Synthetic Melanin Hybrid Patchy Nanoparticle Photocatalysts - The

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Synthetic Melanin Hybrid Patchy Nanoparticle Photocatalysts Yuan Zou, Zhao Wang, Zhan Chen, Quan-Ping Zhang, Qiujing Zhang, Yu Tian, Shijie Ren, and Yiwen Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10469 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 11, 2019

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The Journal of Physical Chemistry

Synthetic Melanin Hybrid Patchy Nanoparticle Photocatalysts Yuan Zou,1 Zhao Wang,2 Zhan Chen,1Quan-Ping Zhang,3Qiujing Zhang,1 Yu Tian,1 Shijie Ren,1 and Yiwen Li1, * 1

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials

Engineering, Sichuan University, Chengdu, 610065, China 2

Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States

3

State Key Laboratory of Environment-friendly Energy Materials, Southwest University of Science and

Technology, Mianyang 621010, China E-mail: [email protected] (Y. L.), Tel: +86 028-85401066.

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ABSTRACT

Rational design and preparation of bioinspired polydopamine (PDA)-based synthetic melanin hybrid materials with well-controlled morphologies and improved properties face a grand challenge at current situation. Herein we report the facile fabrication of photocatalytic CdS@PDA patchy nanoparticles with tunable inorganic patchy densities via different Cd2+-loaded PDA precursors. Both of “post-doping” and “pre-doping” strategies can be used to fabricate CdS@PDA hybrid NPs with distinct CdS patchy densities, which could further induce distinct physical properties and catalytic behaviors. Those resulting functional nanocomposites exhibited significantly enhanced photoactivity and photostability toward the catalytic degradation of methylene blue under visible light irradiation. We believe that the excellent adsorption, redox potential and free radical scavenging properties of PDA substrate could provide the outstanding photoactivity and photostability for CdS photocatalytic reaction. This work could inspire more kinds of synthetic melanin-based functional hybrids for stable, efficient, and sustainable photocatalysis.


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1. INTRODUCTION

Melanin is widely existed in human’s body, which plays an important role in human’s physiological and pathological activity.1-2 In epidermis, melanin protects the skin from ultraviolet penetration through the optical and filtering property. 3 As synthetic analogs of the natural pigment melanin, polydopamine (PDA) nanoparticles (NPs) have been under active investigations in the recent years due to their bioinspired excellent properties,4-8 such as free radical quenching,9 photothermal therapy,10 and metal ion chelation.11 The robust binding affinity of catechol groups to a wide range of metal ions, such as Fe3+,12 Mn2+,13 Gd3+,11 Ti4+,14 and many others,11, 15 allow the facile preparation of various PDA/metal ion complexes with interesting applications in contrast agent12,13,11 antimicrobial16-17 and catalyst.15, 18 More importantly, Gianneschi’s group recently developed a general and efficient methodology (socalled pre-doping strategy) to greatly increase and precisely tune the metal ion loading in PDA NPs,11,12,11 which significantly expands the scope of their utilizations in kinds of new areas. Considering many promising features original from PDA/metal ion complex precursor systems, it could be very interesting to continue to develop new PDA-based synthetic melanin functional hybrid materials with well-controlled morphologies and improved properties.19 Cadmium sulfide (CdS) is one of the most attractive semiconductor with a narrow band gap (~2.4eV) for photocatalytic applications.20-22 However, there are several drawbacks in CdS photocatalytic system. For example, CdS particles tend to aggregate into larger particles easily in aqueous solution, usually resulting in smaller surface area and higher recombination rate of photoinduced electron(e-)-hole(h+) pairs.23-25 Another major concern is related to the photocorrosion issue of CdS by oxidation under visible light.26 Although extensive efforts have been devoted to the use of CdS and other semiconductor heteroarchitectures to address those problems over the past years,27,28 recent attentions turned to employ water-soluble polymers as the good support and protection matrix for CdS.29-30 Notably, the excellent dispersity in water, high surface area, low toxicity and tunable metal loading properties make PDA ACS Paragon Plus Environment

