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Layered Metal Thiophosphite Materials: Magnetic, Electrochemical, and Electronic Properties Carmen C. Mayorga-Martinez,† Zdeněk Sofer,‡ David Sedmidubský,‡ Štěpán Huber,‡ Alex Yong Sheng Eng,† and Martin Pumera*,† †

Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Science, Nanyang Technological University, Singapore 637371, Singapore ‡ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic S Supporting Information *

ABSTRACT: Beyond graphene, transitional metal dichalcogenides, and black phosphorus, there are other layered materials called metal thiophosphites (MPSx), which are recently attracting the attention of scientists. Here we present the synthesis, structural and morphological characterization, magnetic properties, electrochemical performance, and the calculated density of states of different layered metal thiophosphite materials with a general formula MPSx, and as a result of varying the metal component, we obtain CrPS4, MnPS3, FePS3, CoPS3, NiPS3, ZnPS3, CdPS3, GaPS4, SnPS3, and BiPS4. SnPS3, ZnPS3, CdPS3, GaPS4, and BiPS4 exhibit only diamagnetic behavior due to core electrons. By contrast, trisulfides with M = Mn, Fe, Co, and Ni, as well as CrPS4, are paramagnetic at high temperatures and undergo a transition to antiferromagnetic state on cooling. Within the trisulfides series the Néel temperature characterizing the transition from paramagnetic to antiferromagnetic phase increases with the increasing atomic number and the orbital component enhancing the total effective magnetic moment. Interestingly, in terms of catalysis NiPS3, CoPS3, and BiPS4 show the highest efficiency for hydrogen evolution reaction (HER), while for the oxygen evolution reaction (OER) the highest performance is observed for CoPS3. Finally, MnPS3 presents the highest oxygen reduction reaction (ORR) activity compared to the other MPSx studied here. This great catalytic performance reported for these MPSx demonstrates their promising capabilities in energy applications. KEYWORDS: two-dimensional materials, metal phosphorus chalcogenides, hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction

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INTRODUCTION Nowadays, the electrochemical performance of two-dimensional materials allows us to evaluate catalytic activity looking for the future applications in energy alternatives, advanced electronic devices, and (bio)sensing systems.1 Besides graphene materials,2−4 transitional metal dichalcogenides,1,5,6 and black phosphorus,7,8 the attention is currently turned to other 2D materials in search of enhanced electrical and electrochemical properties. Interestingly, the long history of two-dimensional (2D) materials called metal phosphorus chalcogenides with a general formula of MPSx or MPSex9,10 has revealed their following characteristics: (i) anisotropic properties imposed by their layered structure and their ability to act as host lattices for intercalation compounds,11 (ii) magnetic and antiferromagnetic properties, because magnetic moments are localized in the transition metal ions that form a honeycomb network structure,12,13 and (iii) wide-range band gaps that suggest their optoelectronic applications in a broad wavelength range.10 Furthermore, these materials show a layered structure where a single layer is made up of sheets of metal and phosphorus atoms located between two sheets of S atoms, in a similar way to transition metal dichalcogenides.10 © XXXX American Chemical Society

In this work, we synthesized, characterized, and performed a comprehensive study of the magnetic and electrochemical properties of 10 different metal phosphorus chalcogenides that we name hereinafter “metal thiophosphites” with a general formula of MPSx. The metal component (M) will vary by using transitional metals (Cr, Mn, Fe, Co, Ni, Zn, and Cd) or pmetals (Ga, Sn, Bi), and their crystal structures are shown in Scheme 1. The electrochemical performance in terms of inherent electrochemistry, heterogeneous electron transfer, hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction is evaluated, looking for the future application in alternative energy sources and designing an efficient energy-storage device, as well as the development of (bio)sensing system.

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RESULTS AND DISCUSSION In this study, we investigated the electrocatalytic performance of the metal thiophosphites (MPSx) where the metal Received: December 23, 2016 Accepted: March 20, 2017

