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

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 Zuo School of Chemistry Chemical Engineering, Frontiers Science Center Transformative Molecules, State Key Laboratory Metal Matrix Composites, Shanghai Electrical Insulation Thermal Ageing, Jiao Tong University, 200240 Google Scholar More articles by this author , Feng Institute Functional Nano Soft Materials (FUNSOM), Jiangsu Carbon-Based & Devices, Soochow Suzhou 215123 Zhong *Corresponding authors: E-mail Address: [email protected] 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, switchability is a great challenge. Here, address challenge, we extend the concept polymeric unimolecular micelles (a typical self-assembled structure) construction stimuli-responsive recoverable molecular catalyst single-metal atoms that exhibits switchable photocatalytic activity splitting. In micelle-based catalyst, Pt site stabilized pH-sensitive hydrophilic polymer shell, which affords novel excellent dispersibility, unencumbered mass transfer, high active accessibility. Furthermore, pH-responsive shell renders capability evolution via splitting upon visible light irradiation (? > 420 nm). When exposed an acidic medium, exhibit remarkable ability rate 49,465 ?mol g(Pt)?1 h?1, superior those most reported Pt-based photocatalysts. alkaline aggregate together, their “switched off” accordingly. Meanwhile, can be simply recovered from solutions reused in media, allowing recyclable utilization photocatalyst. Download figure PowerPoint Introduction Polymer self-assembly has attracted considerable attention recent decades due its versatile construct diverse delicate nanostructures micelles,1–4 vesicles,5–7 tubes8–10 well potential practical applications these structures many areas.11–18 For example, field, assemblies have proven ideal scaffolds incorporate metal nanoparticles heterogeneous catalysis. The protection effectively alleviate aggregation. associated chains may impart resulting catalysts tailor-made functions, environmental stimuli-responsiveness targeting capability.19–22 However, there also exist number severe challenges hybrid assemblies. First, colloidal stability solvents, especially water, serious problem. loaded mismatched sizes significantly affect behavior polymers, leading aggregation precipitation assemblies.23 Second, recovery highly difficult noncontrol over redispersion solvents.24 Third, important issue, assemblies, suffer low atom efficiency thus limited owing fact only small fraction on surfaces serve sites.25 Recently, various containing suitable supports demonstrated advantage metal–atom economy catalysis.26–32 Inspired appealing concept, here report, first time, nanostructure homogeneous photocatalysis aqueous solution, combines advantages stability, recoverability, catalysis efficiency. are based (UMs) amphiphilic star-shaped (PtTHPP-star-PDMAEMA) consisting 5,10,15,20-tetrakis(4-hydroxyphenyl) platinum porphyrin (PtTHPP) core four poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) arms (Scheme 1). ion PtTHPP was reduced form then solution PDMAEMA shell. resultant denoted Pt-UMs. UM structure provides accessibility, enabling generate ? nm visible-light irradiation. thanks pH-responsiveness shells, Pt-UMs showed dispersion/aggregation through adjusting pH value reaction “on/off” switch recyclability Scheme 1 | Schematic representation single atoms. dispersed UMs catalytically active, whereas aggregated sharply reduced. Results Discussion porphyrin-based star polymer, PtTHPP-star-PDMAEMA, synthesized oxyanionic polymerization ( Supporting Information Figures S1–S6).33 To ensure PtTHPP-star-PDMAEMA average degree 24 arms, total molar around 4.6 kD polydispersity 1.05. Then, ions cores were synthetic procedures characterizations shown S7–S10). First all, dispersibility evaluated. As Figure 1a, self-floated inherent hydrophobicity. contrast, after grafting water-soluble chains, turned could (Figure 1b). Water contact angle analyses further confirmed functionalization-induced hydrophilicity, angles (C.A.) 113° 19°, respectively (Figures 1c 1d; S11). Due feature, self-assemble solution. critical concentration (CAC) determined ca. 0.2 mg/mL at = 2 S12). below CAC, investigated 0.1 mg/mL, is, condition used subsequent evaluation performance. TEM images show molecules formed ultrasmall diameter 2.8 ± 1.0 1e), supported dynamic scattering (DLS) analysis, revealing mean hydrodynamic (Dh) 3.4 1.4 S13). UV–Vis absorption spectra 1f) display no observable change position maximum varying concentration, intensity 403 increases linearly increase (inset 1f). This result line Lambert–Beer law, indicating do not range our experimental concentrations. situation attributed strong intermolecular electrostatic repulsion among suppress molecules. 1. Photographs showing comparison (a) (b) PtTHPP-star-PDMAEMA. measurements (c) (d) (e) image Inset: Statistical size distribution 200 black spots (e). (f) Absorption different linear relation intensities concentrations (red line). identify status species distributed micelles, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) performed. Ultrasmall bright ranging observed without presence or clusters 2a). Pt-loading content 4.1 wt % inductively coupled plasma optical emission spectroscopy (ICP-OES), accords calculated (4 %). Such demonstrates stabilizing probe local atomic level, extended X-ray fine (EXAFS) near-edge (XANES) recorded. EXAFS two notable peaks 1.5 2.5 Å 2b), Pt–N Pt–N–C bonds,29 respectively. While obvious Pt–Cl (?1.9 Å) Pt–Pt (? 