Diverse catalytic behavior of a dye-based polymer metal-free catalyst for hydrogen peroxide photoproduction

Abstract

Metal-free polymer-based catalysts have shown promising photocatalytic performance for efficient hydrogen peroxide (H₂O₂) photoproduction under visible light. However, the structure of the active site and the interface charge transfer of these catalysts have not been revealed clearly. Here, we report the fabrication of four narrow bandgap single-dye-based polymer metal-free catalysts (PPC-X, X = 75, 100, 125, 175). The catalyst PPC-X was synthesized by a hydrothermal method using proanthocyanidin (OPC) as the raw material. The reaction temperature was set at 75 °C, 100 °C, 125 °C, and 175 °C, and finally, the single-dye polymer catalyst was obtained, which was marked as PPC-X, with X representing the polymerization temperature. The PPC-X as catalysts show diverse catalytic behavior for H₂O₂ photoproduction. Under visible light excitation, the PPC-X catalysts (X = 100, 125, 175) reduced O₂ to produce H₂O₂via the two-electron transfer pathway and oxidized water to release O₂ by the four-electron transfer pathway. For the PPC-75 catalyst, O₂ was reduced by photo-generated electrons on the catalyst surface via the two-electron transfer pathway, while the PPC-75 catalyst itself also serves as a sacrificial agent to consume the photo-generated holes, leading to a high H₂O₂ production. The H₂O₂ yield of PPC-75 is 1152 μmol g⁻¹ h⁻¹, and that of PPC-100 is up to 1214 μmol g⁻¹ h⁻¹ at a light intensity of 32.6 mW cm⁻². This work provides a practical research example and idea for the in-depth understanding and design of efficient polymer-based photocatalysts.

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1. Introduction

Hydrogen peroxide (H₂O₂), as a clean high-energy oxidant, has been widely used in medicine, chemistry, and environmental management.^1–5 Moreover, it has attracted more and more attention in the past decade because of its potential to replace fossil fuels.^6–10 The synthesis of H₂O₂ through photocatalysis and direct conversion of solar energy into chemical energy is a cost-effective, safe and green production route.^11–14 Previous studies have shown that metal–semiconductor catalysts have achieved remarkable progress in the photosynthesis of H₂O₂.^15–19 Typically, the catalysts for the photoproduction of H₂O₂ can be divided into metal catalysts and metal-free catalysts. Metal-free catalysts are regarded as promising catalysts due to their low cost, environmental friendliness, and sustainability.^20–25 Metal-free catalysts are mainly carbon-based materials such as g-C₃N₄, GO, CDs, COFs, and metal-free polymer catalysts.^26–31 At present, the generation of H₂O₂ with metal-free photocatalysts represented by modified g-C₃N₄ is much higher among all kinds of semiconductor materials; for example, the H₂O₂ production rate of the catalyst obtained through the hybridization of g-C₃N₄ with CNTs is 32.6 μmol h⁻¹.^31–34 In addition to g-C₃N₄, there are several kinds of metal-free polymer catalysts with excellent photocatalytic performance.^35,36 Shiraishi et al. reported a metal-free catalyst, resorcinol formaldehyde resins, for efficient H₂O₂ photoproduction, which has a strong absorption at 700 nm with a solar to chemical energy conversion rate (SCC) up to 0.5%.³⁷ Liang et al. introduced acetylene or diacetylene moieties into covalent triazine frameworks, which promoted the charge separation in the conjugated structures, subsequently improving the catalytic efficiency.³⁸ Moreover, in the polymer-based metal-free catalyst system, numerous issues, such as the active site structure, reaction mechanism, and catalyst interface charge transfer, have not been revealed clearly.