A

DOI: 10.1021/acsami.6b16553 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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(space group C2) (see the structure in Scheme 1 and Figure 2 as insets). Moreover, the X-ray diffraction patterns (Figure 2) of these transition metal thiophosphites manifest their phase purity and exhibit significant preferential orientation in the (00l) direction due to the layered structure. Besides the monoclinic transitional metal trithiophosphites described before, other metal thiophosphites (CrPS4, SnPS3, GaPS4, and BiPS4) with a layered structure (see the structure in Scheme 1 and in the inset of Figure 2) were identified. Monoclinic CrPS4 (space group C2) also exhibits high preferential orientation in the (00l) direction. The X-ray diffractogram shows that, GaPS4 with a monoclinic structure (space group P21/c) exhibits a preferential orientation in the (h00) direction, SnPS3 crystallizes in the monoclinic structure (space group P21/c) and BiPS4 with an orthorhombic structure (space group Ibca). Structural characterization was further performed by Raman spectroscopy. The MPS3 phases with C2 symmetry (MnPS3, FePS3, CoPS3, NiPS3, ZnPS3, and CdPS3) exhibit eight Raman active modes originating from P2S6 units with D3d symmetry (3A1g and 5Eg Raman active modes).12 Below 150 cm−1, cation vibrations are observed.14,15 The A1g modes are observed around 590, 387, and 255 cm−1 and Eg modes around 560, 283, 235, 180, and 150 cm−1. The differences between wavenumbers of phonon modes for different compounds are up to about 20 cm−1. Let us note that not all phonon modes originating from the symmetry of P2S6 units in MPS3 phases are observed in all investigated compounds. The results are summarized in Table S2. The overtones of A1g modes are observed only in a few MPS3 compounds, especially in NiPS3 at 815 and 1189 cm−1 and around 764 and 1145 cm−1 for MnPS3 and ZnPS3. The Raman spectra are shown in Figure 3. The Raman spectra of SnPS3 are related to the previously discussed MPS3 phases of transition metals. However, due to the differences of lattice structure, the phonon modes of P2S6 units are slightly different. Strong bands below 100 cm−1 originate from tin cation vibrations. The Raman spectra of CrPS4, GaPS4 and BiPS4 consist of several vibration modes of deformed tetrahedron and also cation vibrations at wavenumbers below 100 cm−1. The Raman spectra of CrPS4, GaPS4, BiPS4, and SnPS3 are shown in Figure S4. X-ray photoelectron spectroscopy (XPS) was performed to detect the MPSx surface composition; wide-scan X-ray photoelectron spectra for all the MPSx are illustrated in Figure S5. Besides the presence of C and O in the samples, the signals due to metals (Cr, Fe, Ni, Cd, Sn, Mn, Co, Zn, Ga, and Bi),

Scheme 1. Crystal Structures of Metal Thiophosphite Materialsa

a

The metal component (M) will vary as follows: (A) M = Cr; (B) M = Mn, Fe, Co, Ni, Zn, and Cd; (C) M = Ga; (D) M = Sn and (E) M = Bi.

component is varied using transitional metals CrPS4, MnPS3, FePS3, CoPS3, NiPS3, ZnPS3, CdPS3, and other metals to obtain GaPS4, SnPS3, and BiPS4. Additionally, a systematic characterization of the materials to ascertain their identity is achieved using scanning electron microscopy (SEM), energydispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and vibration sample magnetometry (VSM). The SEM images of MPSx are shown in Figure 1. All of the materials present a characteristic structure of the bulk materials with stacked sheets of varying thicknesses. The chalcogen/ metal (S/M) ratio and phosphorus/metal (P/M) ratio are calculated from the elemental analysis by EDS. Figures S1 and S2 show the EDS mapping from SEM and TEM, respectively. The S/M and P/M ratios for all MPSx materials from the EDS data obtained from SEM images are summarized in Table S1. In all of the materials, the S/M and P/M ratios are very close to the expected ones documenting a successful synthesis of materials. The selective area electron diffraction (SAED) patterns, TEM, and HR-TEM images were performed showing the crystallinity and layered structure of the MPSx materials; see Figure S3 in the Supporting Information (SI). The phase purity and crystal structure of the MPSx materials was subsequently studied using X-ray diffraction. The MPS3 constituted from transitional metals (MnPS3, FePS3, CoPS3, NiPS3, ZnPS3, and CdPS3) crystallize a monoclinic structure

