One-Pot Synthesis of Fe(III)âPolydopamine Complex Nanospheres

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One-Pot Synthesis of Fe(III)-Polydopamine Complex Nanospheres: Morphological Evolution, Mechanism and Application of The Carbonized Hybrid Nanospheres in Catalysis and Zn-Air Battery Jia Ming Ang, Yonghua Du, Boon Ying Tay, Chenyang Zhao, Junhua Kong, Ludger Paul Stubbs, and Xuehong Lu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02331 • Publication Date (Web): 22 Aug 2016 Downloaded from http://pubs.acs.org on August 27, 2016

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One-Pot

Synthesis

of

Fe(III)-Polydopamine

Complex

Nanospheres:

Morphological Evolution, Mechanism and Application of The Carbonized Hybrid Nanospheres in Catalysis and Zn-Air Battery

Jia Ming Ang,a Yonghua Du,b Boon Ying Tay,b Chenyang Zhao,a Junhua Kong,a Ludger Paul Stubbs,b Xuehong Lu*a a

School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang

Avenue, Singapore 639798 b

Institute of Chemical and Engineering Sciences, A*STAR (Agency for Science, Technology

and Research), 1 Pesek Road, Jurong Island, Singapore 627833 E-mail: [email protected]

1 ACS Paragon Plus Environment

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Abstract In this article, we report one-pot synthesis of Fe(III)-polydopamine (PDA) complex nanospheres, their structures, morphology evolution and underlying mechanism. The complex nanospheres were synthesized by introducing ferric ions into the reaction mixture used for polymerization of dopamine. It is verified that both the oxidative polymerization of dopamine and Fe(III)-PDA complexation contribute to the “polymerization” process, in which the ferric ions form coordination bonds with both oxygen and nitrogen, as indicated by X-ray absorption fine-structure spectroscopy. In the “polymerization” process, the morphology of the complex nanostructures is gradually transformed from sheet-like to spherical at the feed Fe(III)/dopamine molar ratio of 1/3. The final size of the complex spheres is much smaller than its neat PDA counterpart. At higher feed Fe(III)/dopamine molar ratios, the final morphology of the “polymerization” products is sheet-like. The results suggest that the formation of spherical morphology is likely to be driven by covalent polymerization-induced decrease of hydrophilic functional groups, which causes re-self-assembly of the PDA oligomers to reduce surface area. We also demonstrate that this one-pot synthesis route for hybrid nanospheres enables the facile construction of carbonized PDA (C-PDA) nanospheres uniformly embedded with Fe3O4 nanoparticles of only 3-5 nm in size. The C-PDA/Fe3O4 nanospheres exhibit catalytic activity towards oxygen reduction reaction and deliver a stable discharge voltage for over 200 h when utilized as the cathode in a primary Zn-air battery, and are also good recyclable catalyst supports.

Keywords: polydopamine, transition metal, complex, self-assembly, oxygen reduction reaction (ORR), catalyst


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Introduction Poly(dopamine) (PDA), a material inspired by mussel adhesives, has attracted great attention since its versatile surface deposition ability was first reported in 2007.1 Lee et al. showed that PDA could be facilely coated on almost all types of surfaces simply by self-polymerization of dopamine hydrochloride (DOPA) in basic aqueous solutions. Since then, PDA-coated bulk substrates and nanostructures have been explored for a wide variety of applications in areas such as

biomedical

science,

sensing,

water

treatment,

polymer

nanocomposites,

energy

storage/conversion, and catalysis.2-8 For example, PDA-coated Fe3O4 microspheres and nanospheres have been demonstrated as recyclable catalyst support because the PDA surface has abundant amino and catechol groups for attaching noble metal catalysts, such as Ag, Pt, Pd and Au,2, 9-12 while the magnetism of the Fe3O4 core allows for facile recovery of the used catalysts. PDA has also been found to be an excellent precursor of graphite-like N-doped carbon.13-14 Carbonized PDA (C-PDA) can provide better chemical stability for use as recyclable catalyst supports and also exhibits high electrical conductivity, facilitating its use in electrodes of energy storage devices or electrocatalysts. 12, 15-17 Apart from the ability to be facilely deposited onto various substrates, PDA could also form colloidal spheres of controlled dimensions. Ju et al. successfully produced PDA nanoparticles through the neutralization of DOPA with sodium hydroxide (NaOH).18 PDA nanospheres with well controlled size were also produced by polymerizing DOPA in a mixture of water, ethanol and ammonia. The size of these nanospheres could be controlled by simply varying the ratio of the aqueous ammonia solution to DOPA in the reaction mixture.19-21 Recently, Yan and coworkers also reported the successful synthesis of mono-dispersed PDA nanospheres with tunable diameters using mixed solvents of only deionized water and alcohol. Core/shell nanostructures,