In this study, we selected the organic dye molecule (proanthocyanidin, OPC) as the raw material and fabricated the narrow bandgap single-dye-based polymer metal-free catalysts (PPC-X; X denotes the reaction temperature) by a hydrothermal self-polymerization method. In the present system, the H₂O₂ yield of PPC-75 is 1152 μmol g⁻¹ h⁻¹, and that of the best sample PPC-100 is 1214 μmol g⁻¹ h⁻¹ at a light intensity of 32.6 mW cm⁻². We used cyclic voltammetry (CV) and transient photovoltage (TPV) tests to reveal the diverse catalytic behavior of dye-based polymer metal-free catalysts for H₂O₂ photoproduction. Under visible light excitation, the catalyst PPC-X (X = 100, 125, 175) reduced O₂ to produce H₂O₂via the two-electron transfer pathway and oxidized water to release O₂ by the four-electron transfer pathway. For the PPC-75 catalyst, the O₂ was reduced by photo-generated electrons on the catalyst surface via the two-electron transfer pathway, while, the PPC-75 catalyst itself also serves as a sacrificial agent for consuming the photo-generated holes, leading to a high H₂O₂ production.

2. Experimental section

2.1 Materials

Proanthocyanidin (AR, 98%) was purchased from Shanghai Yuanye Biotechnology Co, Ltd. Ethanol (AR, 98%) and sulfuric acid (AR, 98%) were purchased from Shanghai Aladdin Biochemical Technology Co, Ltd. All reagents were used without further purification.

2.2 Synthetic strategy

Using an ethanol/water mixture as solvent, the catalyst (PPC-X, X = 75, 100, 125, 175) was synthesized by a hydrothermal method. The specific synthesis process is as follows: first, 0.2 g of proanthocyanidin (OPC) was added to 4.5 mL ultrapure water, followed by 0.5 mL ethanol and 1 mL sulfuric acid solution (3 mol L⁻¹), and sonicated for 20 min. The resulting mixed solution was transferred to a hydrothermal reaction vessel and sealed. Finally, the sealed reaction vessel was hydrothermally reacted at different temperatures for 10 h, and the reaction temperature was set to 75 °C, 100 °C, 125 °C, and 175 °C, respectively. After the reaction was complete, it was allowed to naturally cool to room temperature. The precipitate in the reactor was washed with ultra-pure water and centrifuged, and this was repeated three times. The washed precipitate was then dried at 60 °C for 36 h. Finally, the obtained polymer was ground to obtain a powdery single-dye polymer, labeled PPC-X, where X represents the polymerization temperature.

3. Results and discussion

3.1 Morphology and structure

The morphology of the catalyst was observed by a scanning electron microscope (SEM). It can be seen from Fig. 1(a)–(d) that the polymer PPC-X obtained at different polymerization temperatures is composed of microspheres. However, by comparison, it can be found that the catalyst polymerized at a lower temperature (PPC-75) has better dispersion than that polymerized at higher temperatures. The catalyst polymerized at a high temperature (PPC-175) has a wide range of agglomeration phenomena. Fig. 1(e) shows the X-ray diffraction (XRD) patterns of the single dye polymer catalysts PPC-X, where the diffraction peaks at 21° and 45° demonstrate that PPC-X was amorphous.^39,40

Fig. 1 SEM images of (a) PPC-75, (b) PPC-100, (c) PPC-125 and (d) PPC-175 catalysts. (e) XRD patterns, and (f) Raman spectra of PPC-X catalysts (X = 75, 100, 125, 175).