Figure 1. SEM images of the metal thiophosphite materials. B


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the other hand, two deconvolved contributions that correspond to P 2p regions are observed for all materials (see Table S3), the average position of P 2p3/2 and P 2p1/2 are 131.5 ± 0.9 and 132.5 ± 0.8 eV for all MPSx and can be attributed to P−S bonding.16 Nevertheless, in almost all materials additional peaks were observed, thus referring to oxidation state of P4O10 with an average position of 134.1 ± 0.4 eV.17,18 Only ZnPS3 did not show the oxidation state of P, even though in CoPS3 and SnPS3 the P4O10 peaks have a higher intensity. Finally, three deconvoluted peaks appear in the S 2p region for all MPSx materials except for BiSP4 (Table S3). The first two peaks corresponding to S 2p3/2 and S 2p1/2 with average positions 161.8 ± 0.5 and 162.5 ± 0.4 eV, respectively, give an evidence of the presence of polysulfide.16 However, the last one corresponds to sulfur peak at average position of 163.6 ± 0.5 eV.16 For BiSP4, the regions S 2p and Bi 4f are overlapping, in this way after the deconvolution four peaks appear. The peaks around 158.6 and 163.8 eV correspond to Bi 4f5/2 and Bi 4f7/2, respectively,19 while the peaks at 161.6 and 162.6 eV provide the presence of polysulfides.16 Additionally, another peak is observed at 225.2 eV corresponding to S 2s transition in its survey scan and confirming the presence of sulfur.19 The magnetic properties of all studied MPS3 as well CrPS4, GaPS4, and BiPS4 phases were explored using vibration sample magnetometry. As expected, SnPS3, ZnPS3, CdPS3, GaPS4, and BiPS4 exhibit only diamagnetic behavior due to core electrons. By contrast, trisulfides with M = Mn, Fe, Co, and Ni, as well as CrPS4, are paramagnetic at high temperatures and undergo a transition to antiferromagnetic state on cooling. The essential magnetic characteristics evaluated from the susceptibility data (Figure 5) are summarized in Table 1. Unfortunately, due to narrow range of available data above the Néel temperature, it was not possible to analyze the effective magnetic moment and Weiss temperature of NiPS3. Within the trisulfide series the Néel temperature characterizing the transition from paramagnetic to antiferromagnetic phase increases with increasing atomic number and the orbital component enhancing the total effective magnetic moment (from pure spin moment for Mn to the highest orbital moment for Ni). The obtained data are consistent with those reviewed in ref 20 except for FePS3 which exhibits substantially higher effective moment and lower Weiss temperature. Moreover, a second transition is clearly seen at ∼18 K likely indicating an ordering in the direction perpendicular to layers with weaker exchange coupling. An interesting effect likely connected with two different coupling mechanisms is observed on the magnetization curve of FePS3 (Figure S6), which is linear as expected for a canonical antiferromagnet; however, it changes the slope at a field of 3 T. The Weiss constant is negative for Mn, Co, and Ni,20 while for Fe and CrPS4 its positive value suggests ferromagnetic correlations within the paramagnetic phase. The observed magnetic behavior is generally consistent with the electronic structure calculated for two selected representatives MPS3, M = Mn and Fe (Figure 6), using a GGA+U functional with an additional Coulomb potential U = 3 eV applied on M-3d states. In particular, the antiferromagnetic spin arrangements20 turned out to be more stable compared to ferromagnetic states by 5 and 15 kJ mol−1 for MnPS3 and FePS3, respectively. The density of valence states constitutes of highly localized states in the range 4−16 eV below the Fermi level (EF) represented by molecular orbitals of P2S6−4 clusters and the majority spin M-t2g3 eg2 multiplets strongly hybridized with S-3p states. Moreover, a narrow minority spin band is

Figure 2. X-ray diffraction patterns with schematic crystal structure representations (insets) of metal thiophosphites (MPSx).

phosphorus and sulfur are also detected in the corresponding spectra, thus confirming the existence of respective constituents in the MPSx materials. The chemical bonding characteristics of the MPSx have been investigated by the narrow scanning XPS spectrum (Figure 4). The high-resolution scanning XPS spectrum of metal (Cr 2p, Mn 2p, Fe 2p, Co 2p, Ni 2p, Zn 2p, Cd 3d, Ga 3d, Sn 3d, and Bi 4f), P 2p, and S 2p of the MPSx materials are evaluated. The positions of the respective signals are summarized in Table S3. The peaks developed at the metal component of CrPS4, FePS3, ZnPS3, CdPS3, GaPS4, and BiPS4 can be attributed to pure metallic Cr 2p(3/2,1/2), Fe 2p(3/2,1/2), Zn 2p(3/2,1/2), Cd 3d(5/2,3/2), Ga 3d, and Bi 4f(7/2,5/2), respectively, whereas the peaks developed for MnSP3, CoPS3, NiPS3, and SnPS3 show both the metallic phase (Mn 2p3/2, Co 2p(3/2,1/2), Ni 2p(3/2,1/2), and Sn 3d(5/2,3/2)) and partial oxidation to MnO2 (Mn 2p3/2), CoO (Co 2p(3/2,1/2)), NiO (Ni 2p3/2), and SnO2 (Sn 3d(5/2,3/2)). On C


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Figure 3. Raman spectrum of transition metal MPS3 phases.

