Microwave-assisted route for the preparation of Pd anchored on surfactant functionalized ordered mesoporous carbon and its electrochemical applications

Abstract

A microwave-assisted route for rapidly synthesizing Pd nanoparticles assembled on sodium dodecyl sulphate (SDS)-functionalized ordered mesoporous carbon (Pd-SOMC) hybrid nanocomposites has been reported. The formation of the composite materials was verified by detailed characterization (e.g., energy-dispersive X-ray spectra, X-ray photoelectron spectroscopy, X-ray diffraction, electrochemical impedance spectroscopy and TEM). TEM images reveal that the Pd nanoparticles with an average size of ∼3.82 nm are uniformly dispersed on the surface of OMC. The novel nanohybrids of Pd-SOMC can provide new features of electro-catalytic activities, because of the synergetic effects of Pd nanoparticles and OMC materials. The successful fabrication of Pd-SOMC holds great promise for the design of electrochemical sensors, and is a promising way to promote the development of new electrode materials.

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1. Introduction

In the past few years, with the increasing demand for electrochemical techniques, development for efficient and high catalytic activity electrocatalysts has been attracting increasingly intense attention. Nanostructured carbon materials have been recognized as very important electrode materials.^1–3 Ordered mesoporous carbon (OMC) is one of the most promising sources for both industrial and electrochemical applications among the various types of carbon materials. The OMC exhibits a uniform tailored and extremely well-ordered pore structure, flexible framework composition, high specific surface area, large pore volume, excellent conductivity, good thermal stability and chemical inertness, which make them suitable for applications in electrocatalysis and design of electrochemical sensors.^4–11 It is advantageous for OMC materials not only to be as an electro-catalyst, but also highly promising as a platform for supporting other nanoentities to form novel hybrid nanostructures with synergetic effects.^8,12–16

By now, the combination of noble-metal nanoparticles and OMC is of special interest; it is known to show an obviously enhanced electrocatalytic activity. Accordingly, a great deal of efforts has been aimed at developing more abundant noble-metal nanoparticles and OMC materials. Kim et al. developed a Pd nanoparticles/OMC composite by an incipient wetness impregnation method in the catalytic decomposition of 2,3-dihydrobenzofuran to monocyclic compounds.¹⁷ The synthesis of Pt nanoparticles/OMC nanohybrids based on the OMC with positively poly(diallyldimethylammonium chloride, PDDA) was reported.¹⁸ Banerjee et al. dealt with the hydrogen absorption properties of multiwalled carbon nanotubes doped with Pd nanoparticles by the conventional wet impregnation method and the polyol method.¹⁹ Au-rich bimetallic Au–Pd nanoparticles attached on OMC composite was prepared using an acid pre-treatment method.²⁰ Ye et al. used a co-calcination strategy with simultaneous N-doping, carbon graphitization, and Pd²⁺ reduction under high temperature to derive Pd nanoparticles budded on OMC.²¹ Although the abundant raw materials and methods proposed to prepare noble-metal and OMC over the past few years, the reported methods suffer some drawbacks. In general, the reduction processes of noble-metal particles are complex and mostly required a specific high temperature and long time.

Hydrazine is widely used in areas such as pharmaceutical intermediates, fuel cells, manufacture of antioxidants, rocket propellants, pesticides, explosives, and textile dyes. It has been reported that hydrazine and its derivatives have adverse health effects.^22–26 Therefore, the rapid and accurate detection of hydrazine is of great significance. Electrochemical methods have been recognized as powerful technique for inorganic and organic compound detection due to their advantages such as fast, simple instrumentation, low cost, easy operation and high sensitivity.^27–30

Herein, we report a one-step, microwave-assisted route for rapidly synthesizing Pd nanoparticles ensemble on sodium dodecyl sulphate (SDS)-functionalized OMC (Pd-SOMC) nanocomposite. The supramolecular self-assembly of SDS was used as a soft template.^31–35 The Pd nanoparticles with small size are uniformly dispersed on the surface of SOMC. The as-prepared novel Pd-SOMC nanohybrids extend the applications of support materials and provide new features of electrocatalytic activities.^12,36–39 Hydrazine was selected as marking molecules to evaluate the electrochemical activity of the Pd-SOMC nanocomposite. The electrochemical results showed that the Pd-SOMC exhibited significant electrocatalytic activity towards hydrazine in neutral solution, which proved that the as-synthesized Pd-SOMC could be used as environmental and electrochemical sensors.