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become the promising polymer matrix support for CdS catalyzation in aqueous solution.11, 19 In addition, the presence of CdS nanocrystals in PDA also provides a synergistic effect with photocatalysts via π−π* electron transition so that the photocatalytic activity can be favored.31 Moreover, the excellent free radical scavenging feature of PDA material might be able to (1) block the attack from active species (i.e. OH· radicals) induced from photoinduced electrons,9 and (2) provide electron-rich environment to compensate the photo-induced hole.32 All those issues suggest that the hybridizing CdS with PDA may efficiently reduce the photocorrosion damages under visible light without sacrificing the total photocatalytic activity. And Cd2+-loaded PDA (Cd2+@PDA) NPs seem to be the suitable precursors for further hybrid materials fabrication. In this work, we report the facile solution preparation of CdS-loaded PDA (CdS@PDA) patchy NPs from Cd2+@PDA precursors for the first time. Both of “post-doping” and “pre-doping” polymerization approaches have been used to synthesize the precursors with different Cd2+ loadings, finally resulting in CdS@PDA NPs with distinct inorganic patchy densities (Scheme 1). The total photocatalytic activity of the CdS@PDA NPs is highly depending on the amount of the attached CdS NPs. We believe that the use of “pre-doping” approach can significantly increase the CdS loading, that is critical for increasing the activity at the same amount of materials. The photocatalytic activity of prepared CdS@PDA NPs is further evaluated by investigation on the degradation behaviors of methylene blue (MB) under visible light irradiation. Moreover, the photostability of those hybrid catalysts has also been carefully discussed. 2. EXPERIMENTAL SECTION 2.1 Materials. Dopamine hydrochloride (98%), cadmium (II) sulfate (99%), tris(hydroxymethyl) aminomethane (Tris) (99.5%), ammonia aqueous solution (25 wt%-28 wt%), thioacetamide (TAA) (99%), methylene blue (99%), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na) (99%), tertbutyl alcohol (t-BuOH) (99%) tetrabutylammonium hexafluorophosphate (Bu4NPF6) (99%), ferrocene (98%) and CdS (99.5%) were all purchased from J&K Scientific Ltd. (Beijing, China). All chemicals were freshly used without further purification. Deionized water was used in all experiments. ACS Paragon Plus Environment

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2.2 Synthesis of Cd2+@PDA NPs. (1) Cd2+@PDA-1 was prepared as follows: Firstly, 500mg dopamine hydrochloride was dissolved in a mixture of 100mL deionized water and 40mL ethanol under magnetic stirring at room temperature for 10min. Subsequently, 2.5mL ammonia aqueous solution (25 wt%-28 wt%) was injected into the above mixture solution. The solution immediately turned brown and then turned black gradually in 30min. The reaction proceeded for 12h. The PDA NPs were obtained by centrifugation and washed with deionized water for three times. Then 100μL of Cd2+ solution (30mg/mL) was added to 20mL of PDA NPs solution (3mg/mL) under vigorous stirring. After 3h, the Cd2+@PDA-1 were separated by centrifugation. The isolated Cd2+@PDA-1 was washed with deionized water for five times to remove the residue Cd2+thoroughly. The amount of Cd2+ was 1.5 wt% measured by inductively coupled plasma emission spectrometry (ICP-AES). (2) Cd2+@PDA-2 were prepared according to a pervious method with modifications.12 45mg of dopamine hydrochloride and 5mg Cadmium(II) sulfate were fully dissolved in 100mL deionized water under magnetic stirring at room temperature for 1h. Subsequently, 50mL aqueous solution of 800mg Tris was quickly injected into the established solution (pH=9.35). The color of the solution immediately tuned brown and gradually tuned black after 0.5h. After another 1.5h, the Cd2+@PDA-2 were separated by centrifugation and washed three times with deionized water. The amount of Cd2+ was 8.4 wt% measured by ICP-AES. 2.3 Synthesis of CdS@PDA NPs. Both CdS@PDA-1 and CdS@PDA-2 NPs were prepared through a similar approach. 2mL of TAA (50mg/mL) aqueous solution was added to 10mL of Cd2+@PDA solution (2mg/mL) under vigorous stirring at 80℃. After 2h, the solution was centrifuged at 12000rpm, then the precipitated CdS@PDA NPs were washed with deionized water three times. The CdS@PDA NPs powder was finally obtained by lyophilization. The amount of CdS in CdS@PDA-1 and CdS@PDA-2 was 1.9 wt% and 11.4 wt% measured by ICP-AES, respectively. 2.4 Characterization. Powder X-ray diffraction (XRD) patterns were recorded by an Ultima IV diffractometer with Cu Kα (λ = 0.154 nm) radiation at 40 kV and 40 mA. The experiments were ACS Paragon Plus Environment

