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• In typical advanced oxidation processes (AOPs), reactive oxidizing radicals such as hydroxyl radical (^•OH) and superoxide radical (^•O₂⁻) are formed upon excitation by light and electricity to break chemical bonds of compound molecules [1]. Common AOPs including UV/H₂O₂ [2], electrochemical catalysis [3], and photocatalysis [4,5] exhibit strong pollutant removal efficiencies due to radical production. Understanding the underlying mechanisms of degradation requires differentiating the contributing radicals that play major roles in these processes. Quenching experiments using scavengers are often employed as they do not require expensive reagents [6-8]. The most common scavenger-radical pairs are isopropanol (IPA) for ^•OH, p-benzoquinone (pBQ) for ^•O₂⁻ and so on [9,10]. The reaction between IPA and ^•OH removes the radicals but in the meantime results in the formation of alcohol radicals (HO-R^•) that are often overlooked [8,11,12]. This overlooked generation of alcohol radicals can lead to misinterpretation of degradation efficiency and mechanisms [13,14].

  The ^•OH abstracts a hydrogen atom from the α-carbon of alcohols to generate alcohol radicals, typical carbon-centered radicals that possess reactivity [12]. Alcohol radicals have a strong reducing capacity with a negative reduction potential ranging from −1.39 to −1.18 V, thus promoting the degradation of halogenated pollutants [15-17]. Additionally, alcohol radicals would initiate subsequent reactions, such as reacting with dissolved oxygen (DO) to produce hydrogen peroxide (H₂O₂), a known ^•OH precursor [13]. This might lead to overestimating the contribution of ^•OH in pollutant removal in the UV/H₂O₂ system. Among heterogeneous catalysis, photocatalysis could be the technology of choice to tackle environmental and energy issues, with the generation of highly reactive free radicals due to photogenerated electron (e⁻) - hole (h⁺) pairs in catalysts [18-22]. Moreover, the reactions of alcohols with the photogenerated h⁺ of the catalysts could form alcohol radicals due to the oxidizing properties of h⁺ [23,24]. These alcohol radicals then react with DO to produce H₂O₂, potentially promoting the degradation of pollutants by increasing the concentration of active species. However, the reaction and role of alcohols in photocatalysis systems have not been comprehensively investigated.

  To elucidate the role of alcohols in photocatalysis, a series of catalysts with varying concentrations of photogenerated h⁺ are needed. Bismuth oxybromide (BiOBr) is an attractive candidate for photocatalytic applications (such as oxygen evolution, hydrogen evolution, CO₂ reduction, and pollutant removal) owing to its appealing energy band structure and unique layered structure [25,26]. The [Bi₂O₂]²⁺ layers of BiOBr contain a high density of oxygen atoms, and the weaker Bi-O bond can be stripped of oxygen atoms to create oxygen vacancies (OVs) in the presence of ethylene glycol (EG) [27,28]. Oxygen vacancies act as electron mediators to accelerate the separation of photogenerated carriers, promoting the generation of h⁺ and allowing electrons and holes with powerful oxidation and reduction abilities [29-31]. With adjustable h⁺ characteristics, BiOBr can regulate the production of alcohol radicals in the photocatalytic process.

  Herein, four types of BiOBr with different amounts of OVs were synthesized to explore the role of alcohols in the photocatalytic process. The OVs and active species of BiOBr were characterized by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The effect of IPA in the photocatalytic process was investigated based on the degradation of carbamazepine (CBZ), a commonly used antidepressant, which poses ecological risks. The generation of alcohol radicals and promotional mechanism of IPA was verified by EPR characterizations and scavenging. Furthermore, alcohol structure (including the length of the carbon chain, the position and number of hydroxyl groups) and influencing factors (such as alcohol dosage, DO, and solution pH) were considered while examining the promotional effect of alcohols.

