Single-Metal-Atom Polymeric Unimolecular Micelles for Switchable Photocatalytic H ₂ Evolution

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Jul 2021Single-Metal-Atom Polymeric Unimolecular Micelles for Switchable Photocatalytic H2 Evolution Quan Zuo, Kun Feng, Jun Zhong, Yiyong Mai and Yongfeng Zhou Quan Zuo School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240 , Kun Feng Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 , Jun Zhong Institute of Functional Nano and Soft Materials Laboratory (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou 215123 , Yiyong Mai *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240 and Yongfeng Zhou *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, Frontiers Science Center for Transformative Molecules, State Key Laboratory of Metal Matrix Composites, Shanghai Key Laboratory of Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, Shanghai 200240 https://doi.org/10.31635/ccschem.020.202000486 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Developing "green" catalytic systems with desirable performance such as good water solubility, recyclability, and switchability is a great challenge. Here, to address this challenge, we extend the concept of polymeric unimolecular micelles (a typical self-assembled structure) to the construction of a stimuli-responsive and recoverable molecular catalyst with single-metal atoms that exhibits switchable photocatalytic activity for water splitting. In the unimolecular micelle-based catalyst, the Pt site is stabilized by a pH-sensitive hydrophilic polymer shell, which affords the novel catalyst excellent water dispersibility, unencumbered mass transfer, and high active site accessibility. Furthermore, the pH-responsive polymer shell renders the catalyst with switchable photocatalytic capability for H2 evolution via water splitting upon visible light irradiation (λ > 420 nm). When exposed to an acidic medium, the unimolecular micelles exhibit remarkable photocatalytic ability with a high H2 evolution rate of 49,465 μmol g(Pt)−1 h−1, which is superior to those of most reported Pt-based photocatalysts. When exposed to an alkaline medium, the unimolecular micelles aggregate together, and their catalytic ability is "switched off" accordingly. Meanwhile, the catalyst can be simply recovered from alkaline solutions and reused in acidic media, allowing the recyclable utilization of the photocatalyst. Download figure Download PowerPoint Introduction Polymer self-assembly has attracted considerable attention in recent decades due to its versatile ability to construct diverse delicate nanostructures such as micelles,1–4 vesicles,5–7 and tubes8–10 as well as the potential of practical applications of these self-assembled structures in many areas.11–18 For example, in the catalytic field, polymer assemblies have proven to be ideal scaffolds to incorporate metal nanoparticles for heterogeneous catalysis. The protection of nanoparticles by the polymer assemblies can effectively alleviate their aggregation. Furthermore, the associated polymer chains may impart the resulting catalysts with tailor-made functions, such as environmental stimuli-responsiveness and targeting capability.19–22 However, there also exist a number of severe challenges for the catalytic applications of the reported hybrid polymeric assemblies. First, their colloidal stability in solvents, especially in water, is a serious problem. The loaded metal nanoparticles with mismatched sizes may significantly affect the self-assembly behavior of the associated polymers, leading to the aggregation and precipitation of the resulting hybrid assemblies.23 Second, the recovery of the hybrid catalysts is highly difficult due to noncontrol over precipitation and redispersion of the hybrid assemblies in solvents.24 Third, also the most important issue, in the reported hybrid polymeric assemblies, the loaded metal nanoparticles suffer from low atom utilization efficiency and thus limited catalytic performance owing to the fact that only a small fraction of atoms on the surfaces can serve as the active catalytic sites.25 Recently, various catalysts containing single-metal atoms on suitable supports have demonstrated the great advantage of metal–atom economy for catalysis.26–32 Inspired by this appealing concept, here we report, for the first time, a polymeric unimolecular self-assembled nanostructure containing single-metal atoms for homogeneous photocatalysis in aqueous solution, which combines advantages of excellent colloidal stability, pH-responsive recoverability, and high catalysis efficiency. The polymeric assemblies with single-metal atoms are based on unimolecular micelles (UMs) from an amphiphilic star-shaped polymer (PtTHPP-star-PDMAEMA) consisting of a 5,10,15,20-tetrakis(4-hydroxyphenyl) platinum porphyrin (PtTHPP) core and four hydrophilic poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) arms (Scheme 1). The Pt ion in the PtTHPP core was reduced to its atom form and then stabilized in aqueous solution by the hydrophilic PDMAEMA shell. The resultant catalyst was denoted as Pt-UMs. The UM structure provides the catalyst with excellent water dispersibility, unencumbered mass transfer, and high active site accessibility, thus enabling remarkable photocatalytic performance for water splitting to generate H2 upon λ > 420 nm visible-light irradiation. Furthermore, thanks to the pH-responsiveness of the PDMAEMA shells, Pt-UMs showed switchable dispersion/aggregation behavior through simply adjusting the pH value of the aqueous reaction medium, enabling the "on/off" switch of photocatalytic activity as well as recyclability of the catalyst (Scheme 1). Scheme 1 | Schematic representation of switchable unimolecular micelles with single Pt atoms. The dispersed UMs are catalytically active, whereas the photocatalytic activity of the aggregated UMs is sharply reduced. Download figure Download PowerPoint Results and Discussion The porphyrin-based star polymer, PtTHPP-star-PDMAEMA, was synthesized through the oxyanionic polymerization of PDMAEMA arms from the PtTHPP core ( Supporting Information Figures S1–S6).33 To ensure good water dispersibility, PtTHPP-star-PDMAEMA was synthesized with an average degree of polymerization of 24 for the PDMAEMA arms, and a total molar mass of around 4.6 kD with a polydispersity of 1.05. Then, the Pt ions in the PtTHPP cores were reduced and stabilized in aqueous solution by the hydrophilic PDMAEMA shell. The synthetic procedures and characterizations are shown in the Supporting Information ( Supporting Information Figures S7–S10). First of all, the water dispersibility of PtTHPP-star-PDMAEMA was evaluated. As shown in Figure 1a, PtTHPP aggregated and self-floated on water due to its inherent hydrophobicity. In contrast, after the grafting of the water-soluble PDMAEMA chains, PtTHPP-star-PDMAEMA turned hydrophilic and could be dispersed well in water (Figure 1b). Water contact angle analyses further confirmed the functionalization-induced hydrophilicity, as PtTHPP and PtTHPP-star-PDMAEMA showed water contact angles (C.A.) of 113° and 19°, respectively (Figures 1c and 1d; Supporting Information Figure S11). Due to the amphiphilic feature, PtTHPP-star-PDMAEMA could self-assemble in aqueous solution. The critical aggregation concentration (CAC) of PtTHPP-star-PDMAEMA was determined to be ca. 0.2 mg/mL at pH = 2 ( Supporting Information Figure S12). When the concentration was below CAC, the nanostructures of PtTHPP-star-PDMAEMA were investigated with pH = 2 and a polymer concentration of 0.1 mg/mL, that is, the condition used for the subsequent evaluation of photocatalytic performance. TEM images show that the PtTHPP-star-PDMAEMA molecules formed ultrasmall polymer nanoparticles with an average diameter of 2.8 ± 1.0 nm (Figure 1e), which was further supported by dynamic light scattering (DLS) analysis, revealing a mean hydrodynamic diameter (Dh) of 3.4 ± 1.4 nm ( Supporting Information Figure S13). The UV–Vis absorption spectra of PtTHPP-star-PDMAEMA in solution (Figure 1f) display no observable change in the position of the maximum absorption with varying the polymer concentration, whereas the maximum absorption intensity at 403 nm increases linearly with an increase in the polymer concentration (inset of Figure 1f). This result is in line with the Lambert–Beer law, indicating that the PtTHPP-star-PDMAEMA molecules do not aggregate and exist as unimolecular micelles in the aqueous solution in the range of our experimental concentrations. This situation is attributed to the strong intermolecular electrostatic repulsion among the PDMAEMA chains and their good water solubility, which effectively suppress the aggregation of the PtTHPP-star-PDMAEMA molecules. Figure 1. | Photographs showing the comparison of water dispersibility of (a) PtTHPP and (b) PtTHPP-star-PDMAEMA. Water contact angle measurements of (c) PtTHPP and (d) PtTHPP-star-PDMAEMA. (e) TEM image of Pt-UMs. Inset: Statistical size distribution of 200 black spots in (e). (f) Absorption spectra of Pt-UMs in aqueous solutions with different concentrations. Inset: The linear relation of the absorption intensities at 403 nm of the Pt-UMs solutions and their concentrations (red line). Download figure Download PowerPoint To identify the status of the Pt species distributed in the unimolecular micelles, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was performed. Ultrasmall bright spots with sizes ranging from 0.1 to 0.2 nm were observed without the presence of metal nanoparticles or clusters (Figure 2a). The Pt-loading content was determined to be 4.1 wt % by inductively coupled plasma optical emission spectroscopy (ICP-OES), which accords well with the calculated value (4 wt %). Such a high metal content