Figure 4. High-resolution X-ray photoelectron spectrum and deconvoluted peaks of metal (Cr 2p, Mn 2p, Fe 2p, Co 2p, Ni 2p, Zn 2p, Cd 3d, Ga 3d, Sn 3d, and Bi 4f), P 2p, and S 2p regions of metal thiophosphite (MPSx) materials. D


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peaks. On the other hand, irreversible oxidation peaks of SnPS3, CrPS4, CdPS3, and FePS3 are observed, because after the first scan the peaks disappear from subsequent scans. Moreover, GaPS4 shows an irreversible reduction peak. Finally, in the second scan of NiPS3 and BiPS3, an oxidative peak appears and increases in the third scan. Heterogeneous electron-transfer (HET) processes at nanoscopic size materials play key roles in enhanced effects of electronic structure of electrode materials.21 Moreover, HET of the materials determine their suitability for application in advanced label-free (bio)sensor devices. In this manner, the heterogeneous electron transfer (HET) rates of the MPSx were evaluated. Cyclic voltammograms of redox couple [Fe(CN)6]3/4 were recorded in anodic direction (Figure 8A) to obtain the separations between anodic and cathodic peaks (ΔEp) (see Figure 8B). HET rate constants can be calculated using ΔEp values based on the Nicholson’s method1 and are summarized in the Table S5. BiPS4 exhibits the fastest HET similar to GC, which then decreases in the following order CdPS3 > CrPS4 > SnPS3 > CoPS3. The other materials show a heterogeneous electron transfer rate constant one order of magnitude below. The HET performance is related to the intrinsic orbital orientation of each material. In the case of BiPS4 the inherent electrochemistry study shows that this material is more electroactive than the other MPSx, since this material shows an oxidation peak around the potential window where the HET studies were performed, and this can be the reason for the enhanced HET observed in this material. Moreover, the charge transfer resistance (Rct) was obtained by electrochemical impedance spectroscopy, and the measurements were modeled using Randle’s approach modified with Warburg impedance (Zw). The Faradic reaction that occurs during the electrochemical impedance spectroscopy experiment is shown below:

Figure 5. Magnetic susceptibility of MnPS3, CoPS3, NiPS3, FePS3, and CrPS4 phases measured at a field of 1000 Oe. The inset is the inverse susceptibility representation (without NiPS3).

Table 1. Magnetic data of MnPS3, CoPS3, NiPS3, FePS3, and CrPS4: Effective Magnetic Moment μeff, Apparent Total Spin S Assuming Zero Orbital Moment, Néel Temperature TN, and Weiss Constant Θ MPSx

μeff/μB

S

Θ (K)

TN (K)

CrPS4 MnPS3 FePS3 CoPS3 NiPS3

3.87 5.92 5.41 4.90

1.50 2.50 2.25 2.00

60 −245 16 −223

38 115 120 132 265

pinned to Fermi level in FePS3, consistent with the overall highspin configuration Fe-t2g4 eg2. Unfortunately, the enhanced magnetic moment observed for FePS3 cannot be accounted for in terms of an additional orbital component, since this amounts only 0.02 μB according to our calculations. The semiconducting character of both compounds is manifested by the band gap energies 2.0 and 1.5 eV obtained for MnPS3 and FePS3, respectively. After the characterization of the MPSx materials, the electrochemical fundamental studies were subsequently performed. Cyclic voltammograms (Figure 7) of all materials were carried out to evaluate their inherent electrochemical properties. The innate oxidation and reduction signal of MPSx are summarized in Table S4. As can be seen, MnPS3, CdPS3, CoPS3, and GaPS4 show inherent reduction and oxidation

Fe(CN)−6 4 → Fe(CN)−6 3 + e

(1)

This reaction occur because ferro/ferricyanide redox couple is used as an analyte, commonly employed for electrochemical impedance spectroscopy experiment for sensing purpose. The lowest Rct values are obtained for CdPS3 and BiPS4 and increase in the following order CrPS4 < ZnPS3 < CoPS3 < FePS3 < SnPS3 < GaPS4 < NiPS3 < MnPS3. However, the Rct of GC is in between of CdPS3 and BiPS4, and the higher Rct values observed for almost all materials are related to their semiconducting nature.10 For this reason these materials do