2. Experimental

2.1. Chemical reagents

SDS and Nafion solution (5 wt% in 15–20% water/lower aliphatic alcohols) were used as purchased from Aldrich. PdCl₂ and hydrazine 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). The type of microwave system was used by Galanz 800 W microwave oven. 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. Synthesis of nanohybrids of Pd-SOMC

OMC was prepared according to the method reported by Ryoo et al.⁴⁰ The nanohybrid of Pd-SOMC was synthesized using microwave irradiation method in ethylene glycol. 60 mg of the OMC was dispersed in 60 mL of deionized water with sonication in the form of 40 kHz ultrasonic waves at 100 W output power. 8 mg of SDS was added as a surfactant with magnetic stirring at 50 °C for 1 h. And then, 40 mL ethylene glycol and 15 mg PdCl₂ were added into the suspension with magnetic stirring. The pH of the reaction system was then adjusted to 10.0 by adding 2.0 M of NaOH solution, followed by sonicated for another 10 min, the solution was immediately placed in the microwave oven for 2 min (power: 800 W). The suspension was isolated by centrifugation at 9000 rpm, followed by consecutive washing/centrifugation cycles several times with doubly distilled water. The obtained Pd-SOMC was dry in a vacuum oven at 60 °C for 24 h. Illustration of the preparation of Pd-SOMC is presented in Scheme 1.

Scheme 1 Illustration of the preparation of Pd-SOMC composites.

2.4. 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, 1.5 mg of the electrode materials were dispersed into a mixture of 0.1 mL (5 wt%) Nafion and 0.9 mL doubly distilled water 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.

3. Results and discussion

3.1. Characterization of the as-prepared samples

SDS is a strongly negatively charged and water-soluble polyelectrolyte. After being sonicated with OMC, the strong adsorption of the negatively charged SDS forms a supramolecular self-assembly on the surface of OMC. Fig. 1A and B shows the photographs of OMC and SOMC samples in aqueous solution taken at 0 h (A) and two weeks (B). As can be seen from Fig. 1B, after two weeks, OMC appeared as heterogeneous aggregates, and can be clearly observed at the bottom of the vial. However, a uniform and well-distributed solution of SOMC composites was observed in the aqueous solution to the naked eye. The good stability of the solution of SOMC should originate from the aqueous solubility imparted by the intermolecular electrostatic repulsion of these SOMC composites possessed the negative charge. Pd-SOMC nanocomposites were facilely obtained through heating SOMC solution mixture containing ethylene glycol and Pd²⁺ under a simple microwave-assisted heating method. The negatively charged SDS attracted Pd²⁺ via electrostatic interaction, and the electrostatic attraction can be a driving force for the formation of hybrid materials. Therefore, the Pd nanoparticles can be easily and firmly incorporated on the surface of OMC.

Fig. 1 Photographs of vials containing 1.5 mg mL⁻¹ of OMC and SOMC in aqueous solution taken at 0 h (A) and two weeks (B).

Fig. 2A and B show SEM images of the pure OMC materials. The OMC was made of small grains, which have a submicrometer-scale particle size with the length about of 0.5 μm. TEM images were used to examine the structural order and morphology of the OMC. The cross-sectional TEM image of the OMC viewed along the channels shows the hexagonal arrays of uniform mesopores (Fig. 2C). Bright contrast strips on the planar TEM image represent the pore-wall images, whereas dark contrast cores display empty channels (Fig. 2D). The typical TEM images of Pd-SOMC are presented in Fig. 2E and F. From the Fig. 2E, we can observe a number of black points attached to the gaps. Moreover, for low-scaled image of Pd-SOMC sample (Fig. 2F), it is shown that Pd nanoparticles are uniformly dispersed on the surface of OMC. The average diameter of these Pd nanoparticles determined from a statistical study of 100 nanoparticles is 3.82 nm (Fig. 2G). The high-resolution TEM image of Pd-SOMC shown in Fig. 2H reveals that the spacing of the adjacent fringes along the wire growth direction is 0.223 nm, corresponding to the (111) interplanar distance off ace-centered cubic structure.

