Magnetic field assisted synthesis of Co2P hollow nanoparticles with controllable shell thickness for hydrogen evolution reaction

Magnetic field assisted synthesis of Co2P hollow nanoparticles with controllable shell thickness for hydrogen evolution reaction

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Journal Pre-proof Magnetic field assisted synthesis of Co2P hollow nanoparticles with controllable shell thickness for hydrogen evolution reaction Xiaoyang Wang, Chunhong Liu, Chun Wu, Xiaomin Tian, Kai Wang, Wenli Pei, Qiang Wang PII:

S0013-4686(19)32062-6

DOI:

https://doi.org/10.1016/j.electacta.2019.135191

Reference:

EA 135191

To appear in:

Electrochimica Acta

Received Date: 29 September 2019 Accepted Date: 1 November 2019

Please cite this article as: X. Wang, C. Liu, C. Wu, X. Tian, K. Wang, W. Pei, Q. Wang, Magnetic field assisted synthesis of Co2P hollow nanoparticles with controllable shell thickness for hydrogen evolution reaction, Electrochimica Acta (2019), doi: https://doi.org/10.1016/j.electacta.2019.135191. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Magnetic Field Assisted Synthesis of Co2P Hollow Nanoparticles with Controllable Shell Thickness for Hydrogen Evolution Reaction Xiaoyang Wang1, 2, Chunhong Liu3, Chun Wu4, Xiaomin Tian1, 2, Kai Wang1, Wenli Pei3*, Qiang Wang1* 1

Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China 2

School of Metallurgy, Northeastern University, P. O. Box 314, No. 11, Lane 3, Wenhua Road, Heping District, 110819, Shenyang, China

3

Key Laboratory of Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China

4

School of Materials Science and Engineering, Liaoning Technical University, Fuxin 123000, China.

*Corresponding author’s e-mail: [email protected], [email protected] Abstract: A novel method to prepare hollow Co2P nanoparticles (NPs) with controllable shell thicknesses is proposed. In this method, high magnetic field was employed to tune the shell thickness. The mechanism of shell-tuning by magnetic field was discussed. The formation of Co2P hollow NPs involves two main processes: the dissolution of CoO on the surface of Co NPs, and the Kirkendall effect process. Magnetic field can reduce the particle size and tune the shell thickness by accelerating the dissolution process and slowing down the Kirkendall effect process. The effects of shell thickness on the stability of the hollow NPs were investigated. During the hydrogen evolution reaction (HER) process in alkaline medium, the Co2P hollow NPs with shell thicker than 3.90 nm have good structure stability and perform a good

catalytic performance. Nevertheless, when shell thickness is reduced to 3.32 nm, there is a great change in the hollow structure. It will fracture into small NPs and lead to significant degradation of activity. This work develops a reasonable method to synthesize small-sized transition metal phosphides hollow particles with adjustable shell thickness, and provides a reference for balancing the activity and stability of hollow particles, which is significant to prepare hollow nanoparticles with good catalytic performance. Keywords: Co2P hollow nanoparticles, high magnetic field, Kirkendall effect, shell thickness, hydrogen evolution reaction, stability. 1. Introduction Hollow structures have many fascinating properties, such as larger surface area, low density and high loading capacity, and demonstrate many applications, including energy storages [1-4], catalysis [5, 6] and drug delivery carries [7, 8]. In recent years, hollowed transition metal phosphides (TMPs) nanoparticles attract much attention due to its high catalytic activity for hydrogen evolution reaction (HER) [9-13]. Raymond E. Schaak's group has done some works on the HER performance of TMPs hollow nanostructures [14-16]. For example, Co2P [14], CoP [14, 15] and FeP [16] hollow nanoparticles exhibit really high activity toward HER in acidic media. As we all know, the catalytic performances of hollow nanoparticles are related to their shell thickness. The thinner the shell of hollow NPs means a larger specific surface area, but it also faces the risk of poor structure stability. To the best of our knowledge, there is no experimental data about the effect of shell thickness on the stability of hollow TMPs

