Electrically conducting palladium selenide (Pd₄Se, Pd₁₇Se₁₅, Pd₇Se₄) phases: synthesis and activity towards hydrogen evolution reaction

Abstract

Electrically conducting, continuous films of different phases of palladium selenides are synthesized by the thermolysis of single source molecular precursors. The films are found to be adherent on flat substrates such as glass, indium tin oxide and glassy carbon and are stable under electrochemical conditions. They are electrocatalytically active and in particular, for hydrogen evolution reaction. Catalytic activities with low Tafel slopes of 50–60 mV per decade are observed.

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Hydrogen, as a clean renewable energy source, has attracted wide attention¹ and its global production primarily stems from steam reforming which requires methane and access to high temperatures.² In contrast, water electrolysis represents a clean, sustainable and environmentally friendly approach to generate H₂. Platinum is known to be the most efficient and commonly used electrocatalyst for hydrogen evolution reaction (HER).³ Other metals from the platinum group are also known to be good catalysts and are explored for various reactions involving organic synthesis, oxygen reduction reaction and photocatalysis.⁴ Chalcogenides of palladium have attracted considerable attention in recent times and in particular the synthesis of various palladium sulphides and selenides has been reported.^5–9 The compositions of PdS and Pd₄S have been shown to exhibit good chemical stability and catalytic properties.⁵ Similarly, palladium and selenium form different phases such as PdSe, Pd₁₇Se₁₅, Pd₇Se₄, Pd₄Se and PdSe₂, and various synthetic methods including solid state synthesis, solvothermal method, template assisted synthesis, thermal decomposition of complexes and hydrothermal method have been reported.^6–9

The thermal decomposition of a single molecular precursor is important among the synthetic routes because of its ease and use of relatively low temperatures.^7–9 O'Brien et al. have reported the synthesis of metal selenides based on the low pressure chemical vapour deposition technique using platinum and palladium dithio/diselenoimido-diphosphinato complexes.⁷ Jain et al. have reported the synthesis of palladium selenides by thermal decomposition of metal benzylselenolate, 2-(diethylamino)ethaneselenolate, monoselen-ocarboxylate and 2-(methoxycarbonyl)ethyl-selenolate complexes,⁸ and Singh et al. have explored the synthesis of palladium selenides from the selenoether complexes of palladium.^9a We have recently reported the use of didecylselenium based metal complexes to synthesize palladium selenides.^9b The use of single source precursors to synthesize different phases of selenides is quite demanding as compared to solid state methods.¹⁰

Herein, we present the synthesis of various palladium selenides from the same palladium and selenium precursors containing different mole ratios by thermal decomposition. The films formed on glass substrates are well-characterized using electrical, microscopic and diffraction techniques. The electrocatalytic activity of palladium selenides has not been explored so far. It is found in the present study that they are excellent catalysts for driving electrochemical redox reactions and HER is used to exemplify this point.

Palladium selenide phases are obtained by thermolysis of palladium didecylselenium complexes. The complexes with different compositions are prepared by mixing different mole ratios of palladium acetate and didecyl selenide (DDSe) in toluene for 10 min under ambient conditions. The Se precursor (DDSe) is synthesized by adopting a reported procedure.¹¹ In a typical synthetic procedure for the preparation of the complex, 40 μL of DDSe and 50 mg of Pd(OAc)₂ are stirred for 10 min, at 25 °C. The complexes are represented as B_xy, where x and y denote the moles of Pd(OAc)₂ and DDSe, respectively. The complexes are drop-cast on freshly cleaned glass slides of 1 cm² area and the solvent is evaporated under ambient conditions. Subsequently, the thermal decomposition of the complexes is carried out in a tubular furnace at 250 °C for 1 h under a N₂ atmosphere resulting in the formation of stable, continuous and shiny black colored films. Different phases of palladium selenide are synthesized under different conditions and the details are tabulated in Table S1 (ESI†).