  By varying the ratios of EG to H₂O, BiOBr samples with various amounts of OVs were synthesized and named BiOBr-1, BiOBr-2, BiOBr-3, and BiOBr-4 (Fig. 1a). Representative scanning electron microscopy (SEM) images showed that the morphologies of BiOBr gradually changed from spheres to irregular aggregates, thick squares, and nanosheets as the ratio of EG to H₂O decreased from 70:0 to 0:70 (Fig. S1 in Supporting information). This can be attributed to hydrogen ions generated by bismuth nitrate hydrolysis being adsorbed onto the oxygen atom of the {001} facet, thereby inhibiting the growth direction of BiOBr [32]. The X-ray diffraction (XRD) patterns of the samples depicted in Fig. 1b confirmed that BiOBr was successfully prepared and that all four samples exhibited a tetragonal phase (JCPDS No. 85–0862). The increased intensity of the {001} facet of BiOBr indicated that the facet was more exposed, corresponding to the changes in morphology. The variation of lattice fringe spacing of BiOBr further confirmed the variation of the exposed facet of BiOBr (Fig. S2 in Supporting information). The top facet of BiOBr-4 showed a lattice fringe with a spacing of 0.277 nm, corresponding to the {110} planes of BiOBr (Fig. S2d). Considering the tetragonal symmetry of BiOBr, the highly exposed surfaces of BiOBr-4 are identified as {001} facets, while the lateral surfaces are {110} and {100} facets [33]. XPS was used to analyze the elemental composition and chemical states of BiOBr (Text S7 in Supporting information). As shown in Fig. 1c, the peak at 530.05 eV was attributed to lattice oxygen (O₂⁻), the peak at 532.67 eV was attributed to H₂O and OH⁻ adsorbed on the sample surface, and the peak at 531.69 eV was attributed to OVs [34]. The XPS results confirmed the presence of OVs in BiOBr, and the discrepancy in peak area indicated a variation concentration of OVs. The amounts of OVs in each BiOBr were subsequently determined by EPR. Fig. 1d shows the signal intensity order of OVs as BiOBr-3 > BiOBr-2 > BiOBr-1 > BiOBr-4, confirming that the series of BiOBr possessed different amounts of OVs. Previous research has suggested that OVs affect the amount of effective photogenerated h⁺ of BiOBr, thus influencing the generation of active species [35,36]. Various active species may affect the role of alcohols in heterogeneous catalytic systems.

  Figure 1

  

  Figure 1.  Synthesis, characterizations, and photocatalytic reactivity of BiOBr. (a) Schematic diagram of the hydrothermal synthesis of BiOBr with different ratios of ethylene glycol (EG) to H₂O. (b) XRD, (c) XPS, and (d) EPR patterns of BiOBr. (e) Removal rates of CBZ by BiOBr with and without the presence of IPA. The addition of IPA significantly reduced the catalytic reactivity of BiOBr-1, but boosted the catalytic reactivity of BiOBr-3 (highlighted in red dash boxes). All data are presented as the means ± SD, *P < 0.05 and **P < 0.01 compared to the control. Reaction conditions: [BiOBr]= 0.6 g/L, [CBZ]₀ = 10 mg/L, [IPA]=50 μmol/L. The removal rate of CBZ was calculated based on high-performance liquid chromatography (Text S5 in Supporting information).

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  IPA was found to play different roles in the degradation of CBZ by a series of BiOBr (Fig. 1e). The degradation of CBZ by BiOBr-3 was significantly promoted by IPA (P < 0.05) (highlighted in red dash boxes), increasing by about 10%. In contrast, the removal rate of CBZ by BiOBr-1 was significantly lower than that of the control group without IPA (P < 0.01), decreasing by 10%. The degradation of CBZ by BiOBr-2 and BiOBr-4 was not affected by IPA. This may be related to differences in the types and concentrations of active species produced by BiOBr, as active species oxidize IPA to form alcohol radicals [37,38]. Alcohol radicals could affect the transformation of radicals in the original system, thereby influencing degradation efficiency.

