydrocarbons by BiVO₄ photoanodes, which also highlighted the generation of Cl˙ from Cl₂ homolysis, though they employed Me₄NCl as the chlorine source in purely organic solution. In contrast, our work utilizes a more cost-effective and readily available NaCl aqueous solution as the chloride source. The hydrogen atom transfer reaction, initiated by Cl˙ from Cl₂ homolysis, occurs in the organic phase—requiring prior diffusion of stable Cl₂ across the aqueous–organic interface. This spatial separation between the photoelectrode interface and the hydrocarbon substrates, which are prone to direct oxidation, potentially minimizes overoxidation and improves faradaic efficiency. Recently, two additional studies on PEC C–H bonds chlorination have emerged,^43,80 which complement our work both mechanistically and performance-wise, collectively underscoring the significance of PEC chlorine radical generation.

Photoelectrochemical oxygenation of C(sp³)–H bonds

The generation of carbon radicals via Cl˙-mediated C–H activation opens the possibility of direct oxygenation of C–H bonds using molecular O₂. Compared to traditional oxidants such as hypervalent iodine,⁸¹ S₂O₈ (ref. 2–7) and chromium(VI) salts,⁸² which are often toxic, expensive, and environmentally hazardous, molecular O₂ offers a more sustainable and cost-effective alternative with high atom utilization efficiency. Leveraging our strategy for Cl˙ generation through the PEC process, aldehydes, ketones, and alcohols were produced as oxygenated products under an O₂ atmosphere, with minimal formation of chlorinated byproducts (Table 2).

Table 2 PEC oxidation of arene over BiVO₄/TiO₂/CoNi₂O_x photoanode in O₂^a

a Reaction conditions: 20 mL 0.1 M NaCl (pH = 2) solution, 4 mL arene, white LED (100 mW cm⁻²), 1 atm O₂. b FE and selectivities of oxygenated products were determined by GC and NMR. c Besides alcohol (54%) and ketone (7%), chlorinated products with 39% selectivity were detected.

---

Toluene was selected as the model substrate for the oxidation of C–H bonds (Table 2, entry 1, Fig. S22). The primary products were benzaldehyde and benzyl alcohol, with selectivities of 65% and 22%, respectively, and only small amounts of benzoic acid (8%) and chlorinated products (less than 5%). The FE for the main oxygenated product, benzaldehyde, was approximately 71%. It is likely that benzyl alcohol serves as an intermediate oxygenated product in the PEC oxygenation process; however, due to the excess of toluene, the system appears to reach an equilibrium between benzyl alcohol and benzaldehyde. The alicyclic hydrocarbon cyclohexane was also tested as a substrate but it showed much lower selectivity and hence is not listed.

The substrate scope was subsequently explored with a range of aromatic compounds. As observed in the chlorination reactions, electron-donating (Table 2, entry 2) and electron-withdrawing groups (Table 2, entry 3) did not significantly affect the product selectivity, although 4-chlorotoluene exhibited a lower FE (44% for aldehyde formation) due to its higher BDE. Ethylbenzene, which contains weaker secondary C–H bonds, showed higher selectivity for 1-phenylethanol (58%) compared to acetophenone (36%) with a good overall FE (Table 2, entry 4). This outcome is likely due to the preferential formation of alcohols from the more reactive secondary C–H bond, with ketones forming more slowly. Tetralin, on the other hand, displayed lower selectivity for oxygenated products, with 55% selectivity for alcohols, 8% for ketones, and a significant proportion of chlorinated byproducts (39%). This suggests that electrophilic aromatic substitution by Cl₂ competes with the oxygenation process (Table 2, entry 5). These results highlight the potential of Cl˙ in facilitating selective C(sp³)–H bond oxidation under mild conditions, offering a sustainable and efficient approach to oxygenating aromatic hydrocarbons using molecular O₂ in PEC cells.

