Introduction

With the shortage of fossil energy and more attention to environmental protection, hydrogen energy has attracted more and more interest for its zero emissions to the environment and high energy density [1, 2]. Due to the excellent corrosion resistance, high activity, stability and durability, noble metals are critical industry catalysts and used in many fields, such as catalytic conversion [3], hydrogen evolution reaction (HER) [4] and oxygen reduction reaction (ORR) [5]. During the past decades, electrocatalytic water splitting has been considered as a promising method to produce hydrogen owing to its clean process and high energy conversion efficiency [6]. However, the scarcity and high cost of noble metals impede its application and promotion. To overcome the issues of the noble metal catalysts, various approaches have been explored, such as reducing the noble metals loadings, increasing activity and enhancing durability. The demonstrated pathways to reduce the noble metals loadings and improve Pt utilization include decreasing the size of catalyst particles and constructing core–shell structures with an abundant metal core coated by a noble metal shell [7,8,9]. Regarding of achieving high activity and enhanced durability, considerable effects have been made on alloying Pt with 3d-transition metals [10,11,12,13] and exposing highly active lattice planes on their surface [14,15,16]. Studies show that Pt-M (Fe, Co, Ni, Cu) alloy with a chemically ordered structures performs the better activity and durability [17,18,19]. However, the prepared Pt3Co alloy nanoparticles often have a chemically disordered face-centered cubic (fcc) lattice, whose activity and durability are lower than the chemically ordered structure. In order to obtain chemically ordered structures, high-temperature heat treatment is usually necessary, which is inevitably leading to the aggregation and growth of nanoparticles. Faced with this challenge, many methods have been put forward, such as encapsulating the nanoparticles with oxide like silicon dioxide or magnesium oxide [20,21,22], milling the FePt nanoparticles with sodium chloride to make the nanoparticles monodisperse in the NaCl-matrix [23, 24]. But all of these strategies need to fabricate the corresponding alloy nanoparticles firstly, which is complicated and highly demands for the process.

Here, we report an approach for large-scale production of chemically ordered Pt3Co NPs with an average size of 7.14 nm via a spray paint drying method (SPD method) combined with a post-annealing treatment. In the synthesis, the NaCl-matrix serves as a reactor for alloying of the reduced metal and effectively prevents Pt3Co NPs from coalescence during the annealing [25]. As a result, the chemically ordered Pt3Co NPs exhibit a high mass current density of 667.9 A g−1 (Pt) at − 0.05 V, and only 3.3% loss after 5000 cycles while commercial Pt/C reduces by almost 28.5% for HER in 0.5 M H2SO4.

Experiments

Materials

Cobaltous nitrate (Co(NO3)2·6H2O)(99.99%), sodium chloride (NaCl) (99.99%) and hexadecane thiol (HDT)(97.0% GC) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Oleic acid (OA) and hexachloroplatinic acid (H2PtCl6·6H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Vulcan XC-72R carbon powder was purchased from Cabot Corporation (USA). Commercial Pt/C was purchased from Johnson Matthey.

Synthesis of Pt3Co NPs

The Pt3Co NPs were prepared using a SPD method. In a typical synthesis, 1 g of hexachloroplatinic acid (H2PtCl6·6H2O), 187.3 mg of cobaltous nitrate (Co(NO3)2·6H2O) and 11.87 g of sodium chloride (NaCl) were dissolved in 50 mL deionized water. After ultrasonic dissolving for 10 min, the mixed solution was sprayed on a quartz plate which was kept at 270 °C. As the solvent evaporated off instantly, a molecular level mixture of chemical composition (salt precursor) remained on the quartz plate. Then, the mixture was annealed at 500 and 700 °C under a H2/Ar atmosphere for 2 h. The products are denoted as Pt3Co-500 and Pt3Co-700. Finally, the Pt3Co NPs monodispersed in NaCl-matrix was obtained.

