The Role of the Metal Element in Layered Metal Phosphorus

Research Article Cite This: ACS Catal. 2017, 7, 8159-8170

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The Role of the Metal Element in Layered Metal Phosphorus Triselenides upon Their Electrochemical Sensing and Energy Applications Rui Gusmaõ ,† Zdeněk Sofer,‡ David Sedmidubský,‡ Štěpán Huber,‡ and Martin Pumera*,†,‡ †

Division of Chemistry & Biological Chemistry, School of Physical 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: The number of layered materials seems to be evergrowing, from mono- to multielement, with affiliates and applications being tested continuously. Chalcogenophosphites, also designated as metal phosphorus chalcogenides (MPXn), have attracted great interest because of not only their magnetic properties but also promising capabilities in energy applications. Herein, bulk crystals of different layered metal triselenophosphites, with a general formula MPSe3 (M = Cd, Cr, Fe, Mn, Sn, Zn), were synthesized. Structural and morphological characterization was performed prior to testing their electrochemical performance. From the set of ternary layered materials, FePSe3, followed by MnPSe3, yielded the highest efficiency for the hydrogen evolution reaction (HER) both in acidic and alkaline media with good stability after 100 cycles. MnPSe3 also holds the lowest oxidation potential for cysteine, although this is due to the presence of MnO2 in the structure as detected by X-ray photoelectron spectroscopy. For the oxygen evolution reaction, the best performance was observed for FePSe3, although the stability of the material was not as good as in the case of HER. These findings have profound implications in the application of these layered ternary compounds in energy-related fields. KEYWORDS: layered materials, metal phosphite trichalcogenides, cysteine, hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction

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INTRODUCTION From bulk crystals to a single layer, there has been immense interest in layered materials within the past decade. Initiated by the reintroduction of graphene and graphene oxide, this interest was encouraged by the exciting and highly unusual physical, chemical, and materials properties of graphene (single sheet of graphite).1 There is similar growing attention in layered transition-metal dichalcogenides (TMD), which can be exfoliated to single sheets MX2, where M is a transition metal, such as Mo, and X is a chalcogen such as S or Se.2 These single sheets are not a monoelement, with a metal atom sandwiched between chalcogen atoms in X−M−X configuration. More recently, another elemental layered material reemerged: black phosphorus (BP). As in the case of graphite, BP has a layered structure which consists of a single element and can be easily exfoliated to single- or few-layer structures.3 Arsenene, antimonene, and bismuthene have been theoretically predicted, and the synthesis of antimonene has also been reported.4 One of the reasons sparking the interest in these 2D layered materials is that their electrochemical performance allows for © XXXX American Chemical Society

the evaluation of the catalytic activity looking for future applications in energy alternatives, advanced electronic devices, and (bio)sensing systems.5,6 Beyond those mentioned, there is a new class of ternary layered materials called metal phosphochalcogenides, also designated as metal phosphorus chalcogenides (MPXn), which have recently attracted the attention of scientists.7,8 Centuries old track record of these layered materials has revealed their following distinctive characteristics: (i) anisotropic properties imposed by their layered structure and their ability to act as host lattices for intercalation compounds;9 (ii) magnetic and antiferromagnetic properties, because magnetic moments are localized on the transition-metal ions that form a honeycomb network structure;8,10 and (iii) wide-range band gaps that suggest their optoelectronic applications in a broad wavelength range.7,9,11,12 Each layer is stacked through weak “van der Waals” forces similar to what happens for example with TMDs; hence, it is Received: June 29, 2017 Revised: September 25, 2017

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DOI: 10.1021/acscatal.7b02134 ACS Catal. 2017, 7, 8159−8170

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Figure 1. Schematic side view of the crystal structures of metal phosphorus triselenides. Left panel, layered MPSe3 (M = Mn, Fe, Cr, Zn, Cd); right panel, SnPSe3. Color scheme: P, green; Se, yellow; metal, inside polyhedral.

Figure 2. SEM images of the different MPSe3 layered materials. From left to right (top row): CrPSe3, MnPSe3, and FePSe3; (bottom row) ZnPSe3, CdPSe3, and SnPSe3. Bars in the image represent 1 μm.

transfer, thiol oxidation, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are assessed in view of alternative materials for energy and sensing applications.