To determine whether the active species in the oxygenation reaction are Cl˙ and molecular O₂ rather than superoxide radical (O₂˙⁻) or singlet oxygen (¹O₂), we conducted detection and quenching experiments. We used 9,10-anthracenediylbis(methylene)dimalonic acid (ABDA)⁸³ to detect ¹O₂ (Fig. S30a). The absorption of ABDA showed no significant decrease, confirming the absence of ¹O₂. Additionally, ¹O₂ is typically formed via triplet energy transfer from a triplet excited state photosensitizer, which was absent in our system.⁸⁴ Dihydrorhodamine 123 (DHR123) was used as a fluorescent probe to detect O₂˙⁻.⁸⁵ No significant fluorescence was observed at 550 nm during the reaction, indicating the absence of O₂˙⁻ (Fig. S30b). Furthermore, quenching O₂˙⁻ with 1,4-benzoquinone (BQ) resulted in only minor changes in selectivity and FE of the oxygenation products, possibly due to BQ interfering with Cl₂ absorption rather than quenching a reactive intermediate. When a long-pass filter (λ > 405 nm) was used to suppress Cl˙ formation, a marked decrease in the FE of the oxygenated products was observed (Fig. S30c), indicating that chlorine radicals are the key active species.

To unambiguously identify the oxygen source, we conducted H₂¹⁸O isotope labeling experiments and analyzed the products via GC-MS (Fig. S31). The results revealed negligible incorporation of ¹⁸O into benzaldehyde under H₂¹⁸O conditions, confirming that the oxygen in the product does not originate from water. The minor presence of ¹⁸O-labeled benzaldehyde likely arose from PEC-generated oxygen at the photoanode, consistent with the minor O₂ byproducts observed in Fig. 2e and parallel chlorination experiments. Moreover, by simply changing the reaction atmosphere from Ar to O₂, without altering other parameters, we observed a complete transition from chlorinated products to exclusively oxygenated products. If water had been the primary source, oxygenation products should have appeared, even under the Ar atmosphere. However, control experiments under Ar conditions showed exclusive formation of chlorinated products, further reinforcing O₂ as the dominant oxygen source in our system. Combining these results with the absence of oxygenation products under Ar conditions, we conclude that the active species in the oxygenation reaction are Cl˙ and molecular O₂.

Scheme 2 summarizes the photoelectrochemical chlorination and oxygenation of C(sp³)–H bonds under either inert or oxygen environment, driven by the generation of Cl˙ as the key HAT intermediate. The process begins with the efficient generation of Cl₂ through photoelectrochemical oxidation of Cl⁻ in the NaCl aqueous solution following the absorption of the first photon on the BiVO₄/TiO₂/CoNi₂O_x photoanode (blue arrow). Subsequently, the stable Cl₂ diffuses into the organic phase, where it undergoes photoexcitation (light red arrow) to form Cl˙, which serves as the HAT agent, abstracting a hydrogen atom from the C(sp³)–H bond and generating a carbon-centered radical. This radical then reacts with chlorine species or molecular O₂, completing the chlorination or oxygenation of the C–H bonds (Fig. S32). This two-step coupled photoexcitation process in the PEC cell effectively accumulates free energy, allowing the system to circumvent the high reduction potential (E°(Cl˙/Cl⁻)) and generate the reactive Cl˙ intermediate, enabling efficient C(sp³)–H activation at a low applied bias.

Scheme 2 Schematic illustration of the two-step coupled PEC C–H activation process. E_F is the quasi-Fermi level of the photoanode.

Conclusions

In conclusion, we have developed a high-performance BiVO₄/TiO₂/CoNi₂O_x photoanode for photoelectrochemical C(sp³)–H bond functionalization through a novel two-step coupled process. This approach leverages the PEC-driven chlorine radical (Cl˙) generation to achieve both chlorination and oxygenation of hydrocarbons in a facile and sustainable way. The photoanode first demonstrates excellent Cl₂ evolution efficiency with a photocurrent density of 2.9 mA cm⁻² at a low applied bias of 0.8 V vs. RHE, approximately 0.7 V lower than that required for direct electrochemical oxidation of chlorides. The low applied bias brings the advantage of net energy output rather than net energy consumption, achieving a positive applied bias photon-to-current efficiency (ABPE) of up to 3.20%. In the second step of the two-step coupled process, the evolved Cl₂ is subsequently photolyzed to form Cl˙, which is responsible for C(sp³)–H bond activation. Compared to chlorination reaction by direct excitation of chlorine gas which is usually hazardous, we avoided the direct use of hazardous Cl₂ and external expensive oxidants, achieving chlorination reactions directly from Cl⁻ which is cheaper, easier accessible, and nontoxic.