Preparation of Pt3Co colloid

Hundred and fifty milligrams of salt-nanoparticle complex (containing 5 mg of Pt3Co NPs) was scattered into a solution, which was composed of 30 mL hexane, 150 µL hexadecane thiol (HDT) and 150 µL oleic acid (OA). After being sonicated for 30 min, the NPs were transferred into the oil phase successfully and form a red colloid.

Preparation of carbon-supported Pt3Co catalysts

Twenty milligrams of Vulcan XC-72R was added into the above-prepared colloid, and the mixture was sonicated for 60 min. Then, the mixture was dried and annealed at 500 °C for 2 h to remove the oil. That the carbon-supported Pt3Co (C-Pt3Co) was successfully prepared. Finally, the C-Pt3Co catalyst was dispersed in a mixture of deionized water, isopropanol and Nafion (5%) (v/v/v 3:1:0.05) to form a 2 mg mL−1 catalyst ink. A glassy carbon electrode (5 mm diameter) was polished, and then 20 µL of catalyst ink was deposited on it and dried at ambient condition.

Electrochemical measurements

Electrochemical studies were conducted by a CHI 660D electrochemical workstation with a three-electrode cell at room temperature using a 0.5 M H2SO4 aqueous solution. A platinum wire was used as the counter electrode and Ag/AgCl (3 M KCl) as a reference electrode, which was calibrated with respect to a reversible hydrogen electrode (RHE). The HER polarization curves were recorded by linear sweep voltammetry with scan rate of 20 mV s−1, while the scan rate for durability test was 50 mV s−1 between − 0.3 and 0.9 V potential.

Characterization of samples

The XRD patterns were recorded on an X-ray diffractometer (Rigaka Ultima IV multipurpose X-ray diffractometer) with a Cu-Kα radiation source over the range of 5–95° (scanning rate of 10° min−1). X-ray photoelectron spectroscopy (XPS) studies were carried out on a Phi 5000 Versa Probe Scanning ESCA Microprobe (Ulvac-Phi, Inc., Japan), and all binding energies were referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon to correct the shift caused by charge effect. The magnetic properties were measured using a Superconducting Quantum Interface Device (SQUID) (Quantum Design) with the magnetic field up to 60 kOe at the temperature of 2 K. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on the JEM-2100HR JEOL (Japan) TEM, operating at 200 kV. The electrochemical performance measurements were taken using the electrochemical workstation (CHI 660D, Chenhua Instruments, China).

Results and discussion

The schematic illustration for the fabrication of Pt3Co NPs is shown in Fig. 1. First, a mixed solution of specific proportion of sodium chloride, cobaltous chloride and chloroplatinic acid is prepared (Fig. 1a). Second, the solution was atomized and sprayed on a heated quartz plate (Fig. 1b). With the solvent evaporating instantly, each component precipitate simultaneously forms a uniformly mixed-salt precursor (Fig. 1c). Third, an annealing treatment is utilized to get Pt3Co NPs (Fig. 1d). According to the phase diagram of Pt–Co alloy [26], the ordered primitive cubic phase is more stable than disordered phase (fcc) below 750 °C. But the prepared Pt3Co alloy NPs often has a disordered phase. High-temperature treatment can promote the diffusion of atoms to realize an ordered arrangement [27]. In ordered to optimize the electrochemical performance of Pt3Co NPs, the different phases of Pt3Co NPs can be converted by controlling the annealing temperature.