possible to exfoliate the bulk crystal down to few layers. 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 Se atoms, in a way to similar to that of transition-metal dichalcogenides (Figure 1). Apart from some TMDs,13−15 transition-metal phosphides have attracted enormous attention toward the hydrogen evolution reaction (HER) in the past.16,17 The P sites are postulated to be active for H adsorption and desorption. Hence, it is interesting to explore metal complexes containing both Se and P, which might have good catalytic activity. Calculated band gaps and band edge positions from accurate studies predict that MPS3 (M = Fe, Mn, Ni, Cd, Zn) and MPSe3 (M = Fe, Mn) monolayers are promising candidates as photocatalysts for water splitting.12 Particularly, MnPSe3 monolayer is a direct-band gap semiconductor which exhibits obvious absorption in the visible light spectrum. To the best of our knowledge, only some metal phosphorus thiolates (MPSx) as bulk crystals18 and FePS3 exfoliated12 have been tested for HER performance. In this work, we synthesized, characterized, and tested the electrochemical performance of six different metal phosphorus chalcogenides layered crystals that shall be designated as “metal selenophosphites” with a general formula of MPSe3. The metal component (M) will vary by using nonprecious transitional metals (M = Cr, Mn, Fe, Zn, and Cd) and one case of posttransition metal (M = Sn). The electrochemical performance in terms of inherent electrochemistry, heterogeneous electron

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RESULTS AND DISCUSSION Metal selenophosphites (MPSe3), where the metal component varied using transitional metals CrPSe3, MnPSe3, FePSe3, ZnPSe3, and CdPSe3 and one case of post-transition metal (M = Sn) were synthesized (see details in Experimental Section). Additionally, a systematic characterization of the materials to ascertain their identity is achieved using density functional theory (DFT) calculations of electronic structure, X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and magnetic measurements. Finally, we tested the sensing and electrocatalytic performance of the obtained MPSe3. It is important to highlight that the MPS3 studied here are crystals in the bulk layered form, which have not been subjected to exfoliation. The morphologies of the MPSe3 crystals as observed by SEM images are shown in Figure 2. All the materials present a characteristic microstructure of bulk materials with multiple stacked sheets of various thickness. Lower-magnification images are shown in Figure S1. The chalcogen/metal (Se/M) ratio and phosphorus/metal (P/M) ratio, calculated from the elemental analysis by EDS, are summarized in Table S1 and correspond 8160


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Figure 3. Electronic density of states (DOS) of MPSe3: (a) M = Cr, (b) M= Mn, (c) M = Fe, and (d) M = Sn. Pink dotted line shows the contribution of metal ions.

to the respective values ∼3 and ∼1 as expected from the structure and from the initial batch composition. Figure S2 shows the EDS mapping of the individual elements for all MPSe3 materials. Powder XRD revealed the presence of typical layered structures of M2P2Se6 (M = Cr, Mn, Fe, Zn, Cd) consisting of hexagonal Se2 double layers with metal atoms occupying 2/3 of octahedral positions while the remaining 1/3 is occupied by P2 pairs constituting the characteristic P2Se6 units. The respective structures differ mainly in the relative position of the individual two-dimensional blocks resulting in different space group classification: C2/m for M = Cr; R-3 for M = Mn, Zn, and Cd; and R3 for M = Fe. By contrast, as seen from the right panel in Figure 1, tin exhibits an 8-fold coordination in SnPSe3 with both edge and corner sharing of these polyhedra yielding a rather three-dimensional character of the structure. X-ray diffractograms are shown in Figure S3. Because all selenophosphites with the exception of SnPSe3 exhibit the identical structure, the electrocatalytic and other electrochemical properties originate only from the present metal ions and the electronic structure of such compounds. Although the crystal structures are represented in polyhedral model in Figure 1, they can be also regarded as built up of P2Se64− dimers (hexaselenidodiphosphate anions) and M2+ cations in the ratio 1:2. This simple ionic picture basically correlates with the results of DFT electronic structure calculations performed for four selected phases (M = Cr, Mn, Fe, and Sn) within the GGA+U approximation and APW+lo basis set. The density of states (DOS) below the Fermi level is dominated by a multiplet of narrow bands in the range −17 to −5 eV formed by “molecular orbitals” of P2Se64− dimer, valence

band of prevalent Se 4p character, and the valence states of divalent metal element, 3d orbitals of the transition metal and 5s orbitals of Sn2+ (Figure 3). The Sn 5s2 pair reveals a slight stereoactivity imposing three short, three intermediate, and two long bonds. The transition-metal selenophosphites were considered as spin-polarized with antiferromagnetic (AF) spin arrangement in line with the observed magnetic behavior as discussed below. Because both spin channels have an identical DOS for antiferromagnets (with opposite majority spins for nonequivalent TM atoms), only the spin-up channel is shown in Figure 3a−c. The occupation of 3d states is consistent with the respective high spin electron configurations of M2+, namely t2g3↑ and eg1↑ for Cr2+, eg2↑ for Mn2+ and eg2↑ t2g1↓ for Fe2+. The calculated band gaps are relatively large for M = Mn and M = Sn (1.4 and 1.2 eV, respectively) and narrow M = Cr (0.25 eV), where the octahedral distortion opens a small crystal field gap between two eg states. Similarly, in FePSe3 there is a symmetry-imposed splitting of down-spin t2g states into one doubly degenerate half-filled band and a single empty band separated by a gap of 0.5 eV. However, it should be noted that the reported band gaps can be influenced by the selected Coulomb repulsion parameter U = 3 eV. While the selenophosphites of Zn, Cd, and Sn reveal a clear diamagnetic behavior manifested by a negative slope of linear field dependence of magnetic moment (see the Supporting Information, Figure S4), the mid-row transition-metal compounds of Cr, Mn, and Fe are paramagnetic at room temperature and undergo a transition into an antiferromagnetic state at Neél temperatures ranging from 136 to 78 K (see the summarized magnetic characteristics in Table 1 and the 8161


