Facile and green decoration of Pd nanoparticles on macroporous carbon by polyoxometalate with enhanced electrocatalytic ability

Abstract

A well-defined Pd nanoparticles@polyoxometalates/macroporous carbon (Pd@POMs/MPC) tri-component nanohybrid has been developed using a facile, green, and one-pot synthesis method. The polyoxometalates were used as both reductant and bridging molecules. The novel nanohybrids of Pd@POMs/MPC can provide new features for electro-catalytic applications, because of the synergetic effects of Pd nanoparticles and MPC materials. The successful fabrication of Pd@POMs/MPC holds great promise for the design of electrochemical sensors, and is a promising material to promote the development of new electrode materials.

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1. Introduction

With the ever increasing demand for electrochemical techniques on a global scale, development of efficient and high catalytic activity electrocatalysts has been attracting increasingly intense attention. Nanostructured carbon materials have been recognized as one of the most important electrode materials.^1–5 Especially, the combination of large specific surface area and high electrical conductivity makes nanostructured carbon materials highly promising as electrocatalyst supports.^6–8 Recently, more and more attention has been focused on macroporous carbon (MPC) materials. This is because MPC has been demonstrated to be a very attractive support material due to its exciting properties, such as unique macroporous structure, large specific surface area and pore volume, good thermal as well as mechanical stability, exceptional chemical inertness, and excellent electrical conductivity.^9–11

To date, the most effective electrocatalysts are noble metal–carbon composites.^12–14 The carbon materials may offer a platform for supporting noble metal nanoparticles to form novel hybrid nanostructures with synergetic effects. The combination of noble metal nanoparticles and carbon materials is of special interest; it is known to show an obviously enhanced electrocatalytic activity. Although there have been abundant raw materials and methods proposed to prepare noble-metal/carbon materials over the past few years, the reported methods suffer various drawbacks. In general, the reduction processes of noble metal particles are complex and mostly required a specific high temperature and long time. Also the reaction process is normally not environmentally friendly, which limit the practical applications of noble-metal/carbon materials.

Polyoxometalates (POMs) are a subset of inorganic polynuclear metal–oxygen clusters, which exhibit remarkably rich redox and optical properties, and thus show promising applications in several fields including catalysis, medicine and materials sciences.¹⁵ POMs also are a class of photoactive materials, they can undergo fast, reversible, and stepwise multielectron-transfer reactions without changing their structures.^16–18 Lacunary POMs composed of metal centers in the highest oxidation states have been used as oxidatively resistant pure inorganic multidentate ligands in place of organic ligands for constructing well-defined structural models for photoredox processes.¹⁹ In their reduced forms, their electron and proton transfer and/or storage abilities, may act as efficient donors or acceptors of several electrons. The reduced POMs have been shown to serve as reducing and capping agents for noble-metal nanostructures.^20,21 Moreover, POMs were reported to be adsorbed on various solid materials and this property has recently been exploited for the stabilization of nanoparticles.^22–24

In the current study, relatively uniform Pd nanoparticles attached to MPC were prepared through a facile and green method using H₃PW₁₂O₄₀ (PW12, henceforth POMs for convenience) as both reductant and bridging molecules. The as-prepared novel tri-component nanohybrids of Pd@POMs/MPC extend the applications of support materials and provide new features of electrocatalytic activities. Hydrazine, hydrogen peroxide (H₂O₂) and nitrobenzene (NB) were selected as marking molecules to evaluate the electrochemical activity of the Pd@POMs/MPC-x nanocomposite.

2. Experimental

2.1. Chemical reagents

H₃PW₁₂O₄₀ (POMs), isopropanol, and N,N′-dimethylformamide (DMF) (HPLC grade) were used as purchased from Beijing Chemical Co. Ltd. PdCl₂ was purchased from Sinopharm Chemical Reagent Co. Ltd. Hydrazine, H₂O₂, and NB were obtained from Sigma. The 0.1 M phosphate buffer solution (PBS pH 7.0), which was made up from NaH₂PO₄, Na₂HPO₄, and H₃PO₄, was employed as a supporting electrolyte. All other reagents were of analytical grade, and all solutions were prepared using double distilled water.

