Layered Noble Metal Dichalcogenides: Tailoring ... - ACS Publications

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Layered Noble Metal Dichalcogenides: Tailoring Electrochemical and Catalytic Properties Xinyi Chia,† Zdeněk Sofer,‡ Jan Luxa,‡ and Martin Pumera*,† †

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

ABSTRACT: Owing to the anisotropic nature, layered transition metal dichalcogenides (TMDs) have captured tremendous attention for their promising uses in a plethora of applications. Currently, bulk of the research is centered on Group 6 TMDs. Layered noble metal dichalcogenides, in particular the noble metal tellurides, belong to a subset of Group 10 TMDs, wherein the transition metal is a noble metal of either palladium or platinum. We address here a lack of contemporary knowledge on these compounds by providing a comprehensive study on the electrochemistry of layered noble metal tellurides, PdTe2 and PtTe2, and their efficiency as electrocatalysts toward the hydrogen evolution reaction (HER). Observed parallels in the electrochemical peaks of the noble metal tellurides are traced to the tellurium electrochemistry. PdTe2 and PtTe2 can be discriminated by their distinct reduction peaks in the first cathodic scans. Considering the influence of the metal component, PtTe2 outperforms PdTe2 in aspects of charge transfer and electrocatalysis. The heterogeneous electron transfer (HET) rate of PtTe2 is an order of magnitude faster than PdTe2, and a lower HER overpotential of 0.54 V versus reversible hydrogen electrode (RHE) at a current density of −10 mA cm−2 is evident in PtTe2. On PdTe2 and PtTe2 surfaces, adsorption via the Volmer process has been identified as the limiting step for HER. A general phenomenon for the noble metal tellurides is that faster HET rates are observed upon electrochemical reductive pretreatment, whereas slower HET rates occur when the noble metal tellurides are oxidized during pretreatment. PtTe2 becomes successfully activated for HER when subject to oxidative treatment, whereas oxidized or reduced PdTe2 shows a deactivated HER performance. These findings provide fundamental insights that are pivotal to advancing the field of the underemphasized TMDs. Furthermore, electrochemical tuning as a means to tailor specific properties of the TMDs is advantageous for the development of their future applications. KEYWORDS: palladium telluride, platinum telluride, layered, electrochemical activation, electrochemistry, electron transfer, hydrogen evolution

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INTRODUCTION Two-dimensional (2-D) layered nanomaterials now stand front and center on the research agenda of the materials science community, fueled by their revolutionary technological opportunities associated with their anisotropy. Amidst all 2-D layered materials, research into this class of materials − transition metal dichalcogenides (TMDs) − has propelled well ahead of the pack. Given a general formula of MX2, a TMD comprises a transition metal from groups 4−10 and a chalcogen such as sulfur, selenium, or tellurium, represented as M and X, respectively. Documented in numerous published works, layered TMDs burnish their reputation as a gamut of disciplines ranging from energy storage and conversion to electrochemical sensing and even photovoltaics.1−5 Surprisingly, majority of the progress to date has been sewn up in favor of Group 6 TMDs, notably for the molybdenum and tungsten sulfides, whereas layered TMDs beyond Group 6 TMDs are largely unexplored. Analogous to 1T-MoS2 and 1T© 2017 American Chemical Society

WS2, which adopt a CdI2-type structure, layered Group 10 TMDs exhibit an octahedral structure where the Group 10 transition metal center coordinates to six chalcogen atoms.6 Unlike Group 6 TMDs, it is prudent to note that not all Group 10 TMDs exist in layered octahedral structures. These are limited to NiTe2, PdTe2, and all Pt dichalcogenides.6 Since the early works of Grønvold et al. and Kjekshus et al.,7,8 the band structures of Group 10 TMDs have been widely debated in the past.9−11 Proximate energy levels of the ultimate d orbitals of the transition metal in Group 10 and the outermost p orbitals of the chalcogen promote strong hybridization in these orbitals.11 For this reason, the electronic band structures of Group 10 TMDs diverge from TMDs belonging to earlier groups. Owing to the escalating interest in exploiting TMDs for Received: April 11, 2017 Accepted: July 10, 2017 Published: July 19, 2017 25587

DOI: 10.1021/acsami.7b05083 ACS Appl. Mater. Interfaces 2017, 9, 25587−25599

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic drawing of a PdTe2 and PtTe2 structure (space group P3m ̅ 1; blue ball denotes Pd or Pt atom; red ball denotes Te atom). (b) X-ray diffractograms of PdTe2 (left) and PtTe2 (right).

knowledge that noble metals are, by far, the best HER electrocatalysts with the promising electrocatalytic properties of TMDs, it is astounding that the concept of layered noble metal dichalcogenides for HER has not received much attention. In a bid to facilitate the understanding of layered noble metal dichalcogenides in the area of HER electrocatalysis, a study has been carried out aimed at elucidating their electrocatalytic behavior. Herein, we investigate the electrochemistry of layered noble metal tellurides, specifically PdTe2 and PtTe2. The essence of this work is to understand the significance of the metal type, Pd and Pt, on the electrochemical and electrocatalytic properties of PdTe2 and PtTe2. In addition, the prospect of electrochemical tuning of the PdTe2 and PtTe2 catalytic performance is also explored. In particular, we evaluate the efficacy of electrochemical activation on these noble metal tellurides in terms of the electron transfer rate and electrocatalytic activity toward hydrogen evolution. PdTe2 and PtTe2 were first systematically characterized before any electrochemical measurements.