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as possible surface oxidation which can significantly reduce the possibility of ohmic contact formation between MPSe3 and gold interdigitated contact electrodes. No direct correlation between electrocatalytic activity and material conductivity was observed. Photoelectron spectroscopy (XPS) was employed to characterize the surface composition of the MPSe3 layered crystals. Wide-scan X-ray photoelectron spectra for the different MPSe3 are shown in Figure S5. XPS is a surface-sensitive technique with a maximum depth of approximately 10 nm in the samples, with detection limits at about 0.1 wt % of the material. In all samples, besides the presence of C (carbon tape) and O (adsorbed and/or partial oxidation of the material surface), phosphorus (P) and selenium (Se) were also detected in the corresponding wide-range XPS spectra. For each case, the signals due to expected corresponding metal (Cr, Mn, Fe, Cd, Zn, and Sn) were detected, hence confirming the existence of each element in the MPSe3 layered crystals. The chemical bonding characteristics of the MPSe3 have been investigated by the narrow scanning XPS spectrum (Figure 5). The high-resolution scanning XPS spectrum of each metal (Cr 2p, Mn 2p, Fe 2p, Zn 2p, Cd 3d, and Sn 3d), P 2p, and Se 3p of the MPSe3 materials are evaluated. Deconvolution analysis of the bonding modes and the components’ positions of the respective signals are summarized in Table S2. The peaks in the metallic component of CrPSe3, FePSe3, ZnPSe3, and CdPSe3 can be assigned to pure metallic elements Cr 2p(3/2 and 1/2), Fe 2p(3/2 and 1/2), Zn 2p(3/2 and 1/2), and Cd 3d(5/2 and 3/2), respectively. In contrast, the peaks developed for MnSPe3 and SnPSe3 show both the metallic phase (Mn 2p3/2 and Sn 3d(5/2 and 3/2)) and partial oxidation to MnO2 (Mn 2p3/2) and SnO2 (Sn 3d(5/2 and 3/2)). On the other hand, two deconvolved contributions that correspond to P2p regions are observed for all materials (see Table S2); the average position of P 2p3/2 and P 2p1/2 are 130.6 ± 0.3 and 132.4 ± 0.7 eV for all MPSe3 and can be attributed to P−S bonding.19 Nevertheless, in almost all materials additional peaks were observed, thus referring to oxidation state of P4O10 with an average position of 134.9 ± 0.9 eV,20 most probably due to a partial oxidation of the layered material at the exposed surface.3 Only for ZnPS3 was the oxidation state of P absent. CrPSe3 and MnPSe3 have the P4O10 peaks with a higher intensity. Finally, two deconvoluted peaks appear in the Se 3d region for all MPSe3 (Table S2). The peaks correspond to Se 3d3/2 and Se 3d5/2 with average positions of 53.8 ± 1.6 and 54.6 ± 1.4 eV, respectively. The relatively high error in Se 3d is because the position of the peaks is in this case affected by the partial oxidation of the

temperature dependence of magnetic susceptibility in Figure 4). Table 1. Magnetic Data of MPSe3 (M = Cr, Mn, Fe)a CrPSe3 MnPSe3 FePSe3

μeff/μB

S

θ (K)

TN (K)

3.11 5.65 4.67

1.13 2.37 1.89

−323 −156 32

136 78 112

a Effective magnetic moment, μeff; apparent total spin, S; Weiss constant, θ (all evaluated from high-temperature paramagnetic region, inset in Figure 4); and Néel temperature, TN.