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such as PDA/Fe3O4 and PDA/Ag core/shell nanospheres, can also be synthesized by surface modification of the as-prepared PDA nanospheres.22 Despite the vast potential applications of PDA nanospheres across many fields, their formation mechanisms have not been elucidated owing to the complex formation mechanism and structural diversity of PDA.1, 23-25 It is widely accepted that PDA is formed via the collective action of covalent bonding and non-covalent interactions.24-26 Another interesting property of PDA is that it is also able to form coordination bonds with several transition metal species such as titanium (IV) oxide,27 silver oxide28 and ferric ions29. Early studies by Wilker et al. and Sever et al. showed that the adhesive plaques in marine mussels were formed by the cross-linking of catechol-containing proteins with ferric ions.30-32 The degree of cross-linking between catechol groups and ferric ions was subsequently found be dependent on pH, with one ferric ion chelating with one, two or three catechol groups in pH range of approximately < 5.6, 5.6 - 9.1 and > 9.1, respectively.33-34 Mimicking mussel adhesive proteins, ferric ions have been used to crosslink catechol-containing synthetic polymers to produce self-healing hydrogels/networks.33 It was also reported that in the presence of nickel species or ferric ions in the reaction media, polymerization of DOPA could form Ni(II)-PDA or Fe(III)-PDA complexes.29, 35-36 Zn-air batteries (ZnABs) are a promising solution for clean energy sources due to its high energy density and the low cost of zinc. The performance of primary ZnABs is crucially influenced by the electrocatalyst on the cathode that catalyzes the oxygen reduction reaction (ORR) during the discharge process.37 Precious metal-based catalysts can efficiently catalyze ORR, however they are costly, preventing their widespread use. Transition metal nitrides,38 oxides,39-40 nitrogen doped carbon41-42 and their hybrids36, 43-46 have recently been explored as


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alternative electrocatalysts for ORR and some showed promising catalytic activity for replacing precious metal-based catalysts. Recently, Zhou et al. prepared C-PDA hollow nanospheres containing Fe3O4 nanoparticles by in situ polymerization of DOPA on SiO2 nanospheres in the presence of FeCl3, followed by carbonization and KOH etching of the SiO2 template. Li et al. prepared sheet-like C-PDA embedded with iron metal nanoparticles by polymerization of DOPA in the presence of iron salts, followed by carbonization. These C-PDA-based nanostructures were studied as non-precious metal electrocatalysts for ORR,36 while the spheres with Fe3O4 nanoparticles reported by Zhou et al. showed better performance.36 Inspired by the ability of PDA to self-assemble into nanospheres and Fe(III)-catechol coordination chemistry, in this work, for the first time we synthesized Fe(III)-PDA complex nanospheres via one-pot ferric ion-mediated polymerization of DOPA. This synthesis method is advantageous as it is a single-step template-free process under mild conditions, noting that current methods for producing such hybrid nanospheres involve multi-step reactions or spheres made of other materials as templates. To probe how the ferric ions affect the polymerization of DOPA and self-assembly of PDA, morphological evolution of the Fe(III)-PDA complex and neat PDA nanospheres was monitored. The effect of varying ferric ion/DOPA ratio on the morphology of the Fe(III)-PDA complex nanostructures was also studied. The chemical structures of the Fe(III)-PDA complex nanospheres, in particular the interactions between ferric ion and PDA, were analyzed using various spectroscopic methods, including X-ray absorption fine-structure (XAFS) and X-ray photoelectron spectroscopy (XPS). The studies shed some light on the mechanism of the complex process of ferric ion-mediated polymerization of DOPA, in which covalent bonding, coordination bonding and physical interaction-induced self-assembly take place simultaneously. Furthermore, using this simple one-pot synthesis method, the complex


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nanospheres are embedded with homogeneously distributed ferric ions and the hybrid nanospheres size is much smaller than those of neat PDA nanospheres reported in literatures. Upon heat treatment, the Fe(III)-PDA complex nanospheres can be easily converted to C-PDA nanospheres with evenly distributed Fe3O4 nanoparticles of only 3-5 nm in size, which are potentially good non-precious metal electrocatalyst. Herein we also demonstrate that such Fe3O4/C-PDA nanospheres exhibit ORR catalytic properties and deliver a stable discharge voltage when utilized as the cathode in primary ZnABs, and are also useful recyclable catalyst support.

Experimental Chemicals 3,4-Dihydroxyphenethylamine hydrochloride (DOPA), tris(hydroxymethyl) aminomethane (Tris) and iron(III) chloride (FeCl3) were purchased from Sigma-Aldrich and used without further purification. All solutions were prepared using deionized (DI) water. Preparation of Fe(III)-PDA Complex and Fe3O4/C-PDA Composite Nanospheres DOPA (1 g L-1) was dissolved in 1000 mL of DI water. FeCl3 was then added at various concentrations (5.27, 2.64, 1.76, 1.32, 1.05 and 0.88 mmol L-1) to achieve Fe(III)/DOPA molar ratio of 1:1, 1:2, 1:3, 1:4, 1:5 and 1:6, respectively. The pH of the solution was adjusted by adding in 1.2114 g (10 mmol) of Tris and the reaction mixture stirred for 72 h. The solid products obtained (Fe(III)-PDA complex) were separated from the solutions by centrifugation, washed with DI water and then lyophilized. The products were then annealed in a tube furnace at 650 °C for 3 h under constant argon flow to yield Fe3O4/C-PDA composite nanospheres. 6 ACS Paragon Plus Environment