electrochemical applications, our previous work correlated the electronic properties to the electrochemical aspects of the underrepresented layered Pt dichalcogenides and determined trends resulting from their varied chalcogen type.12 Yet, this previous study falls short of examining the transition metal factor on the electrochemical behavior of layered Group 10 TMDs. Far from nondescript, the transition metal of Group 10 TMDs includes noble metals palladium and platinum. Of interest to us, the genre of layered noble metal dichalcogenides,13 otherwise confined to the noble metal tellurides, is a relatively unusual class of layered TMDs that is seldom studied for its electrochemical applications. Therefore, in this work, we inquire the effect of the metal type on the electrochemical behavior by drawing comparisons between the layered noble metal tellurides, PdTe2 and PtTe2. On a broader context, the development of efficient and green platforms to support the production of renewable energy has posed a longstanding challenge. As hydrogen is proposed as a cleaner fuel than the combustion of hydrocarbon, ample research efforts into layered TMDs have focused on their capabilities as electrocatalysts for the hydrogen evolution reaction (HER) initiated by the success of MoS2.2,14−19 The most effective electrocatalysts for HER at present are recognized to be noble metals like platinum. Combining the

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RESULTS AND DISCUSSION Characterization of Noble Metal Tellurides. The noble metal tellurides were thoroughly characterized by powder X-ray diffraction, transmission electron microscopy (TEM), scanning 25588


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Figure 2. Transmission electron micrographs (left and center), SAED (shown in the inset of the center image), and high-resolution TEM (HRTEM) images (right) of (a) PdTe2 and (b) PtTe2. Scale bars represent 5 nm for the HRTEM images. The orientation of the observed plane is (001) in the case of PdTe2 and (539) in the case of PtTe2.

Figure 3. Scanning electron micrographs of (a) PdTe2 and (b) PtTe2 at a magnification of 5000×. Scale bars represent 1 μm.

Between the two, the edges are the more prominent feature in PtTe2. The micrograph for PdTe2 (Figure 3a) reveals a fair share of basal planes and edges. The elemental composition of the noble metal tellurides as determined by the EDXS analysis unveils chalcogen-to-metal ratios of 2.0 and 2.1 for PdTe2 and PtTe2, respectively (Table S1). These noble metal tellurides fulfill the theoretical chalcogen-to-metal ratio of 2.0; therefore, we confirm their successful synthesis. Inherent Electrochemistry of Noble Metal Tellurides. The inherent electrochemistry of the noble metal tellurides remains hitherto in obscurity. Herein, inherent electrochemistry describes the intrinsic redox behavior of PdTe2 and PtTe2 when an electrochemical potential is applied.3 With awareness of the inherent electrochemical behavior of these noble metal tellurides, their proposed applications can be exercised with much discretion. Cyclic voltammetry in two directions, anodic and cathodic, accompanied by three subsequent scans, was performed for the inherent electrochemical study of noble metal tellurides. The acquired voltammetric profiles provide information about the electroactive surface groups of the

electron microscopy, energy-dispersive X-ray spectroscopy (EDXS), and X-ray photoelectron spectroscopy (XPS). X-ray diffractograms illustrated in Figure 1 show the singlephase composition of both the synthesized noble metal tellurides with a P3̅m1 space group without any additional phases like unreacted metal or tellurium excess. The preferential orientation along the (00l) direction is observed due to the layered structure of tellurides. Figure 2 shows the transmission electron micrographs of PdTe2 and PtTe2 in the form of layered flakes with a particle size up to 1 μm. The selected area electron diffractograms (SAED) of PdTe2 and PtTe2 flakes provide evidence for the hexagonal structure. The EDXS mapping based on TEM depicted in Figure S1 indicates the homogeneous distribution of elements without any visible phase separation. One common trait in the morphologies of PdTe2 and PtTe2 is their layered nature. Distinct layers are evident, and coupled with the sheets of different widths stacked in close proximity, PdTe2 and PtTe2 exist as layered bulk materials. Basal planes and edges are also apparent for both the noble metal tellurides. 25589


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Figure 4. Cyclic voltammograms of PdTe2 beginning in the direction of the (a) anodic potential and (b) cathodic potential and of PtTe2 in the direction of an initial (c) anodic potential and (d) cathodic potential in a PBS electrolyte (50 mM, pH 7.0).

anodic and cathodic scans, PdTe2 demonstrates two oxidative peaks at ca. 0.2 and 0.4 V, whereas PtTe2 shows a lone oxidative peak at ca. 0.4 V. At these oxidizing potentials, oxidative stripping of Te occurs on the noble metal tellurides. Of the possible processes, the anodic wave at ca. 0.2 V corresponds to the oxidation of elemental tellurium to Te(II) states,21 while the oxidative peak at ca. 0.4 V originates from the anodic stripping of Te(0) to Te(IV) species.12,22 Coincident with the oxidative signal in PtTe2, Guascito and co-workers23 observed the oxidation of the Te chalcogen component at 0.38 V versus saturated calomel electrode in Pt/Te microtubes, which results in Te(II) and Te(IV) species. The initial weak cathodic wave at ca. −0.75 V during the anodic sweeps of PdTe2 and PtTe2 became more conspicuous in the successive scans. Because this signal intensifies after ca. 0.4 V, which is the prevailing anodic peak in the noble metal tellurides due to Te oxidation, the cathodic peak at ca. −0.75 V is diagnostic of the Te reduction from the oxidation state of +4 to 0. The intermediate reduction to Te(II) is unlikely because the reduction of Te(IV) tends to proceed to the elemental state with no evidence of reducing Te(IV) to Te(II) in the literature.17 Moreover, the signal of Te(IV) species at −0.8 V reported by Thakkar et al.20 via a differential pulse polarographic method closely approximates that of our experimental value. The noble metal tellurides are discernible on the basis of their cathodic peaks exhibited during the first scans. PdTe2 displays a significant reduction peak at ca. −1.0 V when swept in the cathodic direction and strong separate signals at ca. −0.9 and −1.0 V in the anodic sweep. Regardless of the cathodic or anodic sweeps, PtTe2 maintains a well-defined reduction signal at ca. −1.4 V, concurring with our previous work.12 The provenance of these signals lies in part in the electroactivity of the metal species. Oxidation states of both Pd and Pt in the noble metal tellurides are expected to be +4. Therefore, these cathodic processes largely involve reducing Pd(IV) or Pt(IV)