  EPR was used to identify the active species generated by BiOBr and elucidate the reactions between IPA and active species (Text S8 in Supporting information). As shown in Fig. 2a, a characteristic EPR signal of DMPO–^•O₂⁻ (quartet peaks) was observed across all four types of BiOBr. Additionally, the EPR signal of TEMP-singlet oxygen (¹O₂), with a peak intensity of 1:1:1, was detected (Fig. 2b). These results indicated that BiOBr-1 generated higher concentrations of ^•O₂⁻ and ¹O₂ compared to the other three BiOBr, which might account for its higher degradation efficiency on CBZ as observed in Fig. 1e. BiOBr-1 has the smallest band gap and the strongest light response, which is conducive to the production of high concentrations of ROS (Fig. S5 in Supporting information). The EPR signal of DMPO-^•OH was observed in BiOBr-1 and BiOBr-2, while the signal was not present in BiOBr-3 and BiOBr-4 (Fig. 2c). IPA, a common ^•OH scavenger due to its high reactivity with these radicals [7,8], could inhibit CBZ degradation by quenching ^•OH. Conversely, the absence of ^•OH in BiOBr-3 and BiOBr-4 systems explained why IPA did not affect their degradation performance. Moreover, all BiOBr produced photogenerated h⁺, with BiOBr-3 having the highest yield due to its high concentration of OVs (Fig. 2d). At the same time, BiOBr-3 has the smallest electrical impedance radius and the strongest photocurrent signal, which confirm the excellent separation of photogenerated electron-hole pairs (Fig. S9 in Supporting information). This might partly explain the enhanced CBZ degradation efficiency observed with BiOBr-3 upon the addition of IPA as photogenerated h⁺ can react with alcohol to form alcohol radicals [39]. Furthermore, the promotional effect of IPA also existed in the degradation of oxytetracycline and doxycycline by BiOBr-3, confirming that the promotional role of alcohol was not accidental (Fig. S10 in Supporting information).

  Figure 2

  

  Figure 2.  Identifications of active species generated by BiOBr based on electron paramagnetic resonance (EPR) spectra. EPR spectra of (a) DMPO-^•O₂⁻ (methanol system), (b) TEMP-¹O₂, (c) DMPO-^•OH (water system), and (d) TEMPO-h⁺ generated by BiOBr. Four types of BiOBr showed different levels of active species generated under visible light irradiation. (e) EPR spectra evidenced the alcohol radicals' generation by BiOBr-3 under visible light irradiation after IPA was added. (f) EPR spectra demonstrated that h⁺, ^•O₂⁻, and ¹O₂ all contribute to the generation of alcohol radicals in the presence of IPA, with h⁺ contributing the most. By adding scavengers of any two active species (h⁺, ^•O₂⁻, and ¹O₂) and DMPO for alcohol radicals capture, the contribution of the remaining active species to the generation of alcohol radicals can be determined. Reaction conditions: [BiOBr] =0.6 g/L, [IPA]= 50 μmol/L, [Trapping agent] =50 mmol/L, [FFA]= 0.5 mmol/L, [pBQ]= 0.05 mmol/L, [EDTA-2Na]=0.05 mmol/L.

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  The formation of alcohol radicals by BiOBr-3 with IPA was confirmed by EPR. EPR result revealed the formation of a new DMPO-adduct signal with a peak intensity ratio of 1:1:1:1:1:1 (Fig. 2e), characteristic of DMPO–HO-R^• radicals, as indicated by their hyperfine splitting constants (α_H = 22.7 G, α_N = 15.7 G) [40]. The exclusive appearance of this signal under photoexcitation suggested that IPA was oxidized to alcohol radicals by the active species generated by BiOBr-3. Moreover, IPA could be oxidized into alcohol radicals independently by h⁺, ^•O₂⁻, and ¹O₂ (Fig. 2f). Among these, the DMPO–HO-R^• signal generated by h⁺ was the most prominent, followed by those from ^•O₂⁻ and ¹O₂. This indicated that h⁺ played the most significant role in the production of alcohol radicals among active species generated by BiOBr-3. The reaction between alcohol and h⁺ to form alcohol radicals can be summarized by reaction 1. These generated alcohol radicals, being strong one-electron reductants, rapidly react with DO to form corresponding peroxy free radicals (reaction 2) [39,41]. Subsequently, a reaction between peroxyl alcohol radicals leads to the formation of H₂O₂ and other products (reaction 3) [11,41,42]. The occurrence of these chain reactions is suggested to be a prerequisite for IPA's role as a promoter.