Figure 6. Electronic structure (density of statesDOS vs energy) of antiferromagnetic MnPS3 (A) and FePS3 (B). Dashed and dotted lines represent, respectively, the projected DOS on majority and minority (plotted as negative) spins of a single Mn (Fe) atom. E


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Figure 7. Inherent electrochemistry of the metal thiophosphite materials. Cyclic voltammograms are recorded in supporting electrolyte of purged 50 mM PBS at pH 7.2 and scan rate of 0.1 V/s. All of the scans start at 0.0 V in anodic direction.

Figure 8. Electron transfer kinetics studies in ferro/ferricyanide redox probe 10 mM of the metal thiophosphites: Cyclic voltammograms (A) and the peak separation values obtained from CVs (B). Nyquist plots (C) and the Rct values (D). Error bars correspond to standard deviations based on triplicate measurements.

materials of low cost and high performance even aroused great interest in the scientific community. In line with this effort we carefully evaluated the HER performance of MPSx. The HER polarization curves of MPSx and the summary of the onset potentials at −1 mA/cm2 are presented in Figure 9A and B, respectively. The HER measurements of bare glassy carbon (GC) electrode and that modified with platinum-on-carbon (Pt) are also included in this figure as a reference. We observed the lowest onset potential of approximately −0.40 V vs RHE for BiPS4 that may be attributed to reduction signals observed on this material due to other species in the electrolyte such as oxygen, as well as its inherent reduction. NiPS3 (−0.53 V vs RHE), CoPS3 (−0.59 V vs RHE), and FePS3 (−0.86 V vs RHE) all of them lying in between the onset potentials of Pt

not greatly enhance the electron transfer; rather the electron transfer is worse with regards to the GC electrode. However, some of these materials are not as bad as the others, and this is due to the intrinsic conductivity of each material. Subsequently, the electrocatalytic performance of the MPSx materials was evaluated by testing their catalytic activity in hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). The hydrogen evolution reaction (HER) is a cathodic half reaction of water splitting and is relevant in the applications of energy conversion devices including water electrolysis and artificial photosynthetic cells. Precious metals like platinum22 and more recently 2D materials like TMDs show a high efficiency for HER.1,23−27 However, the search for alternative F


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Figure 9. Hydrogen evolution reaction (HER) performance on metal thiophosphites. Linear sweep voltammograms (A) and the average of the overpotential at a current density of −1 mA/cm2 (B). Tafel plots (C) and Tafel slopes values (D) for metal phosphorus chalcogenides. Error bars correspond to standard deviations based on triplicate measurements.

Figure 10. LSV of oxygen evolution reaction (OER) (A) and LSV of oxygen reduction reaction (ORR) (C). Average of the overpotential at a current density of 10 mA/cm2 for OER (B) and peak reduction potentials for the ORR (D). Error bars correspond to standard deviations based on triplicate measurements.

to have the highest activity for HER.28 Moreover, other compounds that include in their structure Ni and Co show a high HER performace.28−30 Furthermore, Tafel analyses from the polarization curves were subsequently evaluated as a second parameter to elucidate the electrochemical mechanisms of the HER on MPSx (see Figure 9C and D). The general mechanisms for the HER are defined by the following equations:

(−0.103 V vs RHE) and GC (−0.89 V V vs RHE). The HER onset potentials for the other materials (SnPS3, GaPS4, CrPS4, MnPS3, ZnPS3, and CdPS3) are higher than the onset potential of GC. The high HER activity for NiPS3 and CoPS3 may be ascribed to the crystal structure that for this material exhibits significant preferential orientation in (001) (see Figure 2), and for other compounds like Ni2P these crystal planes have been predicted G