The palladium selenide films formed on the glass substrate are found to be very adherent and possess metallic shine with good electronic conductivity. The electrical resistivity values are found to be 30, 50 and 175 μΩ cm for Pd₁₇Se₁₅, Pd₇Se₄ and Pd₄Se, respectively. The thickness of the films ranges from few to several microns (2–7 μm) depending on the volume of the precursor complex used to cover the glass slide. The XRD patterns (Fig. S1, ESI†) of the precursor complex B₁₁ on the glass surface show diffraction peaks at 1.56 and 3.0° that correspond to the (001) and (002) planes, respectively, and the appearance of periodic reflections is indicative of the formation of a lamellar structure. The d-spacing values obtained for the (001) and (002) planes are 56.6 and 29.3 Å that correspond to the bilayer formation of the complex from Pd(OAc)₂ and DDSe similar to the metal thiolates.¹² The formation of the complex is also evident from ⁷⁷Se NMR where a change in the chemical shift value from 159 to 259 ppm is noted for the B₁₁ complex as compared to pure Se. This may be attributed to a decrease in the electron density on Se due to the co-ordination of its lone pair with Pd (Fig. S2, ESI†). The UV-vis absorption spectra as a function of the molar ratios of Pd(OAc)₂ and DDSe (Fig. S3, ESI†) support the conclusions based on NMR and XRD. Pd(OAc)₂ and DDSe show the absorption bands at 400 and 288 nm, respectively. The absorption band obtained for Pd(OAc)₂ is due to the ligand to metal charge transfer transition while the band observed for DDSe is due to the n to σ* transition (Fig. S3(a), ESI†). The DDSe absorption band undergoes a bathochromic shift of 16 nm upon addition of varying amounts of Pd(OAc)₂ [Fig. S3(b) and (c), ESI†]. Apart from this observation, the disappearance of the band corresponding to Pd(OAc)₂ indicates the formation of the complex B_1y. It is reported that the reaction of PdCl₂ with didocosyl selenide results in the formation of a complex of composition Pd[(SeR₂)₂Cl₂].¹³ Similarly, in the present study, the composition of the complex B_1y may be expected to be Pd[(SeR₂)₂(OAc)₂], where the DDSe molecules bind via co-ordination bonds with the Pd center through the lone pair of electrons present on Se atoms. Similarly, in the case of the complex B_x1, the composition is expected to be the same but with excess of Pd(OAc)₂. This is evident from the UV-vis spectrum of the complex wherein the characteristic peak due to Pd(OAc)₂ observed at 400 nm increases with the increasing amount of Pd precursor. This is absent in the case of the complex B_1y (Fig. S3(c), ESI†).

The phase identification of the as-obtained materials formed by the thermal decomposition of B_xy is carried out using XRD. As shown in Fig. S4 (ESI†), the thermolysis of the complex B₁₁ at 250 °C results in the formation of the Pd₇Se₄ phase. The XRD pattern of Pd₇Se₄ corresponds to the orthorhombic system with lattice parameters a = 5.23, b = 6.87 and c = 6.21 Å (JCPDF 31-0939). The crystallite size determined using the Scherrer formula is ∼76 ± 5 nm. Similarly, the formation of the phase Pd₄Se is observed at a high molar ratio of palladium acetate in the complex, for instance B₄₁. The lattice parameters of the pure Pd₄Se phase obtained are a = 5.232 and c = 5.674 Å with tetragonal symmetry as given in Fig. 1(a). A further increase in the mole ratio of the Se precursor in the complex results in the formation of the Pd₁₇Se₁₅ phase (Fig. S4, ESI†). The XRD pattern of Pd₁₇Se₁₅ reveals a cubic phase with the lattice parameter a = 10.06 Å (JCPDF 73-1424). The crystallite size as determined using the Scherrer formula is ∼102 ± 10 nm. The XRD patterns of the phases obtained from the complexes B₂₁ and B₃₁ show the presence of peaks corresponding to both Pd₇Se₄ and Pd₄Se phases. Thus, the thermolysis of B₁₁ and B₄₁ yields the pure Pd₇Se₄ and Pd₄Se phases respectively while the complexes B₂₁ and B₃₁ give mixed phases (Fig. S5, ESI†). In order to obtain further insight into the evolution of palladium selenides, in situ XRD patterns are recorded as a function of temperature. For instance, the formation of a pure phase of Pd₄Se is observed during the course of thermolysis of the complex B₄₁. As shown in Fig. 1(b), the appearance of the Pd₄Se phase is observed at 150 °C and beyond.

