Bismuth oxide nanoparticles as a nanoscale guide to form a silver–polydopamine hybrid electrocatalyst with enhanced activity and stability for the oxygen reduction reaction

Abstract

Highly dispersed Ag nanoparticles (Ag NPs) were successfully synthesized on functionalized polydopamine (PDA)@Bi₂O₃ NPs for use as an electrocatalyst. In the proposed method, a uniform layer of PDA was first coated on Bi₂O₃ NPs. The surface of the PDA@Bi₂O₃ can then be used as a nanoscale guide to deposit Ag NPs and hence for the formation of Ag–PDA@Bi₂O₃ hybrid nanocatalysts. It was found that Ag NPs enhanced the electrocatalytic ability on PDA@Bi₂O₃ by a synergetic effect for direct 4e⁻ transfer in the oxygen reduction reaction (ORR) with a low overpotential. The surface morphology and lattice fringes of Ag NPs of crystalline nature of the obtained Ag–PDA@Bi₂O₃ hybrid nanocatalysts were examined through HR-TEM and SAED patterns. The material's purity and chemical functional groups were identified by FT-IR analysis. This strategy provides new opportunities to design and optimize heterogeneous nanocatalysts with tailored size, morphology, chemical configuration and supporting substrates for metal-catalyzed reactions.

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Introduction

We report here the application of polydopamine (PDA) coating-directed substrate-immobilization of Ag nanoparticles (Ag NPs) as a promising material in the oxygen reduction reaction (ORR). Note that ORR is essential in various applications, such as fuel cells, metal–air batteries, catalysis, enzymatic reactions and estimation of dissolved oxygen in bio/environment samples.^1–6 Recent efforts in ORR electrocatalysis have focused on improving the catalytic activity of Pt alloys, minimizing Pt content by utilizing a core–shell structure, preparing Pt supporting material via synergetic effect and replacing Pt with less expensive materials.^1,7,8 For example, several materials, such as Co,^1,9 metal oxides/composites,^1,3,4 Ag,^10–18 and N, S, B, P-doped carbon based materials,^19–25 were reported to replace Pt for ORR.

Since PDA layer can be formed easily on the surface of Bi₂O₃ NPs, our strategy is to use the PDA@Bi₂O₃ as a nanoscale guide to form uniform Ag NPs on the surface of PDA@Bi₂O₃. Silver can be regarded as a less-expensive and relatively abundant metal, exhibits good ORR activity in alkaline media,^15,16 especially for HO₂⁻ disproportionation.^17,18 In addition, N-containing compounds and polymers were reported to prepare ORR catalyst for tuning the metal nanoparticle size, surface area, complex formation, hetero atom doping with higher stability and durability.^1,2,19,20 Synergetic effects of within metal, metal oxide, carbon nano-materials, mixed composites material, etc. were demonstrated to increase the catalytic activity. Of course, the main advantage of PDA is that it could be easily self-polymerized in basic pH medium on any matrix surfaces with multiple applications in variety of fields.²⁶ It has shown good adhesive and reducing properties with higher amount of nanoparticle loading and different structure nanomaterial formation.²⁷ Meanwhile, both PDA-based materials and bismuth-combined composites have also shown enhanced activity in ORR.^9,28–33

Herein, an easily modified large surface area Non Hazardous Nano Composite (NHNC) material based on Ag–PDA@Bi₂O₃ was demonstrated to have enhanced catalytic behavior towards ORR. A key issue for ORR activity at Ag or other catalysts is whether O₂ is reduced by two electrons to H₂O₂ (or HO₂⁻) or four electrons to water (or OH⁻), i.e., with an n-value of 2 or 4. In general, an n-value of 4 is the preferred pathway, because of both the higher currents available and the unwanted chemical reactivity of H₂O₂ and its decomposition products toward various fuel cell components. In this paper, we thoroughly studied the electrocatalytic ability of the Ag–PDA@Bi₂O₃ for direct 4e⁻ transfer in ORR with a low overpotential. The surface morphology, lattice fringes and crystal plane of the obtained Ag–PDA@Bi₂O₃ hybrid nanocatalysts were examined through HR-TEM and SAED pattern.