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materials. The high OER activity presented by CoPS3 is likely due to the cobalt, since other Co compounds such as CoP nanorods and nanosheets exhibit high OER.35,36 The oxygen reduction reaction (ORR) for proton exchange membrane fuel cells has been thoroughly studied with the cathode Pt-based catalyst showing the best performance, but the low abundance and high cost of this noble metal are the reason for the search for new materials with high ORR activities.37 Hence, the oxygen reduction reaction (ORR) activity of the MPSx was also assessed, as displayed in Figure 10C and D. A distinct reduction peak at about −0.28 V vs RHE is observed for MnPS3, while for Pt the reduction peak is about −0.21 V vs RHE. For the other MPSx materials, the reduction peaks corresponding to ORR were recorded at higher potentials compared to GC electrode. Nevertheless, other nanomaterials like MnOx nanoparticles shows good ORR performance.38,39 In this way the enhanced ORR activity of MnPS3 may be not surprising. These novel class layered materials such as NiPS3, CoPS3, and MnPS3 presented here, shown enhanced HER, ORR, and OER. The HER performances of NiPS3 and CoPS3 were compared with some materials with similar structure of morphology like metal phosphides (NiP,40 CoP, and Co2P nanoparticles29) and TMDs (WS2, MoS2, WSe2, and MoSe2 methyllithium and butyllithium exfoliated,1 pristine monolayer MoS2 with induced defect via plasma treated,26 WS2 nanoflakes25), our materials showing similar or improved HER catalysis. Moreover, the most interesting finding is that CoPS3 also exhibits a high OER performance that is much better than IrO2 reported as reference material35,36 and CoP (nanorods35 and nanosheets36) uses for this aim. On the other hand, it is important to highlight that the MPSx shown here are in the bulk layered state without any exfoliation.

Volmer adsorption step: +

−

H3O + e → Hads + H 2O

b ≈ 120 mV/dec

(2)

Heyrovsky desorption step: Hads + H3O+ + e− → H 2 + H 2O

b ≈ 40 mV/dec (3)

Tafel desorption step: Hads + Hads → H 2

b ≈ 30 mV/dec

(4)

The highest catalytic efficiency was observed for NiPS3 with a Tafel slope at an average of ∼56 mV/dec, a value in agreement with the Heyrovsky mechanism, while ZnPS3 (75 mV/dec), CoPS3 (84 mV/dec), and BiPS4 (102 mV/dec) have Tafel slopes lower than the Volmer step. On the other hand, the Tafel slopes of CdPS3, CrPS3, SnPS3, GaPS4, FePS3, and MnPS3 range from 0.132 mV/dec to 0.318 mV/dec showing the Volmer step as the rate-limiting step. We further performed the stability test of the materials that show the best HER catalysis (NiPS3, CoPS3, and BiPS4) in 0.5 M H2SO4 by cyclic voltammetry (CV) scanning 100 cycles with a scan rate of 100 mV/s (see Figure S7 in SI), at their respective HER onset potential window where the current density reaches at least −0.1 mA/cm2.31 The LSV curve of the CoPS3 HER catalysis was unchanged compared with the initial measure, which indicates that these materials have a good cycle life in acidic media. Meanwhile, the NiPS3 and BiPS4 show that after the CV cycling the LSV curves change in both cases. Nevertheless, the change is more evident for BiPS4. Therefore, XPS was performed for these three materials, and screen printed (SP) electrodes were used (Figure S8 in SI). For NiPS3 and CoPS3 no significant changes were observed, but in the case of BiPS4 the structure of the material changed completely, and the phosphorus component was totally lost. Oxygen evolution reaction (OER) electrolysis represents another important half-reaction involved in water splitting with the application in rechargeable metal−air batteries mainly.32 This reaction has been intensely investigated for decades, and the low overpotentials are shown by IrO2 and RuO2.33 However, the high cost of these materials have led to a search for alternative materials such as transitional metal phosphides.34 Thus, the evaluation of the OER performance of MPSx is very important. Figure 10A shows the polarization curves performed in alkaline media (1 M KOH), and Figure 10B shows the summary of the onset potential at 10 mA/cm2 for all of the MPSx materials evaluated. The measurements were performed in high concentrations of alkali media for stable electrocatalysis until the current density reached 10 mA/cm2 in order to avoid in this way the effect of inherent electrochemistry. As we can see, the best catalytic performance is observed on CoPS3 with an onset potential of 0.84 at 10 mA/cm2, and the current density reached up to 30 mA/cm2. However, it is important to note that the other materials exhibited lower OER activities with onset potentials at 10 mA/cm2 similar to the GC electrode (1.65 V vs RHE). Furthermore, the stability test was performed for the CoPS3 by 100 CV cycles at 100 mV/s at OER onset potential window where the current density reached is at least 0.1 mA/cm2. The LSV curve obtained after the cycling was closely to LSV measured before to CV cycling (see Figure S9 in SI). To evaluate the effect of the OER electrocatalysis on the CoPS3 structure, XPS study were performed and shown in Figure S10. As it can be seen the high-resolution XPS scans do not show any major changes compared with the initial