2.2. Instrumentation

All the electrochemical experiments were performed with a CHI760e electrochemical Analyzer (CH Instruments, Shanghai Chenhua Instrument Corporation, China). Electrochemical impedance spectroscopy (EIS) was conducted using a PARSTAT 2273 Potentiostats-Electrochemistry Workstation (AMETEK Instruments, USA) in a 0.1 M KCl solution containing 5.0 mM K₃Fe(CN)₆/K₄Fe(CN)₆, from 0.1 Hz to 10.0 kHz. X-Ray diffraction (XRD) patterns were obtained on an X-ray D/max-2200vpc (Rigaku Corporation, Japan) instrument operated at 40 kV and 20 mA using Cu Kα radiation (k = 0.15406 nm). Scanning electron microscopy (SEM) image was determined with a Philips XL-30 ESEM, operating at 3.0 kV. Transmission electron microscopy (TEM) images and energy-dispersive X-ray spectra (EDX) were obtained using a JEM-2100F transmission electron microscope JEOL (Japan) operating at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed with a thermo ESCA LAB spectrometer (USA). A conventional three electrode cell was used; the working electrode was glassy carbon electrode (GCE) or the modified electrode; a platinum electrode was used as the counter electrode whereas an Ag/AgCl (in saturated KCl solution) electrode served as a reference electrode. All potentials in this paper were measured and reported versus Ag/AgCl. It is worth mentioning that in this study, all the sample solutions were purged with purified nitrogen for 20 min to remove oxygen prior to the beginning of a series of experiments and all experiments were carried out at laboratory temperature.

2.3. Preparation of the modified electrodes

Prior to the modification, GCE (model CHI104, 3 mm diameter) was polished before each experiment with 1, 0.3 and 0.05 μm alumina power, respectively, rinsed thoroughly with double distilled water between each polishing step, and then sonicated successively in 1: 1 nitric acid, absolute alcohol, double distilled water. The cleaned electrode was dried with nitrogen stream for the next modification. To prepare the modified electrodes, 5 mg of the electrode materials were dispersed into 1 mL DMF to give homogeneous suspension upon bath sonication. A 5 μL of the suspension was dropped onto GCE and the electrode was then dried at room temperature.

2.4. Synthesis of tri-component nanohybrids of MPC and Pd@POMs/MPC-x

The SiO₂ template was prepared by the typical Stöber's method.²⁵ The carbon was introduced into the interstices of the template using the modified method of Jun et al.²⁶ In a typical synthesis, 2.0 g of sucrose was dissolved in 10 mL aqueous solution containing 0.15 mL of 98% H₂SO₄. 2.0 g of SiO₂ template was immersed into sucrose solution and kept in vacuum for 3 h at room temperature for thorough impregnation. Then the mixture was heated at 100 °C for 6 h, followed by heating at 160 °C for a further 6 h for polymerization of sucrose. The solid was subsequently carbonized at 900 °C in N₂ for 3 h in a tube oven. The SiO₂ template was then etched away by overnight dissolution in 10% aqueous HF to leave behind a MPC.

The POMs were firstly reduced photochemically. A 500 W Hg lamp was used as a ultra-violet (UV) light source. In a typical synthesis, POMs (100 mL, 1 mM) were added to a quartz bottle and mixed with isopropanol (700 μL). Then, the resulting solution was irradiated under the UV light for 30 min. This solution of reduced POMs was mixed with prepared MPC suspension (10 mL, 2 mg mL⁻¹) and aqueous solution of PdCl₂ (1 mL) at room temperature, and then stirred for 10 min; the tri-component nanohybrids were prepared. The suspension was isolated by centrifugation at 8000 rpm, followed by consecutive washing/centrifugation cycles several times with doubly distilled water. The obtained Pd@POMs/MPC-x was dried in a vacuum oven at 60 °C for 24 h. For optimization of the nanocomposite, different concentrations of PdCl₂ (2 mM, 20 mM, and 200 mM) were selected to be added to the mixture. The Pd@POMs/MPC-x materials are referred to as Pd@POMs/MPC-1, Pd@POMs/MPC-2, and Pd@POMs/MPC-3, where 1, 2, and 3 represent the different concentrations of PdCl₂ (2 mM, 20 mM, and 200 mM) in the synthesis procedure, respectively. Illustration of the preparation of Pd@POMs/MPC-x is presented in Scheme 1.