The effective paramagnetic moments evaluated from the apparent linear trend of inverse susceptibility versus temperature (inset in Figure 4A) yield the total spin values which are roughly consistent with the respective theoretical values for Mn and Fe; however, the obtained experimental value S = 1.13 in CrPSe3 is highly underestimated compared to the expected spin S = 2 of Cr2+. Considering the strong deflection from the linear trend above TN, the highly negative Curie−Weiss constant suggesting strong AF correlations in PM state, as well as the narrow temperature interval used for the evaluation, we can infer that the resulting value does not correspond to hightemperature PM limit and is likely affected by spin correlations lowering the effective magnetic moment. Whereas the field cooled (FC) and zero-field cooled (ZFC) susceptibility curves coincide for CrPSe3 and MnPSe3, we can observe a pronounced deflection of the ZFC curve of FePSe3 starting at the temperature Tb ∼ 130 K. This behavior, along with a noticeable susceptibility enhancement below 200 K and a small hysteresis superposed on the linear magnetization curve recorded at 4.5 K suggests a frustration in exchange interactions leading either to a canted AF arrangement or to a cluster glass state. Let us note that the DFT calculation of FePSe3 predicts a small net magnetic moment 0.13 μB, while a clear zero moment is obtained for AF-ordered MnPSe3 and CrPSe3. The electrical conductivity of MPSe3 was measured using drop-casted covered gold interdigitated electrodes (10 μm spacing). Good electrical conductivity was observed only for samples CdPSe3 and SnPSe3. Significantly lower conductivity was observed for FePSe3 and CrPSe3. The others (ZnPSe3 and MnPSe3) were nonconductive in the potential range of +2 to −2 V. CrPSe3 exhibits also the oxidation and reduction effects originating from oxidation of Cr2+ and its reduction. The I/V curves are shown in Figure 4B. Significant differences in the conductivity originate from its semiconducting behavior as well

Figure 4. (A) Magnetic susceptibility of MPSe3. For M = Cr or Mn, the field cooled (FC) and zero-field cooled (ZFC) curves are identical, while for M = Fe they start to differ at Tb = 130 K. Inset: linear dependence of inverse magnetic susceptibility in the paramagnetic region. (B) Polarization I/V profiles of MPSe3 measured after material deposition on a Au-IDE. 8162


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Figure 5. High-resolution X-ray photoelectron spectrum and peak deconvolution of the respective metal element (Cr 2p, Mn 2p, Fe 2p, Zn 2p, Cd 3d, and Sn 3d), P 2p, and Se 3d regions of MPSe3 layered materials.

material, furthermore depending on the metal element bonding to Se, and shifts occur in the Se 3d peaks.21,22 Raman spectra of layered selenophosphites are shown in Figure 6. Selenophosphites of composition MPSe3 with C2 symmetry (MnPSe3; FePSe 3; ZnPSe3; CdPSe 3) exhibit structure of Raman modes similar to that of MPS3 phases.18 This originates from the P2Se6 units with D3d symmetry

consisting from 3 A1g and 5 Eg modes. At low wavenumbers, typically below 100 cm−1, phonon modes from cation interactions are observed (denoted as C in Figure 2). These vibration bands with low intensity were observed only for CdPSe3 (94.7 cm−1). The Raman spectra are dominated by vibration bands of A1g and Eg phonon modes of P2Se64− units in the range of 250−100 cm−1. In all these samples, A1g phonon 8163


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Figure 6. Raman spectra of MPSe3 layered materials with an excitation wavelength of 532 nm.

mode located in the range of 213−223 cm−1 (MnPSe3 219.6 cm−1; FePSe3 213.7 cm−1; ZnPSe3 223.0 cm−1; CdPSe3 218.8 cm−1) has a dominating intensity. In addition, two Eg phonon modes in the range of 166−172 cm−1 and 149−158 cm−1 were observed. The MnPSe3 phase exhibits apparently the most complex spectrum with an additional Eg mode at 113 cm−1 and A1g at 147 cm−1. Also, the second Eg mode was shifted to a slightly higher wavenumber (158 cm−1) in comparison with other MPSe3 phases with P2Se64− units. These results are in good agreement with the literature data.23,24 Around 450 and 500 cm−1 were observed weak Raman bands which can be associated with Eg and A1g phonon mode observed on MPS3 phases at slightly higher wavenumbers. These Raman active vibration bands were observed only for the ZnPSe3, CdPSe3, and MnPSe3 samples. Their absence in FePSe3 has also been reported in the literature.24 CrPSe3 and SnPSe3 phases exhibit significantly different Raman spectra because of the differences in structural stacking of P2Se64− units combined with MSe6 octahedra. The Raman spectrum of CrPSe3 is dominated by the strong band at 55 cm−1 originating from cation interactions. The weak cation interaction can be also associated with phonon mode at 111 cm−1 in the SnPSe3 phase. Electrochemical fundamental studies were subsequently performed. Cyclic voltammograms of modified GC electrodes with each MPSe3 material were carried out to evaluate their inherent electrochemical properties. Figure 7 shows the corresponding anodic scans of each MPSe3 layered material. Characteristic of noninert materials, each MPSe3 will withhold distinct native oxidation and reduction signals, as summarized in Table S3. This is highly influenced by the metal element of the MPSe3. Apart from CrPSe3, all the other MPSe3 show inherent reduction and oxidation peaks. On the other hand, irreversible oxidation peaks of MnPSe3, ZnPSe3, and FePSe3 are observed, because the peaks disappear after the first scan. Moreover, ZnPSe3, FePSe3, and CdPSe3 show an irreversible reduction peak, while MnPSe3 has stable reduction peaks throughout the scans. Between the different MPSe3, FePSe3 has the lowest oxidation peak in the series (cathodic branch), i.e., it