further demonstrates the advantage of the unimolecular micelles in stabilizing single Pt atoms. To further probe the local structure of Pt-UMs on an atomic level, the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure spectra (XANES) were recorded. The EXAFS spectra of Pt-UMs exhibit two notable peaks at 1.5 and 2.5 Å (Figure 2b), which are attributed to the Pt–N and Pt–N–C bonds,29 respectively. While no obvious Pt–Cl (≈1.9 Å) or Pt–Pt (≈ 2.7 Å) peaks are detected in comparison with PtCl2 and Pt foil, indicating the atomic dispersion of the Pt species in the unimolecular micelles. In the XANES spectra, it is seen that the white-line peak intensity of Pt-UMs locates between those of Pt foil and PtO2 (Figure 2c), suggesting that the Pt species present positive charges.26 Moreover, Pt-UMs exhibit similar EXAFS and XANE spectra to those of PtTHPP ( Supporting Information Figure S14), demonstrating that the modification of the PDMAEMA polymer chains on PtTHPP has negligible effect on the coordination environment of Pt. The high-resolution Pt 4f X-ray photoelectron spectroscopy (XPS) spectrum of Pt-UMs exhibits a single doublet signal (4f7/2 and 4f5/2) at 72.7 and 76.0 eV, respectively, which is between those of Pt (II) and Pt (0) (Figure 2d), further confirming that the Pt species are partially in an oxidized form.30 Figure 2. | Characterizations of single Pt sites. (a) Aberration-corrected HAADF-STEM image of Pt-UMs. (b) Pt L3-edge EXAFS spectra for Pt-UMs, PtCl2, and Pt foil. (c) The normalized Pt L3-edge XANES spectra for Pt-UMs, PtO2, and Pt foil. (d) High-resolution Pt 4f XPS spectrum of Pt-UMs. Download figure Download PowerPoint To gain insight into the recombination situation of photogenerated electrons and holes, photoluminescence (PL) spectroscopy was performed. As shown in Figure 3a, Pt-UMs show a nearly quenched PL emission, indicating improved photogenerated charge separation and suppressed electrons–holes recombination, which is beneficial to the enhancement of photocatalytic efficiency. Furthermore, the time-resolved PL spectra (Figure 3b) reveal that Pt-UMs exhibit much shorter PL lifetime (3.5 ns) than that of PtTHPP (10.1 ns), suggesting a faster photogenerated electron transfer from the porphyrin ring to the Pt atom.28,31 This result is confirmed by the electrochemical impedance plots, in which Pt-UMs exhibit a smaller arc radius compared with that of PtTHPP (Figure 3c), indicating lower charge transfer resistance between Pt-UMs and the medium. As shown in Figure 3d, the enhanced separation and transfer of the photoinduced charges in Pt-UMs are further supported by their stronger photocurrent response. Accordingly, Pt-UMs with enhanced charge separation and transfer efficiency are expected to possess high photocatalytic activity, profiting from their excellent dispersibility in the aqueous phase. Figure 3. | Photoelectrochemical characterizations of Pt-UMs and PtTHPP. (a) Steady-state PL spectra. (b) Time-resolved PL spectra. (c) Nyquist plots, the inset shows a simulated equivalent electrical circuit, in which Rs and Rt represent the electrolyte solution resistance and the interfacial charge-transfer resistance, respectively. (d) Photocurrent–potential curves for PtTHPP and Pt-UMs, respectively. Download figure Download PowerPoint We then evaluated the photocatalytic performance of Pt-UMs for H2 production via water splitting, with ascorbic acid as the sacrificial agent upon λ > 420 nm visible light irradiation. For comparison, UMs containing Zn sites (denoted as Zn-UMs) were also synthesized by a similar strategy ( Supporting Information Figures S15–S17). Meanwhile, highly hydrophobic PtTHPP was also employed as the control sample. The comparison of the performance of Pt-UMs and Zn-UMs is to verify the important role of the Pt atoms in the photocatalysis, given that Zn-UMs and Pt-UMs have a similar unimolecular micellar structure, but different catalytic sites. As a consequence, Zn-UMs exhibit negligible photocatalytic activity with a H2 generation rate of only 27.7 μmol g−1 h−1 (Figure 4a), probably because of the serious carrier recombination. It therefore reflects that the Pt sites play a crucial role in the photocatalytic process. Similarly, PtTHPP shows poor photocatalytic activity with a low H2 evolution rate of 66 μmol g−1 h−1. Furthermore, we also chose a charged platinum porphyrin complex of PtTCPP [PtII tetrakis(4-carboxyphenyl)porphyrin] as a control sample, which has charged organic substituents on the porphyrin unit and thus enables good water solubility ( Supporting Information Figure S18a). As shown in Supporting Information Figure S18b, PtTCPP exhibits a low H2-production rate of 212 µmol g−1 h−1 and poor H2-production stability with severe performance degradation (from 212 to 88 µmol g−1 h−1 at the seventh hour), due to the absence of the protection of the polymer chains. Moreover, PtTCPP