Fig. 1(f) shows the Raman spectra of catalysts PPC-X, which displays two peaks centered at 1360 cm⁻¹ and 1600 cm⁻¹, corresponding to the D band with sp³ defect and the G band caused by the in-plane vibration of sp² carbon. The ratio of the D band to G band (I_D/I_G) indicates the defect characteristics of the catalyst. As shown in Fig. 1(f), the I_D/I_G ratio increases with the increase in temperature, which indicates that defects are more likely to occur at high temperatures. Therefore, when the temperature is too high, there are likely to be numerous defects, which is not conducive to the catalytic performance of the sample. The Fourier transform infrared spectroscopy in Fig. S1(a) (ESI†) indicates that the single dye polymer catalyst (PPC-X) obtained at different polymerization temperatures has similar infrared spectra, and the surface of the materials contains functional groups such as C–O, C=C, C=O, and C–OH bonds. Fig. S1(b), ESI† shows the FT-IR spectra of OPC and PPC-100. It can be seen from the FT-IR spectrum of OPC that it contains functional groups such as C–O, –Ph, and C–OH bonds. Compared with the FT-IR spectrum of OPC, the FT-IR spectrum of PPC-100 has a slight difference, with the peak at 1650 cm⁻¹ corresponding to the C=O bond of the catalyst. The morphology of the single-dye polymer catalyst was further analyzed by transmission electron microscopy, and the results are shown in Fig. S2(a), ESI.† It can be seen that the surface of the microspheres composed of the polymer PPC-100 is relatively smooth, accompanied by some lamellar structures peeled off from the microspheres in the catalyst. Fig. S2(b) (ESI†) shows the particle size distribution of the single-dye polymer PPC-100, indicating that the average particle size of the polymer is mainly about 2500 nm.

The functional groups and bond energies of the catalysts were further analyzed by X-ray photoelectron spectroscopy (XPS). Fig. 2 shows the C 1s spectra of different catalysts. It can be seen from Fig. 2 that the C 1s spectra of the catalysts obtained at different temperatures can be fitted to four peaks located at 284.6, 285.3, 286.3, and 288.2 eV, corresponding to the C=C, C–C, C–O and C=O of the polymer components.^41,42 From the C 1s spectra of OPC in Fig. S8 (ESI†), it can be seen that the C 1s spectra of the catalysts obtained at different temperatures can be fitted into three peaks located at 284.6, 285.3, and 286.3 eV, corresponding to the C=C, C–C, and C–O components of the polymer. This result is consistent with the FT-IR spectra. The elemental analysis in Table S1 (ESI†) shows that all catalysts were composed of C, H, O, and S, and the S comes from the residual sulfuric acid on the surface of the catalyst. Simultaneously, it can be seen that the content of C in the polymer increases first and then decreases with the increase in the polymerization temperature, and reaches a maximum value when the polymerization temperature is 100 °C. The thermogravimetric spectrum of PPC-100 is shown in Fig. S7 (ESI†). When the temperature reaches 900 °C, the decomposition of PPC-100 is complete, and the carbon content was found to be 27.60%.

Fig. 2 C 1s spectra of PPC-X catalysts. (a), (b), (c) and (d) for PPC-75, PPC-100, PPC-125 and PPC-175, respectively.

3.2 Band structure

To determine the band location of the catalyst, the ionization potential of catalyst PPC-100 was determined by ultraviolet photoelectron spectroscopy (UPS) to further explore the band structure of the sample. As shown in Fig. 3(a), the valence band (VB) position of the catalyst PPC-100 can be obtained at 5.76 eV by subtracting the spectral width of the ultraviolet photoelectron spectrum of He I from the excitation energy (21.22 eV); the conduction band (CB) position can be further calculated at 4.07 eV through the bandgap.⁴³ To better display the relationship between the conduction band/valence band of the catalyst and the redox potential of H₂O and O₂, the unit is converted from eV into electrochemical potential energy V according to the reference standard, where 0 V (vs. RHE) equals −4.44 eV (vs. Evac). As shown in Fig. 3(b), the CB of single-dye polymer PPC-100 is higher than the level of H₂O₂ produced by oxygen reduction, and the VB is lower than the level of O₂ produced by water oxidation, but it cannot reach the level of H₂O₂ produced by water oxidation. Therefore, it can be inferred that the catalyst can theoretically achieve the reduction reaction of O₂ and the oxidation reaction of water. Fig. 3(c) shows the UV-Vis absorption spectra of the catalysts PPC-X. It can be seen from the figure that the as-prepared single-dye polymer photocatalyst PPC-X has strong absorption in the ultraviolet and near-infrared regions, and its absorption value reaches a maximum value at about 600 nm, indicating that the catalyst can effectively utilize solar energy. In addition, it can also be seen from Fig. 3(c) that with the increase in the polymerization temperature, the optimal absorption wavelength of the catalyst first increases and then decreases, and the optimal absorption wavelength can reach 640 nm for PPC-100. The Tauc diagram obtained by the UV-Vis absorption spectrum can be used to calculate the optical band gap of a catalyst.^24,44 The (αhν)²–hν curve obtained by the UV-Vis absorption spectrum is shown in Fig. 3(d). The curve shows a good linear relationship, indicating that photocatalyst PPC-X is a direct bandgap material. The bandgap of photocatalyst PPC-100 is 1.69 eV. The bandgap of photocatalyst PPC-175 is 1.93 eV. A low bandgap is more favorable for the generation of photogenic charge, which improves the photocatalytic activity of the catalyst and completes the conversion of solar energy.