Scheme 1 Illustration of the preparation of Pd@POMs/MPC composites.

For comparison, the nanocomposites without Pd nanoparticles decoration (POMs/MPC) are also prepared. In a typical synthesis, MPC suspension (10 mL, 2 mg mL⁻¹) were added in the initial POMs (100 mL, 1 mM), and then stirred for 10 min, followed by filtering, washing several times and drying in a vacuum oven at 60 °C for 24 h.

3. Results and discussion

3.1. Characterization of the as-prepared samples

The morphologies of MPC and Pd@POMs/MPC-x were initially characterized using SEM and TEM analysis. The as-prepared pristine MPC is seen as a well-defined interconnected macroporous nanostructure (Fig. 1A–C) with a pore size of about 110 nm. The typical TEM image of Pd@POMs/MPC-2 is presented in Fig. 1D, it is shown that Pd nanoparticles are uniformly dispersed on the surface of MPC. The average diameter of these Pd nanoparticles determined from a statistical study of 100 nanoparticles is 5.96 nm (Fig. 1E). The high-resolution TEM image of Pd@POMs/MPC-2 shown in Fig. 1F reveals that the spacing of the adjacent fringes along the wire growth direction is 0.223 nm, corresponding to the (111) interplanar distance of face-centered cubic structure. However, for Pd@POMs/MPC-3, the agglomerates of Pd nanoparticles look totally different (Fig. S1 and S2†). The image clearly illustrates that too much Pd nanoparticles can form Pd cluster, and cannot be uniformly dispersed on the surface of MPC.

Fig. 1 SEM image of MPC (A), TEM images of MPC (B) and (C) TEM image of Pd@POMs/MPC-2 (D), columnar distribution of Pd nanoparticles size for Pd@POMs/MPC-2 (E), and HRTEM image of Pd@POMs/MPC-2 (F).

The composition of as-synthesized Pd@POMs/MPC-2 was confirmed by energy-dispersive X-ray (EDX) spectroscopy, as shown in Fig. 2A. It shows the peaks corresponding to carbon, oxygen, tungsten, and palladium elements (the strong peaks of Cu are from the copper grid), therefore confirming the existence of Pd@POMs in the Pd@POMs/MPC nanohybrids. The nanohybrids were further characterized by X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscopy (XPS) analysis. As shown in the XRD spectra of the Pd@POMs/MPC-2 (Fig. 2B), the characteristic peak at 25° belong to the C (002) plane and other diffraction peaks at 39.9°, 44.9°, 67.2° and 80.5° can be indexed to Pd (111), (200), (220) and (311) planes of face-centered cubic crystalline of Pd, respectively. Moreover, diffraction peaks from the POMs were also observed. Their presence constitutes an unquestionable evidence for the formation of tri-component nanohybrids of Pd@POMs/MPC-2. XPS was carried out to further investigate the surface chemical component of the as-prepared materials (Fig. S3†). Both samples show obvious peaks, which were assigned to the signals of W 4f, W 4d, C 1s, Pd 3d, and O 1s, respectively. The elemental composition of different materials obtained from XPS analysis is shown in Table S1.† The XPS spectrum of the as-prepared Pd@POMs/MPC-2 nanohybrids shows the Pd 3d5/2 and 3d3/2. With the charge effect corrected by fixing the photoelectric peak 1s of carbon at 284.8 eV, the 3d5/2 level is located at 336.1 eV and the 3d3/2 level at 341.6 eV. These values suggest unambiguously that Pd is present only in the metallic form, indicating the formation of Pd nanoparticles on the surface of MPC. The presence of W was also detected by XPS despite the thorough washing of the samples. As shown in Fig. 2D, there is W 4f5/2 and 4f7/2 doublet with the binding energies of 36.1 and 38.2 eV respectively. These values indicate that W is in its full oxidation form in POMs when assembling in the nanohybrids. In addition, the Pd 3d and W 4f doublets of both Pd@POMs/MPC-1 and 3 are presented in Fig. S4.†

Fig. 2 EDX spectra of Pd@POMs/MPC-2 (A). XRD patterns of the Pd@POMs/MPC-2 (B). High-resolution XPS spectra of Pd 3d (C) and W 4f (D) spectra of Pd@POMs/MPC-2 nanohybrids.