Figure 1
figure 1

Schematic illustration for the synthesis of chemically ordered Pt3Co NPs

Full size image

Typical X-ray diffraction (XRD) patterns of the bulk structural information of as-synthesized Pt3Co-500 and Pt3Co-700 NPs are shown in Fig. 2a. As can be seen, seven well-defined diffraction peaks are observed at 2θ values of 40.5°, 47.1°, 68.8°, 83.0° and 87.6°. All of these peaks can be successfully indexed to (111), (200), (220), (311) and (222) plane reflections of the fcc-Pt3Co. Compared with the fcc-Pt3Co, there are two obvious additional diffraction peaks at 23.1° and 32.8° corresponding to (100) and (110) for Pt3Co-700, suggesting that chemically ordered primitive cubic (L12) Pt3Co NPs have been successful prepared after annealing at 700 °C for 2 h in Ar/H2 atmosphere. The more detailed elemental composition of the as-prepared samples is further characterized by X-ray photoemission spectroscopy (XPS), as shown in Fig. 2c, d. The Pt 4f spectrum exhibits two contributions, 4 f7/2 and 4 f5/2 (resulting from the spin–orbit splitting), located at, respectively, 71.40 and 74.47 eV, upshifting of about 0.2 eV from the standard Pt 4 f7/2 and Pt 4 f5/2 peaks at 71.20 and 74.53 eV, indicating that the Pt3Co alloy is successfully prepared [28,29,30,31]. For the Co 2 p3/2, there is a peak at 780.6 eV, indicating that the surface of the NPs is slightly oxidized when the NPs is exposed to the air. Furthermore, the calculated atomic ratio of Pt to Co is approximatively 3 for Pt3Co-500, while 4.67 for Pt3Co-700. As XPS is a surface analysis technique, there probably is a Pt-rich shell encasing the Pt3Co-700 NPs [17, 22, 32]. All the results from XRD and XPS suggest that pure and well crystallized chemically ordered Pt3Co NPs have been obtained by our simply method. In addition, Fig. 2b shows the hysteresis loop of Pt3Co nanocrystals annealed at different temperatures. It can be found that the saturation magnetization increases with the increasing annealing temperature. It can be attributed to the size growing of the NPs resulting from the higher temperature, which is consistent with previous reports [33, 34].

Figure 2
figure 2

a XRD patterns of Pt3Co-500 and Pt3Co-700. b Hysteresis loops of Pt3Co-500 and Pt3Co-700 measured at 2 K. c The Pt 4f high-resolution XPS spectra for Pt3Co-500 and Pt3Co-700. d The Co 2p high-resolution XPS spectra for Pt3Co-500 and Pt3Co-700

Full size image

The morphology and structure of the as-prepared samples are investigated by transmission electron microscopy (TEM). Figure 3a–d shows low- and high-magnification TEM images of the Pt3Co-500 and Pt3Co-700 NPs. The low-magnification images (Fig. 3a, c) indicate that the NPs exhibit a narrow size distribution and well crystallized. Moreover, it can be observed that the Pt3Co-500 NPs show a spherical morphology, while the Pt3Co-700 NPs exhibit polygon morphology under a higher annealing temperature. Clearly, the magnified images (Fig. 3b, d) show that there is one lattice spacing, about 0.22 nm, which corresponds to (111) lattice planes of Pt3Co NPs. Figure 3e shows the size distribution of Pt3Co-500 and Pt3Co-700. It can be seen, the as-synthesized Pt3Co-500 NPs have an average size 5.8 nm with a narrow size distribution, and the Pt3Co-700 is about 7.1 nm. As shown in Fig. 3f, the Pt3Co NPs are monodispersed in the NaCl-matrix with a uniform size during the annealing process, which plays a key factor in inhibiting the growth of the NPs. As a result, the NPs just show slightly growth in size even annealed at 700 °C. It should emphasize that the controlled size is an important way in realizing high utilization of Pt. After transferred into hexane, the Pt3Co NPs are well dispersed forming a red colloid (Fig. 3g), which further confirms that our samples have a small and uniform size.

Figure 3
figure 3

a–b Low- and high-magnification TEM images of Pt3Co-500 NPs annealed at 500 °C. c–d Low- and high-magnification TEM images of Pt3Co-700 NPs annealed at 700 °C. e The size distribution pattern of Pt3Co-500 and Pt3Co-700. f Pt3Co NPs scattered in annealed NaCl-matrix. g Optical image of Pt3Co-700 NPs dispersed in hexane