Fig. 3 (a) UPS spectrum of catalyst PPC-100. (b) Energy band structure of catalyst PPC-100 and the redox energy levels of water and O₂. (c) UV-Vis spectra and (d) the (αhν)²–hν curves of polymer catalysts PPC-X (X = 75, 100, 125, 175).

Due to the existence of defects in carbon materials, they have different band gaps. The oxidation and reduction state of catalysts can be studied by cyclic voltammetry (CV), and the band structure of the catalyst can be further deduced. Fig. 4 shows the CV curve of catalysts PPC-X. It can be seen from Fig. 4(a) that PPC-75 has multiple oxidation peaks, which are 1.66 and 1.91 V. However, other catalysts PPC-100, PPC-125, and PPC-175 have only one, at 1, 68, 1.7 and 1.69 V, respectively. The multiple oxidation levels occurring in the catalyst PPC-75 are due to the unfixed type of defects.

Fig. 4 (a) The CV curve of polymer catalyst PPC-75 (the oxidation states of the PPC-75 catalyst are 1.66 and 1.91 V, and the reduction state of the PPC-75 catalyst is −0.27 V). (b) The CV curve of polymer catalyst PPC-100 (the oxidation state of the PPC-100 catalyst is 1.68 V, and the reduction state of the PPC-100 catalyst is −0.31 V). (c) The CV curve of polymer catalyst PPC-125 (the oxidation state of the PPC-125 catalyst is 1.7 V, and the reduction state of the PPC-125 catalyst is −0.1 V). (d) The CV curve of polymer catalyst PPC-175 (the oxidation state of the PPC-175 catalyst is 1.69 V, and the reduction state of the PPC-175 catalyst is −0.33 V).

3.3 Photochemical properties

As shown in Fig. S3 (ESI†), all PPC-X samples generated photocurrent once exposed to light, indicating that all the polymer catalysts generate photocarriers under light conditions. In addition, by comparing the samples obtained at different temperatures, it was found that catalyst PPC-100 produced the maximum photocurrent under light conditions, while catalyst PPC-175 produced the minimum photocurrent under the light conditions, indicating that catalyst PPC-100 has the best photoresponse performance, which provides a basis for its photocatalytic application.

3.4 The transient photovoltage (TPV) measurements

As shown in Fig. 5(a), PPC-100 has the strongest photovoltage, indicating that after excitation, PPC-100 generates more photo-charge compared to other catalysts, which is conducive to oxygen reduction in the H₂O₂ generation process. As shown in Fig. 5(b), t_max represents the time required from lighting up to maximum charge extraction. There is no significant difference between each catalyst, indicating that there is little difference in the photo-charge transfer rate. Fig. 5(c) shows the maximum charge extraction amount A, which represents the maximum value of photogenerated charge excited on the surface of the catalyst. In Fig. 5(f), the A value of PPC-100 is significantly higher than that of PPC-75 (A₁ = 0.098), PPC-125 (A₃ = 0.108), and PPC-175 (A₄ = 0.087), indicating that PPC-100 had better photo-charge extraction ability. Fig. 5(d) shows the τ values of different catalysts. Here, τ represents the time decay constant of the photo-charge. The larger the τ value is, the longer the lifetime of the photo-charge is.⁴⁵ It can be seen from Fig. 5(d) that PPC-100, PPC-125, and PPC-175 have only one attenuation stage, while PPC-75 has two attenuation stages, which are composed of a fast decay process (τ₁ = 0.066 ms) and a slow decay process (τ₂ = 0.168 ms), which may be related to the heterogeneity of components. The average decay constant of PPC-75 (τ_2avg = 0.081 ms) is calculated by a formula in ESI.†