materials, whereas the nature of the redox process is derived from the three consecutive scans. The voltammetric measurements occur within a potential range of −1.8 to +1.8 V in phosphate buffered saline (PBS) in neutral pH (pH 7.0). For clarity, the reported electrochemical potentials in this work are referenced to the Ag/AgCl electrode unless mentioned otherwise. As illustrated in Figure 4, PdTe2 and PtTe2 elicit distinct electrochemical profiles. These distinctive profiles are a result of the redox processes undergone by their surface electroactive species. Considering the first scans of PdTe2 and PtTe2, an initial reductive sweep is a prerequisite to the emergence of their oxidative peaks. When scanned in the cathodic direction beginning from 0 V, PdTe2 exhibits a sharp reductive signal at ca. −1.0 V that precedes an oxidative peak at ca. 0.2 V (Figure 4b). Yet, when first scanned toward the anodic direction, PdTe2 affords three reductive peaks: a shoulder signal at ca. −0.75 V and two distinctive signals at −0.9 and −1.0 V; with no observable oxidation signals (Figure 4a). Likewise, this phenomenon is also noted for PtTe2. During the cathodic sweep, PtTe2 manifests a mild anodic wave at ca. 0.4 V that succeeds two cathodic signals: a modest peak at ca. −0.75 V and a strong peak at −1.4 V; whereas in the anodic sweep, the anodic wave was absent and only the reductive signal is apparent for PtTe2. Therefore, the absence of oxidative signals in the first anodic scans of the noble metal tellurides suggests that the proclivity of electroactive groups on PdTe2 and PtTe2 for oxidation arises from the initial reduction. Similarities between the voltammetric profiles of the noble metal tellurides are rationally assigned to the inherent electrochemistry of their identical chalcogen component − tellurium. Comparing their electrochemical features to the existing literature,20−25 the electrochemistry of tellurium is clearly accentuated in the noble metal tellurides. For both 25590


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Figure 5. X-ray photoelectron spectra of PdTe2 in high resolution before and after electrochemical oxidation or reduction for the regions: Pd 3d (a) and Te 3d (b).

chalcogen component and point to the reversible nature of tellurium electrochemistry, which has been recorded in an early work by Lingane et al.26 Voltammetric traits of the noble metal tellurides delineate the intrinsic redox processes by their metal and chalcogen constituents. The resemblance in some signals of PdTe2 and PtTe2 boils down to the tellurium electrochemistry, whereas those from their respective metal component render them distinguishable. Characterization of Electrochemically Treated Noble Metal Tellurides. Following the study on the inherent electrochemistry of noble metal tellurides, we endeavor to electrochemically activate PdTe2 and PtTe2. Electrochemical treatment is performed by subjecting the material to an oxidation or reduction at a potential that marginally lies beyond the observed electrochemical signals that are characteristic to the noble metal tellurides. The oxidative potentials chosen for electrotreatment are +0.8 V for both noble metal tellurides, whereas the reduction treatment potentials are identified at −1.2 V for PdTe2 and −1.6 V for PtTe2. Redox reactions of the material at these potentials may induce chemical alterations to the surface structure. In electrochemistry, electron transfer occurs heterogeneously across an interface consisting of an electrolyte and a material surface. Hence, knowledge on surface

species to an oxidation state of +2 or even 0. While either of the reductive signals of ca. −0.9 or −1.0 V in the PdTe2 may correspond to the reduction of the metal species, the other may coincide with Te reduction. Akin to PdTe2, Group 4 and 5 tellurides demonstrate a cathodic wave at ca. −1.0 V possibly due to the reductive stripping of Te(0) to soluble Te2−.16,24 Another likely contribution stems from the Te(VI) species undergoing reduction to the Te(IV) state. Our XPS analysis reveals the presence of the Te(VI) state in PdTe2 (Figure 5), given visible shoulder signals at Te 3d7/2 and Te 3d5/2 binding energies of 577.6 and 588.0 eV, respectively, which virtually disappear when PdTe2 is reduced. These XPS signals of Te(VI) are not detected in PtTe2. As the cathodic peaks are unique to PdTe2, it becomes more compelling to assign the cathodic signal to Te(VI) reduction. Majority of the electrochemical processes of the noble metal tellurides are deemed as chemically irreversible. For the reduction peaks attributed to the Pt or Pd species, there is an apparent diminution of the signal amplitude in the following scans. The diminished current intensity in subsequent scans indicates the chemically irreversible nature of the reduction process. Contrary to this, the noble metal tellurides show unrelenting current intensities of their oxidation signal at ca. 0.4 V and a reduction signal at ca. −0.75 V in the succeeding cathodic and anodic scans. These features arise from the 25591


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Figure 6. X-ray photoelectron spectra of PtTe2 in high resolution before and after electrochemical oxidation or reduction for the regions: Pt 4f (a) and Te 3d (b).