is more easily oxidized than the other MPSe3. Interestingly, compared with the analogous MPS3,18 MPSe3 have ∼100 mV higher potential for the first oxidation peak observed in the anodic branch, which means that MPSe3 can be considered relatively more stable. Because of their inherent electrochemical processes, MPSe3 can be exploited in biosensing schemes, as an electroactive label, as previously reported for different layered materials.25,26 To probe electrochemical performance of MPSe3, cyclic voltammograms of an inner-sphere redox probe ferro/ ferricyanide, [Fe(CN)6]3−/4−, were recorded in the anodic direction (Figure 8A). Analyses of the voltammetric profiles were taken in terms of peak-to-peak potential (ΔEp) of the oxidation and reduction processes, where a ΔEp of 59 mV/e− is the reversible limit; generally, the smaller the ΔEp (up to the reversible limit) the more reversible the electrochemical process is. The bare GC has a ΔEp of 182 mV [cyclic voltammetry (CV) scans not shown), while FePSe3 has the highest registered value (Figure 8B). CrPSe3 exhibits the lowest ΔEp. In the present case, [Fe(CN)6]3−/4− probe is known to be very sensitive to surface and functional groups. In the case of CrPSe3, its inherent electrochemistry study lacked any process within the potential window of the redox process of [Fe(CN)6]3−/4−, which can be the reason for the best ΔEp observed for the set of MPSe3 layered material. The ΔEp values obtained for MPSe3 are lower than the analogous MPS3, except for SnPSe3, the exception being SnPSe3.18 Electrochemical impedance spectroscopy (EIS) was used to obtain the charge-transfer resistance (Rct), and the measurements were modeled using Randle’s approach modified with Warburg impedance (Zw). This reaction occurs because ferro/ ferricyanide redox couple is used as an analyte, commonly employed for EIS experiments for sensing purposes. The lowest Rct values are obtained for CrPSe3, with the remaining series increasing Rct in the remaining series members: SnPSe3 < FePSe3 < ZnPSe3 < MnPSe3 < CdPSe3. Note that the Rct of GCbare is of the same order as for SnPSe3, and the higher Rct values observed for remaining materials are related to their 8164


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Figure 7. Inherent electrochemistry of bare GC and GC modified by the respective MPSe3 layered materials: (A) CrPSe3, (B) MnPSe3, (C) FePSe3, (D) CdPSe3, (E) ZnPSe3, and (F) SnPSe3. Cyclic voltammograms are recorded in purged 0.1 M PBS at pH 7.2 and scan rate of 100 mV/s. CV scans start at 0.0 V (vs Ag/AgCl reference) in anodic direction.

semiconducting nature.27 For this reason these materials do not greatly enhance the electron transfer, but rather the electron transfer is worse with regards to the GC electrode. Compared with published values of the analogous MPS3 Rct, values reported here are higher for MPSe3, except for CrPSe3.18 Next, we tested the different MPSe3 layered materials in sensing application through electrochemical oxidation behavior of cysteine (Cys) using differential pulse voltammetry (DPV) as shown in Figure 9. Cys is a thiol rich amino acid which is a building block of the antioxidant glutathione28 and in metal binding peptides in plants, phytochelatins.29 DPVs were recorded at physiological pH in purged solutions to avoid

Cys readily oxidizing because of the presence of molecular oxygen in the sample and because the pKa of the thiol moiety is approximately neutral. The electrochemical oxidation of Cys is irreversible, diffusion-controlled, and pH-dependent, and particularly in metallic electrodes, problems of the signal attenuation caused by the adsorption of the Cys oxidation product are common.30,31 At the GC electrode, Cys has a high oxidation potential (Ep) of +0.83 V and low peak currents (Ip). Cys has the highest Ip at CdPSe3, but with an Ep comparable with that of GC. MnPSe3 yielded a well-defined peak and electrocatalytic activity with a decrease in the Ep of 220 mV for Cys (vs bare GC). It is predicted that Ep will be displaced in 8165


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Figure 8. Cyclic voltammograms obtained for 1.0 mM ferro/ferricyanide [Fe(CN)6]3−/4− redox probe in KCl 0.1 M for GC electrode and the different MPSe3 (A). Peak-to-peak separation calculated from CV scans (B). All voltammograms recorded at a scan rate of 100 mV/s. Nyquist plots (C) and respective Rct values (D) in the same media as CV scans. Error bars correspond to standard deviations based on triplicate measurements.