nanoparticles, and it is no idea about what will happen when the shell is quite thin. So it’s an important issue to understand the relationship between the stability and the shell thickness. For this purpose, hollow particles with different shell thicknesses should be prepared. However, related reports are rare. For the preparation of TMPs hollow nanoparticles, the Kirkendall effect caused by the asymmetric diffusion of metal atoms and phosphorus atoms is the main method [17-19]. Generally, the transition metal nanoparticles are prepared firstly, and then reacted with phosphorus source, such as trioctylphosphine (TOP). During the phosphating process, metal atoms and P atoms diffuse by means of vacancies. Because the former diffuses faster than the latter, the vacancies migrate inward, and coalesce into holes eventually [20, 21]. So, it’s necessary to control the Kirkendall effect processes to tune the shell thicknesses of the TMPs hollow nanoparticles. Temperature can affect the diffusion process, while the elevated temperature (ca. 300 °C) for the broken of C–P covalent bond [9] and the boiling point of the solvent really limit the adjustability of the temperature. Therefore, new parameter should be employed to control the Kirkendall effect processes. High magnetic field has been widely used in material preparation process as an important parameter [22-27]. Actually, the effects of magnetic field on the diffusion in bulk materials and films have been extensively studied [28-31]. However, the effects of magnetic field on the nanoscale Kirkendall effect to fabricate hollow nanoparticles are rarely reported. Herein, we prepared Co2P hollow nanoparticles with different shell thicknesses by

applying a high magnetic field and revealed the relationship between shell thickness and stability of Co2P hollow nanoparticles. In addition, the formation process of Co2P hollow nanoparticles was studied, and the possible mechanism of shell thickness tuning by magnetic field was proposed. 2. Experimental section 2.1 Materials The reagents used in the experiment are as follows: Cobalt carbonyl [Co2(CO)8], 1-octadecene (ODE, 90%, TCI), Oleylamine (OY, C18-content 80–90%, Aladdin), Oleic acid (OA, C18-content 80–90%, Aladdin), Nonanoic acid (NA, 98%, TCI), Tri-n-octylphosphine (TOP, 97%, STREM), Potassium hydroxide (KOH, 95%, Aladdin) and Nafion (5%, HESEN). All materials were used without further purification. 2.2 Synthesis of Co nanoparticles The Co nanoparticles were prepared by using an adaptation of a previously reported method [15]. In a typical preparation route, a three-neck and round-bottom flask containing ODE (15 mL), OY (6 mL) and NA (2 mL) was heated to 120 °C and kept for 1.5 hours under Ar gas flow. For degassing, the reaction system was stirred by using a magnetic rotor. Then the magnetic rotor was removed out and the flask was heated to 230 °C followed by injection of [Co2(CO)8] (0.2 g) suspended in ODE (5 mL). After injection of cobalt carbonyl, degassed OA (2 mL) was rapid injected into the reaction flask. After the reaction was held at 230 °C for 10 min, the flask was removed out from the heater and cooled to room temperature. The product was

isolated and washed by repeated centrifugation with a small amount of hexanes and excess ethanol. Finally, small part of the product dispersed in hexane and the rest was dispersed in TOP (5 mL) for conversion to Co2P hollow nanoparticles. 2.3 Synthesis of Co2P hollow nanoparticles For the purposes of preparation of Co2P hollow nanoparticles with different size and studying the effects of magnetic field on the nanoscale Kirkendall effect during the wet-chemical synthesis in liquid system, a furnace [32] which can works under the high magnetic field was used. In a typical preparation route, a three-neck and round-bottom flask containing ODE (6 mL) and OY (5 mL) was heated to 120 °C and kept for 1.5 hours under Ar gas flow. Then 4 mL of TOP was injected into the flask and the flask was heated to 290 or 310 °C. After the flask held at reaction temperature for 30 min, the TOP solution containing Co nanoparticles was injected into the flask and the reaction solution was held at 290 or 310 °C for 1 hour. After the reaction solution was cooled to room temperature, the product was isolated and washed by repeated centrifugation with a small amount of hexanes and excess ethanol. Finally, the Co2P nanoparticles were suspended dispersed in hexane. 2.4 Characterizations The morphology and the size of the Co2P hollow nanoparticles were characterized by transmission electron microscopy (TEM, 200 kV, JEM-2100F, JEOL). To calculate the average particle and hole diameters, at least 100 NPs from each sample were measured. The shell thickness (ST) of each particle was calculated by the particle diameter (D) and the hole diameter (DH): ST = (D-DH)/2. Powder X-ray diffraction