Fig. 1 (a) X-ray diffraction patterns of (i) Pd₄Se prepared by the thermal decomposition of the B₄₁ complex at 250 °C for 1 h in the presence of N₂ and (ii) the standard Pd₄Se pattern (JCPDF 73-1386). (b) In situ XRD patterns of the thermal decomposition of the B₄₁ complex at different temperatures (i) 25, (ii) 100, (iii) 150, (iv) 250 and (v) 350 °C [* indicates peaks from the sample holder].

The surface morphology of the Pd-selenides characterized using electron microscopy shows continuous interconnected structures on glass slides (Fig. S6, ESI†). The palladium to selenium ratios from EDX analysis nearly match with the corresponding phases. Fig. 2(a)–(c) shows the high resolution TEM images of the Pd₁₇Se₁₅, Pd₇Se₄ and Pd₄Se phases. The respective d-spacing values calculated from the intensity line profiles are found to be 2.23, 2.64 and 3.19 Å that correspond to the (112), (311) and (113) planes of the three phases. The d-spacing values are in agreement with the high intensity reflections observed in the respective XRD patterns. The discrete spots observed in the SAED patterns indicate the crystalline nature of the materials (Fig. 2(d)–(f)).

Fig. 2 High resolution TEM images of (a) Pd₁₇Se₁₅, (b) Pd₇Se₄ and (c) Pd₄Se [the inset in each image shows intensity line profile images] (d)–(f) the corresponding SAED patterns.

The chemical compositions as determined by the XPS are given in Table S2 (ESI†) and are close to the expected ratios. However, oxygen is also found to be present to a certain extent in the as-prepared material. The observation of randomly present carbon is likely to be due to the decomposition of alkyl chains in the palladium organoselenolate complex. The high resolution spectra of Pd-3d and Se-3d are shown in Fig. 3. The deconvolution of the Pd-3d region displays a shift of 0.7 eV towards higher binding energies with respect to Pd(0) indicating the presence of a residual positive charge on Pd in the case of the Pd₇Se₄ phase. The binding energy (BE) difference between Pd 3d_5/2 and 3d_3/2 is found to be 5.2 eV due to the spin–orbit coupling. The region corresponding to the Pd-3p level overlaps with that of the O1s region (Fig. S20, ESI†). Once the sample is etched, the O1s peak is found to be absent indicating that the oxygen is adventitious in nature while others are retained in the same ratios (Fig. S21 and S22, ESI†). The peaks corresponding to Se-3d shift to lower BE values as compared to those of Se(0), resulting in a residual negative charge on Se as reported for many metal selenides such as CoSe, NiSe and CuSe.¹⁴ The Se-3d deconvoluted spectrum shows a spin–orbit coupling value of 0.8 eV. The XPS data for the other two phases, namely Pd₁₇Se₁₅ and Pd₄Se, are shown in Fig. 3 and Table S2 (ESI†). As observed, when Pd content increases from Pd₁₇Se₁₅ to Pd₄Se, the BE values of the Pd-3d peak shift to lower values. The BE values for Pd in Pd₁₇Se₁₅ (336.3 eV) do not exactly match with those of Pd⁰ (335.1 eV). The oxidation state of Pd is found to be between 0 and +2 in all three phases.

Fig. 3 High resolution deconvoluted XPS spectra of the Pd-3d and Se-3d regions of (a) and (b) Pd₁₇Se₁₅, (c) and (d) Pd₇Se₄ and (e) and (f) Pd₄Se.

The palladium selenide phases prepared in the current study are highly conducting and possess work function values ranging from 4.3 to 4.7 eV. The electrical resistivity measurements indicate that these phases are metallic in nature (Fig. S8, ESI†). Hence, they can act as good electrode materials for driving redox reactions. Just to confirm the amenability of the materials for electrochemical studies, a standard redox couple involving ferrocyanide/ferricyanide [Fe(II)/Fe(III)] has been studied in a standard three electrode cell. Fig. S9(a) (ESI†) displays the cyclic voltammograms that reveal reversible peaks on the palladium selenide phases. The linear variation of peak currents with concentration of redox species is observed as well.