Mechanistic studies revealed that Cl˙ is the key intermediate responsible for initiating C–H bond activation, as confirmed by transient absorption spectroscopy, electron spin resonance and controlled experiments with scavengers. Selective chlorination of aliphatic and aromatic hydrocarbons was achieved under mild conditions with high faradaic efficiency. The system exhibited good selectivity when mediated by Cl˙ rather than Cl₂ or Cl₂˙⁻. Additionally, under O₂ conditions, Cl˙ facilitated the oxygenation of C–H bonds, producing aldehydes, ketones, and alcohols in an environmentally friendly and sustainable manner using molecular O₂. This study offers a promising strategy for advancing solar-driven fine chemical synthesis, providing an efficient route to value-added products from simple hydrocarbons. The low applied bias and high atom utilization efficiency make this approach a sustainable alternative to traditional methods, demonstrating the potential for broader applications in solar-driven energy-demanding synthesis.

Author contributions

W. Yang and Y. Han synthesized the materials. W. Yang and P. Li performed the experiments. W. Yang and Y. Han wrote the manuscript. Z. Zhao, L. Tian and M. G. Humphrey modified the figures and the manuscript. Z. Zhang and Y. Han contributed to the characterizations. K. Hu and C. Zhang conceptualised and supervised the research and contributed to reviewing and editing the manuscript. All authors discussed the results and contributed to the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

Supplementary information: detailed materials and methods; SEM, HRTEM, XRD, XPS spectra; IMPS, EIS, ABPE and LSV curves; NMR, GC and GC-MS spectra. See DOI: https://doi.org/10.1039/d5sc05195a.

Acknowledgements

This study is sponsored by National Key R&D Program of China (2023YFE0124100), National Natural Science Foundation of China (22173022), Fundamental Research Funds for the Central Universities, and the China Postdoctoral Science Foundation (2025T180305, 2024M760508, GZB20240159).