composition is a first rung on the ladder toward understanding the electrochemical attributes of the noble metal tellurides. XPS was carried out to confirm any surface composition changes to the noble metal tellurides as a consequence of electrochemically oxidizing or reducing them at selected potentials. High resolution XPS spectra of core level Pd 3d and Te 3d regions of PdTe2 and core level regions for Pt 4f and Te 3d of PtTe2 before and after electrochemical treatment are depicted in Figures 5 and 6. Derived from the high resolution spectra, atomic percentages of the deconvoluted states for the metal component are recorded in Tables S3 and S4. Tables S3 and S4 summarize the relative variations in the oxidation states of the respective metal in the noble metal tellurides upon electrochemical treatment. Before electrochemical treatment, the principal oxidation state for the metal in both noble metal tellurides runs contrary to the expected +4. PtTe2 has a primary oxidation state of +2, whereas Pd(II) and Pd(0) states are equally dominant in PdTe2. PtTe2 reveals a pair of signals at 72.5 and 75.9 eV that are assigned to Pt 4f7/2 and 4f5/2 binding energies of the Pt(II) state, respectively.27 Deconvolution analysis of the Pd bonding modes in PdTe2 shows a pair of doublets (3d5/2 and 3d3/2) originating from the Pd(II) state at 336.2 and 341.5 eV and also from the Pd elemental state at 335.4 and 340.7 eV.28 Likewise, the main Te chalcogen signal for untreated PdTe2 and PtTe2 deviates from the anticipated Te(−II) state. Instead, the Te chalcogen signal of PdTe2 comprises three observed features: a

conspicuous Te 3d5/2 peak at 575.9 eV that corresponds to Te(IV) species; a broad shoulder Te 3d5/2 peak at 577.6 eV that stems from Te(VI) species; and a modest Te 3d5/2 peak at 573.1 eV that arises from the Te(0) state. PtTe2 demonstrates a pair of distinct doublet Te signals (3d5/2 and 3d3/2) at binding energies of 572.8 and 583.2 eV, and 575.2 and 585.6 eV, which are respectively indexed to Te(0) and Te(IV) states.12,29,30 The discrepancy in the oxidation states of metal and chalcogen from the ideal states is rationalized by the electronic band structure of PdTe2 and PtTe2. Both noble metal tellurides display strong orbital mixing in p and d bands, anomalous to the energetically separated bands reported for layered TMDs belonging to earlier groups.31 Compared with earlier groups, Group 10 tellurides are equipped with more metal d electrons and smaller electronegative difference between the elemental constituents. In turn, the close proximity in the energy levels of the d orbital of Pt or Pd metal and the p orbital of tellurium chalcogen undermines the ionic character of the Pd−Te or Pt− Te bond while undergirding their covalent nature.32 As a result of the immense covalent character of the Pd−Te and Pt−Te bonds, the electronic structures for PdTe2 and PtTe2 share an atypical description. In particular, both Pd and Pt metals undertake an oxidation state between 0 and +2 instead of +4, whereas the chalcogen acquires a higher state than −2.9,11,33 Moreover, a massive chalcogenide, like the tellurium anion, possesses a unique polymerizing ability, whereby the anionic behavior partially oxidizes the tellurium.34 In doing so, the Te 25592


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Figure 7. Cyclic voltammograms of 5 mM [Fe(CN)6]3−/4− on untreated, electrochemically oxidized or reduced (a) PdTe2 and (b) PtTe2. (c) Histograms detailing the peak-to-peak separations of noble metal tellurides and their electrochemically treated counterparts along with the error bars.

persists as the prevailing oxidation state and contributes to approximately 74−79% of the Pt composition. Prevalent Pt 4f signals (4f7/2 and 4f5/2) remain unwavered at 72.5 and 75.9 eV. Between untreated and oxidized PtTe2, the Pt composition hardly changed. Proportions of Pt(0), Pt(II), and Pt(IV) are mostly comparable. Slight variations in the proportion of Pt(0) are noted in PtTe2 after electrochemical reduction. When reduced, PtTe2 has a considerable increase in Pt(0) by 10.0%. In correlation to this increment, the Pt(II) doublet signals appear less defined and a noticeable shoulder emerges at the lower binding energy. Upon deconvolution of the signals, we obtain a modest pair of signals at Pt 4f7/2 and 4f5/2 binding energies of 70.7 and 74.1 eV that is attributed to the elemental Pt species.12,27 On the basis of the distinct variations in the Te 3d signals, both noble metal tellurides demonstrate visibly more pronounced differences in the Te chalcogen than the Pd or Pt metal component when subject to electrochemical treatment. At one glance, the shape profiles of the Te 3d region for PdTe2 evolved from a pair of central Te 3d signals indexed to the Te(IV) species in the untreated state to a distinct pair of doublets of electrochemically reduced PdTe2. Deconvolution analysis of this pair of doublets in reduced PdTe2 unveils the presence of three major oxidation species. Focusing on 3d5/2 binding energies, the pair of peaks of the reduced PdTe2 occurring at 575.5 and 572.3 eV ascribes to Te(IV) and combined Te(0) and Te(−II) states in sequence. Substantial amounts of Te(0) and Te(−II) assigned at 3d5/2 binding energies of 572.8 and 572.0 eV merge into a peak at 572.3 eV.12,24,29 Similarly, electrochemical treatment of PtTe2 also engendered marked shifts in the Te 3d signals. Untreated PtTe2 bears a pair of doublets arising primarily from Te(0) and

chalcogen in PdTe2 and PtTe2 is conferred a higher oxidation state than −2. Congruent with the above reasoning, our XPS spectra demonstrate the predominant oxidation states in PdTe2 to be Te(IV) and a comparable Pd(0) and Pd(II), and in PtTe2 to be Pt(II) and an equal dominance for Te(0) and Te(IV). Judging from the prominent shifts in the primary Pd 3d binding energies in Figure 5, the Pd composition in PdTe2 yields strong dependence on electrochemical treatment. In the untreated form, PdTe2 shows a pair of signals (Pd 3d5/2 and 3d3/2) at 335.9 and 341.1 eV with equivalent contribution by the Pd(0) and Pd(II) states. Upon electrochemical oxidation, the pair of signals is shifted toward higher binding energies at 336.5 and 341.8 eV and the Pd(II) gains dominance over the Pd(0) state. Deconvolution of the bonding modes in oxidized PdTe2 further reveals the moderate presence of the Pd(IV) state that is quantified to assume 26.6% of the Pd composition. There is also a surge in the proportion of Pd(II) to 66.0% in oxidized PdTe2 from an initial 47.5% in untreated PdTe2. Complementary to this increment is the attrition in the amount of Pd(0) from 50.6% when untreated to 7.6% once oxidized. Hence, the drastic decline in Pd(0) endorses the oxidation of Pd(0) to Pd(II) as well as to Pd(IV) in PdTe2. When PdTe2 is electrochemically reduced, Pd(0) becomes the major species as depicted in the lower Pd 3d5/2 and 3d3/2 binding energies at 335.5 and 340.7 eV, respectively. This is accompanied by a spike in the amount of Pd(0) to 65.0% and a decline in Pd(II) contribution to 31.6% in reduced PdTe2. In stark contrast to PdTe2, wherein the Pd composition fluctuates with redox treatment, PtTe2 largely preserves the Pt composition, evident in the fairly unperturbed Pt 4f signals (Figure 6) regardless of electrochemical treatment. Across untreated, electrochemically oxidized or reduced PtTe2, Pt(II) 25593