Electrochemical impedance spectra (EIS) experiments can provide useful information on the impedance changes of the electrode surface. The charge transfer resistance (R_ct) at the electrode surface can be used to describe the interface properties of the electrode. Fig. S5† shows the Nyquist plots of the EIS for the bare GCE, MPC–GCE, and Pd@POMs/MPC-1, 2, and 3-GCE. The R_ct values for the Fe[(CN)₆]^4−/3− couple at different electrodes were recorded and are shown in Table S2.† It demonstrates that MPC can form a fast electron pathway between the electrode and the electrolyte and can therefore serve as a good platform for sensing applications. However, it is worth mentioning that the amount of Pd nanoparticles and its quality of dispersion are the key factors to the rate of electron transfer. At the Pd@POMs/MPC-1-GCE, the amount of Pd is relatively low, so that the electron transfer rate cannot achieve the best results. As far as Pd@POMs/MPC-3-GCE is concerned, the too big amount of Pd nanoparticles not only raises the costs of production, it may also cause the nanoparticles to agglomerate, and this makes it ineffective to the electron transfer. Compared with the two nanohybrids, Pd@POMs/MPC-2 possesses reasonable coverage amount and well dispersion of Pd nanoparticles on the MPC, which can facilitate the rate of electron transfer. This is the reason why it has the best electrocatalytic ability among the Pd@POMs/MPC samples investigated in this study, as shown in the following section.

3.2. Electrocatalysis of hydrazine, H₂O₂ and NB and their detection

The present study reports on the use of Pd@POMs/MPC as effective sensing templates for enhanced hydrazine, H₂O₂ and NB electrochemical detection for the first time. In Fig. 3A, the CVs for hydrazine oxidation at different electrodes were compared. It shows a weak electrocatalytic oxidation current towards hydrazine at bare GCE and MPC–GCE. Interestingly, the oxidation current of hydrazine at the Pd@POMs/MPC-2-GCE exhibits an increased signal than that of the bare GCE and MPC–GCE, which may have resulted from the excellent conductivity of MPC with large surface area and unique electrocatalytic properties of Pd nanoparticles. Compared with the Pd@POMs/MPC-1 and 3 electrocatalysts (Fig. S6†), the much higher current densities on Pd@POMs/MPC-2, which can be ascribed to the well dispersion of Pd nanoparticles on the MPC and the suitable configurations of Pd nanoparticles in the Pd@POMs/MPC-2 nanohybrids. Fig. 3B displays the current–time responses of Pd@POMs/MPC-2-GCE for hydrazine detection at pH = 7.0 with the applied potential of 0 V. Inset a of Fig. 3B shows the amperometric response of low concentration of hydrazine at Pd@POMs/MPC-2-GCE. The current response of Pd@POMs/MPC-2-GCE generally reached a steady-state level within 4 s after the hydrazine addition (inset b of Fig. 3B). The corresponding calibration plot for the reduction of hydrazine at Pd@POMs/MPC-2-GCE is shown in Fig. 3C. The hydrazine sensor displays a linear range of 2–2450 μM (R² = 0.999) with a sensitivity of 62.8 μA mM⁻¹. The detection limit was calculated to be 0.82 μM with the signal to noise ratio of three (S/N = 3). The reproducibility of the sensor was also investigated by current–time method for five repetitive measurements with additions of 100 μM hydrazine at 0 V (pH = 7.0). The RSD of the sensitivity was less than 3.0%. When the Pd@POMs/MPC-2-GCE was stored at 4 °C for two weeks, the current response to 100 μM hydrazine remained 93.6% of its original value, suggesting the long-term stability of the modified electrode. The performance of the Pd@POMs/MPC-2-GCE was also compared with other hydrazine sensors (Table S3†).