Experimental

Dopamine hydrochloride was received from Sigma Aldrich. Sodium dihydrogen phosphate, disodium hydrogen phosphates were purchased from SHOWA chemicals, Japan. Silver nitrate (AgNO₃), Tris–HCl, nano MnO₂ and bismuth oxide were of ACS grade chemicals. The disposable Screen Printed Carbon Electrode (SPCE) (geometrical area 0.2 cm²) and Ring Disk SPCE (RDSPCE) were purchased from Zensor R&D laboratory, Taichung, Taiwan. Millipore deionized water (18 MΩ cm⁻¹) was used for throughout this experiments. All chemicals were used without further purifications.

To prepare the PDA@Bi₂O₃ composite, 100 mg Bi₂O₃ was dispersed in 100 mL of Tris–HCl (10 mM) under ultra-sonication for 10 min. Then, 200 mg dopamine–HCl was introduced into the solution and allowed to continuous stirring for 12 h. The obtained precipitate was centrifuged, washed with distilled water and dried at 60 °C. As to the preparation of Ag–PDA@Bi₂O₃ composite, 5 mg of PDA@Bi₂O₃ composite was dispersed into 9 mL of water under ultra-sonication for 5 min and then 1 mL (from 20 mM) of AgNO₃ (2 mM) solution added into solution under continuous stirring 2 h. Finally, the obtained precipitate was centrifuged, washed with distilled and followed by drying at 60 °C, which is used for further experiments and characterization (Scheme: Fig. S1†).

The coating solution was prepared by dispersing 2 mg of catalyst (Ag–PDA@Bi₂O₃) in 1 mL water by ultra-sonication for 5 min. Then, 8 μL of coating solution was drop-coated on SPCE or disk surface of RDSPCE and dried at room temperature. The ring electrode of RDSPCE was modified by 5 μL MnO₂ coating solution prepared by adding 5 mg MnO₂ in 1 mL water and dried at room temperature. The same procedure was followed to prepare Bi₂O₃/SPCE, PDA@Bi₂O₃/SPCE, Ag–PDA/SPCE and Ag–Bi₂O₃/SPCE.

Functionalized PDA was examined by FT-IR (JASCO FT-IR4100, Japan) in the range from 550 to 4000 cm⁻¹. The morphology particle size and lattice fringes were analyzed through High Resolution Transmission Electron Microscopy (HR-TEM) measurements with Surface Area Diffraction Pattern (SAED) (JEM 2010 instrument operating at an accelerating voltage of 200 kV). Dissolved oxygen meter (EUTECH) was used to quantify the amount of oxygen in O₂- and Ar-saturated solution.

Electrochemical experiment was carried out using CHI727e and CHI900 for flow injection analysis work station. Three electrode systems were used for electrochemical work station, catalyst modified electrode as a working electrode, Pt wire as an auxiliary electrode and Ag/AgCl as a reference electrode. Cyclic voltammetry experiment was performed in 0.1 M, pH 7.4 Phosphate Buffer Solution (PBS) at a scan rate of 20 mV s⁻¹. Catalyst-modified glassy carbon rotating disk electrode (GCRDE) was used in O₂-saturated PBS with sweep rate 20 mV s⁻¹ in LSV. Flow injection analysis experiment was carried out by our group previously reported RDSPCE cell model.⁴⁰ An Ar-saturated PBS was used in FIA, flow rate 0.3 mL min⁻¹ and 20 μL of O₂ saturated solution injected into the flow for the H₂O₂ intermediate monitoring.