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Figure 8. Polarization curves for the HER in 0.5 M H2SO4 for the untreated, electrochemically oxidized and reduced (a) PdTe2 and (c) PtTe2. Tafel graphs for (b) PdTe2 and (d) PtTe2.

Therefore, employing PtTe2 as a prospective electrode material is preferred to PdTe2 for electrochemical sensing applications. Both PdTe2 and PtTe2 unearth a trend such that their charge-transfer rates are effectively tailored by electrochemical treatment. Figure 7 reflects slower HET rates in oxidized noble metal tellurides and faster HET rates when they were electrochemically reduced. This observation of the noble metal tellurides emulates that of our previous works on electrochemically treated Pt dichalcogenides12 and Group 6 TMDs.14,36 For n-type semiconductors like MoS2 and PtS2, electrons are the main charge carriers.37,38 When these materials are subject to electrochemical oxidation, assuming at a potential above their flat band potential, electrons are conducted away from the interface, and this develops a depletion region. As the electrolyte−electrode interface becomes electron deficient, the transfer of electron from the oxidized material to the electrolyte is encumbered. In the same manner, an accumulation region ensues upon electrochemical reduction. The electron-rich interface promotes electron transfer given by the hastened HET rates of the reduced material. Divergent from semiconductors, metallic PdTe2 and PtTe2 have an infinitesimally thick accumulation region that provides free electron transfer across the electrode−electrolyte interface. The observed HET behavior of the electrochemically oxidized and reduced noble metal tellurides may be also associated to the variation in the metal and chalcogen composition in both noble metal tellurides due to electrochemical treatment. The influence of the type of electrochemical treatment on the HET rates of noble metal tellurides are further emphasized in PdTe2 compared to that in PtTe2. After electrochemical oxidation, k0obs is computed to be 3.4 × 10−6 cm s−1 in oxidized PdTe2, whereas oxidized PtTe2 takes on a k0obs of 4.3 × 10−5 cm s−1. Although both of them show a general drop in the observed HET rates, the decline is more substantial in PdTe2. The electron transfer rate in oxidized PdTe2 is 19.1 times

Te(IV). The intensities of the peaks strongly correlate to the type of electrotreatment. Once PtTe2 is reduced, Te(0) predominates Te(IV), whereas the opposite happens in oxidized PtTe2. As ascertained from the XPS study, electrochemical treatment governs the composition of elemental constituents in noble metal tellurides. The Pd metal and Te chalcogen have been documented to be sensitive toward electrochemical treatment, given the shifted signals. Conversely, the Pt composition of PtTe2 remains relatively unchanged and is inferred to be independent of electrotreatment. Thus, the effect of electrotreatment on the composition is more highlighted in PdTe2 than in PtTe2. Heterogeneous Electron Transfer (HET) Ability of Electrochemically Treated Noble Metal Tellurides. In most electrochemical applications, the HET rate of an electrode material is one of the important metrics for assessing its prospective use. Broadly, a fast HET rate decreases the overpotential required for an electrochemical reaction and is therefore favored in electrode materials for sensing applications. The Nicholson approach correlates the peak-to-peak separation (ΔE) with the observed HET rate constant (k0obs). By this relation, a narrower ΔE translates into a faster HET rate. We studied the HET rates of PdTe2 and PtTe2 in the presence of an [Fe(CN)6]3−/4− redox probe, which has been known for its surface sensitivity.35 Cyclic voltammograms of [Fe(CN)6]3−/4− on noble metal tellurides and their oxidized or reduced forms via electrochemical means are collated in Figure 7. Computed k0obs values for the noble metal tellurides before and after electrochemical treatment are recorded in Table S5. Considering the effect of the Pt or Pd metal component in the tellurides, PtTe2 demonstrates a significantly higher HET rate than PdTe2. In the untreated states, PdTe2 and PtTe2 show k0obs of 6.5 × 10−5 and 2.5 × 10−4 cm s−1, respectively. Clearly, PtTe2 displays an order of magnitude faster in the observed HET rate at 10−4 cm s−1 than in PdTe2 at 10−5 cm s−1. 25594