Figure 9. DPVs of bare GC and GC modified with the different MPSe3 in 1.0 mM cysteine, 0.1 M PBS pH 7.2 (A). Average Ep and Ip for the oxidation process of Cys at bare GC and the modified electrodes (B). Error bars correspond to standard deviations based on triplicate measurements. Scan rate: 5 mV/s.

for MnO2 and MnPSe3, the former has slightly lower oxidation potentials and comparable peak currents (Figure S6). Thus, in this case, the specific interaction between the thiol group of Cys and the partially oxidized MnPSe3, in the Mn(IV) oxidation state, favors the electrochemical oxidation process. Energy-related applications of MPSe3 layered materials were evaluated by testing their catalytic activity for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. HER is a cathodic half reaction of water splitting, and the applications of energy conversion devices include water electrolysis and artificial photosynthetic cells. Noble metals such as Pt show the highest efficiency for HER. Nevertheless, the need for alternative low-cost and efficient

function of metallic element in the layered MPSe3, based on the Irving−Williams series, suggesting that the stability of complexes formed by divalent first-row transition-metal ions generally increases across the period.32 Thus, a weaker interaction between Mn of the layered MnPSe3 and Cys in solution causes a negative shift of Ep , i.e., an easier oxidation. MnPSe3 seems therefore to be a good catalyst for the electrochemical oxidation of Cys. We investigated whether this improvement is because of its partial oxidation of Mn to MnO2 of the MnPSe3, as detected in the XPS analysis (Figure 5). Manganese dioxide has been found to exhibit excellent catalytic activity toward Cys and other thiols’ electrochemical oxidation.33−36 Indeed, if Cys oxidation signals are compared 8166


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Figure 10. LSV corresponding to HER in 0.5 M H2SO4 for the different MPSe3 (A) and the corresponding average of the overpotential at a current density of −10 mA/cm2 (B). LSV of HER in 1.0 M KOH (C) and the average overpotential at a current density of −10 mA/cm2 (D). Error bars correspond to standard deviations based on triplicate measurements. Scan rate: 5 mV/s.

electrocatalysis until the current density reached −10 mA/cm2 in order to avoid the effect of inherent electrochemistry in this way. The onset potentials obtained (Figure 10D) were higher than in acidic media, but FePSe3 and MnPSe3 had nevertheless the best performance in the series. With respect to their stability, MPSe3 were tested in both media by measuring HER polarization curves before and after 100 CV scans with a scan rate of 100 mV/s (see Experimental Section and Figure S7). LSV curve of FePSe3 HER catalysis in H2SO4 (Figure S7A) was practically unaffected compared with the initial measurement, while MnPSe3 revealed enhanced onset potentials by ca. 200 mV, indicating a good cycle life of this material in acidic media. For the remaining MPSe3, their performance was not stable. In alkaline media the stability of the materials worsened (Figure S7B), but FePSe3 and MnPSe3 still outperformed the others, making them the most interesting of the series. Oxygen evolution reaction electrolysis represents another important half-reaction involved in water splitting with application mainly in rechargeable metal−air batteries.23 This reaction has been intensely investigated for decades, and low overpotentials are shown by IrO2 and RuO2.38 Again, the high cost of such materials has led to a search for alternative materials such as transition-metal phosphides. Thus, the evaluation of the OER performance of MPSe3 is relevant. The polarization curves performed in alkaline media are shown in Figure 10A, and the OER onset potentials at +15 mA/cm2 are shown in Figure 11B. The best catalytic performance of the MPSe3 materials is observed for FePSe3 with an onset potential of +1.8 V vs RHE, which is much higher than the one observed

materials caused an ongoing race in the scientific community. In line with this effort we evaluated the HER performance of MPSe3. Furthermore, MnPSe3 has been predicted to be a promising candidate as photocatalyst for water splitting; thus, it is wise to test this and other MPSe3 for HER and OER. The HER polarization curves of MPSe3 and the summary of the onset potentials are presented in panels A and B of Figure 10, respectively. HER measurements of bare GC electrode and Pt are also shown as performance references. Among the explored MPSe3, FePSe3 and MnPSe3 have the lowest onset potential of approximately −0.91 V vs RHE, followed by CrPS3, CdPSe3, and SnPSe3, all of them framed between the onset potentials of Pt (−0.09 V vs RHE) and GC (−1.20 V vs RHE). The HER onset potential for CdPSe3 is even more negative than that of GC. In general, the layered MPSe3 have lower HER onset values than the analogous bulk MPSx in acidic conditions.18 Lower onset potentials have been reported for few-layer FePS3,37 which is expected because of the creation of a higher number of higher active sites as the result of the exfoliation process. Searching for non-noble-metal or earth abundant metal HER electrocatalysts with high efficiency and excellent stability in a wide pH range is a great challenge for the hydrogen-based energy industry. Only a few catalysts are known to exhibit good activity both in acidic and alkaline conditions. Some transitionmetal phosphides (e.g., Ni2P, MoP) are found to show efficient catalytic activity toward HER in alkaline medium, but they deteriorate rapidly.17 The performance of MPSe3 was therefore tested in alkaline media (Figure 10C). The measurements were performed in high concentrations of alkali media for stable 8167