Notes and references

1. D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, A. F. Carley, A. A. Herzing, M. Watanabe, C. J. Kiely, D. W. Knight and G. J. Hutchings, Science, 2006, 311, 362–365.
2. X. Cao, Z. Chen, R. Lin, W.-C. Cheong, S. Liu, J. Zhang, Q. Peng, C. Chen, T. Han, X. Tong, Y. Wang, R. Shen, W. Zhu, D. Wang and Y. Li, Nat. Catal., 2018, 1, 704–710.
3. R. G. Bergman, Nature, 2007, 446, 391–393.
4. D. Cantillo, Curr. Opin. Electrochem., 2024, 44, 101459.
5. P. S. Baran and Y.-L. Zhong, J. Am. Chem. Soc., 2001, 123, 3183–3185.
6. Z. Zhang, X. Song, G. Li, X. Li, D. Zheng, X. Zhao, H. Miao, G. Zhang and L. Liu, Chin. Chem. Lett., 2021, 32, 1423–1426.
7. C. Huang, J.-H. Wang, J. Qiao, X.-W. Fan, B. Chen, C.-H. Tung and L.-Z. Wu, J. Org. Chem., 2019, 84, 12904–12912.
8. L. Ding, J. Tang, M. Cui, C. Bo, X. Chen and X. Qiao, Ind. Eng. Chem. Res., 2011, 50, 11143–11152.
9. S. M. Maddox, C. J. Nalbandian, D. E. Smith and J. L. Gustafson, Org. Lett., 2015, 17, 1042–1045.
10. S. Song, X. Li, J. Wei, W. Wang, Y. Zhang, L. Ai, Y. Zhu, X. Shi, X. Zhang and N. Jiao, Nat. Catal., 2020, 3, 107–115.
11. W. Liu and J. T. Groves, J. Am. Chem. Soc., 2010, 132, 12847–12849.
12. L. Kesavan, R. Tiruvalam, M. H. A. Rahim, M. I. bin Saiman, D. I. Enache, R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor, D. W. Knight, C. J. Kiely and G. J. Hutchings, Science, 2011, 331, 195–199.
13. A. K. Cook, S. D. Schimler, A. J. Matzger and M. S. Sanford, Science, 2016, 351, 1421–1424.
14. R. A. Periana, O. Mironov, D. Taube, G. Bhalla and C. J. Jones, Science, 2003, 301, 814–818.
15. X. Yang, Q.-L. Yang, X.-Y. Wang, H.-H. Xu, T.-S. Mei, Y. Huang and P. Fang, J. Org. Chem., 2020, 85, 3497–3507.
16. Y.-C. Wu, R.-J. Song and J.-H. Li, Org. Chem. Front., 2020, 7, 1895–1902.
17. L. Buglioni, F. Raymenants, A. Slattery, S. D. A. Zondag and T. Noël, Chem. Rev., 2022, 122, 2752–2906.
18. L. Qian and M. Shi, Chem. Commun., 2023, 59, 3487–3506.
19. D. M. Schultz and T. P. Yoon, Science, 2014, 343, 1239176.
20. Y.-J. Chen, T. Lei, H.-L. Hu, H.-L. Wu, S. Zhou, X.-B. Li, B. Chen, C.-H. Tung and L.-Z. Wu, Matter, 2021, 4, 2354–2366.
21. J. P. Barham and B. König, Angew. Chem., Int. Ed., 2020, 59, 11732–11747.
22. T. Tong, M. Zhang, W. Chen, X. Huo, F. Xu, H. Yan, C. Lai, W. Wang, S. Hu, L. Qin and D. Huang, Coord. Chem. Rev., 2024, 500, 215498.
23. L. Zhang, L. Liardet, J. Luo, D. Ren, M. Grätzel and X. Hu, Nat. Catal., 2019, 2, 366–373.
24. T. Li, T. Kasahara, J. He, K. E. Dettelbach, G. M. Sammis and C. P. Berlinguette, Nat. Commun., 2017, 8, 390.
25. H. Tateno, S. Iguchi, Y. Miseki and K. Sayama, Angew. Chem., Int. Ed., 2018, 57, 11238–11241.
26. J.-H. Wang, X.-B. Li, J. Li, T. Lei, H.-L. Wu, X.-L. Nan, C.-H. Tung and L.-Z. Wu, Chem. Commun., 2019, 55, 10376–10379.
27. Z. Li, L. Luo, M. Li, W. Chen, Y. Liu, J. Yang, S.-M. Xu, H. Zhou, L. Ma, M. Xu, X. Kong and H. Duan, Nat. Commun., 2021, 12, 6698.
28. Y. He, Z. Huang, K. Wu, J. Ma, Y.-G. Zhou and Z. Yu, Chem. Soc. Rev., 2022, 51, 2759–2852.
29. S. K. Kariofillis and A. G. Doyle, Acc. Chem. Res., 2021, 54, 988–1000.