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treatment worsens the HER performance of PdTe2, electrochemical oxidation effectively activates the catalytic property of PtTe2 to attain a lower overpotential required for HER than before. Oxidized PtTe2 affords an HER overpotential of 0.40 V versus RHE at −10 mA cm−2. In contrast, electrochemical reduction did not generate any notable differences in the catalytic property of PtTe2. Measured to be 0.53 V versus RHE, the HER overpotential of reduced PtTe2 closely approximates that of its untreated form. Taking into account the HER overpotentials of the noble metal tellurides, it is also worth mentioning that other members of the TMD family, such as the extensively studied Group 6 TMDs, have been shown to behave as better electrocatalysts for HER, particularly when thinned down to a monolayer. MoS2, the prototype TMD that also belongs to Group 6, requires overpotentials over a range from 0.12 to 0.62 V versus RHE for nanosheets to bulk structures at a current density of −10 mA cm−2.36,39−41 Low HER overpotentials for other Group 6 TMDs including WS2 and MoSe2 nanosheets have also been reported at ca. 0.15 V.42,43 Using chemical vapor deposition, a single crystal of VS2 synthesized by Lou and co-workers demonstrated an overpotential of as low as 0.06 V versus RHE for hydrogen evolution.44 The HER overpotentials of the noble metal tellurides in the bulk state, even in the case of the bestperforming oxidized PtTe2, are generally higher than TMDs such as MoS2, WS2, MoSe2, and VS2 nanosheets. In addition to the HER overpotential at −10 mA cm−2, the Tafel slope is also a performance indicator for HER. More extensively, the Tafel slope illuminates the electrochemical mechanism of the material in question. A small Tafel slope is deemed a desirable attribute in a prospective HER electrocatalyst because this indicates that a low overpotential is necessary to elicit a rise in current density. Tafel slope analysis offers insights into the reaction pathway undertaken by different material surfaces for hydrogen evolution where the rate-determining step is then elucidated. The key steps are as such45,46 1. Adsorption via the Volmer process

slower, whereas oxidized PtTe2 is 5.8 times slower than that before treatment. The HET rates of both noble metal tellurides become activated after reductive treatment. In particular, the electrochemical activation in HET rates is most strongly encountered by PdTe2 than by PtTe2. Despite the overall accelerated HET rates in both noble metal tellurides with k0obs lying in the region of 10−3 cm s−1 after electrochemical reduction, reduced PdTe2 (k0obs = 3.5 × 10−3 cm s−1) showcases an increment in the HET rate that is equivalent to 53.8 times that of its untreated state, whereas in reduced PtTe2 (k0obs = 2.5 × 10−3 cm s−1), the HET rate improves by a smaller extent of merely 10 times that of its untreated counterpart. On the basis of the HET rates of the noble metal tellurides, electrochemical treatment of PdTe2 elicits a more pronounced response than PtTe2. Among all electrochemically treated noble metal tellurides, reduced PdTe2 yields the fastest electron transfer rate, whereas the HET rate of PdTe2 is most severely deactivated by an oxidative treatment. HER Ability of Electrochemically Treated Noble Metal Tellurides. Next, we investigate the electrocatalytic properties of the noble metal tellurides toward the HER. Of paramount interest, we discuss the feasibility of tuning the catalytic properties of PdTe2 and PtTe2 via electrochemical means. Moreover, our previous attempt with TMDs, including MoS2 and WS2, had been fruitful at improving their HER performance.14,36 Earlier, we have established the trend that a reductive treatment enhances the HET rates of the noble metal tellurides, whereas an oxidative treatment decreases the rate. The optimistic prospect of altering the electrochemical properties of PdTe2 and PtTe2 spurs us to explore further, this time with their electrocatalytic performance. Linear sweep measurements were performed in N2-saturated 0.5 M H2SO4 to capture the HER performance of PdTe2 and PtTe2, with their polarization curves before and after electrochemical treatment for each noble metal telluride reflected in Figure 8. An alongside comparison of the HER performance attributed to the metal dependence of the noble metal tellurides is provided in Figure S5 for untreated PdTe2 and PtTe2. The polarization curve of platinum on carbon (Pt/ C), which is the top electrocatalyst to date, has been included as a benchmark to gauge the HER efficiency of the noble metal tellurides. A bare glassy carbon (GC) electrode has also been demonstrated for reference. Figure S5 presents a comprehensive comparison of the HER parameters across all electrochemically treated materials. Overpotentials at a current density of −10 mA cm−2 of the noble metal tellurides represent a first point of reference in the assessment of their electrocatalytic performance for HER. This particular current density of −10 mA cm −2 typically corresponds to a tangible level of H2 production and has been frequently used for drawing comparisons in the literature. Linear sweep voltammograms (Figure S5) of the untreated noble metal tellurides disclose that PtTe2 is the stronger performing electrocatalyst of the two. At a current density of −10 mA cm−2, the HER overpotential of PdTe2 is recorded to be 0.74 V versus RHE; and PtTe2, at 0.54 V versus RHE. Thus, it is evident that PtTe2 demonstrates a lower HER overpotential than PdTe2. Figure 8 evinces that the electrocatalytic behavior of the noble metal tellurides can be wrought by electrochemical treatment. Electrochemically reducing or oxidizing PdTe2 deteriorates the HER performance of PdTe2 as manifested in higher overpotentials of ca. 0.83 versus RHE in both oxidized and reduced PdTe2. Even though electrochemical

H3O+ + e → M − H + H 2O, b ≈ 120 mV dec−1

2. Desorption via the Heyrovsky process M − H + H3O+ + e b ≈ 40 mV dec−1 → H 2 + H 2O + M*,

or desorption via the Tafel process 2M − H → H 2 + 2M*, b ≈ 30 mV dec−1

Of all experimented materials, Pt/C exemplifies the smallest Tafel slope of 37 mV dec−1. The desorption step for Pt/C is rate limiting in consonance with the low Tafel values between 30 and 40 mV dec−1 gathered from the literature.47 Despite the Pt metal constituent in PtTe2, the Tafel slope of PtTe2 and its electrochemically treated forms are unable to rival that of Pt/C. Before any treatment, PtTe2 yields a Tafel slope of 110 mV dec−1, whereas that for PdTe2 is derived to be lower at 92 mV dec−1. Both Tafel slopes signify adsorption as the slow step. The marginally lower Tafel slope suggests faster HER kinetics on PdTe2 compared to that of PtTe2. This kinetic consideration stands antithesis of the thermodynamic aspect, wherein PtTe2 requires a lower HER overpotential than PdTe2. The two facets 25595