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CONCLUSION In summary, we have synthesized, characterized, and performed a comprehensive electrochemical study of layered metal thiophosphite materials (MPSx). The SEM images showed typical structures of layered materials, and the structural characterization performed via EDS, XRD, and XPS spectroscopy demonstrated the successful synthesis of MPSx. Magnetic properties were studied, and we found that in the studied series of MPS3 (M = Mn, Fe, Co,Ni) as well as CrPS4 the Néel temperature characterizing the transition from paramagnetic to antiferromagnetic phase increases with an increasing atomic number. By contrast, SnPS3, ZnPS3, CdPS3, GaPS4, and BiPS4 exhibit only diamagnetic behavior due to core electrons. The observed magnetic behavior is generally consistent with the electronic structure calculated for two selected representatives of MPS3. High electrochemical performances in catalytic terms were observed for HER (NiPS3 and CoPS33), OER (CoPS3), and ORR (MnPS3), which suggests that these materials can be used as efficient electrocatalysts for key industrially important reactions.

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EXPERIMENTAL SECTION

Reagents. Red phosphorus (99.999%) and sulfur (99.999%) were obtained from STREM, Germany. The elements used for synthesis were in a powder form with the exception of low melting point metals (Zn, Ga, Sn, and Bi). Zn, Ga, Cd, Sn, and Bi granules with purity 99.999% were obtained from STREM, Germany. Chromium (99.9%), manganese (99.9%), iron (99.9%), cobalt (99.9%), and nickel (99.9%) in powder form (−100 mesh) were obtained from Alfa Aesar, H


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ACS Applied Materials & Interfaces Germany. Potassium ferrocyanide, potassium chloride, potassium hydroxide, phosphate dibasic, sodium phosphate monobasic, sodium chloride, sulfuric acid, and 20 wt % platinum on graphitized carbon were purchased from Sigma-Aldrich. Synthesis of Metal Thiophosphite Materials. The stoichiometric amount of metal, phosphorus, and sulfur corresponding to 5 g of thiophosphite was placed in a quartz glass ampule (18 × 100 mm; wall thickness 2 mm) and sealed under high vacuum (below 5 × 10−3 Pa) using oxygen/hydrogen welding torch. The ampules were placed in the muffle furnace and heated on 650 °C for 120 h. The heating rate was 5 °C/min, and the cooling rate was 1 °C/min. Some of the samples were additionally treated in order to complete the reaction. ZnPS3 sample was additionally heated at 750 °C for 168 h (heating and cooling rate was 5 °C/min), while CoPS3 and NiPS3 were additionally heated for 300 h at 550 °C (heating and cooling rate was 5 °C/min). Structural and Morphological Characterization. To obtain SEM micrographs, scanning electron microscopy (JEOL 7600F, Japan) was used in gentle-bean mode at 2 kV. Transmission electron microscopy (TEM) and SAED patterns were performed using an EFTEM Jeol 2200 FS microscope (Jeol, Japan). EDS measurement was performed with X-MaxN 80 T detector from Oxford Instruments. A 200 keV acceleration voltage was used for measurement. Sample preparation was attained by drop casting the suspension (1 mg mL−1 in water) on a TEM grid (Cu; 200 mesh; Formvar/carbon) and drying at 60 °C for 12 h. The XPS spectra were obtained using X-ray photoelectron Phoibos 100 spectrometer (SPECS, Germany) with a monochromatic Mg Kα radiation as the X-ray source; the spectra were calibrated to the C 1s peak at 284.5 eV. InVia Raman microscope (Renishaw, England) was used for Raman spectroscopy measurements in backscattering geometry with a CCD detector. Nd:YAG laser (532 nm, 50 mW) and 50× objectives were used for the measurement. The instrument calibration was achieved using a silicon reference which gave the peak position at 520 cm−1 and a resolution of less than 1 cm−1. To ensure a sufficiently strong signal and to avoid radiation damage to the samples, the laser power used for these measurements was 5 mW. Magnetic measurement was performed using PPMS Evercool system from Quantum Design (USA). The vibration sample magnetometer (VSM) insert was used for measurement of magnetization curves, zero-field cooled (ZFC), and field-cooled (FC) susceptibility curves (recorded at a field of 1000 Oe) in the temperature range of 4−300 K and field up to 7 T. DFT Calculations. The electronic structure of MnPS3 and FePS3 was calculated within density functional theory (DFT) using APW+lo basis set41 and generalized gradient approximation (GGA, PBE96 parametrization scheme42) for the exchange correlation potential as implemented in the Wien2k software package. Moreover, an additional Coulomb potential U = 3 eV was applied on transition metal 3d states to properly treat the electron correlations. Spin orbit coupling was considered for Fe within a second-variational procedure applied on nonrelativistic Fe-3d states. The plane wave cutoff energy of 320 eV and the tetrahedron method with the k-mesh 11 × 7 × 7 were used. Electrochemical Measurements. Electrochemical measurements were carried out at room temperature using a Autolab PGSTAT204/ FRA32 M (Eco Chemie, Utrecht, The Netherlands) controlled by NOVA Version 1.1 software (Eco Chemie) and three-electrode system. This system is composed by glassy carbon GC as a working electrode modified with 3 μL of metal thiophosphites solution by drop casting for obtain a film of 15 μg of the desired material, Pt counter electrode, and Ag/AgCl reference electrode. The electrodes were purchased from CH Instruments, Texas, USA. Each material was dispersed in deionized water at a concentration of 5 mg/mL and sonicated for 4 h. Studies on inherent electrochemistry of MPSx on the modified electrodes were made in a 50 mM phosphate-buffered saline (PBS, pH 7.2) solution using cyclic voltammetry at a scan rate of 0.1 V/s in a potential window of −1.8 to 1.8 V. Solutions were purged with nitrogen gas before measurements. The heterogeneous electron transfer rates measurements were done using KCl solution 0.1 M as