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Characterization UV-vis analysis of the solutions was conducted on a Shimadzu UV-vis spectrometer (UV2700). The morphologies of the samples were examined using a field emission scanning electron microscope (FESEM, JEOL 7600F) at an accelerating voltage of 5 kV and a transmission electron microscope (TEM, JEOL 2010) with accelerating voltage of 200 kV. A X-ray diffractometer (XRD, Bruker D8 Discover) with Cu Kα (λ = 1.5406 Å) radiation generated at 40 mA and 40 kV was used to investigate the structure of the nanospheres in 2θ range of 5 to 90º. Scan rate used was 1º min-1 with a step of 0.02 º. Raman spectra were collected using a confocal Raman microscope (Renishaw InVia Raman Microscope) in back scattering configuration (Leica N Plain EP1 100X objective lens, NA 0.85) equipped with a charge coupling device (CCD). The laser source used was Argon ion laser with a wavelength of 785 nm. Fourier transform infrared spectroscopy (FTIR) measurements were collected using a Perkin-Elmer (Spectrum GX FTIR) spectrometer at room temperature from 500 to 4000 cm-1. X-ray absorption fine-structure (XAFS) spectroscopy was recorded at the XAFCA beamline47 at the Singapore Synchrotron Light Source (SSLS). X-ray photoelectron spectroscopy (XPS) measurements were collected on a Kratos Analytical AXIS His spectrometer with a monochromatized Al Kα X-ray source (1486.6 eV photons). Room temperature magnetization curves of the product were measured using a vibrating sample magnetometer (Lakeshore, VSM-7404). Electrode Preparation and Electrochemical Measurements Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were conducted on an Autolab potentiostat/galvanostat (PGSTAT302N) station attached to a rotating disk electrode (RDE) using 0.1 M KOH electrolyte saturated with O2 or N2. Ag/AgCl electrode (saturated with 3 M KCl) and a Pt foil were used as the reference and counter electrodes, respectively. The 7 ACS Paragon Plus Environment

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working electrode was prepared by dispersing 20 mg of Fe3O4/C-PDA in 1 ml aqueous solution of Nafion (1 wt.%, diluted from 5 wt.%, Aldrich) by ultra-sonicating for 15 min to obtain a consistent catalyst ink. The catalyst ink was pipetted onto a glassy carbon electrode (GCE, 5 mm in diameter) and allowed to dry in air overnight. The loading of catalyst was fixed at 0.5 mg cm-2. The number of transferred electrons (η) per O2 molecule in ORR was calculated by Koutecky-Levich (K-L) equations:

= + =

= 0.2 ( )

+

(1)

(2)

=

(3)

where J is the measured current density, JK and JL are the kinetic-limiting and diffusion-limiting current density, respectively; ω is the disks’ angular velocity, n is the number of electrons transferred per O2 molecule in ORR, F is Faraday constant, CO is the bulk concentration of O2, DO is the diffusion coefficient of oxygen (O2), v is the electrolytes’ kinematic viscosity and k is the electron transfer rate constant. Assembly and Test of Zinc Air Battery The performance of the primary ZnAB was tested using a self-assembled Zn-air cell. A twoelectrode configuration was used by pairing Fe3O4/C-PDA loaded carbon paper electrode (loading of 1 mg cm-2) and a polished zinc plate in 6 M KOH. The discharge tests were performed at room temperature under atmospheric condition.


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Results and Discussion Chemical structures of the Fe(III)-PDA complexes In this work, the Fe(III)-PDA complexes were synthesized by firstly dissolving a certain amount of FeCl3 into an aqueous solution of DOPA to form Fe(III)-DOPA complexes and then adding in Tris buffer to trigger the polymerization of DOPA. Although previous studies showed that ferric ions could effectively cross-link mussel adhesive proteins and catechol-grafted synthetic polymers via Fe(III)-catechol coordination bonds,30-33 so far how ferric ions would be incorporated into PDA in this one-pot polymerization process has not been clarified. In particular, the self-polymerization of DOPA occurs under basic conditions, and both DOPA and PDA are redox active, which may convert ferric ions to species such as iron hydroxides or oxides. UV-vis, FTIR, XAFS and XPS studies were thus conducted to probe the chemical structures of the hybrid obtained. For chemical analysis, the feed molar ratio of ferric ions to DOPA was fixed at 1 to 3. In the preparation process, upon the addition of FeCl3, a Lewis acid, into the DOPA solution, the pH of the solution dropped from 5.7 to 2.8 and the color of the solution changed to green, suggesting the formation of Fe(III)-catechol mono-complex.48 Absorption maxima at about 400 and 740 nm were observed in the UV-vis spectrum of the solution, which are consistent with literatures for the formation of mono-complex.33-34, 48 With the addition of Tris buffer, instantly the solution turned to purple (inset of Figure 1) and the pH was increased to 7.7. From the corresponding UV-vis spectrum (Figure 1), we can observe that the absorption maximum undergoes a blue shift to 558 nm.34 The UV-vis spectra and the corresponding pH values indicate that with the addition of Tris buffer, the solution may consist of a mixture of bis- and triscomplex Fe(III)-catechol, while the tris-complex is formed predominately.33-34 In the beginning stage of the polymerization process, no solid products could be obtained by centrifuge. By 9 ACS Paragon Plus Environment