Research Article

ACS Applied Materials & Interfaces can be resolved as such; in comparison to PtTe2, the rate of HER occurs faster on PdTe2 but requires a larger overpotential for the HER to proceed. Regardless of the electrochemical treatment, adsorption via the Volmer process remains the ratedetermining step for the electrochemically treated noble metal tellurides. The Tafel slopes of these materials approximate 120 mV dec−1, where they exist within the range of 93 to 130 mV dec−1. The onset potential is another parameter employed for evaluating the HER performance. In this work, we determine the onset potential for HER to be equivalent to the potential measured at a current density of −0.1 mA cm−2. Complying with the trend based on HER overpotentials, the onset potential for PtTe2 occurs substantially earlier at −0.17 V versus RHE than PdTe2 in which HER begins at more negative potentials at −0.43 V versus RHE. Mirroring this, the onset of HER for electrochemically treated PdTe2 occurs much later than the electrochemically treated PtTe2. Perusal of the onset potentials reveals slight differences among the materials upon electrochemical treatment. In particular, the onset potentials of oxidized PtTe2 and reduced PtTe2 are measured at −0.14 and −0.19 V versus RHE, respectively. For PdTe2, there is no change to the onset of HER when reduced, whereas the HER onset becomes delayed to −0.46 V versus RHE in oxidized PdTe2. The HER onset potentials of the noble metal tellurides before and after electrochemical treatment differed by no more than 0.03 V. In light of all performance indicators, electrochemical treatment of the noble metal tellurides exerts most clout on their HER overpotential at a tangible level of H2 production. In terms of the HER overpotential, PtTe2 is found to be more responsive toward electrochemical treatment compared with PdTe2. Upon electrochemical oxidation, PtTe2 depicted markedly lower HER overpotential by 26%, thereby conferring oxidized PtTe2 as the leading electrocatalyst choice for hydrogen evolution. The successful activation of PtTe2 for HER electrochemically contrasts the deactivation of PdTe2. Performing either oxidative or reductive treatment on PdTe2 renders an undermined HER performance by 12%. According to these findings, the metal type in the noble metal telluride dictates the extent of the implication on HER behavior stemming from electrochemical treatment. The upshot of this is clear: Pt > Pd, whereby oxidized PtTe2 exhibits an activated electrocatalytic performance to a greater extent than the deactivated HER behavior in oxidized or reduced PdTe2.

PtTe2 requires a lower overpotential at 0.54 V versus RHE but a larger Tafel slope of 110 mV dec−1 relative to PdTe2, which has an overpotential of 0.74 V versus RHE and a Tafel slope of 92 mV dec−1. Furthermore, efforts to tune the electrochemical and electrocatalytic attributes using electrochemical means disclose several interesting observations. Chief among these are conclusions on the effect of electrochemical treatment of the noble metal tellurides on the HET rate and the HER performance. Electrochemical reduction effectively activates their electrochemical property as substantiated by the enhanced HET rates in their reduced forms. Unlike electrochemical reduction, subjecting both noble metal tellurides to oxidation impeded their electron transfer. The electron transfer of PdTe2 responds more dramatically to electrochemical treatment. However, success in electrochemical activation toward HER is exclusive to PtTe2 via oxidation given the significantly lowered HER overpotential to 0.4 V versus RHE at a current density of −10 mA cm−2. Notwithstanding the type of electrochemical treatment, PdTe2 in reduced or oxidized forms display higher overpotentials of ca. 0.83 V versus RHE than formerly. Hence, it is deduced that the electrochemical redox treatment of PdTe2 deactivates its electrocatalytic performance. Whereas the electron transfer property of PdTe2 is more receptive to electrochemical treatment, the electrocatalytic property of PtTe 2 is more sensitive to electrochemical treatment. Dissimilar performance in the HET and HER aspects of the noble metal tellurides originates from the intrinsic response of the electrotreated material to the type of electrochemical application and leaves no room for worry. There is no immediate correlation between HET and HER concepts as they are founded on fundamentally different mechanisms. Evidently, electrochemical treatment governs the electrochemical and electrocatalytic properties of the noble metal tellurides. The distinct responses from electrochemically treated PdTe2 and PtTe2 highlight the importance of the metal dependence. With regard to HET, the Pd metal is more receptive than Pt. For HER, the Pt metal is more sensitive. These findings are beneficial in the current understanding of noble metal dichalcogenides, imparting to us the fundamentals of their electrochemistry as well as the potential of electrochemically tweaking their properties to achieve desired objectives.

CONCLUSIONS In summary, we have unraveled the electrochemical and electrocatalytic properties of noble metal tellurides in the bulk form, namely, PdTe2 and PtTe2. Both PdTe2 and PtTe2 have been determined to be electroactive as gleaned from their inherent electrochemical signals. For instance, the oxidative peak at ca. 0.4 V versus Ag/AgCl is a shared feature of the noble metal tellurides and is therefore traced to the intrinsic electrochemistry of the tellurium chalcogen. Yet, the cathodic signals characteristic to the respective metal components of the noble metal tellurides differentiates them. Moreover, we have shown that PtTe2 possesses a faster HET rate than PdTe2, thereby deeming PtTe2 a better electrode candidate in electrochemical sensing applications. In the aspect of a HER electrocatalyst, both the noble metal tellurides exhibit reasonably competent electrocatalytic properties, with PtTe2 being the stronger performer due to its lower overpotential.

Materials. Palladium (99.9%) and platinum (99.9%) powders were obtained from Sigma-Aldrich, Czech Republic. Tellurium (99.999%) was obtained from Mateck, Germany. Platinum (Pt), GC (3 mm diameter), and Ag/AgCl electrodes were procured from CH Instruments, TX. Platinum on carbon, sulfuric acid, potassium ferrocyanide, sodium phosphate monobasic, potassium chloride, sodium chloride, potassium phosphate dibasic, and N,N-dimethylformamide were commercially obtained from Sigma-Aldrich, Singapore. Synthesis. PdTe 2 . A quartz glass ampoule containing a stoichiometric amount of Pd and Te to yield 2 g of PdTe2 was evacuated on base pressure 1 × 10−3 Pa with the help of a diffusion pump. The quartz glass ampoule (15 × 100 mm2; 2 mm wall thickness) was melt sealed by an oxygen−hydrogen welding torch. The ampoule was heated on 1200 °C at a rate of 1 °C min−1. After 2 h on dwell temperature, it was cooled at a cooling rate of 1 °C min−1 to room temperature. PtTe2. A quartz glass ampoule containing a stoichiometric amount of Pt and Te to yield 2 g of PtTe2 was evacuated on base pressure 1 × 10−3 Pa with the help of a diffusion pump. The quartz glass ampoule

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Research Article

ACS Applied Materials & Interfaces (15 × 100 mm2; 2 mm wall thickness) was melt sealed by an oxygen− hydrogen welding torch. The ampoule was heated on 800 °C (heating rate 1 °C min−1). After 24 h on dwell temperature, it was cooled at a cooling rate of 0.1 °C min−1 to room temperature. Apparatus. XPS measurements were executed on a Phoibos 100 spectrometer (SPECS, Germany) using a monochromatic Mg Kα radiation. In the deconvolution of HR spectra of Pd 3d, Pt 4f, and Te 3d regions, relative sensitivity factors were taken into account to achieve best fit curves, using specific area ratios of 3d5/2 and 3d3/2 as 3:2 and 4f7/2 and 4f5/2 as 4:3. Scanning electron micrographs were captured at an accelerating voltage of 5 kV on a field-emission scanning electron microscope (7600F; JEOL, Japan). EDXS analysis was performed using an EDXS analyzer (Oxford Instruments) at 15 kV. Voltammetry experiments were measured on an electrochemical analyzer (μAutolab III; Eco Chemie, The Netherlands) using the NOVA software program in version 1.8. Experiments for both noble metal tellurides (PdTe2 and PtTe2) were performed in a 3 mL electrolytic cell at 25 °C in a widely adopted three-electrode system. Pt, GC, and Ag/AgCl electrodes served respectively as the counter, working, and reference electrodes. X-ray diffraction (XRD) was conducted under Bragg−Brentano parafocusing geometry on a Bruker diffractometer (D8 Discoverer). A Cu Kα radiation was the X-ray source. The diffraction patterns were collected between 10 and 80° of 2θ. HighScore Plus 3.0e software was employed to evaluate the XRD data. HRTEM was performed on an EFTEM Jeol 2200 FS microscope (Jeol, Japan). 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. Procedures. All electrochemical procedures were performed in a manner similar to our previous works.12,14 For the inherent electrochemical study of the noble metal tellurides, cyclic voltammetry was performed in an electrolyte of PBS (50 mM, pH 7.0) at 100 mV s−1. Starting at 0 V, this potential is assumed to be devoid of redox reactions;48 the voltammetric scan progresses toward +1.8 V, followed by a backward sweep to −1.8 V, which then returns to 0 V for a complete anodic cycle. The cathodic cycle proceeds first in the reductive potentials toward −1.8 V before a reversal to +1.8 V and finally returning to the initial 0 V. In a glass vial with N,Ndimethylformamide, PdTe2 and PtTe2 were each in concentrations of 2 mg mL−1. To attain uniform dispersions for the first time, both samples were ultrasonicated for 1.5 h. Before any voltammetric measurement, an ultrasonication of 10 min was obligatory to maintain homogeneity of the dispersion. On a clean and polished GC electrode surface, 2.0 μL of the dispersion ink was drop casted and then left to dry under the lamp. Electrotreatment was performed by subjecting the PdTe2- or PtTe2modified GC surface to an oxidative or reductive potential for 5 min to acquire their respective oxidized or reduced states. The pretreatment was conducted in pH 7.0 PBS at a particular potential as ascertained by the redox signals characteristic of their intrinsic electroactivity. PdTe2 was subject to redox potentials at −1.2 and +0.8 V, whereas PtTe2 was treated at −1.6 and +0.8 V. The electrochemically treated materials were then examined for their electron transfer and electrocatalytic HER properties. During the study of HET at PdTe2 and PtTe2 surfaces, cyclic voltammetric scans were recorded at 100 mV s−1 in the presence of a potassium ferrocyanide redox probe (5 mM) prepared in a potassium chloride (0.1 M) electrolyte. We compute the k0obs values in accordance to the Nicholson method49 using the reported diffusion coefficient for the ferri/ferrocyanide redox probe, D = 7.26 × 10−6 cm2 s−1.50 Subsequent scans were used in the calculation of the k0obs values to eradicate any interference due to intrinsic electroactivity of the noble metal tellurides. In the assessment of the HER performance of PdTe2 and PtTe2, linear sweeps were performed at 2 mV s−1 in the H2SO4 electrolyte (0.5 M). Linear sweep voltammograms are plotted with respect to the

RHE using equation51 ERHE = EAg/AgCl + 0.059 × pH + E0Ag/AgCl to calibrate the measured Ag/AgCl potentials to RHE potentials.

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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.7b05083. Energy-dispersive X-ray maps; wide-scan X-ray photoelectron spectra; tabulated chalcogen-to-metal ratios; tabulated HET rate constants; linear sweep voltammograms (PDF)

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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 M.P. acknowledges funding from Ministry of Education (Singapore) from Tier 1 RGT1/13. X.C. acknowledges the support from the Nanyang President Graduate Scholarship. Z.S. and J.L. were supported by Czech Science Foundation (GACR No. 17-11456S) and supported by specific university research (MSMT No. 20-SVV/2017). This work was created with the financial support of the Neuron Foundation for science support.

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REFERENCES

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