the supporting electrolyte for 10 mM of the ferro/ferricyanide redox probe. Cyclic voltammetry at a fixed scan rate of 0.1 V and electrochemical impedance spectroscopy in a frequency range of 1 mHz to 100 kHz experiments applying 20 mV AC potential perturbation (root-mean-square) overlapping with 0.2 V DC potential were performed. The calculation of the HET rate constant (k0obs) was achieved using the Nicholson method,1 relating the observed ΔEp to a dimensionless parameter, ψ, and consequently to k0obs. The roughness factor was not taken into account in this case. The [Fe(CN)6]3−/4− diffusion coefficient of 7.26 × 10−6 cm2/s was used for calculation.1 The hydrogen evolution reaction (HER), oxygen reaction reduction (ORR), and oxygen evolution reaction (OER) were performed by linear sweep voltammetry at a scan rate of 0.005 V/s. The HER was performed in 0.5 M H2SO4, and ORR and OER were done in 1 M KOH. Control experiments were also performed for HER and ORR using platinum-on-carbon instead MPSx materials. The platinum-oncarbon powder was prepared in concentrations of 5 mg/mL in deionized water and subject to sonication to obtain dispersed suspension; 3 μL of this solution was drop cast onto the working electrode for obtain a film of 15 μg. The stability tests of HER in 0.5 M H2SO4 and OER in 1 M KOH were evaluated, before and after 100 CV scanning at 100 mV/s. The CV scan windows corresponds to HER and OER onset region where the current density is at least −0.1 mA/cm2 and 0.1 mA/cm2, respectively. For the HER of NiPS3, CoPS3, and BiPS3 the cyclic voltammetric scan window is from −0.065 V to −0.465 V vs RHE. Moreover, for OER of CoPS3 the scan range is from 0.385 to 0.785 V vs RHE. For the evaluation of the structure after the stability test by XPS analysis, the measurement was performed in a SPE under the experimental condition.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16553. EDS mapping and the table with the atomic composition and S/M and P/M ratios for all MPSx. Raman spectrum and its tabulated peak position data. Wide-scan X-ray photoelectron spectrum and a table of peak position from the high-resolution X-ray photoelectron spectrum deconvoluted. Susceptibility and magnetization curves of FePS3. Tables of oxidation and reduction peaks position from inherent electrochemistry study and heterogeneous electron transfer rate constant of the MPSx. Stability tests for HER and OER (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M. Pumera). ORCID

Alex Yong Sheng Eng: 0000-0001-9577-1681 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.P. acknowledges a Tier 2 grant (MOE2013-T2-1-056; ARC 35/13) from the Ministry of Education, Singapore. Z.S., D.S., and Š.H. were supported by Czech Science Foundation (GACR No. 16-05167S) and by specific university research (MSMT No. 20-SVV/2016). I


Research Article


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