Results and discussion

Characterization of the Ag–PDA@Bi₂O₃ nanocomposite

The formation of PDA layer on Bi₂O₃ surface was first characterized by FT-IR spectrum. As shown in Fig. 1, compared to those of DA and PDA spectrum, the stretching vibration at 3200–3500 cm⁻¹ corresponds to N–H and O–H groups. Whereas, the peak at 1602 and 1497 cm⁻¹ corresponds to indole and indoline aromatic ring center, respectively. Finally, the broad peak ranging from 1240 to 1282 cm⁻¹ comes from the C–O asymmetric stretching vibration and C–OH asymmetric bending vibration. This is in consistent with previous studies for characterization of PDA thin films deposited by autoxidation of DA.^34,35 The fact that there are no any other extra peaks observed at PDA and PDA@Bi₂O₃ also confirmed the purity of the as-prepared materials. Overall, melanin like structure PDA layer was strongly formed on the surface of the Bi₂O₃ NPs.²⁷

Fig. 1 Characterization of DA (a), PDA (b) and PDA@Bi₂O₃ (c) by FT-IR spectroscopy.

Since we propose to use the Bi₂O₃ NPs as a nanoscale guide for Ag NPs formation, the surface morphology of the catalyst by HR-TEM can provide a direct evidence for such a purpose. At first, as shown in Fig. 2a a pure nanostructured spherical Bi₂O₃ NPs was observed. Note that the Bi₂O₃ particle formation density was varied at different particle with higher density particle showing very dark and sharp edges and lower density particle of grey color (Fig. S2a†). Simply by mixing the Bi₂O₃ NPs with dopamine solution in Tris–HCl, the polymer matrix was effectively coated on the surface of the Bi₂O₃ NPs, as confirmed from Fig. 2b. Note that this is similar to the polymer matrix layer formed on Bi₂O₃ NPs to that of previously reported PDA layer formation on Fe₃O₄.³⁶ The polymer coated thickness of PDA was measured as 50 nm. As shown in Fig. 2c, in the absence of PDA, no obvious Ag NPs were observed for the Ag–Bi₂O₃ composites. Yet, small amount of silver layer was found on the edges of Bi₂O₃ surfaces. Note that it could also be visibly observed as yellow color Bi₂O₃ solution turned into slightly whitish yellow while synthesis the composite (Fig. 2c and S2b†). In other words, the existence of PDA is essential for the successfully loading of large amount of Ag NPs. To prove this, we purposely prepare an Ag–PDA composite via AgNO₃ solution mixed with PDA powder under stirring condition. As reported earlier, Ag cations can be reduced by PDA powder from metal ions to metal nanoparticles.^26,27,37 As shown in Fig. 2d, large amount of Ag NPs were indeed formed under the PDA condition.

Fig. 2 HR-TEM images of Bi₂O₃ (a), PDA@Bi₂O₃ (b), Ag–Bi₂O₃ (c) and Ag–PDA (d).

We next improve the preparing method simply by mixing the PDA@Bi₂O₃ with AgNO₃. The surface morphology of the as-prepared Ag–PDA@Bi₂O₃ composite was further studied by HR-TEM. As shown in Fig. 3 and S3,† there is a layer of mixture of Ag film and Ag NPs on the PDA@Bi₂O₃ surface. The Ag NPs was found to form on PDA@Bi₂O₃ surface with spherical shape but in different size. The average particles size of Ag NPs were ∼34 nm (Table S1†).³⁷ Note that higher amount Ag NPs were formed on PDA@Bi₂O₃ surface due to PDA reducing properties.^27,37 The crystal nature of the Ag NPs was studied by SAED image. Fig. 3c and d show the lattice fringes and SAED pattern of Ag NPs in Ag–PDA@Bi₂O₃ composite. It is clear that {111}, {220}, {200} lattice plane of face center cubic (fcc) crystal nature of Ag NPs were formed at the Ag–PDA@Bi₂O₃ composite. This is advantageous since the pattern is similar to the Ag NPs added to improve the performance of a zinc oxide nanowire ultraviolet photodetector.³⁸

Fig. 3 HR-TEM images of Ag–PDA@Bi₂O₃ (a), (b) and lattice fringes of silver in Ag–PDA@Bi₂O₃ (c) and SAED image for crystal nature characterization of Ag NPs in Ag–PDA@Bi₂O₃ (d).