Fig. 2 SEM images of OMC (A and B), TEM images of OMC (C and D), TEM images of Pd-SOMC (E and F), columnar distribution of Pd nanoparticles size for Pd-SOMC (G), and HRTEM image of Pd-SOMC (H).

The composition of as-synthesized Pd-SOMC composites was confirmed by EDX spectroscopy, as shown in Fig. 3A. It shows the peaks corresponding to carbon, oxygen, and palladium elements (the strong peaks of Cu are from the copper grid), therefore confirming the Pd nanoparticles on the surface of the Pd-SOMC nanocomposites. The sample was further characterized by XRD patterns and XPS analysis. As shown in the XRD spectra of the Pd-SOMC (Fig. 3B), the characteristic peak at 25° belong to the C(002) plane and other diffraction peaks at 40.1°, 45.6°, 68.2° and 81.5° can be indexed to Pd(111), (200), (220) and (311) planes of face-centered cubic crystalline of Pd, respectively. The average Pd nanoparticle size also can be estimated by using the Scherer equation.⁴¹ The equation can be expressed as follows:

where D is the average particle size (nm), k = 0.89, λ is the wavelength of X-ray (0.15406 nm), θ is the angle at the peak maximum, and β is the half-height peak width. The calculated mean sizes according to the diffraction peak of Pd(111) are found to be 3.68 nm for Pd-SOMC nanocomposites. This result is very consistent with the TEM image. The XPS spectrum of the as-prepared Pd-SOMC nanocomposites shows the Pd 3d_5/2 and 3d_3/2 (Fig. 3C). The 3d_5/2 level is located at 335.8 eV and the 3d_3/2 level at 341.2 eV, respectively. These values suggest unambiguously that Pd is present only in the metallic form, indicating the formation of Pd nanoparticles on the surface of OMC. EIS was employed to investigate the interface properties of various modified electrodes. Fig. 3D shows the results of EIS for the bare GCE, OMC–GCE, and Pd-SOMC–GCE. It can be seen that bare GCE exhibits a semicircle part at high frequency (black curve). After being modified with OMC, the diameter of the semicircle markedly decreases, indicating that OMC can form good electron pathways between the electrode and electrolyte and can be expected to be a good electrochemical platform. Moreover, the charge-transfer resistance of Pd-SOMC–GCE decreases, indicating that the electron transfer ability of Pd nanoparticles has been greatly improved by incorporating OMC.

Fig. 3 EDX spectra of Pd-SOMC (A). XRD patterns of the Pd-SOMC (B). High-resolution XPS spectra of Pd 3d (C). EIS of the bare GCE (black), OMC–GCE (blue) and Pd-SOMC–GCE 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 (D).

3.2. Electrocatalysis of hydrazine and its detection

The present study reports on the use of Pd-SOMC as effective sensing templates for enhanced hydrazine electrochemical detection for the first time. In Fig. 4A, the CVs for hydrazine oxidation at different electrodes were compared. It shows a weak electrocatalytic oxidation current towards hydrazine at bare GCE (a). However, it exhibits an increase in catalytic current for hydrazine oxidation at OMC–GCE (b) compared with bare GCE. Interestingly, the oxidation current of hydrazine at the Pd-SOMC–GCE (c) exhibits an increased signal than that of the bare GCE and OMC–GCE, which may have resulted from the excellent conductivity of OMC with large surface area and unique electrocatalytic properties of Pd nanoparticles. The reproducibility of catalytic current for hydrazine at different electrode was measured (Fig. 4A-d). Fig. 4B displays the current–time responses of Pd-SOMC–GCE for hydrazine detection at pH = 7.0 with the applied potential of 0.1 V. Inset of Fig. 4B shows the amperometric response of low concentration of hydrazine at Pd-SOMC–GCE. The corresponding calibration plot for the reduction of hydrazine at Pd-SOMC–GCE is shown in Fig. 4C. The hydrazine sensor displays a linear range of 2.99–1034.35 μM (R² = 0.997) with a sensitivity of 8.50 μA mM⁻¹ and from 1034.35 to 61 768.56 μM (R² = 0.992) with a sensitivity of 1.33 μA mM⁻¹. The detection limit was calculated to be 1.16 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 0.1 V (pH = 7.0). The RSD of the sensitivity was less than 3.0%. The performance of the Pd-SOMC–GCE was also compared with other hydrazine sensors (Table 1).