Fig. 3 (A) CVs of bare GCE, MPC–GCE, and Pd@POMs/MPC-2-GCE in the presence in the presence of 100 μM hydrazine. Scan rate: 50 mV s⁻¹; pH = 7.0. (B) Typical amperometric current–time curve of Pd@POMs/MPC-2-GCE with successive additions of hydrazine (pH = 7.0). (C) Relationship between hydrazine concentration and current signal for Pd@POMs/MPC-2-GCE.

Fig. 4A displays the CVs of different electrodes in the presence of H₂O₂ (1.0 mM). There is a small electrochemical response at bare GCE. However, it exhibits an increase in catalytic current for H₂O₂ reduction at MPC–GCE compared with bare GCE. More interestingly, Pd@POMs/MPC-2-GCE shows an obvious decrease in overpotential as well as response current increase for H₂O₂ reduction compared with bare GCE and MPC–GCE. The CVs of the Pd@POMs/MPC-1 and 3-GCE for H₂O₂ reduction are presented in Fig. S7.† In this study, the current–time method was employed to detect H₂O₂ at Pd@POMs/MPC-2-GCE (Fig. 4B). Inset a of Fig. 4B shows the amperometric response of low concentration of H₂O₂ at Pd@POMs/MPC-2-GCE. It is very clear that the current response of Pd@POMs/MPC-2-GCE generally reached a steady-state level within 2 s after the H₂O₂ addition (inset b of Fig. 4B). The calibration curve of reduction current is depicted in Fig. 4C, it exhibits steady amperometric response towards H₂O₂ in the linear concentration range of 1–110 μM (R² = 0.998) with a sensitivity of 98.5 μA mM⁻¹ (inset of Fig. 4C) and from 110 to 1710 μM (R² = 0.998) with a sensitivity of 76.2 μA mM⁻¹. The detection limit is 0.36 μM. The reproducibility of the sensor was also investigated by current–time method. The RSD of current signal for 200 μM H₂O₂ was less than 5.2% for five measurements, for the same electrode. After being stored at 4 °C for two weeks, 7.2% current loss was observed at Pd@POMs/MPC-2-GCE on the amperometric response of 200 μM H₂O₂. The detailed comparison of H₂O₂ detection performance using different H₂O₂ sensors is summarized in Table S4.†

Fig. 4 (A) CVs of bare GCE, MPC–GCE, and Pd@POMs/MPC-2-GCE in the presence in the presence of 1.0 mM H₂O₂. Scan rate: 50 mV s⁻¹; pH = 7.0. (B) Typical amperometric current–time curve of Pd@POMs/MPC-2-GCE with successive additions of H₂O₂ (pH = 7.0). (C) Relationship between H₂O₂ concentration and current signal for Pd@POMs/MPC-2-GCE.

Fig. 5A displays the CVs behaviors of the bare GCE, MPC–GCE, and Pd@POMs/MPC-2-GCE in 0.1 M PBS (pH = 7.0) solution in the presence of 100 μM NB. Clearly, there is a small electrochemical response at bare GCE. However, a significantly enhanced electrochemical response towards NB reduction is obtained after modification by the MPC. Pd@POMs/MPC-2-GCE shows an obvious decrease in overpotential as well as response current increase for NB reduction compared with bare GCE and MPC–GCE. The CVs of the Pd@POMs/MPC-1 and 3-GCE for NB reduction are presented in Fig. S8.† It is shown that, among the synthesized Pd@POMs/MPC electrode materials, Pd@POMs/MPC-2 sample possesses the best electrocatalytic activity towards NB reduction. The differential pulse voltammetry (DPV) was employed to detect NB at Pd@POMs/MPC-2-GCE in this study (Fig. 5B). Clearly, a series of the DPV curves (a→n) were obtained from different concentrations of NB. Inset of Fig. 5B shows the DPV response of low concentration of NB at Pd@POMs/MPC-2-GCE. A plot of I versus concentration of NB exhibited linear relationship (Fig. 5C). The NB sensor displays a linear range of 1–70 μM (R² = 0.998) with a sensitivity of 943.3 μA mM⁻¹ and from 70 to 1000 μM (R² = 0.998) with a sensitivity of 706.8 μA mM⁻¹. The detection limit is calculated as 0.42 μM. The reproducibility of the sensor was also investigated by DPV method. The RSD of current signal for 100 μM NB was less than 4.1% for five measurements for the same electrode. After being stored at 4 °C for two weeks, 6.6% current loss at Pd@POMs/MPC-2-GCE was obtained by the amperometric response of 100 μM NB. The detailed comparison of NB detection performance using different NB sensors is summarized in Table S5.†