Electrocatalytic activity of the Ag–PDA@Bi₂O₃ composite electrode for ORR

Fig. 4 compares the electrochemical behavior of different composite electrodes towards ORR. As can be seen, in the absence of Ag NPs, both Bi₂O₃ and PDA@Bi₂O₃ electrodes revealed very low catalytic behavior for the reduction of O₂. On the other hand, both Ag NPs on Ag–PDA and Ag layer on Ag–Bi₂O₃ modified electrodes exhibited good electrocatalytic behavior at ∼−0.6 V vs. Ag/AgCl.³⁷ As to the Ag–PDA@Bi₂O₃ composite electrode, a lower reduction potential at −0.38 V with an enhanced catalytic activity in ORR was observed. The observation of a totally different cyclic voltammograms also indicates that there is a synergetic effect among PDA, Ag NPs and Bi₂O₃ enhance the electrocatalytic ability and hence a lower potential for ORR. Compared to those of Ag layer on Bi₂O₃ and Ag NPs on PDA@Bi₂O₃, the Ag NPs on PDA with Bi₂O₃ supporting substrate improved the catalytic activity with a lower reduction potential for ORR. To elucidate this, the ORR electrocatalytic activity of the Ag–PDA@Bi₂O₃ was investigated more detail in O₂-saturated 0.1 M, pH 7.4 PBS using a rotating disk electrode system to verify whether O₂ is reduced with an n-value of 2 or 4.

Fig. 4 Cyclic voltammograms of different composite-modified SPCE for ORR in atmospheric (dashed line), anaerobic (dotted line) and O₂-saturated (solid line) PBS (pH = 7.4), respectively.

We purposely compare different electrochemical behavior of ORR at three representative electrodes of Au–RDE, GC-RDE and Ag–PDA@Bi₂O₃ modified GC-RDE. It is well known that GC electrode catalyzes the ORR with an n-value of 2 and nano-Au electrode involves two step 2e⁻ transfer for ORR and H₂O₂ reduction, respectively. As shown in Fig. 5a, at the Au–RDE, two step 2e⁻ transfer was truly observed at ∼−0.37 V for ORR and at −0.7 V for H₂O₂ reduction. The GC-RDE, on the other hand, only shows a 2e⁻ transfer ORR at −0.7 V. It is interesting that the Ag–PDA@Bi₂O₃ modified electrode shows a single and sharp ORR peak at −0.38 V. In order to quantitatively evaluate the ORR kinetics (including n-values), analysis of the RDE data was done using the Koutecky–Levich (K–L) formalism as given by:²⁰ 1/j = 1/j_k + 1/(Bω)^1/2, in which B = 0.62nFC_oD_o^2/3/v^1/6 where j is the measured current density, j_k is the kinetic current density, ω is the rotation rate, n is the overall number of electrons transferred in oxygen reduction, F is the Faraday constant (F = 96 485 C mol⁻¹), C_o is the bulk concentration of O₂ (0.25 × 10⁻³ mol L⁻¹), D_o is the diffusion coefficient of O₂ in the electrolyte (2.51 × 10⁻⁵ cm² s⁻¹),⁶ and v is the viscosity of the electrolyte. Active surface area can be calculated by Randles–Sevcik equation (Fig. S5 and Table S2†). Fig. 5b and c show the ORR polarization curve at different rotation rate ranging from 150 to 1000 rpm. The ORR current responses were found to increase with the rotation rate, and this process represented a diffusion control process. Fig. 5d express the K–L plot via the inverse current density (j⁻¹) as a function of inverse of the square root of the rotation rate. According to the K–L equation (Table S2†), the n of ORR was calculated to be 3.69 at −0.38 V and j_k value of 18.18 mA cm⁻². The corresponding K–L curves at various potentials exhibit good linearity, and the slopes remain almost unchanged over the potential range from −0.38 to −0.78 V (Fig. S6†), suggesting that the electron transfer numbers for oxygen reduction at different potentials are similar. The linearity and parallelism of the plots are considered as an indication of first-order reaction kinetics with respect to the concentration of dissolved O₂.²⁰ This value is good compared to previously reported Ag-based ORR catalyst^10–13,16 and other non-Pt based catalyst.^{3,9,19–25,29–31}

Fig. 5 (a) Cyclic voltammograms for ORR at different RDEs. RDE voltammetry curve for ORR on GCRDE (b) and Ag–PDA@Bi₂O₃-modified GCRDE (c) in 0.1 M O₂-saturated PBS (pH 7.4) at various rotation rates. (c) RDE diffusion curve of different electrodes (GC at −0.9 V, Ag–PDA@Bi₂O₃ at −0.38 V). (d) K–L plots.