The electrocatalytic activity of palladium selenides towards HER has been followed using the as-obtained films on glass slides with copper wire as the contact. Polarization measurements are carried out in N₂-saturated 0.5 M H₂SO₄ and the potentials are reported with respect to the reversible hydrogen electrode (RHE, Fig. S10, ESI†). The IR corrected linear sweep voltammograms at a scan rate of 2 mV s⁻¹, obtained using Pd₁₇Se₁₅, Pd₇Se₄ and Pd₄Se, display the onset potentials of −0.08, −0.07, −0.03 vs. RHE respectively (Fig. S11(a), ESI†) for HER. Beyond the onset potential, the cathodic currents rise rapidly. The overpotentials of −94 mV, −162 mV and −182 mV are observed to attain a current density of 10 mA cm⁻², for Pd₄Se, Pd₇Se₄ and Pd₁₇Se₁₅ respectively. In comparison to other well-known HER catalysts such as MoS₂, MoSe₂ and MoSSe,¹⁵ the palladium selenide phases require low overpotentials for hydrogen evolution. The adherence and stability of palladium selenide films towards continuous hydrogen evolution are improved considerably when Toray carbon paper (Fig. S11, ESI†) is used as the substrate. The presence of three-dimensional fibrous carbon structures with roughness along with the high surface area of Toray carbon holds the selenides strongly and high HER activities can be obtained. Toray carbon itself is inactive towards HER (Fig. S11(b), ESI†). The exchange current densities observed on Pt/C, Pd₄Se, Pd₇Se₄ and Pd₁₇Se₁₅ are 0.8 × 10⁻³ A cm⁻²,¹⁷ 2.3 × 10⁻⁴ A cm⁻², 2.5 × 10⁻⁵ A cm⁻² and 6.6 × 10⁻⁶ A cm⁻² respectively (Fig. 4(a)). The apparent Tafel slopes are observed to be ∼50, ∼56 and ∼57 mV per decade for Pd₄Se, Pd₇Se₄ and Pd₁₇Se₁₅ over 70–100 mV over potentials, respectively, whereas the measured Tafel slope for Pt/C under identical conditions is 31 mV dec⁻¹ (Fig. 4(b)).¹⁶ Tafel slopes are used only for comparing the activities of different selenides.

Fig. 4 (a) IR corrected linear sweep voltammetric curves of (i) Pd₁₇Se₁₅, (ii) Pd₇Se₄ and (iii) Pd₄Se towards HER and (b) the enlarged portion of the onset potentials. (c) Tafel-plots for (i) Pd₁₇Se₁₅, (ii) Pd₇Se₄, (iii) Pd₄Se, and (iv) Pt for catalyst loading of 0.28 mg cm⁻². The scan rate used is 2 mV s⁻¹. (d) XRD patterns of Pd₄Se before (i) and after (ii) 3000 cycles, at a scan rate of 100 mV s⁻¹. The inset in (d) shows the stability of currents at −0.4 V as a function of cycling.