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Figure 11. LSV of OER in 1.0 M KOH (A) and the average overpotential at a current density of 10 mA/cm2 (B). LSV of ORR in 1.0 M KOH (C) and the average overpotential at a current density of +15 mA/cm2 (D). All error bars correspond to standard deviations based on triplicate measurements. Scan rate: 5 mV/s.

state at Neél temperatures ranging from 136 to 78 K. In addition, FePSe3 showed at low temperature frustration in exchange interactions which led to the canted antiferromagnetic arrangement or a cluster glass state. This is in good agreement with performed DFT calculations showing an antiferromagnetic (AF) spin arrangement. The calculated band gaps are relatively large for M = Mn and Sn (1.4 and 1.2 eV, respectively) and narrow for M = Cr and Sn (0.25 and 0.5 eV, respectively). MnPSe3 had the lowest oxidation potential for Cys, although this is due to the partial oxidation of the metal element in the layered structure in the form of MnO2 as detected by XPS. The electrochemical properties and electrocatalytic activity were investigated in detail for industrially important reactions, as well as for the detection of biologically active species. From the set of ternary layered materials, FePSe3 yielded the highest efficiency for HER and OER. With respect to their stability as electrocatalysts, FePSe3 and MnPSe3 yield very good stability after 100 cycles for HER in a wide pH range, but not for OER. This is most likely due to irreversible oxidation of MPSe3 at the anodic limit for OER. Moreover, the layered bulk MPSe3 have lower HER onset values than the corresponding bulk MPSx in acidic conditions,18 being here tested for alkaline HER for the first time. Lower onset potentials for acid HER have been reported for few-layer FePS3,37 which if applied to the different bulk MPSe3 is also expected to improve their performance because of the creation of higher active sites as the result of exfoliation process. The set of findings can have profound

for Pt (+1.3 V vs RHE). The remaining MPSe3 layered materials exhibited comparable OER onset potentials at ca. +1.8 V vs RHE, almost indistinct from the bare GC electrode. It is interesting to note that FePSe3 shows better OER performance than the one reported for FePS3, with the remaining MPSe3 having indistinct OER performance when compared with MPS3.18 All of the MPSe3 present poor stability, and their catalytic effect for OER is almost lost after 100 CV scans (Figure S7C), which is most likely due to oxidation of the materials after the anodic limit is reached on the initial LSV for OER. The layered materials were also tested for oxygen reduction reaction as displayed in Figure 11C,D. For the set of MPSe3 layered materials, the corresponding ORR peaks were recorded at very similar potentials of the bare GC electrode, which means that in this case the reaction is not significantly catalyzed.

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CONCLUSIONS In summary, we have synthesized, characterized, and performed a comprehensive electrochemical study of layered metal selenophosphite materials (MPSe3). The SEM images showed typical structures of layered materials, and the structural characterization performed via EDS, XRD, and Raman and XPS spectroscopies demonstrated the successful synthesis of MPSe3. Detailed investigation of MPSe3 magnetic properties showed paramagnetism of CrPSe3, MnPSe3, and FePSe3 at room temperature and a transition into an antiferromagnetic 8168