(XRD) patterns of the Co2P hollow nanoparticles were collected using a Rigaku SmartLab diffractometer equipped with a Cu Kα X-ray source. The X-ray photoelectron spectroscopy (XPS, ESCALAB250) was used to characterize the chemical composition and elemental valence states. The inductively coupled plasma mass spectrometry (ICP-MS, Optima 8300DV) was used to quantify the atomic ratio of P/Co of the nanoparticles. 2.5 Electrochemical performances All electrochemical measurements were performed using a three electrodes system equipped with an electrochemical workstation (VSP, Bio-Logic) and a rotating electrode device. Ag/AgCl electrode (1 M KCl) and a Pt grid were employed as reference and counter electrode, respectively. 1M KOH aqueous solution (pH=13.7) was used as the electrolyte. For the preparation of working electrode, 5 mg of the samples powder was dispersed in 1 mL of ethanol and then 50 µL of Nafion solution was added. After the mixed was sonicated for 60 min, 10 µL of the suspension was dropped on a glassy carbon disk electrode (5 mm in diameter) to achieve a mass loading of 0.25 mg•cm-2. To obtain a stable current, cyclic voltammetry (CV) test was performed firstly at the potential range from -0.9 to -1.4 V (vs. Ag/AgCl) with a potential sweep rate of 50 mV•s-1 for 30 cycles. Then, linear sweep voltammetry (LSV) measurements were carried out at the same potential range with a scan rate of 2 mV• s-1 to obtain the polarization curves. All polarization curves were iR-corrected. Stability tests were conducted by potential cycling between -0.9 and -1.4 V (vs. Ag/AgCl) at a sweep rate of 100 mV•s-1 for 500 cycles. In the all tests, the rotating

electrode device worked at a rotational speed of 1600 rpm. In this paper, all reference potentials have been converted to relatively reversible hydrogen electrode potentials (RHE). The calibration was carried out in 1M KOH aqueous solution with two Pt wires as working and counter electrode, respectively. The average of the two potentials at which the current crossed zero in the CV curve (Fig S1) was taken to be the thermodynamic potential for the hydrogen electrode reactions, and the RHE potential was calculated as E (RHE) = E (vs. Ag/AgCl) + 1.00 V. 3. Results and analysis For the preparation of Co2P nanoparticles (NPs), Co nanoparticles with uniform shape and size (Spheres with an average size of 12.58 nm, see Fig S2) were synthesized firstly by the decomposition of [Co2(CO)8]. Then the Co2P NPs were formed by reacting Co NPs with TOP at 290 °C for 1 h. The morphology of Co2P NPs prepared under different intensities of magnetic field (0, 2, 4 and 6 T) was characterized by transmission electron microscopy (TEM). As depicted in Fig 1, all particles demonstrate a multi-faced and hollow structure. The Kirkendall effect caused by asymmetric diffusion of Co atoms and P atoms could be responsible for the formation of the hollow structure. The statistics of particle diameter (D), hole diameter (DH) and shell thickness (ST) are given in Fig S3, and the average values are summarized in Fig 2a. The D, DH and ST values of the particles prepared without magnetic field (B= 0 T) are 14.10, 4.80 and 4.65 nm, respectively. When a magnetic field is applied, the D, DH and ST values decrease. In detail, for B= 2 T, the D, DH and ST values are 12.29, 4.50 and 3.9 nm, respectively. At B= 4 T, the D, DH and ST values become 11.31, 3.94 and

3.68 nm, respectively. When the magnetic field intensity is up to 6 T, the D, DH and ST values decrease to 10.18, 3.20 and 3.49 nm, respectively. It indicates that the introduction of the magnetic field can reduce the particle diameter, hole diameter and shell thickness of the Co2P NPs. In addition, the inductively coupled plasma mass spectrometry (ICP-MS) (Fig 2b) suggests the atomic ratio of P/Co for the conditions of 0, 2, 4 and 6 T are 0.47, 0.46, 0.38 and 0.33, respectively. Obviously, the hollowed NPs obtained under 4 and 6 T magnetic field show a Co-rich structure. From the results, it can be determined that the magnetic field reduces the content of P in the particles, which indicates the inward diffusion of P atoms are inhibited, namely the Kirkendall effect process is slowed down. (a)

(b)

(c)

(d)

Fig 1 TEM images of Co2P hollow NPs prepared under different magnetic field at 290°C for 60 min: (a) 0 T, (b) 2 T, (c) 4 T and (d) 6 T

(a) 16

Particle diameter Hole diameter Shell thickness

14

P/Co atomic ratio

Size (nm)

12

(b)

10

5 4

0.50

0.45

0.40

0.35

3 2

0

2

4

Magnetic field (T)

6

0.30

0

2

4

6

Magnetic field (T)

Fig 2 (a) Particle diameter, hole diameter and shell thickness, (b) P/Co atomic ratios (ICP-MS) of Co2P hollow NPs prepared under various magnetic field.