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performed in the diffraction angle range of 2θ = 10-70°. The surface morphology of the samples was acquired on scanning electronic microscope (SEM) of FEI Quanta 250. A Hitachi HD-2300 Dual EDS Cryo STEM was used for EDS mapping images. The samples were prepared on the mica sheet (1mm × 1mm) by spin-coater (KW-4A, institute of microelectronics, Chinese Academy of Science). The mica surface was freshly cleaved by lifting off a thin layer of mica sheet with an adhesive tape. The microstructures were obtained by transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) with a JEOL JEM-2100 microscope at an accelerating voltage of 200kV. Sample were prepared by drying a drop of the solution on a copper grid and allowed to dry completely at room temperature. UV-vis diffuse reflectance spectra were collected on a Shimadzu UV3600

UV-vis

spectrometer.

Deionized

water

was

used

as

the

reference.

Dynamic light scattering (DLS) and zeta potential measurements were characterized by a Malvern Zetasizer NanoZS particle analyzer. Samples were dispersed in water after sonification for DLS measurements. A folded capillary cell equipped with gold electrodes was used for Zeta potential measurements. The temperature was kept at 25.0℃. The waiting time before recording the measurement was 2min. Total organic carbon (TOC) was monitored using a Shimadzu TOC-500 Total Organic Carbon analysis system. EPR (Electron Paramagnetic Resonance) testing of PNs was performed on Bruker EPR EMX_Plus. The spectrometer was operated at X-Band (9.85GHz) and the spectra were obtained with 100 kHz field modulation at 0.1 mW power. X-ray photoelectron spectroscopy (XPS) measurements were conducted with a VG ESCALAB MKII spectrometer. Cyclic voltammogram (CV) and impedance measurements were conducted on a CHI760E electrochemical workstation using sample film coated indium tin oxide (ITO) as the working electrode, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode in a 0.1M tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. The ferrocene/ferrocenium (Fc/Fc+) couple was used as an internal reference. 2.5 Determination of Cd (II) Concentration. To determine Cd (II) concentration, the metal was first stripped from the as-prepared samples using the following procedure. To an aliquot of each sample (10μL) was added 1% HNO3 in deionized water (9.90mL). Each mixture was then stirred for 12h. The ACS Paragon Plus Environment

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amount of cadmium was then determined by inductively coupled plasma emission spectrometry (ICPAES) of IRIS Adv in college of Chemistry, Sichuan University. 2.6 Photocatalytic Reaction. A 300W Xe arc lamp (PLS-SXE 300C, Beijing Perfectlight Co. Ltd) with an UV cut-off filter (λ≥420nm) was used as a visible light source for MB degradation. The optical density of light source was 300 mW/cm2. The illuminated area was a circle with a radius of 2.6 cm. The distance between the liquid level and light source was 10 cm. In a typical photocatalytic experiment, a suspension of 3mg catalyst and a fresh MB aqueous solution (100mL, 15mg/L) were stirred magnetically in the dark for 10h to reach the adsorption equilibrium prior to the photocatalytic reaction. Then, an aliquot of 5mL sample was collected at 20 min intervals after irradiating and the photocatalyst was removed by centrifugation. The concentration of the residue MB was determined by the maximum absorption band (662 nm) by ultraviolet–visible (UV–vis) diffuse reflectance spectra. The active species generated during the photocatalytic experiment were detected through trapping by EDTA-2Na as holes (+) scavenger and t-BuOH as hydroxyl radicals (·OH) scavenger. The concentration of the residue MB was measured by UV-vis in the same way. In a typical photostability experiment, a suspension containing 3mg CdS, CdS@PDA-1 or CdS@PDA-2 was used. After the photocatalytic reaction for 2h, the photocatalysts were separated from the reaction system by centrifugation, then washed by deionized water and ethanol and collected by dried at 50℃。The photocatalysts were reused for photocatalytic degradation reaction to evaluate the photostability under the same conditions.