Fig. 5 (a) Comparison of the TPV with PPC-75, PPC-100, PPC-125 and PPC-175. (b) Maximum charge extraction time (t_max) of PPC-75, PPC-100, PPC-125 and PPC-175. (c) Charge extraction of PPC-75, PPC-100, PPC-125 and PPC-175. (d) The charge recombination of PPC-75, PPC-100, PPC- 125 and PPC-175. (e) Maximum charge extraction time (t_max). (f) The maximum charge extraction (A). (g) Charge attenuation time (τ). (h) Surface effective charge (n_e).

The signal characteristics of TPV indicate that it is a typical non-static signal. Fig. 6(a) shows the FFT curve of PPC-100. It can be seen that the curve reaches its maximum amplitude when the frequency is close to zero, which is a typical non-static signal characteristic. To further analyze the frequency and time characteristics of this non-static signal, we adopted the continuous wavelet transform (CWT) based on Bior3.9 to analyze both time and frequency scales.⁴⁶ We selected three frequencies f = 2 Hz, f = 4 Hz, and f = 10 Hz, and the CWT signal in the whole-time scale is shown in Fig. 6(c). When f = 2 Hz, it has signal distribution in the time range from 0 to 5 ms, indicating that both the fast and slow process of charge transfer plays an important role in the overall process. When f = 4 Hz, the CWT signal range is 0–1.4 ms, indicating that the fast electron transfer process occupies the main part of charge transfer. When f = 10 Hz, the CWT signal only exists in the range of 0–0.4 ms, which proves that the fast electron transfer process only exists in the initial stage of the charge transfer process. Fig. 8(d) shows CWT signals at t = 0.21 ms, t = 0.75 ms, and t = 4.0 ms in different frequency ranges. When t = 0.21 ms, signals exist in the range of 0–12 Hz, indicating that charge transfer is composed of both fast and slow processes. However, when t = 0.75 ms, the signal range is reduced to 0–8 Hz, and the signal at this time is mainly a medium and low-frequency signal. As time continues to increase to t = 4 ms, only the low-frequency part (0–2 Hz) of the signal in Fig. 8(d) is left, and almost the whole charge transmission process is composed of the slow process.

Fig. 6 (a) FT spectrum, (b) 2D CWT spectrum, (c) Intensity–frequency curves (t = 0.21, 0.75 and 4.0 ms) and (d) Intensity–time curves (f = 2, 4, and 10 Hz) of PPC-100, (e) Intensity–time curves of PPC-100, (f) Peak delay time (Δt) at different frequencies (f = 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17 and 18 Hz); here, the peak time of sample PPC-100 was used as the reference value.

We used the peak delay time (ΔT) of the intensity time curve to study the charge transfer process between the catalyst interfaces and determined the speed of the charge transfer process. Fig. 6(e) shows the comparison between the peak position of PPC-75 and PPC-100 at f = 2 Hz, and we observed the time scale delay of the peak position of PPC-100.⁴⁷Fig. 6(f) shows the relationship between its peak delay time (ΔT) and frequency (see ESI† for the specific calculation method). It can be seen that the peak delay time (ΔT) is greater than zero in all frequency ranges, and the peak delay time (ΔT) of PPC-100, PPC125 and PPC-175 is also greater than zero in all frequency ranges (Fig. S4–S6, ESI†). These results indicate that the charge transfer process of PPC-100 is slower than that of the catalysts at other temperatures, and the slower charge transfer is helpful to the improvement of photogenic electron stability and catalytic activity.