  $ \begin{aligned} \text { HO-} \mathrm{RH}+\mathrm{h}^{+} \rightarrow & \text { HO-R}^{•}+ \text { other products, } \\ & k＜6.2 \times 10^8 \mathrm{~L} \mathrm{~mol}^{-1} \mathrm{~s}^{-1} \end{aligned} $

  (1)

  $ \text { HO-}\mathrm{R}^{•}+\mathrm{O}_2 \rightarrow \text { HO-}\mathrm{ROO}^{•}, \mathrm{k} \approx 4.9 \times 109 \mathrm{~L} \mathrm{~mol}^{-1} \mathrm{~s}^{-1} $

  (2)

  $ 2 \text { HO-} \mathrm{ROO}^• \rightarrow \mathrm{H}_2 \mathrm{O}_2+\text { other products } \mathrm{k} \approx 2.1 \times 109 \mathrm{~L} \mathrm{~mol}^{-1} \mathrm{~s}^{-1} $

  (3)

  To validate the feasibility of these pathways, the effect of varying alcohol dosages (12.5–200 µmol/L) on the degradation of CBZ by BiOBr-3 was examined. As shown in Fig. 3a, the removal rates of CBZ initially increased, followed by a decline, and eventually reached a plateau as the IPA dosage increased from 12.5 µmol/L to 200 µmol/L. The removal rate of CBZ by BiOBr-3 was notably enhanced compared to the control when the IPA dose reached 50 µmol/L. Beyond 50 µmol/L, further increase in IPA dosage resulted in a diminished promotional effect on CBZ degradation by BiOBr-3, stabilizing at a certain level. These results confirmed that IPA acted as a promoter within a certain dosage range in photocatalysis. The inhibitory effect observed at low IPA dosages might be attributed to a reduction in H₂O₂ generation. At high IPA dosages, the promotional effect weakened, likely due to the produced H₂O₂ being preferentially quenched by the excess alcohols instead of interacting with pollutants. To substantiate this hypothesis, the yield of H₂O₂ by BiOBr-3 at varying IPA concentrations was evaluated (Text S6 in Supporting information). The trend in H₂O₂ production (Fig. 3b) mirrored that of the CBZ removal rate, indicating that IPA influences CBZ degradation by affecting H₂O₂ production. This confirmed the crucial role of alcohol and its dosage in initiating chain reactions.

  Figure 3

  

  Figure 3.  Factors affecting the degradation of CBZ by BiOBr-3 with the presence of IPA. Influences of IPA concentrations (0–200 µmol/L) on the (a) removal rates of CBZ and (b) yield of H₂O₂ by BiOBr-3. Influences of dissolved oxygen on the (c) removal rates of CBZ and (d) yield of H₂O₂ by BiOBr-3. IPA and dissolved oxygen are the key factors affecting the reaction of H₂O₂ formation from alcohols. EPR spectra of (e) DMPO-^•O₂⁻ (methanol system) and (f) TEMP-¹O₂ generated by BiOBr-3 with and without IPA added. The addition of IPA enabled BiOBr-3 to produce more ^•O₂⁻ and ¹O₂. All data are presented as the means ± SD, P < 0.05, **P < 0.01, and ***P < 0.001 compared to control. µM = µmol/L in (b, d). Reaction conditions: [BiOBr-3] = 0.6 g/L, [CBZ]₀ = 10 mg/L, [IPA]= 50 μmol/L, [Trapping agent] = 50 mmol/L.