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and Ag/AgCl as reference electrode (2 mm diameter from CH Instruments, Texas, United States). Each material was dispersed in deionized water at a concentration of 5 mg/mL and sonicated initially for 1 h in an ultrasonic ice bath (T < 20 °C). Prior to GC modification, each suspension was mechanically stirred for 1 min using a vortex. Working electrode modification was done by drop casting 2 μL of each MPSe3 suspension. Studies of the inherent electrochemistry of MPSe3-modified electrodes were made in 0.1 M phosphate buffered solution (PBS, pH 7.2) using cyclic voltammetry at 0.1 V/s scan rate, in a potential window of −1.8 to 1.8 V. Solutions were purged with nitrogen gas before measurements. The heterogeneous electron transfer (HET) rate measurements were done at a scan rate of 0.1 V/s, for 1.0 mM of the ferro/ferricyanide redox probe in a 0.1 M KCl solution. Electrochemical impedance spectroscopy experiments were performed in a similar medium, in a frequency range of 1−100 kHz experiments applying 20 mV AC potential perturbation (root-mean-square) overlapping with 0.2 V DC potential. The cysteine oxidation studies were done by differential pulse voltammetry with a voltage step of 5 mV/s, for 1.0 mM of Cys in 10 mL of purged 0.1 M PBS at pH 7.2. The hydrogen evolution reaction, oxygen reaction reduction, and oxygen evolution reaction were performed by linear sweep voltammetry (LSV) at a scan rate of 5 mV/s. The HER was performed in 0.5 M H2SO4 and 1.0 M KOH, while ORR and OER were done in 1.0 M KOH (O2 saturated). The stability tests of HER in the same acidic and alkaline conditions were also evaluated, before and after 100 CV scans at 0.1 V/s scan rate. The potential windows selected for HER and OER onset was the current density ca. ±0.1 mA/cm2 for each case. For the HER, MPSe3 modified electrodes were scanned from 0 to −0.5 V vs RHE for both media. On the other hand, for the OER stability test, the MPSe3-modified electrodes were cycled from 0 to +0.5 V vs RHE. I/V measurements were carried out by first depositing 2 mL of the material suspension onto an interdigitated gold electrode with a gap of 10 μm. The solvent was then evaporated under a lamp for 20 min to leave a randomly deposited film on the interdigitated area. I/V curves were obtained by LSV (scan rate, 20 mV/s). DFT Calculations. The electronic structure of MPSe3 (M = Cr, Mn, Fe, and Sn) was calculated within density functional theory using APW+lo basis set and generalized gradient approximation (GGA, PBE96 parametrization scheme) for 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. A spin polarized state and an antiferromagnetic arrangement were considered for the transition-metal compounds. The plane wave cutoff energy of 320 eV and the tetrahedron method (k-mesh ranging from 800 to 1000 points per the first Brillouin zone) were used.

implications in the application of these layered ternary compounds in energy-related fields.

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EXPERIMENTAL SECTION Reagents. Red phosphorus (99.999%) and selenium (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 and Sn). Zn, Cd, and Sn granules with purity 99.999% were obtained from STREM, Germany. Chromium (99.9%), manganese (99.9%), and iron (99.9%) in powder form (−100 mesh) were obtained from Alfa Aesar, Germany. Potassium ferrocyanide, potassium chloride, potassium hydroxide, phosphate dibasic, sodium phosphate monobasic, sodium chloride, sulfuric acid, cysteine, manganese dioxide, and platinum on carbon were purchased from Sigma-Aldrich. Synthesis of Metal Selenophosphite Materials. The stoichiometric amount of metal, phosphorus, and selenium corresponding to 5 g of selenophosphite 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 240 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. The ZnPSe3 sample was synthesized at 370 °C for 240 h with an addition of 50 mg of iodine, and on the opposite site of ampules were formed ZnPSe3 crystals. Structural and Morphological Characterization. To obtain SEM micrographs, scanning electron microscopy (JEOL 7600F, Japan) was used in gentle-beam mode at 2 kV. The XPS spectra were obtained using an 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. An 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. X-ray diffraction (XRD) was done with a Bruker D8 Discoverer diffractometer in Bragg−Brentano parafocusing geometry. Cu Kα radiation was used. The diffraction pattern were collected between 10° and 80° of 2θ. The obtained data were evaluated using HighScore Plus 3.0e software. Magnetic measurements were performed using a PPMS Evercool system from Quantum Design (United States). The vibration sample magnetometer (VSM) insert was used for measurement of magnetization curves (field range from −7 to 7 T), 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. Electrochemical Measurements. Electrochemical measurements were carried out at room temperature using an Autolab PGSTAT204 (Eco Chemie, Utrecht, The Netherlands) controlled by NOVA Version 2.1 software (Eco Chemie) and a three-electrode arrangement. Glassy carbon GC electrode (3 mm diameter from CH Instruments, Texas, United States) was used as a working and counter electrode

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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/acscatal.7b02134. EDS mapping and table with atomic composition and Se/M and P/M ratios for all MPSe3, Raman spectrum and its tabulated peak position data, wide-scan X-ray photoelectron spectrum and table of peak position from 8169


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the high-resolution X-ray photoelectron spectrum deconvoluted, tables of oxidation and reduction peak position from inherent electrochemistry study, stability curves of MPSe3 for HER and OER (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Z.S., D.S., and Š.H. were supported by Czech Science Foundation (GACR No. 17-11456S and GACR No. 1605167S). This work was created with the financial support of the Neuron Foundation for science support. This work was supported by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR).

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The Role of the Metal Element in Layered Metal Phosphorus

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