3.5 Photocatalytic experiments

In the photocatalytic experiments, a multi-channel photocatalytic reaction system was used to evaluate the photocatalytic activity of the catalyst in a 40 mL glass bottle reactor with a visible light source, and the wavelength of visible light was λ ≥ 420 nm, without adding any sacrificial agent or cocatalyst. In a typical catalytic reaction process, 10 mg catalyst was dispersed in 20 mL ultrapure water for 10 min by ultrasound, and then the reactor was placed in a multi-channel photocatalytic reaction system and exposed to visible light for 6 h at room temperature. After the reaction, the reaction solution was centrifuged and filtered, and then the H₂O₂ in the filtrate was quantitatively analyzed.

For the stability of the catalyst catalyzed reaction, the reactor was shaded and settled for 12 h after each illumination. After that, 10 mL supernatant was filtered to measure its hydrogen peroxide yield. The remaining solution and catalyst were dried thoroughly and then the cycling experiment was carried out. The cycle was repeated 5 times, each time under the same conditions, and the quantitative determination of hydrogen peroxide was carried out after each experiment.

According to the influence of the wavelength of light on the yield of H₂O₂ catalyzed by the catalyst, light source with different wavelengths was used to illuminate the catalyst, and the wavelengths were 365, 420, 485, 595, and 620 nm respectively. The reaction system comprised 10 mg PPC-100 dispersed in 20 mg ultrapure water, and the reaction time was 6 h. After the reaction, the yield of H₂O₂ in the system was quantitatively determined. To study the effects of different atmospheres on the yield of H₂O₂, gas bubbles were used to create different atmospheres. The reaction system with 10 mg catalyst was bubbled with N₂ and O₂ for 20 min to create an N₂ and O₂ saturated atmosphere. Then, the reaction system was sealed and placed in a multi-channel photocatalytic reaction system for 6 h. H₂O₂ in the system was quantitatively determined after the reaction. To analyze the effect of adding a sacrificial agent on H₂O₂ production. Electron sacrificial agent, hole sacrificial agent, superoxide free radical capture agent, and hydroxyl free radical capture agent were added to the reaction system, respectively. During the reaction, 0.2 mmol of silver nitrate (AgNO₃), methanol (CH₃OH), benzoquinone (BQ), and tert-butanol (TBA) were added to the reaction system. H₂O₂ in the system was quantitatively determined after 6 h illumination. To validate the effect of the reaction environment on the catalytic activity of the catalyst, N₂, and O₂ bubbles were used to create different reaction environments. Taking N₂ as an example, the reaction system with dispersed catalyst PPC-100 was bubbled with N₂ for 30 min, so that the solution reached N₂ saturation. The reactor was then sealed and placed under visible light for 6 h. After the reaction, the H₂O₂ in the filtrate was quantitatively analyzed. For all the above methods, the quantitative determination of H₂O₂ was carried out using KMnO₄ titration. During the titration process, 10 mL of the filtered solution after completion of the reaction, was collected and titrated with 0.02 mol L⁻¹ of KMnO₄ standard solution (KMnO₄ : H₂SO₄ = 1 : 1) until the solution turned red and did not fade for 30 s. Finally, the amount of H₂O₂ produced in the catalytic system was calculated according to the volume of potassium permanganate used in the titration.