Fig 3a shows the XRD patterns of Co2P NPs obtained under 0 and 6 T magnetic fields. When there is no magnetic field, a main diffraction peak located at 40.7° can be observed, which indexed to the diffraction from (121) plane of orthorhombic Co2P (PDF 32-0306). When a 6 T magnetic field was introduced, the diffraction peak located at 40.7° becomes weaker and broader, which reveals a decrease in grain size. Then the XPS measurements were carried out to characterize the chemical states of Co and P. The survey spectrums shown in Fig 3b indicate the existence of elements Co and P. The peak at 778.3 eV in the high resolution scan of the Co 2p (Fig 3c) originates from Coδ+ in Co2P [33-36], while the peaks at higher binding energy of 781.4 eV and 780.5 eV (for 0 and 6 T, respectively) can be attributed to the Co oxidation state (Co2+ and Co3+, respectively) in cobalt phosphate formed on the surface of the NPs [37, 38]. Additionally, two peaks exist in the high resolution scan of the P 2p shown in Fig 3d. The peaks at 129.5 eV can be attributed to Pδ- in Co2P [33-36], and the peaks at 133.3 eV and 132.9 eV (for 0 and 6 T, respectively) can be indexed to the P species in cobalt phosphate [37, 38]. It further confirms that the samples consisted of cobalt phosphide.

P 2p3/2

C 1s P 2s

Co 3p

0T 6T O 1s

• (320)

• (002)

• (121)

Intensity (a.u.)

(b)

0T 6T

Co 2p1/2 Co 2p3/2 Co LMM

• Co2P

Intensity (a.u.)

(a)

PDF#32-0306 30

40

50

60

1000

800

2θ θ (degree)

(c)

778.3 eV

6T

600

400

200

0

Binding Energy (eV) Co 2p

(d)

6T

132.9 eV

129.5 eV P 2p

0T

790

781.4 eV

788

786

784

782

778.3 eV

780

778

Binding Energy (eV)

776

774

Co 2p

772

Intensity (a.u.)

Intensity (a.u.)

780.5 eV

0T

133.3 eV

P 2p 129.5 eV

138

136

134

132

130

128

126

Binding Energy (eV)

Fig 3 (a) Experimental XRD patterns, (b) XPS survey spectra, (c) high resolution scan of the Co 2p, and (d) high resolution scan of the P 2p of the Co2P hollow NPs obtained under 0 and 6T.

To investigate how magnetic field affects the size of Co2P NPs, experiments with shorter reaction time at 290 °C were carried out. Fig 4 shows the TEM images of nanoparticles obtained at reaction times of 10 and 30 min with and without a 6 T magnetic field. It can be seen that all particles are solid, suggesting that the Kirkendall effect has not produced a significant effect yet due to the lack of P ions liberated from TOP in the initial stage of the reaction. When no magnetic field is applied, the NPs as prepared become irregular (Fig 4a, 4c), the average size for 10 min and 30 min are 12.99 and 12.48 nm, respectively. When a 6 T magnetic field is introduced, the NPs remain in a uniform spherical shape (Fig 4b, 4d). At reaction time of 10 min, the average size of the prepared NPs is 12.44 nm (Fig 4e), which is close to the size of Co NPs. Interestingly, the particle size decreases to 8.99 nm (Fig 4f) as the reaction time increases to 30 min. We believe that the reduction in particles size in the early stages

of the reaction gives rise to the reduction in the size of the Co2P hollow NPs formed in the subsequent reaction. (b)

(e)

0 T-12.99 nm 6 T-12.44 nm

25

Frequency (%)

(a)

20 15 10 5

(d)

(f)

0 0 T-12.48 nm 6 T-8.99 nm

25

Frequency (%)

(c)

20 15 10 5 0

4

6

8

10

12

14

16

18

20

Particle size (nm)

Fig 4 TEM images of the particles prepared at 290 °C: (a) 0 T/ 10 min, (b) 6 T/ 10 min, (c) 0 T/ 30min, (d) 6 T/ 30 min; Corresponding particle size distributions: (e) 0, 6 T/10 min, (f) 0, 6 T/30 min.