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  To investigate the role of DO in the promotional pathway of alcohols, the effect of DO on removal rates of CBZ and yield of H₂O₂ by BiOBr-3 were studied. As shown in Fig. 3c, the removal rate of CBZ by BiOBr-3 increased in the IPA+O₂ group (high-DO) and decreased in the IPA+N₂ group (low-DO) compared to the IPA group (medium-DO) (P < 0.05). This finding suggested that higher O₂ levels enhanced the promotional effect of IPA on CBZ degradation by BiOBr-3, possibly because increased DO facilitated the conversion of alcohol radicals to H₂O₂. The H₂O₂ yield was higher in the IPA+O₂ group and lower in the IPA+N₂ group than in the IPA group (Fig. 3d), demonstrating that DO play a significant role in converting alcohol radicals into H₂O₂. The consistency between the trends in H₂O₂ generation and CBZ removal rates indicated that H₂O₂ production significantly influenced CBZ degradation by BiOBr-3.

  Given that the quantity of reactive oxygen species (ROS) determined the degradation efficiency of pollutants in catalytic systems [43], it was reasonable to speculate that H₂O₂ generated through chain reactions involving IPA led to increased ROS production. EPR spectroscopy was performed to compare the signal intensity of ROS in BiOBr-3 with and without the presence of IPA. The addition of IPA resulted in stronger signals for both DMPO-^•O₂⁻ and TEMP-¹O₂ (Figs. 3e and f). This indicated that the sequential reactions among alcohol, h⁺, and O₂ enhanced the production of ^•O₂⁻ and ¹O₂, as O₂ could react with alcohol and water to produce ^•O₂⁻ (as shown in reactions 4 and 5) [14]. Moreover, ^•O₂⁻ could react with photogenerated h⁺ to form ¹O₂ (reaction 6) [44]. Additionally, radical quenching experiments identified the dominant reactive species of BiOBr-3 involved in CBZ degradation. Furfuryl alcohol (FFA), p-benzoquinone (pBQ), and ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) were used as radical scavengers due to their higher reaction constants with ¹O₂, ^•O₂⁻, and h⁺ [43,45]. The removal rate of CBZ decreased from 49.70% to 37.05%, 4.85%, and 13.65% upon the addition of 0.05 mmol/L FFA, pBQ, and EDTA-2Na, respectively (Fig. S11 in Supporting information). This indicated that ^•O₂⁻ was the dominant reactive species for CBZ degradation. The chain reactions between alcohol and h⁺ led to the generation of ^•O₂⁻, which was the primary reason IPA promoted CBZ degradation by BiOBr-3.

  $ \text{HO-}\mathrm{ROO}^{•} \rightarrow \mathrm{CH}_2 \mathrm{O}+{ }^{•} \mathrm{O}_2^{-}+\mathrm{H}^{+} $

  (4)

  $ ^• \mathrm{OH}+\mathrm{H}_2 \mathrm{O}_2 \rightarrow {^•\mathrm{O}_2^{-}}+\mathrm{H}_2 \mathrm{O} $

  (5)

  $ ^• \mathrm{O}_2^{-}+\mathrm{h}^{+} \rightarrow{ }^1 \mathrm{O}_2 $

  (6)

  Thus, IPA promoted the photocatalytic degradation of CBZ by BiOBr-3 due to the following reasons: (1) BiOBr-3 did not produce ^•OH under photoexcitation; (2) h⁺ contributed most significantly to the generation of alcohol radicals; (3) the reaction between IPA and h⁺ induced the formation of ^•O₂⁻, the dominant radical for CBZ degradation. This analysis also explains IPA did not affect CBZ degradation by BiOBr-2 and BiOBr-4. The quenching of ^•OH by IPA was compensated by ROS generated from h⁺ reacting with IPA due to a higher concentration of h⁺ in BiOBr-2 compared to BiOBr-1 (Fig. 2d). Since BiOBr-4 did not produce ^•OH, IPA did not inhibit CBZ degradation efficiency via ^•OH quenching. Furthermore, the lower h⁺ production by BiOBr-4 compared to BiOBr-3 explains why IPA did not enhance CBZ degradation efficiency in BiOBr-4.