3. RESULTS AND DISCUSSION

3.1 Fabrication and Characterization of CdS@PDA In general, the CdS patchy density of CdS@PDA NPs highly relied on the Cd2+ loading amount in Cd2+@PDA precursors, which can be easily tuned by using different metal ion doping strategies.12 In this study, we employed two typical doping strategies, called “post-doping” (Scheme 1a) and “preACS Paragon Plus Environment

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doping” (Scheme 1b) polymerization approaches, to successfully prepare two kinds of Cd2+@PDA precursors with Cd2+ loading ~ 1.5% (named Cd2+@PDA-1) and ~ 8.4% (named Cd2+@PDA-2), respectively. Note that the Cd2+ loading in NPs can be continuously tuned by simply adjusting the monomer concentrations of Cd (II) (dopamine)233 and free dopamine via “pre-doping” polymerization method (Scheme 1b). The resulting Cd2+@PDA precursors can further transfer to CdS@PDA hybrids by using thioacetamide as the S2- source.34 The S2- was released from thioacetamide at 80℃ in aqueous solution, and diffused into PDA matrix to complex with loaded Cd2+. CdS nanocrystals were then in-situ nucleated and grown from PDA NP matrix, finally resulting in the CdS@PDA patchy particles (called CdS@PDA-1 and CdS@PDA-2). The result showed that we were able to get 5 times higher CdS loading by using the pre-doping strategy (11.4%) compared to post-doped strategy (1.9%), determined by ICP-AES. All the byproducts can be easily removed by repeated centrifugations. Notably, those two complementary methods enable the successful fabrication of CdS@PDA with a wide range of inorganic content within the patchy particles. Newly formed CdS@PDA-1and CdS@PDA-2 patchy NPs were characterized by transmission electron microscopy (TEM) (Figure 1a and S1a) and scanning electron microscopy (SEM) (Figure S1b-c) to quantify their morphologies and uniformity. The combination of TEM (Figure 1a and S1a) and SEM (Figure S1b-c) data shows uniform patchy NPs with diameters ~ 190 nm for CdS@PDA-1 and ~ 140 nm for CdS@PDA-2, which are in good agreement with the results from DLS measurements (Figure S2). Both TEM (Figure 1a and S1a) and SEM (Figure S1b-c) results also clearly confirm the random distribution of CdS nanoparticle within PDA NP surface without aggregation. Additionally, the presence of CdS nanocrystals within the PDA NPs was further evidenced by the results from high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) (Figure 1b), high-resolution TEM (HRTEM) (Figure 1c and S1d), and X-ray powder diffraction (XRD) (Figure 1d). For example, selected area HAADF-STEM coupled with energy dispersive X-ray spectroscopy (EDS) elemental mapping analysis result suggested that the content of Cd and S in the testing area of CdS@PDA-2 were significantly higher than that on the grid surface background (Figure 1b). HRTEM images clearly showed the lattice fringes with d-spacing of 0.203nm (Figure 1c) and ACS Paragon Plus Environment

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0.334nm (Figure S1d) in CdS region, which can be assigned to the (220) and (111) lattice planes of cubic phase CdS crystal, respectively.35 Those observations matched well with the XRD testing results from CdS@PDA samples, which exhibit several characteristic peaks corresponding to (002), (110), (112) planes of CdS nanocrystals at 2θ=26.5°, 43.7° and 51.8° (JCPDS No. 65-2887). All results demonstrated the well installation of CdS nanocrystals into PDA NP matrix. Moreover, the Zetapotential testing showed intrinsic negative surface charges (Table S1) for the resulting CdS@PDA patchy NPs, confirming their high stability in the aqueous solution. Notable, the UV-vis spectra showed that both CdS@PDA-1 and CdS@PDA-2 samples possess strong absorption across the visible light range, which allows their photocatalytic performance under visible light irradiation (Figure S3). Nitrogen adsorption measurement was employed to estimate the surface area of the pure CdS, CdS@PDA-1 and CdS@PDA-2, and the results were shown in Figure 2. The nitrogen uptake of CdS@PDA samples was increased at high relative pressure P/P0 =0.9-1.0 and a hysteresis loop was obtained, indicating the microporous structural feature of those hybrid patchy particles. The BET surface area and pore volume data of CdS@PDA-1, CdS@PDA-2 and CdS were summarized in Table 1. Compared to pure CdS, both CdS@PDA samples possess large BET surface area and pore volume, suggesting the efficient absorption property of CdS@PDA hybrid materials. Furthermore, electrochemical impedance spectra of CdS@PDA samples were shown in Figure S4. The semicircle portion at high frequencies represented the electron transfer limited process while the linear part at the low frequencies was related to the diffusion limited electrochemical process.36 The diameter of semicircle of CdS@PDA-2 is smaller than that of CdS@PDA-1, indicating that CdS@PDA-2 has less electron transfer resistance and better electron transfer capability for photocatalytic application.