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freeze-drying the reaction solution, some sheet-like solid products were obtained (Figure S1) but they could be completely dissolved in DI water, showing that there was no sufficient covalent polymerization of DOPA and π-π stacking of oligomers, i.e., the product was mainly composed of uncrosslinked complex moieties. After polymerizing for 72 h, a dark colored suspension was formed and the pH was reduced to 6.5. The UV-vis spectrum of the suspension, with an absorption band at 350 nm, suggests the formation of PDA oligomers and other small oxidation products.49 The products obtained by centrifuge (Fe(III)-PDA hybrid) are insoluble in water presumably owing to the cross-linking by both covalent and coordination bonds as well as π-π stacking of the oligomers.

Figure 1. UV-vis spectra of the solution before and immediately after addition of Tris, and the suspension after the addition of Tris for 72 hrs (inset: picture of the solution immediately after the addition of Tris).

In the FTIR spectrum of PDA (Figure 2a), the bands at 1508 and 1620 cm-1 can be attributed to the stretching vibration of indoline and indole structure of PDA. For the hybrid nanospheres, the band at 1508 cm-1 is split into two bands at 1483 and 1546 cm-1, and the intensity of the band


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at 1620 cm-1 is lowered. The decrease in intensity and splitting of band suggests the formation of a Fe(III)-PDA complex in a manner similar to previously reported work.31, 35

Figure 2. a) FTIR spectrum of PDA and Fe(III)-PDA, b)XANES spectra of Fe(III)-PDA, Fe2O3 and Fe(OH)3, c) Fourier transformed EXAFS spectra of Fe(III)-PDA, d)XPS spectra of PDA and Fe(III)-PDA , e) O1s XPS spectra of PDA and Fe(III)-PDA and f) N1s XPS spectra of PDA and Fe(III)-PDA.


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Table 1. EXAFS fitting result for Fe K-edge of Fe(III)-PDA. d is bond distances; CN is coordination number; and σ2 is Debye-Waller factor.

Path Fe – O Fe – N

d (Å) 2.01 ± 0.02 1.65 ± 0.02

CN 4.3 ± 0.4 1.0 ± 0.1

σ2 (Å2) 0.007 ± 0.001 0.005 ± 0.001

Fe K-edge XAFS was employed to confirm the chemical state and coordination environment of the complex nanospheres. By comparing the results with known standards (Figure 2b), it can be determined that Fe in the sample exists in the trivalent state and not in the form of iron oxide or iron hydroxide. In Figure 2c, the Fourier transform of the EXAFS oscillations χ(k)*k3 of Fe K-edge into R space in the k range of 2 to 12 Å-1 are shown. Curve fitting was performed and the corresponding data are shown in Table 1. A strong peak in the R range between 0.9 and 1.9 Å can be attributed to photoelectron backscattering on the nearest neighbor around Fe. The fitting results suggest that the ferric ions in the complex nanospheres form coordination bonds with both oxygen and nitrogen with bond length of 2.01 and 1.65 Å, respectively. The ratio of Fe-O and Fe-N coordination numbers is roughly 4.3 to 1.0, indicating that similar to the complexes in the solution, the ferric ions in the nanospheres predominately form coordination bonds with catechol groups of PDA, while some Fe-N coordination bonds may form in the polymerization process. XPS analysis was employed to quantitatively determine the chemical compositions of the Fe(III)-PDA complex. As shown in Figure 2d, the peak related to Fe 2p appears in the spectra of Fe(III)-PDA and accounts for 1.90 at.% of the Fe(III)-PDA complex. This translates to approximately 8 wt.% of Fe in the Fe(III)-PDA complex, which is close to the value estimated using TGA (10 wt.%, Figure S2). The measured Fe contents are slightly lower than the Fe content calculated based on the feed molar ratio of Fe:DOPA (Fe:DOPA molar ratio of 1:3 is equivalent to about 10.9 wt.% Fe), implying that polymerization may occur between the complex


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moieties and some free DOPA monomers that are not bonded to ferric ions. In Figures 2e and 2f, the binding energies of O 1s and N 1s bands of neat PDA are around 533.5 and 402.2 eV, respectively, whereas for Fe(III)-PDA complex, the O 1s band shifts to about 531.6 eV and the N 1s bands to about 400.2 eV, also indicating that the ferric ions are bonded to both O and N atoms of PDA, corroborating the data obtained from XAFS.