To further investigate the effects of the magnetic field, XRD measurements were carried out. As shown in Fig S4, for the conditions of 0 T/10 min and 6 T/10 min, there is a broad peak around 43°, which could be attributed to the overlapping of the diffraction peaks of (002) plane of ε-Co and (200) plane of CoO. The appearance of the CoO diffraction peak could be arose from the crystallization of cobalt oxide amorphous layer on Co NPs surface after being injected into the reaction solution at high temperature [39, 40]. In addition, another peak located at 36.5° can be observed under the condition of 6 T/10 min, which can be indexed to the diffraction from (111) plane of CoO. It should be the preferred orientation along the easy magnetic axes induced by magnetic field [25, 41, 42]. At reaction time of 30 min, when no magnetic field is applied, the broad peak around 43° can still be divided into two peaks, the (002) plane of ε-Co and the (200) plane of CoO. However, when there is a 6 T magnetic field, the diffraction peaks of CoO can’t be observed any more. Additionally,

Fig 5 shows the HRTEM images of the NPs as prepared. At 10 min, there is a cobalt oxide amorphous layer on the surface of the NPs, and localized area has been transformed into crystallized CoO (Fig 5a, b). At 30 min, the cobalt oxide layer on the surface of NPs is reduced, and the particles become irregular (Fig 5c). After applying a 6 T magnetic field, almost no oxide layer is observed, and the particles remain in a uniform spherical shape with a significant decrease in size (Fig 5d). These observations indicate the reduction or disappearance of the cobalt oxide layer on the surface of the NPs, resulting in changes in shape and decrease in size. (a)

(b)

(c)

(d)

Fig 5 HRTEM images of the particles prepared at 290 °C: (a) 0 T/ 10 min, (b) 6 T/ 10 min, (c) 0 T/ 30min, (d) 6 T/ 30 min.

The reduction or disappearance of CoO can be attributed to the dissolution of the CoO due to the solubility in such a liquid system [43- 47]. Fig S5 shows photographs of the reaction solution after centrifugation with 8000 rpm for 5 times. For the condition of 30 min/ 0 T, the reaction solution is light yellow (left), which is the colour of oleylamine solution. When the condition is 30 min/ 6 T, the reaction solution is light pink (right), which can be considered as the colour of Co2+, further confirming the

dissolution of the CoO. Moreover, the ICP-MS measurements demonstrate a Co2+ concentration of 0.1724 mg/mL, which is much higher than the value (0.01898 mg/mL) for the conditions of 30 min/ 0 T. These results lead us to conclude that the oxide layer on the surface of Co NPs will dissolve in the initial stage of the phosphating reaction, and the introduction of the magnetic field will accelerate the dissolution process, resulting in a more reduction in particle size. It has been reported that the dissolution of the magnetic metal oxide nanoparticles can be accelerated by the magnetic free energy produced by the external magnetic field [43]. Finally, the entire phosphating reaction process is summarized in Fig 6. It includes two main processes, the dissolution of the cobalt oxide layer on the surface of Co NPs and the Kirkendall effect. For the first one, in the initial stage of reaction, the amount of P ions released from the TOP in the solution is too small to drive the Kirkendall effect. Due to the solubility, the oxide layer on the surface of Co NPs will dissolve, resulting in irregular shape and a decrease in size. When a magnetic field is applied, the dissolution process is accelerated, resulting in a larger reduction in size, but maintaining a uniform spherical shape. With increasing reaction time, the P ions in the solution increase and the Kirkendall effect occurs. Subsequently, the Co nanoparticles are converted to Co2P via the inward diffusion of P atoms, accompanied by the formation of hollow structures and an increase in size. The undissolved CoO is converted to Co2P via a diffusion-mediated anion exchange pathway [45-47]. With the introduction of magnetic field, the Kirkendall effect process is slowed down, resulting in a smaller hole diameter, thinner shell and a lower P/Co atomic ratio. In essence, the

inhibition of the Kirkendall effect process caused by the magnetic field should be considered as the influence of the magnetic field on the diffusion of Co and P atoms. According to the theory based on plasmamagnetohydrodynamic developed by Youdelis et al [48], the magnetic field would decrease the diffusivity by the factor 1/(1 + ωce2/νe2), where ωce is cyclotron frequency and νe is collision frequency. Therefore, the decreasing in frequency factor caused by magnetic field might be the reason for the decrease in the diffusion rate of Co and P atoms.

Fig 6 Schematic illustration of the phosphating reaction processes with (bottom) and without (top) a magnetic field.