Based on the Tafel slopes, it is likely that the HER mechanism probably follows Volmer–Heyrovsky steps in the case of selenides. Among the three phases, Pd₄Se shows the lowest Tafel slope with the highest exchange current density. The kinetic parameters obtained for the palladium selenide phases are better than those of the other known catalysts such as MoS₃ thin films (1.3 × 10⁻⁷ A cm⁻²), MoS₂ nanoparticles (1.3–3.1 × 10⁻⁷ A cm⁻²), FeP nanoparticles (1.7 × 10⁻⁵ A cm⁻²) and Mo₂C bulk (3.79 × 10⁻⁶ A cm⁻²).¹⁸ The turnover frequency (TOF) values for Pd₄Se, Pd₇Se₄ and Pd₁₇Se₁₅ are determined to be 0.47 s⁻¹, 0.35 s⁻¹ and 0.2 s⁻¹ (at 0.05 V vs. RHE) respectively, while the reported value for platinum is 0.9 s⁻¹ (at 0 V vs. RHE). The TOF values of Pd-selenides are higher than that of Ni₂P (0.015 s⁻¹, at −100 mV vs. RHE) and MoS₂ (0.02 s⁻¹, at 0 V vs. RHE) (Table S3, ESI†).¹⁹ Fig. S12 (ESI†) shows the impedance spectra along with the equivalent circuit used to fit the data. The R_ct values obtained are 86, 100 and 780 Ω for Pd₄Se, Pd₇Se₄ and Pd₁₇Se₁₅ respectively. The stability of the electrode materials has been checked under different conditions and found to be very good (Fig. 4(d) and Fig. S14, S15, S19 and S21 (ESI†)). The activities of electrocatalysts also depend on the number of active sites for H-adsorption which in turn depends on the electrochemical surface area of the material. The active surface areas of the three phases are determined using underpotential deposition of copper. The electrochemical surface areas of the Pd₁₇Se₁₅, Pd₇Se₄ and Pd₄Se phases are found to be 0.048, 0.72 and 1 m² g⁻¹ respectively (Fig. S16, ESI†). To understand and compare the activities of different phases of palladium selenides, density functional theory (DFT) calculations are carried out^20,21 and the details are given in the ESI.† The availability of the d-band density of states (DOS) at the Fermi level is generally correlated with the electrocatalytic activity.²² It is found that the Pd d-band DOS are found to be higher and placed exactly above the Fermi level in the case of Pd₄Se, while in the other two cases, it is not (Fig. S17 and S18, ESI†). The selenium contribution at the Fermi level is found to be very small for Pd₄Se, while it becomes fairly considerable in the case of the other two phases. Additionally, the positive charge on Pd (as given in XPS data earlier) is smaller in the case of Pd₄Se as compared to the other two, based on the shift in binding energy values (Table S2, ESI†). The binding energy value for Pd(0) is 335.1 eV, while the values are 335.3 eV for Pd₄Se and 336.3 and 336.7 eV for the other two phases. This is in parallel to the observation based on the charge density analysis performed on the three selenides. The average charge on Pd is smaller in the case of Pd₄Se (0.05) and increases when the phases are Pd₇Se₄ (0.12) and subsequently Pd₁₇Se₁₅ (0.17) (Table S5, ESI†). It should be noted that there are Pd atoms with different environment in the structures of the palladium selenides,²³ and hence an average charge is given for comparison purposes. As a consequence, the oxidation state of Pd in Pd₄Se is smaller than that of the other two phases. This will possibly help in the effective adsorption of protons on Pd₄Se as compared to Pd₇Se₄ and Pd₁₇Se₁₅. Of course, this is true when the adsorption sites are composed only of palladium. It is likely so since palladium(0) is a known hydrogen adsorption material. Additionally, the structural analysis reveals that the palladium sites are relatively open in the case of Pd₄Se and it is crowded as the composition becomes Pd₇Se₄ and Pd₁₇Se₁₅. All the above arguments lead to the fact that Pd₄Se is a better electrocatalyst than the other two members studied, Pd₇Se₄ and Pd₁₇Se₁₅. Low Tafel slope and low charge transfer resistance and high exchange current density and high electrochemical surface area make Pd₄Se an excellent electrocatalyst as compared to the other two phases. An important point to highlight is that the adsorption/desorption peaks associated with H atoms, before the onset of hydrogen gas evolution as observed on the pure Pt and Pd phases, are not observed in the case of palladium selenides. This requires further investigation.

In summary, three different palladium selenide phases Pd₁₇Se₁₅, Pd₇Se₄ and Pd₄Se are prepared by the thermolysis of different palladium organoselenolate complexes. Among the three phases, Pd₄Se shows very good electrocatalytic activity and this opens up ways of reducing the noble metal content in catalysts of interest.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures for the preparation of different palladium selenides, NMR, UV-vis and XPS quantification, SEM images, and XRD patterns before and after stability. See DOI: 10.1039/c5cc06730h

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