  To further assess the promotional effect of alcohols on CBZ degradation by BiOBr-3, the influence of the length of carbon chain, the numbers of hydroxyl groups and their configurations, and the solution pH were investigated. Methanol (MeOH), ethanol (EtOH), propanol (PA), and butanol (BA) were found to affect the removal rate of CBZ by BiOBr-3 and the yield of the H₂O₂ (Figs. S12a and b in Supporting information). It is suggested that the carbon chain length of alcohol affects its action. Moreover, the removal rate of CBZ exhibited a gradual decrease with increasing carbon chain lengths of alcohols, and a similar trend was observed in the yield of H₂O₂ (Fig. 4a). These results confirmed that other alcohols also affected the degradation of CBZ by BiOBr-3 by influencing the formation of H₂O₂. The promotional effect of various alcohols may be related to their bond dissociation energy (BDE) of the C–H bond adjacent to the hydroxyl group [46], which determines the reactivity of alcohol radicals. As shown in Fig. 4d, the BDE of the C–H bond decreased with increasing carbon chain lengths of alcohols, which was consistent with the sequence of CBZ degradation (MeOH > EtOH > PA > BA). This indicated that the carbon chain length of alcohols regularly affected the promotional effects through the BDE of the C–H bond. The larger the BDE of the C–H bond in alcohols, the stronger the reactivity of alcohol radicals, and the more H₂O₂ produced, resulting in a greater promotional effect.

  Figure 4

  

  Figure 4.  Delineating the effects of different alcohols on their promotional effects. (a) The effects of increasing carbon chain lengths of alcohols on their promotional effects. (b) The effects of primary, secondary, and tertiary alcohols on their promotional effects. (c) The effects of numbers of hydroxyl groups on promotional effects of alcohols. (d) The effects of methanol, ethanol, propanol, and butanol on the removal rates of CBZ by BiOBr-3 were attributed to the C–H bond dissociation energy of alcohols. (e) The effects of n-butanol, sec–butanol, and tert–butanol on the removal rates of CBZ by BiOBr-3 were partly attributed to the C–H bond dissociation energy of alcohols. µM = µmol/L in (a-c). Reaction conditions: [BiOBr-3] = 0.6 g/L, [CBZ]₀ = 10 mg/L, [Alcohols]= 50 μmol/L.

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  n-Butanol (BA), sec–butanol (SBA), and tert–butanol (TBA) were selected to assess the effects of the position of the hydroxyl group on the promotional effects of alcohols. As shown in Fig. S13a (Supporting information), the removal rates of CBZ by BiOBr-3 were significantly higher than that of the control group (P < 0.05) with the presence of SBA and TBA, while BA had little influence on the removal rates of CBZ by BiOBr-3. This might be attributed that SBA and TBA significantly promoted the yield of H₂O₂ (P < 0.05), while BA had little effect (Fig. S13b in Supporting information). Both the removal rate of CBZ and the yield of H₂O₂ exhibited a gradual increase as the hydroxyl group position of the alcohols changed from primary to secondary to tertiary carbon (Fig. 4b). This demonstrated that the hydroxyl group position of alcohols regularly affected the promotional effect by influencing the formation of H₂O₂, and the order of promotional effect was TBA > SBA > BA. However, this order was slightly different from the order of BDE (TBA > BA > SBA) in corresponding alcohols (Fig. 4e). This confirmed that additional factors were involved in the process beyond the BDE of the C–H bond. It might be the result of the steric hindrance effect [47], affecting the proximity of the active species to the C–H bond. In conclusion, the promotional effect of alcohols with different hydroxyl group positions was governed by the combined effect of BDE and steric hindrance.