Instead of using RDE system, our group previously reported a simple way to identify the 4e⁻ transfer using a flow injection system.^39,40 Fig. S7† illustrates the instrumental setup and the working principle. Note that MnO₂ NPs is a well-known catalyst for H₂O₂ oxidation. As shown in Fig. 6, by injection 20 μL of O₂-sat. solution, the ORR reaction was found to occur at applied potential −0.38 V on the Ag–PDA@Bi₂O₃ modified disk electrode and only little amount of H₂O₂ produced from this reaction was monitored at the MnO₂ NPs modified ring electrode at oxidation potential of 0.8 V.³⁹ Based on the results, the achievement of 4e⁻ transfer ORR was as high as 98% since only ∼2% H₂O₂ oxidation was observed. Note that, by calculating from the H₂O₂ formation percentage, n = 3.92 is a further evidence of 4e⁻ transfer in ORR. Since the Ag–PDA@Bi₂O₃ possessed good electrocatalytic ability towards ORR, its application for use as a sensor for detecting dissolved oxygen was also evaluated.^6,14 Fig. S8† shows voltammetric behavior of dissolved oxygen in Ar-saturated 0.1 M PBS (pH 7.4) and a wide linear range from 0.0841–6.048 mg L⁻¹ with a sensitivity of 2.1 μA mg⁻¹ was achieved. It is thus also suitable for application in environmental analysis. Stability is one of the crucial factors in fuel cell and sensor application. The Ag–PDA@Bi₂O₃ composite also showed good stability in O₂-sat. PBS. After 2500 cycles under a scan rate of 200 mV s⁻¹, the maximum current remained similarly except the onset potential shifted positive slightly. This catalyst shows superior stability without any binding material (like Nafion) to modify the electrode due to the superior PDA adhesion properties. Long-term stability of this composite is also excellent to retain 99% of activity towards ORR after 57 days.

Fig. 6 Flow injection analysis of H₂O₂ oxidation on nano MnO₂ catalyst on ring at an operating potential 0.8 V vs. Ag/AgCl and ORR on Ag–PDA@Bi₂O₃ catalyst coated on disk at detection potential −0.38 V vs. Ag/AgCl with flow rate 0.4 mL min⁻¹.

Conclusions

In summary, we have demonstrated successful synthesis a simple way of Ag–PDA@Bi₂O₃ nanocomposite with good stability. Face centered cubic lattice crystal nature Ag NPs can be easily formed on the PDA@Bi₂O₃. The Bi₂O₃ NPs helped to enhance the catalytic activity of Ag NPs with a lower potential in 4e⁻ transfer pathway of ORR. Electrocatalytic tests for ORR indicated that the synthesized Ag–PDA@Bi₂O₃ nanocomposite displayed superior activity. The enhanced ORR activity is attributed to the synergistic effects of the composition and dominant face centered cubic lattice crystal nature Ag NPs. This work would be highly impactful in the rational design of future bimetallic alloy nanostructures with high catalytic activity for fuel cell systems.

Acknowledgements

The authors gratefully acknowledge financial support from the Ministry of Science and Technology of Taiwan. The author heartedly acknowledge to Prof. Annamalai Senthil Kumar, Environmental and Analytical Chemistry Division, Vellore Institute of Technology University, Vellore, India and Dr Ting-Hao Yang, Department of Soil and Environmental Science, National Chung Hsing University, Taichung, Taiwan for given some useful guidance to carry out this work.

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Footnote

† Electronic supplementary information (ESI) available: Scheme of the work, preparation of Ag–Bi₂O₃, Ag–PDA, Randles–Sevcik equation for surface area calculation, K–L plot comparison table for n-value calculation, K–L plot diagram of catalyst at different potential, schematic diagram of FIA, oxygen sensor and stability experiment. See DOI: 10.1039/c4ra12989j

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