3.2 Photocatalytic Performance of CdS@PDA The photocatalytic efficiency of prepared CdS@PDA NPs was further determined by studying the degradation behavior of MB in aqueous solution under visible light irradiation (λ≥420nm). The pure ACS Paragon Plus Environment

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CdS and PDA NPs were used as the reference, and all the testing samples were dispersed in the MB aqueous solution with stirring for 600 mins to reach the adsorption equilibrium (Figure S5). It was observed that both CdS@PDA samples can adsorb large amounts of MB, which provided good supplements for the high photocatalytic activity of the hybrid photocatalysts. For example, the decrease of UV-vis absorbance of MB solution suggested that MB was quickly degraded in keeping with the lighter of the color of MB solution (Figure 3a). The normalized temporal MB concentration changes (C/C0) during the degradation were further employed to investigate the photocatalytic activities, and their results are presented in Figure. 3b. The blank experiment indicated that only a very small portion of MB can be degraded under visible light without any catalysts, and pure PDA NP solution showed a similar degradation rate as blank experiment. Both of CdS@PDA-1 and CdS@PDA-2 demonstrated better degradation activity for MB degradation than pure CdS (Figure 3b). All of their photodegradation processes followed pseudo-first-order kinetics and their reaction rate constants k are ~ 0.0035 min-1 (CdS), ~ 0.0046 min-1 (CdS@PDA-1) and ~0.0065 (CdS@PDA-2) min-1, respectively. Notably, only 0.03mg mL-1 CdS@PDA catalysts were used for the photocatalytic experiment, indicating that the used contents of CdS in both CdS@PDA-1 and 2 (1.9 wt% and 11.4 wt%, respectively) were much lower than the pure CdS reference. Interestingly, both CdS@PDA samples still performed much better degradation rate than pure CdS. Therefore, the degradation results suggested that the existence of PDA matrix was able to improve the catalytic activity remarkably. To further investigate the photocatalytic ability of CdS@PDA catalysts, total organic carbon (TOC) measurement was carried out and the result was shown in Figure S6. After 3h photocatalytic reaction, 33% and 67% TOC was removed with CdS@PDA-1 and CdS@PDA-2 at the same weight of catalysts (0.03mg/mL), respectively. However, the removal of TOC was only 15% with pure CdS within 3h. It could be clearly concluded that most MB could be efficiently converted into small molecules such as CO2, H2O and NO3- by both CdS@PDA samples. All the discussions above confirmed that the introduction of PDA substrate enhanced the catalytic ability of CdS for MB photodegradation. The photostability and reusability of both CdS@PDA catalysts were further evaluated by collecting and reusing them for three cycles. As shown in Figure 4aACS Paragon Plus Environment

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b, two kinds of CdS@PDA samples maintained their photocatalytic activities even after three rounds of testing, and the slight decrease in photocatalytic activity might be caused by the loss of materials during the centrifuging and washing at each round. All the evidences above clearly indicated that the PDA matrix could protect CdS semiconductor nanocrystals from photocorrosion significantly. This conclusion was also fully confirmed by the XRD results from those three catalysts before and after three cycles of photocatalytic reactions (Figure 4c-d, Figure S7). For example, the characteristic diffraction peaks of CdS in the XRD pattern are almost missing after three photocatalytic recycles (0.15mg mL-1), suggesting severe photocorrosions. This was also evidenced by the complete disappearance of yellow color in CdS solution (0.03mg mL-1) after irradiation for 2h by visible light (Figure S8). However, both XRD patterns of CdS@PDA-2 (Figure 4d) and CdS@PDA-1 (Figure S7) had no notable differences before and after the photocatalytic recycles, which indicates that they are photostable and not photocorroded. After three cycles of photocatalytic experiments, it was found that the both CdS@PDA samples almost kept the same morphology as the unreacted ones (Figure S9). From the SEM and TEM images, it could be found that both of the PDA substrate and CdS patchy were not degraded. This is consistent with the UV-Vis testing result, where the spectra of CdS@PDA samples after three cycles were almost the same as the spectra of unreacted ones (Figure S10). EPR experiments were also carried out to evaluate the stability of PDA substrate and the results were shown in Figure S11.14 After 3 cycles of photocatalytic reactions, the peak shape of both CdS@PDA samples in EPR spectra were maintained as same as the unreacted ones, and no new peaks occurred in the spectra.14 All those observations again revealed the PDA substrate could effectively suppress the photocorrosion damage to the whole patchy NPs. Moreover, many other types of photocatalytic reactions, such as photodegradation of RhB, are also important in the photocatalytic reactions.37 Further experiments to elucidate wide application of CdS@PDA-2 in those photocatalytic reactions will also be performed. 3.3 Photocatalytic Mechanism Investigation