30. Z. Chen, M.-Y. Rong, J. Nie, X.-F. Zhu, B.-F. Shi and J.-A. Ma, Chem. Soc. Rev., 2019, 48, 4921–4942.
31. Z. Yang, W. Shi, H. Alhumade, H. Yi and A. Lei, Nat. Synth., 2023, 2, 217–230.
32. Y. Kawamata, M. Yan, Z. Liu, D.-H. Bao, J. Chen, J. T. Starr and P. S. Baran, J. Am. Chem. Soc., 2017, 139, 7448–7451.
33. F. Wang and S. S. Stahl, Acc. Chem. Res., 2020, 53, 561–574.
34. H. Gao, Z. Zha, Z. Zhang, H. Ma and Z. Wang, Chem. Commun., 2014, 50, 5034–5036.
35. E. J. Horn, B. R. Rosen, Y. Chen, J. Tang, K. Chen, M. D. Eastgate and P. S. Baran, Nature, 2016, 533, 77–81.
36. L. Niu, C. Jiang, Y. Liang, D. Liu, F. Bu, R. Shi, H. Chen, A. D. Chowdhury and A. Lei, J. Am. Chem. Soc., 2020, 142, 17693–17702.
37. H. Huang, Z. M. Strater and T. H. Lambert, J. Am. Chem. Soc., 2020, 142, 1698–1703.
38. L. Zhang, Y. Fu, Y. Shen, C. Liu, M. Sun, R. Cheng, W. Zhu, X. Qian, Y. Ma and J. Ye, Nat. Commun., 2022, 13, 4138.
39. D. E. Essayan, M. J. Schubach, J. M. Smoot, T. Puri and S. V. Pronin, J. Am. Chem. Soc., 2024, 146, 18224–18229.
40. C. Yang, L. A. Farmer, E. C. McFee, R. K. Jha, S. Maldonado, D. A. Pratt and C. R. J. Stephenson, Angew. Chem., Int. Ed., 2024, 63, e202315917.
41. S. Han, C. Cheng, M. He, R. Li, Y. Gao, Y. Yu, B. Zhang and C. Liu, Angew. Chem., Int. Ed., 2023, 62, e202216581.
42. G.-X. Dong, M.-R. Zhang, S.-X. Yuan, M. Zhang and T.-B. Lu, Angew. Chem., Int. Ed., 2025, 64, e202510993.
43. Y. Tao, J. Ding, Z. Teng, Q. Xu, W. Ou, H. Sun, S. Li, L. Yu, G. Li, B. Liu and C. Su, J. Am. Chem. Soc., 2025, 147, 18814–18825.
44. Q. Yang, Y.-H. Wang, Y. Qiao, M. Gau, P. J. Carroll, P. J. Walsh and E. J. Schelter, Science, 2021, 372, 847–852.
45. M. I. Gonzalez, D. Gygi, Y. Qin, Q. Zhu, E. J. Johnson, Y.-S. Chen and D. G. Nocera, J. Am. Chem. Soc., 2022, 144, 1464–1472.
46. T. Liang, Z. Lyu, Y. Wang, W. Zhao, R. Sang, G.-J. Cheng and F. Ye, Nat. Chem., 2025, 17, 598–605.
47. L. Troian-Gautier, M. D. Turlington, S. A. M. Wehlin, A. B. Maurer, M. D. Brady, W. B. Swords and G. J. Meyer, Chem. Rev., 2019, 119, 4628–4683.
48. W. Li, J. Liu, M. Zhou, L. Ma and M. Zhang, Org. Biomol. Chem., 2022, 20, 6667–6672.
49. Q. Zhang, B. An, Y. Lei, Z. Gao, H. Zhang, S. Xue, X. Jin, W. Xu, Z. Wu, M. Wu, X. Yang and W. Wu, Angew. Chem., Int. Ed., 2023, 62, e202304699.
50. R. Sun, Z. Zhu, N. Tian, Y. Zhang and H. Huang, Angew. Chem., Int. Ed., 2024, 63, e202408862.
51. H. Wang, Z. Chen, Y. Shang, C. Lv, X. Zhang, F. Li, Q. Huang, X. Liu, W. Liu, L. Zhao, L. Ye, H. Xie and X. Jin, ACS Catal., 2024, 14, 5779–5787.
52. T. W. Kim and K.-S. Choi, Science, 2014, 343, 990–994.
53. D. Liu, J.-C. Liu, W. Cai, J. Ma, H. B. Yang, H. Xiao, J. Li, Y. Xiong, Y. Huang and B. Liu, Nat. Commun., 2019, 10, 1779.
54. E. Usman, M. Barzgar Vishlaghi, A. Kahraman, N. Solati and S. Kaya, ACS Appl. Mater. Interfaces, 2021, 13, 60602–60611.
55. Q. Wang, T. Li, C. Yang, M. Chen, A. Guan, L. Yang, S. Li, X. Lv, Y. Wang and G. Zheng, Angew. Chem., Int. Ed., 2021, 60, 17398–17403.
56. C. Zachäus, F. F. Abdi, L. M. Peter and R. van de Krol, Chem. Sci., 2017, 8, 3712–3719.