has difficulties in recyclable utilization.34 These results indicate that the polymer modification to construct UMs is an effective strategy for achieving efficient and recyclable photocatalysts containing single-metal atoms. Figure 4. | (a) Photocatalytic H2 production rates of Pt-UMs and the control samples upon visible light irradiation. (b) TEM image of the polymer aggregates formed under pH = 10 and a polymer concentration of 0.1 mg/mL. Inset: The corresponding HAADF-STEM image. (c) Photograph of the pH-responsive behavior and the recycling use of Pt-UMs. (d) Switchable photocatalytic activity of Pt-UMs at pH = 2 and 10 aqueous solutions. The duration time of each cycle is 5 h, the blue line represents the different pH values of the solution (pH = 2 or 10), the black line expresses the photocatalytic H2-production performance of the catalyst at the corresponding pH. Download figure Download PowerPoint Benefiting from the advantages of the ultrasmall particle size and excellent dispersibility in aqueous solution, reactants can have unencumbered access to the Pt reaction center. Moreover, as demonstrated in previous studies on porphyrin-based photocatalysts for H2 evolution,27,28 in the photocatalytic process, organic porphyrin molecules with π conjugation as light harvester can be photoexcited to produce photogenerated electrons. Then, the photogenerated electrons are transferred to the cocatalyst for reduction reactions. In our system, the Pt atoms with a low overpotential are the cocatalyst, which can directly trap photogenerated electrons from the intimate contacted porphyrin rings and behave as the proton reduction sites for H2 production. As a consequence, Pt-UMs exhibit the best photocatalytic activity with a high H2 generation rate of 2028 μmol g−1 h−1 based on the total mass of UMs (or 49,465 μmol g(Pt)−1 h−1 based on only the Pt mass). This rate value is superior to those of most reported Pt-based photocatalysts (see Supporting Information Table S1 for example). Furthermore, the over 30-fold enhancement in H2 evolution by Pt-UMs compared with that by PtTHPP further corroborates the effectiveness of the UM-based supports for building highly efficient catalysts. As another control experiment, a PDMAEMA polymer with an average degree of polymerization of 84 was synthesized, and it was mixed directly with PtTHPP in aqueous solution with pH = 2.0 ( Supporting Information Figure S19). However, after dispersion under similar conditions, the mixture suffered from severe aggregation and precipitation after standing for several minutes, indicating the instability of the mixture. As a result, this mixture exhibited negligible photocatalytic activity toward H2 production (only 26.3 µmol g−1 h−1) and poor cycling stability with severe performance degradation. This result demonstrated the necessity of the covalent linkage of PtTHPP with PDMAEMA to form the UMs for achieving efficient photocatalysts. The aggregation state of catalysts can significantly affect the mass transfer, and so it can be utilized to control the catalytic rates, achieving switchable catalysis.35,36 In our catalytic system, the introduction of the pH-responsive PDMAEMA chains allows the change of the hydrophilicity of the polymer by simply adjusting the pH value of the solution, thus achieving the control over the dispersion–aggregation state of the catalyst and consequently switchable activation–deactivation of its photocatalytic ability. We therefore investigated the pH-responsive dynamic aggregation behavior of Pt-UMs and their switchable catalytic performance. Under an acidic condition (pH = 2), the PtTHPP-star-PDMAEMA molecules were protonated and soluble, behaving as UMs, in which reactants could freely reach the catalyst surfaces. Thus, they exhibited high photocatalytic activity. When the pH value increased to 10, that is, above the pKa (∼7.5) of PDMAEMA,37 the PDMAEMA chains were deprotonated and became hydrophobic. It led to the aggregation of the PtTHPP-star-PDMAEMA molecules. The formation of the aggregates was confirmed by TEM and HAAD-STEM observations (Figure 4b, Supporting Information Figures S20 and S21), which revealed much larger particles with an average diameter of 110 ± 37 nm (further supported by DLS analysis giving an average Dh of 136 ± 28 nm, Supporting Information Figure S22). The resulting massive aggregates suffered from hindered mass transfer, undesirable charge recombination, and limited accessibility of active sites, and thus significantly reduced the catalytic activity, giving a very low hydrogen generation rate of 172 μmol g−1 h−1. Moreover, the polymer aggregates could be redispersed in an aqueous solution with pH = 2 (Figure 4c). The hydrogen generation rate displays a reversible pattern in response to the switch between acidic and alkali mediums (Figure 4d), indicating an excellent switchability of Pt-UMs for