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To further study the photocatalytic mechanism of the resulting hybrid patchy particle, the main active species (photogenerated holes and electrons) in the photocatalytic system were discussed. The main oxidative species were trapped by 1mmol EDTA-2Na (general hole scavenger) and 1mmol t-BuOH (general radical scavenger), respectively.24,

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Under illumination by visible light, electrons (e-) are

excited and leaving photogenerated holes (h+) as oxidative species. On the other hand, exited electrons can react with oxygen and produced superoxide radical anion O2·- as the oxidizing agent.39-41 Their degradation rates were measured as the normalized temporal MB concentration changes in per minute. As shown in Figure 5a, the degradation rates showed a decrease in both CdS and CdS@PDA systems after addition of EDTA-2Na or t-BuOH. Notably, the photodegradation efficiency decelerated more when EDTA-2Na was used as the trapping agent compared with t-BuOH used as the trapping agent. Therefore, we believed the photogenerated holes instead of photogenerated electrons are the main oxidants. In CdS@PDA system, the PDA surface could gather the MB molecules where CdS is located, resulting in the direct oxidation of MB by photogenerated holes, and the holes-induced photocorrosion might also be the dominant photocorrosion suppression in this system As is known to all, pure CdS particle is usually unstable in aqueous solution under visible light via photoinduced holes mechanism primarily.26, 42The photocorrosion process can be depicted as follows: CdS + hv → h+ + e-

(Eq. 1)

CdS + 2 h+ → Cd2+ + S

(Eq. 2)

In addition, the MB molecules are mainly oxidized by the photoinduced holes, as discussed above. So, a competing behavior is existed between photocatalytic MB degradation process and CdS photocorrosion process. MB is a typical cationic dye, that can be quickly adsorbed onto PDA matrix surface due to the electrostatic and π-π stacking interactions43,44. As Figure 5b shown, the conduction band (CB) potential and valence band (VB) potential of CdS in aqueous solution are -0.66V and +1.70V (pH=7), respectively. The latter is much higher than its decomposition potential (Ede = +0.32V, pH=7) so that the ACS Paragon Plus Environment

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photocorrosion is prone to occuring by the oxidation of photoinduced holes.26 Nevertheless, the oxidation potential of MB (+0.01V, pH=7) is lower than the Ede of CdS, so MB is prior to be oxidized first by photoinduced holes in presence of high concentration MB on PDA surface. This directly offered a way to prevent the anodic photocorrosion damage of CdS. Additionally, the free radical scavenging feature of PDA makes it easily prevent the attack from ·OH radicals generated from photoinduced electrons and provide electron-rich environment to compensate the photo-induced hole,45 leading a synergistic effect for effectively suppressing the photocorrosion mainly via redox potential mechanism (Figure 5b). The reaction mechanism was further investigated. As shown in Figure 6a, the onset oxidation potential (Eox) and onset reduction potential (Ered) of PDA were measured to be 0.39eV and 0.49eV by electrochemical cyclic voltammetry (CV) measurements, with Ag/AgCl as reference electrode and Fc/Fc+ couple as an internal reference. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels of the PDA (vs. SHE) were estimated to be 0.03eV and -0.85eV, respectively. (In our electrochemical workstation system, EFc/Fc+ was measured to be 0.36 vs. Ag/AgCl (Figure S12). Additionally, the XPS VB measurements were also performed to measure the VB of CdS@PDA-1 (0.08 eV) and CdS@PDA-2 (0.16eV), indicating that the VB value of the CdS@PDA samples could increase as the concentration of CdS (1.70eV) increasing (Figure S13). Therefore, the schematic illustration of the possible reaction mechanism for degradation of MB under visible light over CdS@PDA was proposed in Figure 6b. Under visible light irradiation, PDA was apt to absorb the light due to its narrow band gap (0.88eV). The electrons on HOMO of the PDA were excited to LUMO by absorbing enough visible light energy, leaving the photoinduced hole in HOMO of PDA. The CdS was also excited by the visible light and the electrons of VB were more easily to transfer to the HOMO of PDA so that the MB could be oxidized by the photoinduced holes of CdS. As all mentioned above, PDA matrix seems to be a quite appropriate substrate for photocorrosion suppressing of CdS regions. 4. Conclusions ACS Paragon Plus Environment

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In summary, photocatalytic synthetic melanin hybrid patchy NPs with tunable inorganic patchy densities were successfully achieved from two kinds of Cd2+@PDA precursors. Besides the superior photocatalytic activity, the photostability of those photocatalysts was significantly improved by efficiently suppressing the photocorrosion of CdS in the PDA matrix via redox potential and free radical scavenging mechanism. The PDA substrate could significantly enhance the photostability of the CdS and maintain relatively stable CdS crystal structure within the hybrid patchy particles. The easily recoverable and reusable feature are also valuable for potential application in water purification. This work inspires us to rationally design new fabrication strategy towards new synthetic melanin-based functional hybrids for stable, efficient, and sustainable photocatalysis. ASSOCIATED CONTENT Supporting Information. SEM, TEM/HRTEM, DLS, zeta potential, UV-vis spectra, and electrochemical impedance spectra of as-prepared nanoparticles, Changes of MB concentration in CdS@PDA-1 and CdS@PDA-2 NPs photocatalytic system in the dark, XRD patterns, EPR spectra, SEM and TEM images of CdS@PDA-1 and CdS@PDA-2 before and after 3 times photocatalytic degradation, 0.03mg/mL CdS aqueous solution before (left) and after (right) visible light irradiation for 2h. Cyclic voltammetry measurement of Fc/Fc+ couple, and XPS VB spectra of CdS@PDA-1 and CdS@PDA-2. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.L.)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT ACS Paragon Plus Environment

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This work was supported by National Natural Science Foundation of China (21774079 and 21574087). Y. L. also thanks the financial support from State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology (17kffk07), and State Key Laboratory of Polymer Materials Engineering, Sichuan University (sklpme2018-2-04).

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FIGURES

Scheme 1. (a) Post-doping approach towards CdS@PDA-1. (b) Pre-doping approach towards CdS@PDA-2. (TAA: thioacetamide)

Figure 1. TEM (a) and HRTEM (c) image of CdS@PDA-2. (b)HAADF-STEM image of CdS@PDA-2 and its corresponding element mapping recorded by an EDS spectrometry. (d) XRD patterns of CdS@PDA-1, CdS@PDA-2 and PDA NPs.


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Figure 2. N2 sorption isotherms of CdS@PDA-1, CdS@PDA-2 and CdS.

Table1. BET surface area and pore volume of CdS@PDA-1, CdS@PDA-2 and CdS. Samples

BET surface area (m2 g-1)

Pore volume (cm3 g-1)

CdS@PDA-1

18.293

0.066

CdS@PDA-2

48.138

0.290

CdS

0.225

0.0001

Figure 3. Photocatalytic degradation of MB over the as-prepared samples under visible light irradiation (λ ≥ 420nm). (a) Time-dependent UV-vis absorbance for the catalytic degradation of MB by CdS@PDA-2 in 2h. The inserted photograph shows the color change after 2h irradiation. (b) the normalized temporal MB concentration changes (C/C0) (The experiments were proceeded in the same concentration: 30mg L-1, and MB concentration:15mg L-1. C is MB concentration change, C0 is the initial MB concentration).


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Figure 4. Photocatalytic recycle degradation of MB over CdS@PDA-1, CdS@PDA-2and CdS under visible light irradiation (λ ≥ 420nm). (a) The MB concentration(C)changes (b) the normalized temporal MB concentration changes (C/C0)(The experiments were performed at the same material concentration: 30mg L-1 and MB concentration: 15mg L-1). XRD patterns of CdS (c) and CdS@PDA-2 (d) before and after 3 recycle of photocatalytic reactions.

Figure 5. (a) The plots of photogenerated carrier trapping in the system of photodegradation of MB over CdS and CdS@PDA-2 (EDTA-2Na and t-BuOH: 1mmol). (b) Schematic illustration of the decomposition potential for CdS and redox potentials of the related reactions with respect to the position of the CdS band edges.


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Figure 6. (a) Cyclic voltammetry measurement of PDA. (b) Schematic illustration of the possible reaction mechanism for degradation of MB under visible light over CdS@PDA.


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TOC Graphic


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Synthetic Melanin Hybrid Patchy Nanoparticle Photocatalysts - The

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