Fig. 4 (A) CVs of bare GCE (a), OMC–GCE (b), and Pd-SOMC–GCE (c) in the presence of 100 μM hydrazine. Scan rate: 50 mV s⁻¹; pH = 7.0. Reproducibility of catalytic current for hydrazine at different electrode (d). (B) Typical amperometric current–time curve of Pd-SOMC–GCE with successive additions of hydrazine (pH = 7.0). (C) Relationship between hydrazine concentration and current signal for Pd-SOMC–GCE.

Table 1 Comparison of the performance of the Pd-SOMC–GCE for the electrochemical detection of hydrazine with that of other modified electrodes

Working electrode	Potential (V)	Linear range (μM)	Sensitivity (μA mM^−1)	Limit of detection (μM)	Reference
a Reduced graphene oxide nanosheets/ZnO microspheres–Au nanoparticles modified GCE.b Polyamide 6/polyaniline (PA6/PANI) electrospun nanofibers decorated with ZnO nanoparticles modified fluorine doped tin oxide electrode.c Ag/zeolitic imidazolate frameworks nanocomposite modified GCE.d Porous Co3O4 nanowire modified GCE.e Nafion-coated titanium oxide nanoparticle deposition on carbon nanotube surfaces modified GCE.f Graphene functionalized by benzylamine molecules and subsequently palladium modified GCE.g Co3O4 nanoparticles decorated on the multi-walled carbon nanotubes modified GCE.h Pt–Cu nanoalloy was supported on the surface of porous silicon modified carbon ionic liquid electrode.i Poly(5,10,15,20-tetra(4-sulfophenyl)porphyrin–nickel) modified GCE.j Poly(4,5-dihydro-1,3-thiazol-2-ylsulfanyl-3-methyl-1,2-benzenediol)–gold nanoparticles film on multi-walled carbon nanotubes modified GCE.k Prussian blue/silver nanoparticles modified freestanding graphite felt.l Saturated calomel electrode.
RGO/ZnO–Au/GCE^a	0.1 (Ag/AgCl)	0.05–5	393.34	0.018	42
PA6/PANI_ZnO/FTO^b	0.19 (Ag/AgCl)	0.5–5000	61.77	0.35	43
Ag/ZIF-8/CPE^c	−0.05 (Ag/AgCl)	6–5000	3.87	1.57	44
Co3O4 NWs/GCE^d	0.5 (Ag/AgCl)	20–700	28.63	0.5	45
Nafion–TiO2–CNT/GCE^e	0.4 (Ag/AgCl)	0.35–162	58	0.22	46
rGO–PxDA–Pd/GCE^f	0.1 (Ag/AgCl)	1–7433	15.2	0.17	22
Co3O4/MWCNTs/GCE^g	0.5 (SCE^l)	20–1100	34.5	0.8	23
Pt–Cu@PSi/CILE^h	0 (Ag/AgCl)	0.2–1680	10.35	0.05	47
PNi–TPPS4–NPs/GCE^i	0.55 (Ag/AgCl)	1–400	0.99	0.11	48
Au/PDTYB/MWCNTs/GCE^j	0.08 (Ag/AgCl)	2–350	41.63	0.6	49
PB@Ag/GF^k	0.3 (Ag/AgCl)	0.5–8.5	26.06	0.49	50
SOMC–GCE	0.13 (Ag/AgCl)	2.99–1034.35	8.50	1.16	This work
		1034.35–61 768.56	1.33

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4. Conclusions

In conclusion, this reports on the preparation of a novel Pd-SOMC composite for the first time using a facile and one-pot microwave-assisted synthesis method. The supramolecular self-assembly of SDS was used as a soft template. The OMC can offer a platform for supporting Pd nanoparticles to form novel hybrid nanostructures with synergetic effects. The unique architecture of the Pd nanoparticles 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 nanoparticles obtained by the attachment of OMC greatly improved the electrochemical activity of the composite. A sensitive electrochemical sensor for hydrazine was developed based on the Pd-SOMC–GCE, which showed wide linear range, low detection limit, high sensitivity, and good stability. In point of fact, the successful fabrication of Pd-SOMC holds great promise for the design of electrochemical sensors, 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 No. 15ZF055), and the Natural Science Foundation of Hebei Province (No. B2016201018).

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