photocatalytic water splitting. In addition to the evaluation of photocatalytic performance in the HCl/NaOH aqueous solutions, the performance of Pt-UMs in buffer solutions (H3PO4−NaH2PO4 or NaHCO3−Na2CO3), which may provide stable pH values for the reaction system, was also evaluated ( Supporting Information Figure S23). As a consequence, Pt-UMs exhibited similar photocatalytic performance for H2 production, compared with that of Pt-UMs in the HCl/NaOH aqueous solutions. This result confirms the excellent pH-responsive switchability of Pt-UMs for photocatalytic water splitting. A typical drawback of the practical implementation of well-dispersed molecular catalysts lies in the difficulty of their recyclable uses.38 Impressively, we found that our water-soluble photocatalyst could be simply recovered from the reaction media under alkaline conditions, which led to the coagulation and precipitation of the polymer (Figure 4c). The photocatalysts were then collected simply by centrifugation and redissolved in acidic aqueous solutions. With this simple protocol, the photocatalysts were repeatedly used in water for many catalytic cycles. As shown in Figure 4d, no obvious deterioration of the photocatalytic activity was observed for Pt-UMs during the catalytic cycling, and 95% of the initial activity was retained after three 5 h catalytic cycles. Moreover, the 1H NMR and TEM results confirmed the retention of the chemical composition and the self-assembled structures of Pt-UMs after the catalytic cycles under different pH values ( Supporting Information Figures S24–S27), indicating their excellent chemical and structural stability. The Pt content in Pt-UMs after the photocatalytic reaction was measured to be ca. 4.1 wt % by ICP-OES, further validating their excellent stability without an obvious loss of the Pt content. All these results demonstrate that our catalyst can be efficiently recycled for repeated utilization without compromising their dispersibility and catalytic activity. The pH-switchable Pt-UM photocatalyst also holds promise in gas therapy. For example, the tumor microenvironment is characterized by low pH; thus it can be exploited as endogenous stimuli for targeted H2 generation as well as tumor inhibition by using the Pt-UM catalyst.39 Meanwhile, the H2 production from Pt-UMs could be inhibited in healthy normal cells due to their weak alkaline environment, thus protecting the normal cells from the damage of H2. Such a cancer-selectivity effect of catalysts is highly desirable, endowing our proposed gas therapy advantages over traditional therapeutic strategies. Conclusion In summary, polymeric UMs containing single-metal atoms are prepared for photocatalytic water splitting, which combine advantages of stimuli-responsive polymer assemblies and highly efficient single-atom catalytic system. The water affinity of the polymer chains guarantees the catalyst with excellent dispersibility in an aqueous medium, which circumvents the mass transfer limitation and improves the accessibility of active catalytic sites, thus boosting the photocatalytic water-splitting efficiency. Moreover, such a self-assembled UM catalyst with single-metal atoms exhibits a pH-responsive reversible change in their aggregation state, allowing a reversible "on/off" switch of the photocatalytic ability and consequently recyclability of the catalyst. We believe that our study will widen the application horizon of molecular catalysts and polymer assemblies to the construction of smart and efficient homogeneous catalytic systems for a wide range of applications, including green organic synthesis, enzyme mimics, artificial photosynthesis, and targeted gas therapy. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21774076, 21890730, 21890733, and 51773115), the Program for Basic of Shanghai Science and and the Program of Shanghai and the Shanghai The Shanghai and for the The also the Jiangsu Center of Functional Materials for great 1. Zhou for and to 2. as Unimolecular 3. of and Unimolecular Micelles in 4. Zhou and and 10, and of for on the of Zhou with Polymeric Zhou of and Jiao by of 10, Mai Zhou Zhou by in Feng of by Functional Polymer toward Zhou of with and for with Mai of in in of and Polymer into and Metal in and of Metal for of Zuo Mai Zhou with of Pt for Photocatalytic H2 Jiao Pt into a for to with and in 10, Pt for Tong of and for Photocatalytic Zhou on for the of to Zhou Micelles for a Water at pH by a 10, Switchable for Switchable to and Polymeric for from to Information & Chemical work was financially supported by the National Natural Science Foundation of China (21774076, 21890730, 21890733, and 51773115), the Program for Basic of Shanghai Science and and the Program of Shanghai and the Shanghai The Shanghai and for the The also the Jiangsu Center of Functional Materials for great

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