2.7 detected PtCl2 foil, dispersion micelles. XANES spectra, it seen white-line peak locates between foil PtO2 2c), suggesting present positive charges.26 Moreover, similar XANE S14), demonstrating modification negligible effect coordination environment Pt. high-resolution 4f photoelectron (XPS) spectrum doublet signal (4f7/2 4f5/2) 72.7 76.0 eV, respectively, (II) (0) 2d), confirming partially oxidized form.30 2. Characterizations sites. Aberration-corrected HAADF-STEM L3-edge Pt-UMs, PtCl2, foil. normalized PtO2, High-resolution XPS gain insight into recombination photogenerated electrons holes, photoluminescence (PL) 3a, nearly quenched PL emission, improved charge separation suppressed electrons–holes recombination, beneficial enhancement time-resolved 3b) reveal much shorter lifetime (3.5 ns) than (10.1 ns), faster transfer ring atom.28,31 electrochemical impedance plots, smaller arc radius compared 3c), lower resistance medium. 3d, enhanced photoinduced charges stronger photocurrent response. Accordingly, expected possess activity, profiting phase. 3. Photoelectrochemical PtTHPP. Steady-state spectra. Time-resolved Nyquist inset shows simulated equivalent electrical circuit, Rs Rt represent electrolyte interfacial charge-transfer resistance, Photocurrent–potential curves We evaluated production splitting, ascorbic acid sacrificial agent comparison, Zn sites (denoted Zn-UMs) strategy S15–S17). hydrophobic employed control sample. Zn-UMs verify role photocatalysis, given micellar structure, but consequence, generation 27.7 g?1 h?1 4a), probably because carrier recombination. It therefore reflects play crucial process. Similarly, poor 66 h?1. chose charged complex PtTCPP [PtII tetrakis(4-carboxyphenyl)porphyrin] sample, organic substituents unit enables solubility S18a). S18b, H2-production 212 µmol degradation (from 88 seventh hour), absence chains. difficulties utilization.34 These results indicate effective achieving efficient photocatalysts 4. rates samples aggregates under 10 mg/mL. corresponding image. Photograph recycling use solutions. duration time each cycle 5 h, blue represents values (pH 10), expresses pH. Benefiting particle reactants access center. previous studies evolution,27,28 process, ? conjugation harvester photoexcited produce electrons. transferred cocatalyst reduction reactions. system, overpotential cocatalyst, directly trap intimate contacted rings behave proton production. best 2028 (or mass). (see Table S1 example). 30-fold corroborates effectiveness UM-based building catalysts. another experiment, 84 synthesized, mixed 2.0 S19). conditions, mixture suffered standing several minutes, instability mixture. result, exhibited toward (only 26.3 h?1) cycling degradation. necessity covalent linkage state so utilized rates, catalysis.35,36 introduction allows hydrophilicity dispersion–aggregation consequently activation–deactivation ability. Under 2), protonated soluble, behaving UMs, freely reach surfaces. Thus, they activity. increased 10, above pKa (?7.5) PDMAEMA,37 deprotonated became hydrophobic. led formation HAAD-STEM observations 4b, S20 S21), revealed larger particles 110 37 (further DLS analysis giving Dh 136 28 nm, S22). massive hindered undesirable accessibility sites, very hydrogen 172 redispersed 4c). displays reversible pattern response alkali mediums 4d), addition HCl/NaOH solutions, buffer (H3PO4?NaH2PO4 NaHCO3?Na2CO3), provide stable S23). production, confirms A drawback implementation well-dispersed lies difficulty uses.38 Impressively, found photocatalyst media coagulation collected centrifugation redissolved With simple protocol, repeatedly cycles. 4d, deterioration during cycling, 95% initial retained three h 1H NMR retention chemical composition cycles S24–S27), structural stability. measured ICP-OES, validating loss content. All demonstrate efficiently recycled repeated compromising pH-switchable Pt-UM holds promise gas therapy. tumor microenvironment characterized pH; exploited endogenous stimuli targeted inhibition using catalyst.39 inhibited healthy normal cells weak environment, protecting damage H2. cancer-selectivity desirable, endowing proposed therapy traditional therapeutic strategies. Conclusion summary, prepared combine single-atom system. affinity guarantees circumvents limitation improves boosting water-splitting state, catalyst. believe study will widen application horizon smart wide applications, including green synthesis, enzyme mimics, artificial photosynthesis, available. Conflict Interest There conflict interest report. Acknowledgments work financially National Natural Foundation China (21774076, 21890730, 21890733, 51773115), Program Basic Research Technology Commission (17JC1403200, 19JC1410400, 19JC1410404), Academic Leader (19XD1421700), Eastern Program. authors appreciate Synchrotron Radiation Facility (SSRF) (Beamline BL14W1 BL11B) synchrotron beam time. thank Collaborative Innovation Biomedical support. References Qi M.; Y.Multimicelle Aggregate Mechanism Spherical Multimolecular Micelles: From Theories, Characteristics Properties Applications.Mater. Chem. Front.2019, 3, 1994–2009. Pang X.; Zhao L.; Akinc Kim J.; Lin Z.Novel Amphiphilic Multi-Arm, Star-Like Block Copolymers Micelles.Macromolecules2011, 44, 3746–3752. Wang Li He W.; Wu C.Formation Hyperbranched Terpolymers One-Pot Copolymerization.Macromolecules2015, 48, 7327–7334. Zhang Xu Chen D.; Shi Y.; Shen Z.Polymeric Supramolecular Systems: Design, Assembly Functions.Acta Polym. Sin.2019, 50, 973–987. 5. 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