57. H. Iwami, M. Okamura, M. Kondo and S. Masaoka, Angew. Chem., Int. Ed., 2021, 60, 5965–5969.
58. M. Kan, D. Xue, A. Jia, X. Qian, D. Yue, J. Jia and Y. Zhao, Appl. Catal., B, 2018, 225, 504–511.
59. Z. Chen, T. F. Jaramillo, T. G. Deutsch, A. Kleiman-Shwarsctein, A. J. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E. W. McFarland, K. Domen, E. L. Miller, J. A. Turner and H. N. Dinh, J. Mater. Res., 2010, 25, 3–16.
60. R. K. Quinn, Z. A. Könst, S. E. Michalak, Y. Schmidt, A. R. Szklarski, A. R. Flores, S. Nam, D. A. Horne, C. D. Vanderwal and E. J. Alexanian, J. Am. Chem. Soc., 2016, 138, 696–702.
61. P. Xu, P.-Y. Chen and H.-C. Xu, Angew. Chem., Int. Ed., 2020, 59, 14275–14280.
62. N. Fu, G. S. Sauer and S. Lin, J. Am. Chem. Soc., 2017, 139, 15548–15553.
63. G. Zhang, Z.-A. Lan, L. Lin, S. Lin and X. Wang, Chem. Sci., 2016, 7, 3062–3066.
64. S. Chen, J. Lin, J. Huang, T. Pang, Q. Ye, Y. Zheng, X. Li, Y. Yu, B. Zhuang and D. Chen, Adv. Funct. Mater., 2024, 34, 2309293.
65. V. Nagarajan and R. W. Fessenden, J. Phys. Chem., 1985, 89, 2330–2335.
66. X.-Y. Yu and J. R. Barker, J. Phys. Chem. A, 2003, 107, 1313–1324.
67. M. L. Alegre, M. Geronés, J. A. Rosso, S. G. Bertolotti, A. M. Braun, D. O. Mártire and M. C. Gonzalez, J. Phys. Chem. A, 2000, 104, 3117–3125.
68. Y. Lei, S. Cheng, N. Luo, X. Yang and T. An, Environ. Sci. Technol., 2019, 53, 11170–11182.
69. K. Zhang and K. M. Parker, Environ. Sci. Technol., 2018, 52, 9579–9594.
70. D. O. Mártire, J. A. Rosso, S. Bertolotti, G. C. Le Roux, A. M. Braun and M. C. Gonzalez, J. Phys. Chem. A, 2001, 105, 5385–5392.
71. M. L. Alegre, M. Geronés, J. A. Rosso, S. G. Bertolotti, A. M. Braun, D. O. Mártire and M. C. Gonzalez, J. Phys. Chem. A, 2000, 104, 3117–3125.
72. Y. Lei, X. Lei, P. Westerhoff, X. Zhang and X. Yang, Environ. Sci. Technol., 2021, 55, 689–699.
73. Q. An, Y.-Y. Xing, R. Pu, M. Jia, Y. Chen, A. Hu, S.-Q. Zhang, N. Yu, J. Du, Y. Zhang, J. Chen, W. Liu, X. Hong and Z. Zuo, J. Am. Chem. Soc., 2023, 145, 359–376.
74. S. Förgeteg and T. Bérces, J. Photochem. Photobiol. A, 1993, 73, 187–195.
75. Y. Zhang, J. Li, J. Bai, Z. Shen, L. Li, L. Xia, S. Chen and B. Zhou, Environ. Sci. Technol., 2018, 52, 1413–1420.
76. R. Song, H. Wang, M. Zhang, Y. Liu, X. Meng, S. Zhai, C.-c. Wang, T. Gong, Y. Wu, X. Jiang and W. Bu, Angew. Chem., Int. Ed., 2020, 59, 21032–21040.
77. T. Li, Y. Jiang, X. An, H. Liu, C. Hu and J. Qu, Water Res., 2016, 102, 421–427.
78. L. Wang, J. Guan, H. Han, M. Yao, J. Kang, M. Peng, D. Wang, J. Xu and J. Hao, Appl. Catal., B, 2022, 306, 121130.
79. A. Shi, P. Xie, Y. Wang and Y. Qiu, Nat. Commun., 2025, 16, 2322.
80. S. Y. Chae, A. Mehmood and E. D. Park, J. Am. Chem. Soc., 2025, 147, 19472–19477.
81. M. Uyanik, M. Akakura and K. Ishihara, J. Am. Chem. Soc., 2009, 131, 251–262.
82. A. K. Das, Coord. Chem. Rev., 2004, 248, 81–99.
83. C. Felip-León, M. Puche, J. F. Miravet, F. Galindo and M. Feliz, Mater. Lett., 2019, 251, 45–51.
84. Y. Wang, Y. Lin, S. He, S. Wu and C. Yang, J. Hazard. Mater., 2024, 461, 132538.
85. L. Yu, Y. Xu, Z. Pu, H. Kang, M. Li, J. L. Sessler and J. S. Kim, J. Am. Chem. Soc., 2022, 144, 11326–11337.

---