  Propanol, propanediol, and glycerol were selected to examine the effects of the number of hydroxyl groups on their promotional effects. The removal rate of CBZ was found to be significantly increased in the presence of propanediol (P < 0.01), while no significant effect was observed with propanol and glycerol (Fig. S14a in Supporting information). It indicated that the number of hydroxyl groups affects the action of alcohol. As the number of hydroxyl groups increases, the promotional effect of alcohols on the degradation of CBZ by BiOBr-3 initially increased and then decreased, while the yield of H₂O₂ remained constant (Fig. 4c). This indicated that the effect of the quantity of hydroxyl group on CBZ degradation was not attributed to the yield of H₂O₂ and thus cannot be explained by the BDE value of the C–H bond. A previous study found that the reaction efficiency of alcohols with h⁺ increased with the number of anchoring hydroxyl groups [48]. However, the promotional effect of alcohol did not increase with the increase of the hydroxyl group quantity in this study. It might be attributed that the generated radicals were not alcohol radicals, which was supported by the fact that the radicals rarely induced changes in H₂O₂ production (Fig. S14b in Supporting information). In conclusion, the promotional effect of polyhydric alcohol in heterogeneous catalysis is independent of the alcohol radicals conversion.

  To investigate the effect of the solution pH on the promotional effect of alcohol, the changes in CBZ removal rate by BiOBr-3 under different solution pH conditions were compared. Fig. S15a (Supporting information) showed the removal rate of CBZ by BiOBr-3 was significantly affected by the solution pH (P < 0.05). IPA promoted the degradation of CBZ by BiOBr-3 at a pH of 7, whereas inhibited the degradation of CBZ under acidic or alkaline conditions. Correspondingly, the yield of H₂O₂ was also affected by the pH value of the solution (Fig. S15b in Supporting information). These phenomena could be explained by the strong susceptibility of radicals to H⁺/OH⁻ [6,24]. For instance, peroxy radicals were more inclined to react with hydroxide ions under alkaline conditions, thus reducing the generation of H₂O₂ (reaction 7) [49]. In acidic conditions, ^•O₂⁻ was protonated to form its conjugate acid (reaction 8), the hydroperoxygen radical (HO₂^•) [50].

  $ \text{HO-}\mathrm{ROO}^{•-}+\mathrm{OH}^{-} \rightarrow \mathrm{HR}(=\mathrm{O}) \mathrm{H}+{ }^{•} \mathrm{O}_2^{-}+\mathrm{H}_2 \mathrm{O} $

  (7)

  $ ^• \mathrm{O}_2^{-}+\mathrm{H}^{+} \leftrightarrow {\mathrm{HO}_2}^{•} $

  (8)

  In conclusion, this study demonstrated a promotional pathway for the effect of IPA on the degradation of CBZ by BiOBr-3. This pathway was initiated by alcohol radicals formed from the reaction of alcohol with active species (especially h⁺) of BiOBr in the absence of ^•OH. The formation of H₂O₂ by alcohol radical and O₂ promoted the formation of the dominant radical (^•O₂⁻) for CBZ degradation. In addition, the promotional effect was also observed with other alcohols, the extent of promotion was determined by the bond dissociation energy of the C–H bond adjacent to the hydroxyl group in alcohols. The greater the BDE of the C–H bond in alcohols, the greater the promotional effect. Moreover, the promotional effect of alcohol was favored at a neutral pH, medium alcohol dosage, and high-DO conditions. These results provide new insight into the use of alcohols in heterogeneous photocatalysis. It is necessary to consider the characteristics of the catalyst and alcohol to decide the use of alcohol as a scavenger or promoter in heterogeneous systems.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Shuangyu Wu: Writing – original draft, Methodology, Investigation, Formal analysis, Data curation. Jian Peng: Investigation, Formal analysis, Data curation. Yue Jiang: Validation, Investigation. Sijie Lin: Writing – review & editing, Supervision, Project administration, Funding acquisition, Conceptualization.

  Acknowledgments

  This work was funded by the National Natural Science Foundation of China (No. 22176150), the Shanghai Pilot Program for Basic Research, and the Fundamental Research Funds for the Central Universities. Y. Jiang acknowledges the Fellowship of China National Postdoctoral Program for Innovative Talents (No. BX20230262), the Fellowship of China Postdoctoral Science Foundation (No. 2023M732636), and the Shanghai Post-doctoral Excellence Program (No. 2023755).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110819.