Full size image

To explore the structure effect on the catalytic performance, the synthesized Pt3Co-500 and Pt3Co-700 NPs are incorporated into carbon black and applied as electrocatalysts for the HER. The HER performance of as-synthesized Pt3Co NPs and the contrastive commercial Pt/C are measured in 0.5 M H2SO4 solution by a three-electrode system with a platinum sheet counter electrode. The normalized HER polarization curves of different catalysts are displayed in Fig. 4. As expected, the Pt3Co-700 NPs own the best electrocatalytic activity. Pt3Co-700 NPs exhibit a small onset potential in Fig. 4a. The overpotential is only − 32.6 and − 34.0 mV on the Pt3Co-700 NPs and Pt3Co-500 NPs, respectively, 3 mV and 1.6 mV lower than that on commercial Pt/C at a current density of 10 mA cm−2. Furthermore, the HER activities at the potential of − 0.05 V for the Pt3Co-700 NPs, Pt3Co-500 NPs and commercial Pt/C are 24.8, 21.1 and 19.3 mA/cm2, respectively. As shown in Fig. 4b, the Tafel slope of the Pt3Co-700 NPs is 28.6 mV decade−1, even lower than that of commercial Pt/C 35.2 mV decade−1, suggesting the HER rate of the Pt3Co-700 NPs acquires a more rapid increase with overpotential decreasing. These reflect the best HER activity of the Pt3Co-700 NPs. It is worth mentioning that the mass current density of the Pt3Co-700 NPs could be much higher than commercial Pt/C (473.8 A g−1) and fcc-Pt3Co NPs (569.5 A g−1, up to (667.9 A g−1) (Pt) at − 0.05 V (Fig. 4d).

Figure 4
figure 4

a The HER polarization curves of the commercial Pt/C, Pt3Co-500 NPs and Pt3Co-700 NPs normalized by electrode area, acquired by linear sweep voltammetry with a scan rate of 20 mV s−1 in 0.5 M H2SO4 solution at room temperature. b Durability test of the commercial Pt/C, Pt3Co-500 NPs and Pt3Co-700 NPs. c Corresponding Tafel plots obtained from polarization curves of above catalysts. d HER mass activity normalized by mass of Pt at – 0.05 V

Full size image

To further evaluate the application potential of the as-synthesized NPs as high-performance catalysts for HER, the durability of the Pt3Co-700 NPs were executed by voltammetry (CV) sweeps between − 0.3 and 0.6 V for 5000 cycles. Clearly, the Pt3Co-700 NPs do not show any obvious activity attenuation, as low as 1.5 mV after 5000 CV cycles at a current density of 10 mA cm−2. However, the commercial Pt/C shows approximately 9 mV negative shift, as shown in Fig. 4c. A more significant difference in the stability is shown in Fig. 4d, and the Pt3Co-700 NPs suffers from only 3.3% loss of the initial current density after 5000 cycles while commercial Pt/C reduces by almost 28.5% at − 0.05 V. The Pt3Co-700 NPs perform enhanced HER activity and durability, due to the surface segregation effect which results in a Pt-rich shell on the surface of Pt3Co-700 NPs. Furthermore, the chemically ordered structure exposes ideal catalytic lattice planes on their surface (Fig. 3c, d) and suppresses the cobalt etching in the acid [18]. Besides, previous modeling studies show that, within the chemically ordered L12-Pt3Co structure, synergy arising from the electronic spin–orbit coupling between Co and Pt makes the L12-Pt3Co chemically much more active and stable [35,36,37].

Conclusion

In summary, we present a new SPD method combined with a post-annealing treatment to realize the monodispersing of the chemically ordered Pt3Co NPs. In the synthesis, NaCl-matrix serves as a reactor for alloying of the reduced metal and prevents the NPs from sintering in high annealing temperature. Benefiting from the rational structural features, the chemically ordered Pt3Co NPs show outstanding HER catalytic activity and excellent chemical stability against Co etching in the acid solution compared with chemically disordered Pt3Co NPs. Such simply and conveniently synthetic strategy by NaCl-matrix packaging the as-grown precursor NPs during the annealing process is not limited to fabricate Pt3Co NPs, but also provide a versatile approach to high-heat treatment needed NPs for important energy conversion applications.