It can be seen from the above results that the magnetic field can tune the shell thickness of the hollow particles, but also reduces the P/Co atomic ratios. Therefore, more experiments should be carried out to obtain the hollow NPs with similar P/Co atomic ratios but different thicknesses. Of course, we can increase the P/Co ratio by prolonging the reaction time at 290 °C, but this will inevitably lead to an increase in the shell thickness (Fig S6). In fact, we are more interested in preparing hollow particles with thinner shells. Based on the effects of magnetic field on the phosphating reaction process, we prepare hollow NPs with a thinner shell by increasing the temperature while shortening the reaction time.

Fig 7 shows the TEM images of the particles obtained at 310 °C with different reaction times. Due to the higher reaction temperature, about 50% of the NPs formed hollow structure at reaction time of 10 min, while 12% when a 6 T magnetic field is applied. It further confirms the inhibition effect of magnetic field on the Kirkendall effect. With increasing reaction time to 30 min, almost all NPs are hollow, and the statistics of particle diameter, hole diameter and shell thickness are shown in Fig S7. When B= 0 T, the D, DH and ST values are 15.35, 5.59 and 4.88 nm, respectively. When B= 6 T, the D, DH and ST values are 10.64, 4.00 and 3.32 nm, respectively. Similarly, the reduction in these values should be attributed to the effects of the magnetic field on the phosphating reaction process, i.e. accelerating the dissolution of the oxide layer and slowing down the Kirkendall effect process. The ICP-MS measurements (Fig S8a) demonstrate a P/Co atomic ratio of 0.53 and 0.45 for 0 and 6 T, respectively. XRD measurements (Fig S8b) suggest the hollow NPs prepared under 0 and 6 T both are Co2P phase. As expected, with the assistance of a magnetic field, the hollow NPs with thinner shell are successfully prepared, which also have a similar P/Co atomic ratio to the particles obtained at the conditions of 290°C/60 min/0 T and 290°C/60 min/2 T. In addition, by analyzing the size parameters of the NPs obtained at different reaction times at 290 and 310 °C, it can be found that it is difficult to control the shell thickness of the Co2P hollow NPs by changing the temperature or reaction time in the absence of the magnetic field (Table S1), which just illustrates the advantages of the magnetic field in the controllable synthesis of the Co2P hollow NPs with different shell thicknesses.

(a)

(b)

(c)

(d)

Fig 7 TEM images of the particles prepared at 310 °C with different reaction times under 0 and 6 T magnetic fields. 0 T: (a) 10 min, (c) 30 min; 6 T: (b) 10 min, (d) 30 min.

For the purpose of studying the influence of shell thickness on the electrochemical performances of the Co2P hollow NPs, several samples with similar P/Co atomic ratio but different shell thicknesses have been chosen for the HER performance testing. The reaction conditions and size parameters of Co2P hollow NPs used for electrochemical measurements are listed in table 1. Table 1 Reaction conditions and size parameters of Co2P hollow NPs used for electrochemical measurements. Particle

Hole

P/Co Shell thickness

Samples

Reaction condition

diameter

diameter

Atomic (nm)

(nm)

(nm)

ratio

ST-4.65

290°C/60 min/0 T

14.12

4.80

4.65

0.47

ST-3.90

290°C/60 min/2 T

12.23

4.50

3.90

0.46

ST-3.32

310°C/30 min/6 T

10.64

4.00

3.32

0.45

For simplicity, the samples with shell thickness of 4.65, 3.90 and 3.32 nm were named as ST-4.65, ST-3.90 and ST-3.32, respectively. The HER performances testing was performed in a typical three-electrode system. Fig 8a shows the linear sweep voltammetry (LSV) curves obtained in 1M KOH aqueous solution with a scan rate of

2 mV•s-1. For comparison, performance of 20% Pt/C with same mass loading was also tested. It can be seen that the 20% Pt/C shows the highest catalytic activity and displays a small overpotential of 50 mV to achieve a current density of 10 mA•cm-2 (η10 = 50 mV). Obviously, the samples of ST-4.65 and ST-3.90 show similar HER activity, and the values of η10 are 131 and 133 mV, respectively. However, the sample of ST-3.32 exhibits much lower activity and the η10 value is 195 mV. Fig 8b shows the Tafel curves obtained from the LSV curves in Fig 8a. As expected, the 20% Pt/C shows the minimum Tafel slope of 30 mV•dec-1. The Tafel slopes of ST-4.65 and ST-3.90 are 36 and 33 mV•dec-1, respectively. The similar Tafel slopes indicate they have similar reaction kinetics in alkaline media. The sample of ST-3.32 demonstrates a large Tafel slope of 49 mV•dec-1, indicating a slower reaction kinetic. To investigate the charge transfer kinetics on the surface of working electrode, electrochemical impedance spectroscopy (EIS) techniques were performed at the overpotential of 150 mV. Fig 8c shows the Nyquist plots of the samples. The equivalent circuit used to fit the EIS data is inserted in the Fig 8c, in which, Rs is the solution resistance, CPE and Rct are the constant phase element and charge transfer resistance catalysts/ electrolyte interface, respectively. The small Rct indicates a fast charge transfer, which is beneficial to the superior catalytic activity. Obviously, the samples of ST-4.65 and ST-3.90 have similar Rct with a small value of 9.7 and 12.0, respectively. However, the sample of ST-3.32 shows a large Rct with a value of 144.3 Ω. It is believed that samples of ST-4.65 and ST-3.90 possess faster charge transfer compared to the sample of ST-3.32, which is highly consistent with their higher HER