Morphological evolution of Fe(III)-PDA complex nanostructures After polymerization for 72 hrs, both the neat PDA and Fe(III)-PDA complex obtained from the respective suspension exhibit spherical morphology as disclosed by FESEM studies, while the size of the complex spheres is only about 80 nm, which is much smaller than that of its neat PDA counterpart (Figure 3). In an attempt to understand the formation process of the complex nanospheres and the aforementioned size difference, a TEM study was conducted to monitor the growth processes of both PDA and Fe(III)-PDA complex nanostuctures and the evolution of the nanostructure morphology over time. 10 mL of the respective reaction solutions was extracted at various time inteval during the polymerization process and freezed with liquid nitrogen to immediately stop the polymerization process. The frozen sample was lyophilized before TEM observation. Figures 4a and 4b show the morphology evolution of the PDA and Fe(III)-PDA complex (Fe:DOPA feed molar ratio = 1:3) nanostructures, respectively. For both systems, the morphology of the nanostructures is transformed from the initial stacked nanosheets to interconnected nanospheres on sheets and finally individual nanospheres. The observation of an intermediate state (nanospheres connected on sheets) between the sheet-like morphology and nanospheres suggests that the formation of spherical morphology is likely to be driven by covalent bonding-induced decrease of hydrophilic functional groups, which causes re-self13 ACS Paragon Plus Environment

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assembly of the stacked oligomers to reduce specific surface area. The inter-connected nanospheres observed at the intermediate stage could be at a stage where the rearragement of the oligomers is still ongoing, where some stacked sheets have rearranged into small spheres while some are still stacked together in the sheet form. The complex nanospheres are much smaller probably because they have less covalent bonds and more functional groups on their surfaces, and hence more hydrophilic and tend to achieve larger specific surface area to interact with the aqueous medium. By contrast, in the absence of ferric ions in the reaction system, the functional groups are not consumed by complexation with Fe(III) and hence there are larger amounts of free DOPA monomers and oligomers in the reaction system, which may increase the extent of covalent polymerization. This may make the neat PDA nanospheres more hydrophobic and hence tend to have smaller specific surface area in the aqueous medium.

Figure 3. FESEM micrograph of (a) PDA and (b) Fe(III)-PDA complex nanospheres.


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Figure 4. TEM micrographs showing morphologies of (a) PDA and (b) Fe(III)-PDA complex nanostructures at different reaction time: (a1 & b1) 3 h, (a2 & b2) 12 h and (a3 & b3) 24 h.


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Morphologies of Fe(III)-PDA complex nanostructures at different Fe3+/DOPA feed ratios To further investigate the formation mechanism of the Fe(III)-PDA complex nanospheres, complex nanostructures were also synthesized by adding different amounts of FeCl3 to the aqueous solution of DOPA to achieve Fe3+/DOPA molar ratio of 1:1, 1:2, 1:3, 1:4, 1:5 and 1:6, respectively. A fixed amount of Tris buffer was then added to the solutions and the reaction left to stir for 72 h. Upon the addition of FeCl3 into the DOPA solution, the color of all the solutions changed to green and the pH values of all the solutions were found to be in the range of 2 to 3 (Table 2), suggesting the formation of mono-complex. An absorption maximum at about 740 nm corresponding to mono-complex was also observed in the UV-vis spectra of all the solutions.33-34 With the addition of the Tris buffer, the pH was increased to 4.7, 7.2, 7.7 and 8.0 for the Fe3+/DOPA molar ratio of 1:1, 1:2, 1:3 and 1:4 respectively (Table 2), and the solutions turned to colors ranging from blue to wine red for the Fe3+/DOPA molar ratio of 1:1, 1:2, 1:3 and 1:4 respectively (inset of Figure 5). From the corresponding UV-vis spectra (Figure 5), we can observe that the absorption maximum undergoes a blue shift to 620, 579, 558 and 513 nm for Fe3+/DOPA ratio of 1:1, 1:2, 1:3 and 1:4 respectively.34 The UV-vis spectra and the corresponding pH value indicate that with the addition of Tris buffer, a mixture of bis- and triscomplex is formed in the various solutions and the content of the tris-complex increases with pH. To ascertain that the formation of the various complexes is dominated by the pH of the solution rather than the Fe3+/DOPA ratio, we adjusted the pH of the solutions with the Fe3+/DOPA molar ratios of 1:1 and 1:2 to approximately 8.5 by the addition of extra Tris and observed the UV-vis spectra of the solutions (Figure S3a and b). The UV-vis spectra of both solutions show a blue shift of the absorption maximum when the pH is adjusted to 8.5, confirming that the formation of the various complexes is dominated by the pH of the solution, rather than the Fe/DOPA ratio.


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Table 2. pH values of the various solution before and after addition of tris

Fe3+-DOPA molar ratio Pure Dopamine 1–1

pH value prior to the pH value immediately pH value after 72 hours of addition of Tris after the addition of Tris polymerization 5.7

8.5

7.6

2.4

4.7

3.9

1–2

2.6

7.2

5.2

1–3

2.8

7.7

6.5

1–4

2.9

8.0

7.4

Figure 5. UV-vis spectra of the various samples (inset: picture of the solution with different ratio of ferric ion/DOPA taken immediately after the addition of tris).

The polymerization of DOPA with different amounts of ferric ions (and the fixed amount of Tris buffer) resulted in Fe(III)-PDA complex nanostructures of different morphologies, as shown 17 ACS Paragon Plus Environment

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in the TEM images in Figure 6. In all cases, no aggregated Fe species could be observed in the nanostructures, implying that the ferric ions are uniformly distributed in PDA owing to the formation of complexes. However, Figures 6a and b show that when the feed molar ratio of ferric ions to DOPA is kept at 1:1 and 1:2, the Fe(III)-PDA complex has sheet-like morphology. As the feed molar ratio of ferric ions to DOPA increases to 1:3, TEM micrograph (figure 6c) shows that the morphology evolves into that of a sphere with an average diameter of about 80 nm. Figures 6d, e and f show that as the feed ratio of ferric ions to DOPA is further increased to 1:4, 1:5 and 1:6, the size of the Fe(III)-PDA complex nanospheres formed increases significantly to an average diameter of about 200, 250 and 300 nm, respectively, and the nanospheres become more and more inter-connected. The formation of sheet-like morphology at the Fe:DOPA molar ratio of 1:1 and 1:2 is not related to the presence of bis-complex in the initial solutions because even though tris-complex is predominately formed at pH = 8.5, the final morphology obtained from the solution with ferric ion:DOPA ratio of 1:1 and pH of 8.5 is still sheet-like, as observed from the TEM micrograph (Figure S4). A plausible explanation is that the formation of the sheetlike morphology is mainly due to the stacking of planar oligomers formed by complexation and slight covalent bonding, whereas the spherical morphology observed when the molar ratio of ferric ion to DOPA is increased to 1:3 and above may be attributed to a higher extent of covalent polymerization due to the presence of free DOPA (not bonded to Fe) in the reaction solution. The molar ratio of 1:3 could be the critical ratio whereby there is a sufficient amount of free DOPA monomers in the solution after all of the ferric ions have formed bis- and tris- complexes. These free DOPA monomers may form covalent bonds with the oligomers, consuming hydrophilic functional groups and making the nanostructures more hydrophobic. Thus the selfassembled nanostructures tend to rearrange themselves in a manner to reduce their specific


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surface area, eventually leading to spherical morphology. The increase in size of the nanospheres with increasing Fe/DOPA ratio could be due to the presence of higher amounts of free DOPA monomers, which increases the degree of covalent polymerization and hence makes the resultant spheres more hydrophobic, leading to larger, inter-connected spheres (insets of Figures 6e and 5f) with even smaller specific surface area. TGA data (Figure S2) show that the samples with feed Fe(III):DOPA molar ratios of 1:1, 1:2 and 1:3 have similar Fe content (approximately 10 wt.%), whereas the samples with feed Fe(III):DOPA molar ratios of 1:4, 1:5 and 1:6 have lower Fe contents, indicating that free ferric ions, which have no coordination bonds with DOPA, could not be incoporated into the hybrid nanostructures, whereas free DOPA monomers do take part in the polymerisation process.


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Figure 6. TEM micrographs of Fe3+/PDA complex nanostructures with Fe(III):DOPA feed molar ratios of a) 1:1, b) 1:2, c) 1:3, d) 1:4, e) 1:5 and f) 1:6.


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Structure, morphology and magnetic properties of Fe3O4/C-PDA nanospheres To demonstrate the usefulness of this simple one-pot synthesis method, Fe3O4/C-PDA composite nanospheres were prepared by annealing the Fe(III)-PDA complex nanospheres with Fe(III)/DOPA feed ratio of 1:3. Figure 7a shows the TEM micrograph of the composite nanospheres obtained by annealing at 650 °C for 2 h. There is no significant change in the size of the nanospheres after the annealing process, while homogeneously distributed nanoparticles with size of only about 3-5 nm are observed. These nanoparticles show characteristic X-ray diffraction peaks at 2θ = 18.3°, 30.1°, 35.5°, 37.1°, 43.3°, 53.5°, 57.3° and 62.7° (Figure 7b), corresponding to the (111), (220), (311), (222), (400), (422), (511) and (440) planes of Fe3O4, respectively.50 The magnetization curve of the Fe3O4/C-PDA nanospheres is shown in Figure 7b. The saturation magnetization value (Ms) of the composite nanospheres is about 15 emu g-1, and there is almost no hysteresis loop found in the magnetization curve, suggesting the superparamagnetic property of the Fe3O4/C-PDA nanospheres. The magnetic property makes the Fe3O4/C-PDA nanospheres an ideal candidate for recyclable catalyst support. The conversion of neat PDA nanospheres to C-PDA nanospheres by annealing was investigated through X-ray diffraction and Raman spectroscopy studies. Neat PDA, after annealing, exhibits diffraction peaks at 2θ = 23.4°, 43.7° (Figure 7c), corresponding to (002) and (100) plane of carbon.51 However, such diffraction peaks are not visible in the XRD pattern of Fe3O4/C-PDA due to the presence of intense peaks of Fe3O4. However, different from that of the Fe(III)-PDA complex nanospheres, the Raman spectrum of the Fe3O4/C-PDA composite nanospheres show well defined D and G band of carbon at 1325 and 1581 cm-1, respectively (Figure 7d), confirming the formation of C-PDA in Fe3O4/C-PDA.13 It is worth noting that several previous studies have shown that the aggregation of metal species and evaporation of


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organic volatiles during the carbonization process could create mesopores in the C-PDA matrix,35-36 allowing the embedded inorganic nanoparticles be exposed to the surrounding media. Nitrogen adsorption-desorption isotherms of Fe3O4/C-PDA shows a relatively high BET specific surface area of 475 m2 g-1 (Figure S5). The pore size distribution curve (inset in Figure S5) shows the presence of pores with sizes of about 2 and 9 nm, which are created during the carbonization process. The pore size distribution peak at about 44 nm could be attributed to the inter-sphere spaces among the stacked nanospheres.36 In addition, it is also widely reported that C-PDA is highly graphitized and doped with a substantial amount of pyridinic and graphitic N.3, 19, 36 These features of C-PDA, together with the abundant extremely small Fe3O4 nanoparticles embedded in C-PDA, make the Fe3O4/C-PDA nanospheres a good candidate for ORR electrocatalysts.

Figure 7. a) TEM micrographs, b) VSM curve of Fe3O4/C-PDA composite nanospheres , c) XRD patterns of C-PDA and Fe3O4/C-PDA , and d) Raman spectra of Fe3O4/C-PDA composite and Fe(III)-PDA.


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ORR catalytic activity and Zinc-Air Battery Performance of Fe3O4/C-PDA nanospheres The electrochemical performance of the composite nanospheres was evaluated by LSV, CV and RDE measurements in 0.1 M KOH electrolyte purged with O2 or N2. In CV measurements, no obvious reduction peak is observed for the N2-saturated aqueous KOH electrolyte. A change to the O2-saturated electrolyte leads to the appearance of a cathodic peak at -0.27 V, representing an ORR activity (Figure 8a). To have a deeper understanding of the electrocatalytic behaviors of Fe3O4/C-PDA, RDE voltammograms were recorded alongside with readings from C-PDA and Pt/C catalyst (20 wt.% Pt on carbon black) at a rotation speed of 1600 rpm (Figure 8b). Fe3O4/CPDA composite nanospheres exhibit enhanced catalytic activity to ORR when compared with pristine C-PDA nanospheres. This improvement could be brought about by the good electrocatalytic activity of Fe3O4 towards ORR,52 large surface area of the Fe3O4 particles brought by their very small size and uniform dispersion in C-PDA, and the synergistic effect of Fe3O4 and C-PDA, such as the close contact of Fe3O4 with electrically conductive C-PDA and the presence of pyridinic and graphitic N surrounding Fe3O4.3, 15-16, 19, 36 The onset potentials for C-PDA, Fe3O4/C-PDA and Pt/C are -0.27, -0.14 and 0.03 V, respectively. It is clear that in contrast to C-PDA, marked improvement in onset potential is seen for Fe3O4/C-PDA. However, when compared to Pt/C, Fe3O4/C-PDA shows a slightly negative ORR onset potential. The ORR catalytic activity of Fe3O4/C-PDA was also examined with Koutecky-Levich plots (inset of Figure 8c) derived from the RDE curves at electrode potential range of -0.4 to -0.7 V (Figure 8c). The good linearity and almost constant gradient can be distinctly observed from the plots, suggesting typical first-order kinetics with respect to the concentration of dissolved O2. The number of electrons transferred (n) per oxygen molecule was calculated to be between 3.30 – 3.58, suggesting that to a large extent, Fe3O4/C-PDA promoted ORR in the desirable 4-electron


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transfer process. With the aforementioned synergistic effect of Fe3O4 and C-PDA, the Fe3O4/CPDA nanospheres could be a good candidate for use in catalysis of oxygen reduction reaction.

Figure 8. CV curve of Fe3O4/C-PDA in O2- and N2-purged in 0.1 M KOH, b) LSV curves of C-PDA, Fe3O4/CPDA compared with Pt/C for ORR at a rotation speed of 1600 rpm, c) RDE data of Fe3O4/C-PDA (inset: K-L plots and fitting curves for Fe3O4/C-PDA) and d) voltage profile of a Fe3O4/C-PDA based ZnAB when fully discharged at a current density of 5 mA cm-2 (inset: voltage profile showing voltage difference when fully discharged at current density of 5 mA cm-2 and 20 mA cm-2, respectively).

Fe3O4/C-PDA was also tested as an ORR catalyst for a primary ZnAB, using Fe3O4/C-PDA loaded carbon paper as an air cathode, zinc plate as the anode and 6 M aqueous KOH as electrolyte. The Fe3O4/C-PDA based battery was discharged at a constant current density of 5 mA cm-2 and was able to continuously discharge over a period of 250 h, with voltage value above 1.10 V for the first 200 h (Figure 8d). The stable discharge voltage could be attributed to 24 ACS Paragon Plus Environment

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the stability of Fe3O4/C-PDA when used as an ORR catalyst as the Fe3O4 nanoparticles are well protected and separated by C-PDA. As the discharge current was raised to 20 mA cm-2, the discharge voltage of the Fe3O4/C-PDA based battery dropped to about 0.90 V.

Fe3O4/C-PDA nanospheres as recyclable catalyst support To showcase its versatility, the Fe3O4/C-PDA nanospheres were also evaluated as a recyclable support for catalyst. Pt nanoparticles were deposited on the surface of the composite nanospheres. TEM micrograph (Figure 9a) shows the successful attachment of Pt nanoparticles with average size of about 3 nm on the surface of the composite nanospheres. XRD analysis (Figure S6a) confirms the presence of Pt nanoparticles, with characteristic peaks of Pt observed. Saturation magnetization value (Ms) of Fe3O4/C-PDA/Pt was measured to be 7 emu g-1 (Figure S6b). The reduction of p-nitrophenol by NaBH4 is used as a model reaction to demonstrate the catalytic function of Fe3O4/C-PDA/Pt.35 The reduction process was monitored by measuring the UV-vis absorption spectra of the solution at various time interval, as shown in Figure 9b. In the absence of any catalyst, the characteristic absorption band at 400 nm indicative of p-nitrophenol remains even with the addition of high content of NaBH4. With the addition of Fe3O4/C-PDA/Pt, the intensity of the band at 400 nm progressively decreases and a new band at 295 nm emerges, indicating the formation of p-aminophenol. The bright yellow solution becomes colorless within a short time span of 20 min, which is accompanied by the complete disappearance of the band at 400 nm, indicating the complete reduction of p-nitrophenol to p-aminophenol. Fe3O4/C-PDA/Pt could be easily recycled using a magnet owing to its superparamagnetic property. The stability and activity of the catalyst was studied by repeating the reduction process using the same batch of catalyst for eight cycles (inset of Figure 9b). It is found that Fe3O4/C-PDA/Pt is still highly 25 ACS Paragon Plus Environment

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active at the eighth cycle with no significant change in morphology (inset in Figure 9b). The facile attachment of Pt nanoparticles on the surface of the composite nanospheres can be attributed to the abundant functional groups of PDA retained on the surface of the nanospheres after the annealing process.15 These functional groups are also responsible for the stability of the catalyst, preventing the agglomeration and leaching of the Pt nanoparticles.

Figure 9. a) TEM micrograph of Fe3O4/C-PDA/Pt, b) UV-vis absorption spectra of the reduction of pnitrophenol by NaBH4 in the presence of Fe3O4/C-PDA/Pt (inset: Activity of catalyst after 8 cycles and TEM micrograph of Fe3O4/C-PDA/Pt after the catalytic reaction).

Conclusions In this work, for the first time we demonstrated a facile one-pot method for synthesis of Fe(III)-polydopamine (PDA) complex nanospheres. The results show that Fe(III)-caterchol complexation, covalent bonding and self-assembly take place simultaneously in the “polymerization” process, and the Fe(III)-PDA complex formed gradually transforms from sheet-like to spherical morphology. As the feed ratio of ferric ion to DOPA decreases, the final morphology of the Fe(III)-PDA complex also changes from sheet-like and spherical morphology. The formation of nanospheres is likely to be driven by the covalent polymerization-induced 26 ACS Paragon Plus Environment

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decrease of hydrophilic functional groups, which causes re-self-assembly of the stacked oligomers to reduce specific surface area. The complex nanospheres can be easily converted to C-PDA nanospheres with embedded Fe3O4 nanoparticales with size of only about 3-5 nm via annealing. Electrochemical studies showed the improved ORR activty of Fe3O4/C-PDA compared to the neat C-PDA. More importantly, a stable discharge voltage can be delivered for over 200 h when it was used as the cathode for a primary ZnAB.

Acknowledgements Jia Ming Ang thanks Nanyang Technological University, Singapore, for providing his PhD scholarship during the course of this work.

Supporting Information Available TEM micrograph of PDA at initial stage of polymerization, TGA of Fe(III)-PDA complex at different feed ratio, UV-vis spectra of ferric ion and DOPA solution at different feed ratio and pH, TEM micrograph of Fe(III)-PDA complex with adjusted pH, Nitrogen adsorption-desorption isotherm and XRD and VSM of Fe3O4/C-PDA/Pt.


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One-Pot Synthesis of Fe(III)âPolydopamine Complex Nanospheres

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