  
  
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  Figure 1  Synthesis, characterizations, and photocatalytic reactivity of BiOBr. (a) Schematic diagram of the hydrothermal synthesis of BiOBr with different ratios of ethylene glycol (EG) to H₂O. (b) XRD, (c) XPS, and (d) EPR patterns of BiOBr. (e) Removal rates of CBZ by BiOBr with and without the presence of IPA. The addition of IPA significantly reduced the catalytic reactivity of BiOBr-1, but boosted the catalytic reactivity of BiOBr-3 (highlighted in red dash boxes). All data are presented as the means ± SD, *P < 0.05 and **P < 0.01 compared to the control. Reaction conditions: [BiOBr]= 0.6 g/L, [CBZ]₀ = 10 mg/L, [IPA]=50 μmol/L. The removal rate of CBZ was calculated based on high-performance liquid chromatography (Text S5 in Supporting information).

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  Figure 2  Identifications of active species generated by BiOBr based on electron paramagnetic resonance (EPR) spectra. EPR spectra of (a) DMPO-^•O₂⁻ (methanol system), (b) TEMP-¹O₂, (c) DMPO-^•OH (water system), and (d) TEMPO-h⁺ generated by BiOBr. Four types of BiOBr showed different levels of active species generated under visible light irradiation. (e) EPR spectra evidenced the alcohol radicals' generation by BiOBr-3 under visible light irradiation after IPA was added. (f) EPR spectra demonstrated that h⁺, ^•O₂⁻, and ¹O₂ all contribute to the generation of alcohol radicals in the presence of IPA, with h⁺ contributing the most. By adding scavengers of any two active species (h⁺, ^•O₂⁻, and ¹O₂) and DMPO for alcohol radicals capture, the contribution of the remaining active species to the generation of alcohol radicals can be determined. Reaction conditions: [BiOBr] =0.6 g/L, [IPA]= 50 μmol/L, [Trapping agent] =50 mmol/L, [FFA]= 0.5 mmol/L, [pBQ]= 0.05 mmol/L, [EDTA-2Na]=0.05 mmol/L.

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  Figure 3  Factors affecting the degradation of CBZ by BiOBr-3 with the presence of IPA. Influences of IPA concentrations (0–200 µmol/L) on the (a) removal rates of CBZ and (b) yield of H₂O₂ by BiOBr-3. Influences of dissolved oxygen on the (c) removal rates of CBZ and (d) yield of H₂O₂ by BiOBr-3. IPA and dissolved oxygen are the key factors affecting the reaction of H₂O₂ formation from alcohols. EPR spectra of (e) DMPO-^•O₂⁻ (methanol system) and (f) TEMP-¹O₂ generated by BiOBr-3 with and without IPA added. The addition of IPA enabled BiOBr-3 to produce more ^•O₂⁻ and ¹O₂. All data are presented as the means ± SD, P < 0.05, **P < 0.01, and ***P < 0.001 compared to control. µM = µmol/L in (b, d). Reaction conditions: [BiOBr-3] = 0.6 g/L, [CBZ]₀ = 10 mg/L, [IPA]= 50 μmol/L, [Trapping agent] = 50 mmol/L.

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  Figure 4  Delineating the effects of different alcohols on their promotional effects. (a) The effects of increasing carbon chain lengths of alcohols on their promotional effects. (b) The effects of primary, secondary, and tertiary alcohols on their promotional effects. (c) The effects of numbers of hydroxyl groups on promotional effects of alcohols. (d) The effects of methanol, ethanol, propanol, and butanol on the removal rates of CBZ by BiOBr-3 were attributed to the C–H bond dissociation energy of alcohols. (e) The effects of n-butanol, sec–butanol, and tert–butanol on the removal rates of CBZ by BiOBr-3 were partly attributed to the C–H bond dissociation energy of alcohols. µM = µmol/L in (a-c). Reaction conditions: [BiOBr-3] = 0.6 g/L, [CBZ]₀ = 10 mg/L, [Alcohols]= 50 μmol/L.

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