catalytic activity. 0 -2

j (mA/cm2)

(b)

ST-4.65 ST-3.90 ST-3.32 20% Pt/C

-4 -6

0.20 49 mV/dec

0.15

Overpotential (V)

(a)

33 mV/dec 0.10

36 mV/dec

0.05

-8

30 mV/dec

-10 -0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

0.00

0.0

0.5

Potential (V vs. RHE)

20

CPE Rs

0

10

20

30

Rct

40

50

-0.75 0.00

-0.10

-0.15

-0.25

-0.20

-0.15

-0.10

Potential (V vs. RHE)

-0.05

2.8

0.00

200 150

2

m F/c 0m

F/c 1.04 m

0.04

0.08

0.12

2

m

0.16

0.20

Scan rates (V/s)

300 250

Overpotential (mV)

j (mA/cm2ECSA)

(f) ST-4.65 ST-3.90 ST-3.32

cm F/ 0m

-0.45 -0.60

0.00

-0.20 -0.30

3.6

-0.30

Z' (Ω Ω)

-0.05

2

-0.15

j (mA/cm2)

-Z'' (Ω Ω)

η=150 mV

10

(e)

1.5

(d) 0.00 ST-4.65 ST-3.90 ST-3.32

30

0

1.0

Log (j) (mA/cm2)

40

(c)

ST-4.65 ST-3.90 ST-3.32 20% Pt/C

η @ j=10 mA/cm2

248

Initial After 500 cycles 131 137

133

195

147

100 50 0

4. 6 ST-

5

3.90 ST-

3. 3 ST-

2

Fig 8 Electrochemical performances of the Co2P hollow NPs with different shell thicknesses in 1 M KOH solution: (a) Polarization (LSV) curves, (b) Tafel curves, (c) Nyquist plots at the overpotentials of 150 mV, inset c is the equivalent circuit, (d) Plots of double-layer capacitance (Cdl), (e) Specific activities normalized by ECSA, and (f) Overpotentials at 10 mA/cm2 obtained before (solid color) and after (with pattern) 500 cycles.

Fig 8d shows the plots of double-layer capacitance (Cdl), which demonstrate a Cdl value of 2.80, 3.60 and 1.04 mF•cm-2 for the sample of ST-4.65, ST-3.90 and ST-3.32, respectively. In addition, we calculate the electrochemical active surface area (ECSA) by using the general specific capacitance of Cs = 0.04 mF•cm−2 [49, 50], and the specific activities normalized by ECSA is shown in Fig 8e. It can be found that the sample of ST-4.65 still shows the highest activity, indicating the intrinsic activities of

its active sites are higher than those of other two. Analysis of their TEM images (Fig 1a, Fig 1b and Fig 7d for ST-4.65, ST-3.90 and ST-3.32, respectively) reveals that the hollow NPs in the sample of ST-4.65 are more irregular than those in the other two samples. It is well known that the irregular morphology can provide more active sites, which usually have higher catalytic activity. So the irregular morphology should be responsible for the higher HER activity of ST-4.65. Base on the above results, it raises a question related to the electrochemical performances of ST-3.32. Because of the smaller size and thinner shell, ST-3.32 should have possessed a larger Cdl value and higher catalytic activity, but it is not. Therefore, further researches are needed. Fig S10 shows the LSV curves after 500 cycles. It can be seen that different decay of activity occurred for the samples with different shell thicknesses. The values of η10 obtained before (solid color) and after (with pattern) 500 cycles can be found in Fig 8d. After 500 cycles, overpotentials increased by approximately 6 and 14 mV for the samples of ST-4.65 and ST-3.90, respectively. This slight increase in overpotential may attribute to the agglomeration of particles during the HER process. As for the sample of ST-3.32, there is a significant catalytic activity fading, and the overpotential increased by approximately 53 mV. It indicates that the Co2P NPs with a shell thickness of 3.32 nm suffer from a poor stability. In addition, there is no detectable Pt in the sample after 500 CV cycles (Fig S11), indicating that no electrochemical deposition of Pt occurred. To get insight into stability of the Co2P hollow NPs with different shell thicknesses, the samples after long-term stability tests were characterized by TEM. As shown in