3.6 Evaluation of catalytic activity

First, the photocatalytic activity of PPC-X was studied under different polymerization temperatures. As shown in Fig. 7(a), the catalyst PPC-100 obtained at the polymerization temperature of 100 °C had the highest catalytic activity, and its yield reached 1214 μmol g⁻¹ h⁻¹. With the increase in the polymerization temperature, the catalytic activity of the obtained catalyst decreased. Among the as-prepared catalysts, catalyst PPC-175 had the lowest activity. It is worth noting that the H₂O₂ yield of PPC-75 is not significantly different from that of PPC-100, which is inconsistent with its less effective charge (n_e). Combined with the CV curve, we propose that the partially unoxidized part may act as a sacrificial agent, leading to higher H₂O₂ production. In addition, the conversion efficiency of solar energy to chemical energy (SCC) was calculated, and the SCC of the PPC-100 catalyst reached 0.15% (see ESI† for calculation details). Fig. 7(b) shows the cycling stability of the catalyst. It can be seen from Fig. 7(b) that the activity of the catalyst remains unchanged within the fifth cycle. In the fifth cycle, the H₂O₂ synthesis rate of catalyst PPC-100 was maintained at 1021 μmol g⁻¹ h⁻¹. The results indicate that the single dye polymer PPC-100 has good photocatalytic stability. The FT-IR spectra of fresh PPC-100 and recovered PPC-100 are shown in Fig. S1(c) (ESI†). It can be seen that compared with the fresh PPC-100, the structure/functional groups of recovered PPC-100 have not changed. Fig. 7(c) shows the relationship between the amount of H₂O₂ synthesis and the time of catalyst PPC-100 in the photocatalytic reaction. As shown in Fig. 7(c), no H₂O₂ was detected under dark conditions, indicating the result of the photocatalytic reaction of catalyst when H₂O₂ is generated. Under light conditions, the yield of H₂O₂ increases linearly with the increase in time, indicating the dependence of the catalytic reaction on light, and also indicating the stability and persistence of the catalytic reaction. Fig. 9(d) shows the calculated apparent quantum yield of single-dye polymer PPC-100 at different wavelengths. It can be seen from the figure that the AQY value of catalyst PPC-100 at the wavelength 595 nm is the largest, reaching up to 0.43% (see ESI† for calculation details). In addition, by comparing the UV-visible absorption spectra of AQY with the catalyst at different wavelengths, it can be seen that the variation trend of AQY is consistent with the UV absorption, indicating that the catalyst carries out the photocatalytic reaction through bandgap excitation.

Fig. 7 (a) H₂O₂ production rates of different PPC-X catalysts. (b) Cyclic stability of catalyst PPC-100. (c) Relation of H₂O₂ yield of catalyst PPC-100 with time. (d) AQY of catalyst PPC-100 and its UV-Vis spectrum.

Fig. 8(a) reveals the yield of H₂O₂ under different sacrificial additives. Among them, AgNO₃, CH₃OH, benzoquinone (BQ), and tert-butanol (TBA) were used as the trapping agents of electrons (e⁻), hole (h⁺), superoxide radical (˙O₂⁻), and hydroxyl radical (˙OH), respectively.^48,49 As shown in the figure, the yield of H₂O₂ decreased significantly with the addition of AgNO₃. The yield of H₂O₂ increased with the addition of CH₃OH. It can be said that H₂O₂ is formed by the reaction in which electrons are involved. When electrons are consumed, the activity decreases, while when holes are captured, the separation efficiency of photogenerated charge is improved, resulting in a higher H₂O₂ yield. In addition, when benzoquinone was added, the content of H₂O₂ in the reaction solution was very low, and the addition of tert-butanol had almost no effect on the catalytic activity of PPC-100. These results indicate that when catalyst PPC-100 performs photocatalytic reaction, photogenerated electrons reduce O₂, producing H₂O₂ (O₂ → ˙O₂⁻ → H₂O₂) and photogenerated holes participate in the water oxidation reaction, releasing O₂ (H₂O → O₂).

Fig. 8 (a) H₂O₂ yield of catalyst PPC-100 which was added with different sacrificial agents. (b) The catalytic activity of catalysts under different atmospheres. (c) EPR spectra of catalyst PPC-100 to ˙O₂⁻ under dark and light conditions. (d) The current–time curves of catalyst PPC-100.

The catalytic activity of single dye catalyst PPC-100 in different atmospheres was studied. As shown in Fig. 8(b), the yield of H₂O₂ increased in the O₂ environment, while that in nitrogen was greatly reduced. This is because, in the O₂ saturated environment, sufficient O₂ ensures the oxygen reduction reaction, improving the electron utilization efficiency, and then increasing the yield of H₂O₂. In the N₂ saturated environment, O₂ is mainly generated by the oxidation reaction of water, and this O₂ then participates in the oxygen reduction reaction to produce H₂O₂. Due to the limitation of the water oxidation rate and oxygen content, the catalytic activity of the catalyst is limited, and the yield of H₂O₂ decreases. Therefore, it can be preliminarily inferred that the photocatalytic reaction is the oxygen reduction reaction involving O₂.

The free radicals in the reaction were further confirmed by electron paramagnetic resonance. As shown in Fig. 8(c), under dark conditions, no superoxide radical was detected, while under light conditions, four distinct superoxide radical signals were observed. The results show that the photocatalytic reaction of PPC-100 is the reaction of H₂O₂ production oxygen reduction. The process of H₂O₂ production was further confirmed by the rotating ring electrode. As shown in Fig. 8(d), in pure water saturated with N₂, the disk current of the rotating ring disk electrode increased significantly after the addition of light, while the ring current did not change after the addition of light. Alternatively, during the oxidation of water, no H₂O₂ is produced. The number of electrons transferred in the process of water oxidation is 4.⁵⁰ The results show that the oxidation of water only releases O₂ but does not produce H₂O₂.

Based on the above results and analysis, a catalytic mechanism for single dye polymer catalysts PPC-75 and PPC-100 was proposed. As shown in Fig. 9(a), the catalyst generates a photogenerated charge under light excitation. The conduction band electrons react with O₂ in the water, reducing O₂ to H₂O₂. The holes in the valence band participate in the oxidation reaction of water, releasing O₂. The reaction mechanism of PPC-75 is shown in Fig. 9(b). Oxygen reduction produces hydrogen peroxide in the same way as PPC-100, but the reaction of water oxidation depletion holes is partially replaced by catalyst self-sacrifice, which corresponds to a two-stage attenuation in the TPV test, as shown in Fig. 7(a). Moreover, due to the presence of the self-sacrificial reaction, the hydrogen peroxide production of PPC-75 increases significantly, second only to that of PPC-100.

Fig. 9 (a) The mechanism diagram of photocatalytic H₂O₂ production over the PPC-100 photocatalyst. (b) Possible mechanism diagram of photocatalytic H₂O₂ production over the PPC-75 photocatalyst.

4. Conclusions

In this study, single-dye polymer catalyst PPC-X was designed and synthesized by a hydrothermal polymerization method, in which, the catalyst PPC-100 shows the best catalytic activity for H₂O₂ photoproduction. The conversion efficiency of solar energy to hydrogen energy of the PPC-100 catalyst reached 0.15% at the light intensity of 32.6 mW cm⁻², and the production rate of H₂O₂ reached 1214 μmol g⁻¹ h⁻¹. Combined with various characterization and transient photovoltage tests, the diverse catalytic behavior of the dye-based polymer metal-free catalysts as well as the photocatalytic reaction mechanism was demonstrated. Under light excitation, for PPC-100, PPC-125, and PPC-175, the catalyst reduced O₂ to produce H₂O₂ through the two-electron transfer pathway and oxidized water to release O₂ through the four-electron transfer pathway. For the PPC-75 catalyst, the O₂ was reduced by photo-generated electrons on the catalyst surface via the two-electron transfer pathway, while the PPC-75 catalyst itself also served as a sacrificial agent. This work provides a practical research example and idea for an in-depth understanding and design of efficient polymer-based photocatalysts.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work is supported by National Key R&D Program of China (2020YFA0406104, 2020YFA0406101), National MCF Energy R&D Program of China (2018YFE0306105), Innovative Research Group Project of the National Natural Science Foundation of China (51821002), National Natural Science Foundation of China (51725204, 21771132, 51972216, 52041202), Natural Science Foundation of Jiangsu Province (BK20190041), Key-Area Research and Development Program of Guangdong Province (2019B010933001), Collaborative Innovation Center of Suzhou Nano Science & Technology, the 111 Project, and Suzhou Key Laboratory of Functional Nano & Soft Materials.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ma00162d

‡ These authors contributed equally.

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