Fig. 5 (A) CVs of bare GCE, MPC–GCE, and Pd@POMs/MPC-2-GCE in the presence in the presence of 100 μM NB. Scan rate: 50 mV s⁻¹; pH = 7.0. (B) DPV curves of NB in PBS (0.1 M, pH = 7.0) at the Pd@POMs/MPC-2-GCE. (C) The linear dependence of the current response with the different concentration of NB.

The values of overpotential and peak current of Pd@POMs/MPC-x-GCE for hydrazine, H₂O₂, and NB are presented in Table S6.† The nanocomposites without Pd nanoparticles decoration (POMs/MPC) as the electrocatalyst for hydrazine, H₂O₂, and NB are recorded (Fig. S9†). It shows that the response current and overpotential for hydrazine (A), H₂O₂ (B), and NB (C) at POMs/MPC–GCE are consistent with that of the MPC–GCE. The results indicate that Pd nanoparticles play an important role in the Pd@POMs/MPC tri-component nanohybrids. In addition, the nanocomposites without MPC (Pd@POMs) as the electrocatalyst for hydrazine, H₂O₂, and NB are also recorded (Fig. S10†). It shows that the response current for hydrazine (A), H₂O₂ (B), and NB (C) at Pd@POMs/MPC-2-GCE is much higher than that of the Pd@POMs–GCE. Interestingly, the catalytic activity of Pd@POMs/MPC–GCE is evident from a response current increase for these electroactive moleculars compared with POMs/MPC–GCE and Pd@POMs–GCE. The novel nanohybrids of Pd@POMs/MPC can provide new features of electro-catalytic activities, because of the synergetic effects of Pd nanoparticles and MPC materials.

By and large, from these findings of electrochemical experiments, the Pd@POMs/MPC-2 tri-component nanohybrids displayed the best electrocatalytic activity among the as-synthesized samples investigated in this study. The results indicate that the presence of Pd@POMs/MPC-2 made the electron transfer much easier compared with that of other Pd@POMs/MPC-x samples. Therefore, the Pd@POMs/MPC-2 sample could offer a favorite microenvironment for transferring species in solution, and would also be beneficial for accelerating electron transfer between the electrode and species in solution. Hence, the decrease in the overvoltage and a marked increase in peak current for the hydrazine, H₂O₂ and NB reaction allow the convenient electrochemical detection at the Pd@POMs/MPC-2-GCE.

4. Conclusions

In conclusion, this communication reports on the preparation of a novel Pd@POMs/MPC composite for the first time using a facile, green, and one-pot synthesis method. The POMs were used as both the reductant and bridging molecules. The MPC can offer a platform for supporting Pd@POMs to form novel hybrid nanostructures with synergetic effects. The unique architecture of the Pd@POMs with a uniform size distribution facilitates the mass transport and electron conductivity, leading to improved sensing performance. Through the analyses of the characterization and electrochemical experiments, we found that the nanosized Pd@POMs obtained by the attachment of MPC greatly improved the electrochemical activity of the composite. Pd@POMs/MPC-2 tri-component nanohybrids presented the best electrocatalytic activity among the as-synthesized samples. A sensitive biosensor for hydrazine, H₂O₂ and NB was developed based on the Pd@POMs/MPC-2-GCE, which showed wide linear range, low detection limit, high sensitivity, and good stability. In point of fact, the successful fabrication of Pd@POMs/MPC holds great promise for the design of biosensors, and is a promising way to promote the development of new electrode materials.

Acknowledgements

The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 21505031), science technology research and development guidance programme project of Baoding City (No. 15ZG006) and colleges and universities science technology research project of Hebei Province (No. Z2015096).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06643g

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