Fig 9a-b, after 500 cycles, there are serious agglomerations of particles in the samples of ST-4.65 and ST-3.90, while the hollow structures still can be observed. The corresponding HRTEM images shown in Fig 9e-f further confirm the particles are hollow, suggesting that the Co2P NPs with shell thicker than 3.90 nm have good structural stability during HER process. However, the Co2P hollow NPs in sample of ST-3.32 transformed into solid particles with a significant increase in size (Fig 9c) after 500 cycles. Furthermore, we also detected the sample after the 1 cycle, which is the first one in the 30 cycles performed before the LSV tests. The TEM image shown in Fig 9g demonstrates that there are a large number of NPs with small size (~ 3.98 nm), indicating that the Co2P hollow NPs fractured into smaller pieces after the first CV cycle. During subsequent electrochemical testing, these small particles aggregated and grew, resulting in a fading in HER activity. This explains why the η10 of the ST-3.32 is much larger while the Cdl value is much smaller than those of other samples, as well as the activity decayed significantly after 500 cycles.

(a)

(b)

(c)

(e)

(f)

(g)

Fig 9 TEM images of the samples after 500 cycles: (a) ST-4.65, (b) ST-3.90, (c) ST-3.32; HRTEM images of hollow NPs in the corresponding samples after 500 cycles: (e) ST-4.65, (f) ST-3.90, (g) TEM image of sample ST-3.32 after 1 cycle, the inset is the HRTEM image of the NP.

These experimental results lead us to conclude that the shell thickness has a significant effect on the stability of the Co2P hollow NPs, which in turn leads to the change of catalytic activity. Theoretically, the hollow NPs with thinner shell thickness have larger specific surface area, so they should exhibit higher catalytic activity. However, in the electrochemical process, hollow NPs with thin shell, such as 3.32 nm, can’t exist stably. They are easy to break into fine particles, followed by the aggregation of the fine particles and increase in size, resulting in a rapid decline in catalytic activity. Only when the shell of the hollow NPs exceeds a certain thickness, such as 3.90 nm, can maintain good structural stability and provide stable catalytic activity. 4. Conclusions In summary, Co2P hollow nanoparticles with different shell thicknesses were successfully prepared by a magnetic field assisted wet-chemical method. The effects

of magnetic field on the formation of Co2P NPs were investigated. The phosphating reaction involves two main processes: the dissolution of CoO layer on the surface of Co NPs, and the formation of Co2P hollow structures (Kirkendall effect). The magnetic field not only accelerate the dissolution process, resulting in a larger reduction in particle diameter, but also slow down the Kirkendall effect process, make it more controllable in shell thickness, achieving the purpose of tuning the shell thickness of Co2P hollow NPs. Such method provides a new strategy for the preparation of small-sized transition metal phosphides hollow particles with adjustable shell thickness. The effect of shell thickness on the stability of the Co2P hollow NPs was revealed. For the Co2P hollow NPs, the thinner the shell is not always better. When the shell is thicker than 3.90 nm, the stability decreases slightly with the decrease of the thickness, and exhibits good structure stability during HER process in alkaline medium. However, when the shell thickness is reduced from 3.90 nm to 3.32 nm, there is an upheaval in the hollow structure. It will fracture into small pieces in the initial stage of the HER process, resulting in a significant activity fading. These findings provide a reference value of shell thickness for balancing the activity and stability of hollow particles, and are beneficial for the preparation of high efficiency hydrogen evolution catalyst. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51425401, 51404060, 51690161, 51871045), Liaoning Innovative

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Highlights: 1) Magnetic field makes it more controllable in shell thickness 2) The thinner the shell is not always better for Co2P hollow NPs 3) Co2P hollow NPs with shell thicker than 3.90 nm have good structure stability 4) It fracture easily into small pieces when the shell thickness is reduced to 3.32 nm 5) The fracture of Co2P hollow NPs will lead to a rapid fading in activity

Declaration of interests The authors declare that they have no known competing financialinterestsor personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: