Chemical Vapor Deposition Grown Large-Scale Atomically Thin

Jun 24, 2019 - (42) Relatively high HER performances were achieved with a Tafel slope of 140–41 mV/dec. However, the construction process of ...
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Chemical Vapor Deposition Grown LargeScale Atomically Thin Platinum Diselenide with Semimetal−Semiconductor Transition Jianping Shi,†,‡,§,∥ Yahuan Huan,†,‡ Min Hong,†,‡ Runzhang Xu,⊥ Pengfei Yang,†,‡ Zhepeng Zhang,†,‡ Xiaolong Zou,⊥ and Yanfeng Zhang*,†,‡ Downloaded via UNIV OF SOUTHERN INDIANA on July 21, 2019 at 17:11:49 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China Center for Nanochemistry (CNC), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China ∥ Collaborative Innovation Center of Quantum Matter, Peking University, Beijing 100871, China ⊥ Shenzhen Geim Graphene Center (SGC), Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen, Guangdong 518055, China ‡

S Supporting Information *

ABSTRACT: Among two-dimensional (2D) transition-metal dichalcogenides (TMDCs), platinum diselenide (PtSe2) stands in a distinct place due to its fancy transition from type-II Dirac semimetal to semiconductor with a thickness variation from bulk to monolayer (1 ML) and the related versatile applications especially in mid-infrared detectors. However, achieving atomically thin PtSe2 is still a challenging issue. Herein, we have designed a facile chemical vapor deposition (CVD) method to achieve the synthesis of atomically thin 1T-PtSe2 on an electrode material of Au foil. Thanks to the high crystalline quality, we have confirmed the complete transition from semimetal to semiconductor from trilayer (3 ML) to 1 ML 1T-PtSe2. More importantly, we have found that such atomically thin 1T-PtSe2 can serve as perfect electrocatalysts, featured with a record high hydrogen evolution reaction (HER) efficiency (comparable to traditional Pt catalyst). Our work is helpful toward the large-scale synthesis, exotic physical property exploration, and intriguing application development of atomically thin TMDCs. KEYWORDS: platinum diselenide, chemical vapor deposition, semimetal−semiconductor transition, scanning tunneling microscopy/spectroscopy, hydrogen evolution reaction Recently, both theoretical14 and experimental15 results have revealed that bulk 1T-PtSe2 is a type-II Dirac semimetal, owning a type Lorentz-violating Dirac Fermion and topological surface state. For bulk 1T-PtSe2, various physical phenomena and applications have been predicted, such as the generation of valley polarized current and quantum valley Hall effect and the potential application in topological quantum computing.16 Intriguingly, recent density functional theory (DFT) calculations have predicted that the 1 ML 1T-PtSe2 is a semiconductor with an indirect bandgap of ∼1.20 eV,16 and subsequent angle-resolved photoemission spectroscopy (ARPES)16 and electrical transport17 results verified this

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wo-dimensional (2D) transition-metal dichalcogenides (TMDCs) have fueled vigorous scientific inquiry due to their exceptional physical and chemical properties that are distinct from their bulk analogues.1,2 All of this makes them perfect candidates for exploring both fundamental sciences and technological applications.3−6 Notably, the bandgap of 2D TMDCs is a crucial parameter that determines the optical and electronical properties as well as the on/off ratio of the related field-effect transistors.7,8 In view of their ultrathin thicknesses, the bandgap modulation can be implemented through strain,9 pressure,10 chemical doping,11 and applied electrical field.12 Encouragingly, the bandgap of TMDCs can also be tuned by changing their layer thicknesses, for instance, a tunable bandgap (from ∼1.30 eV indirect bandgap to ∼1.80 eV direct bandgap) has been reported from bulk to monolayer (1 ML) MoS2.13 © XXXX American Chemical Society

Received: June 3, 2019 Accepted: June 24, 2019 Published: June 24, 2019 A

DOI: 10.1021/acsnano.9b04312 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Domain size and thickness tunable growth of atomically thin 1T-PtSe2 on Au foils by an APCVD route. (a) Schematic of the APCVD growth process. (b) XPS signals regarding Pt and Se elements in the as-grown sample. (c) XRD pattern of transferred PtSe2 on SiO2/Si showing its 1T phase feature. Data from JCPDS card no. 18-0970 are shown for comparison. (d−f) SEM images of as-grown 1TPtSe2 on Au foils showing variable domain sizes (5−150 μm), as synthesized at Tp ∼ 800 °C for ∼5, ∼18, and ∼20 min, respectively. (g) Plot of the domain sizes of 1T-PtSe2 flakes as a function of growth time. (h−j) AFM images and corresponding height profiles of transferred 1TPtSe2 flakes on SiO2/Si showing variable thicknesses from 1 to 3 ML, as synthesized at Tp ∼800, ∼820, and ∼850 °C for ∼10 min, respectively. (k) Invariable Raman spectra of freshly synthesized and 3 months aged 1 ML PtSe2 on Au foils (in air) showing its robust stability.

temperature (∼300 °C) by using H2PtCl6 and Se as the precursors with domain sizes 100 were also frequently observed, as shown in Figure S8. To evaluate the stability of CVD-derived 1T-PtSe2, Raman spectra and SEM images of freshly prepared 1, 2, and 3 ML 1T-PtSe2 flakes on Au foils and those obtained after 3 months aging under ambient conditions were deliberately collected (Figure 1k and Figure S9). The nearly unchanged peak position/intensity collectively addresses the excellent stability

RESULTS AND DISCUSSION A facile APCVD route was designed for directly synthesizing atomically thin 1T-PtSe2 with solid PtCl2 and Se as precursors (the schematic view is shown in Figure 1a). An Au foil was selected as the growth substrate in view of its chemical inertness toward Se precursors, surface catalytic activity for growing TMDCs, compatibility with STM/STS characterization, and direct HER application. To remove the impurities and reconstruct the single crystalline surfaces, the Au foil substrates were first annealed at ∼980 °C for 5 h under ambient conditions. In order to determine the chemical composition of as-grown samples, X-ray photoemission spectroscopy (XPS) measurements were first performed, with the results shown in Figure 1b and Figure S1. The absence of O signal in the XPS spectrum suggests the high stability of the CVD-derived PtSe2, even after a long time maintenance in the atmospheric condition. The Pt 4f7/2 (73.4 eV) and 4f5/2 (76.7 eV) peaks are attributed to Pt4+, while the Se 3d5/2 (54.7 eV) and 3d7/2 (55.5 eV) peaks are assigned to Se2− (Figure 1b), consistent with the XPS data of 1 ML 1T-PtSe2 obtained by the direct selenization of Pt foil.16 The Pt and Se atomic ratio C

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Figure 2. On-site STM/STS characterizations of the atomic structures and electronic properties of 1 ML 1T-PtSe2 on Au foils. (a) Largescale STM image of 1 ML 1T-PtSe2 on Au foils (VT = 1.00 V, IT = 0.20 nA, T = 78 K). (b) Corresponding atomic-resolution STM image (0.75 V, 0.23 nA, 78 K). (c) Four typical STS spectra (1.20 V, 0.20 pA, 78 K, Vrms = 10 mV, f = 932 Hz) recorded on the 1 ML 1T-PtSe2/Au foil. The STS spectra are plotted in both linear scale (upper panel) and logarithmic scale (lower panel), showing almost no bandgap around the Fermi energy. (d) Calculated DOS of freestanding 1 ML 1T-PtSe2 and 1 ML 1T-PtSe2/Au(111), respectively. The calculated 1 ML 1TPtSe2/Au(111) system shows almost no bandgap. (e) Atomic-resolution STM image (0.90 V, 0.20 nA, 78 K) of 1 ML 1T-PtSe2 on Au foils showing both atomic-scale and moiré-scale patterns. The unit cell of the moiré pattern is highlighted by a blue rhombus. Inset is the corresponding FFT pattern. (f) Simulation of the moiré pattern arising from the rotation of 1T-PtSe2 lattice with regard to that of Au(111) and the lattice mismatch effect.

decreased due to the strong interaction between 1T-PtSe2 and Au substrate. Furthermore, a large-scale atomic-resolution STM image of 1 ML 1T-PtSe2 is shown in Figure 2e. A moiré structure with a period of ∼1.10 nm is also achieved, as marked by a blue rhombus. Such a superstructure is derived from the lattice mismatch and the relative rotation between 1 ML 1T-PtSe2 and Au(111) (a1T‑PtSe2 = 0.37 nm and aAu(111) = 0.29 nm). Notably, the relative rotational angle (φ) between 1T-PtSe2 and Au(111) can be calculated using the following equation:45

of CVD-derived 1T-PtSe2. In this regard, ultrastable, largedomain, and thickness-tunable 1T-PtSe2 have been successfully synthesized on Au foils by a facile APCVD route, which possibly follows a surface-catalyzed growth mechanism in the 1 ML regime, along with a metal-precursor segregation-assisted growth mechanism for more layers, as similarly proposed in the CVD-grown few-layer MoS243 and graphene44 on Au films and Ni foils, respectively. To explore the crystalline quality and the band structure of CVD-derived 1 ML 1T-PtSe2, the as-grown samples were then characterized by on-site STM/STS. A large-scale STM image shows the covering of 1 ML 1T-PtSe2 on Au terraces (Figure 2a). A typical atomic-resolution STM image of such a region reveals a honeycomb lattice with an interatomic distance ∼0.35 nm (Figure 2b), similar to the interatomic spacing of Se atoms in the (0001) basal plane of bulk PtSe2.16 To determine the band structure of CVD-synthesized 1 ML 1T-PtSe2, lowtemperature STS were further performed on the as-grown sample, with the data shown in both linear scale (upper panel) and logarithmic scale (lower panel) in Figure 2c. The corresponding atomic-resolution STM image is also shown in Figure S10. Interestingly, the density of states (DOS) within the −0.8 to +0.8 V bias range show nonzero values, in line with almost no bandgap for 1 ML 1T-PtSe2 on Au(111). To understand this, DFT calculations were finished to achieve the electronic property of freestanding 1 ML 1T-PtSe2 and 1 ML 1T-PtSe2/Au(111) (Figure 2d), respectively. The corresponding bandgap values are deduced as ∼1.24 and ∼0 eV for the two cases. In short, the bandgap of 1 ML 1T-PtSe2 is highly

cos φ = 1 −

(1 + δ)2 a 2 − λ 2δ 2 2λ 2(1 + δ)

where δ is the lattice mismatch between 1T-PtSe2 and Au(111), λ is the period of a moiré pattern, and a is the lattice constant of 1T-PtSe2. From the above equation, the φ for the moiré pattern is calculated as ∼11°. To confirm this, the simulated pattern generated from 1 ML 1T-PtSe2 stacking on Au(111) is depicted in Figure 2f with a relative rotation angle ∼11°. Interestingly, the hexagonal moiré fringe fits well with that of the STM result. The formation of such moiré superstructure convinces, to some extent, the ultraclean interface between 1 ML 1T-PtSe2 and Au substrate. According to DFT calculations, the bandgap of 1T-PtSe2 can be modulated from semimetal to semiconductor by decreasing the layer thickness from bulk to 1 ML.16 However, the corresponding experimental result is still absent due to the huge difficulty in preparing 2D 1T-PtSe2 down to the 1 ML limit. In this research, the transferred 1T-PtSe2 flakes on SiO2/ D

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Figure 3. Bandgap tunability of 1T-PtSe2 from bulk to 1 ML achieved by various spectroscopy methods. (a−d) OM images of transferred 1 ML, 2 ML, 3 ML, and bulk 1T-PtSe2 on SiO2/Si, respectively. (e) Thickness-dependent Raman spectra of 1 ML, 2 ML, 3 ML, and bulk 1TPtSe2 on SiO2/Si. (f) Corresponding intensity ratio and frequency difference between A1g and Eg modes as a function of thickness, respectively. (g) The Tauc plots of 1 ML, 2 ML, 3 ML, and bulk 1T-PtSe2 transferred on quartz revealing the semimetal−semiconductor transition with decreasing layer thickness. (h) Secondary electron cutoff regions of 1 ML, 2 ML, 3 ML, and bulk 1T-PtSe2 on Si showing the energy differences between the Fermi level (EF) and the EVBM, respectively. (i) Thickness-dependent CBM, EF, and VBM of 1T-PtSe2.

Figure 4. Phase structure and defect type identification of 1 ML 1T-PtSe2 by TEM and STEM. (a) Low-magnification TEM image of a transferred 1 ML 1T-PtSe2 flake. (b) Magnified TEM image captured from the flake edge showing its 1 ML feature. (c) Corresponding SAED pattern captured from (a) (within a 500 × 500 nm2 area) showing its single crystalline feature. (d) Atomic-resolution TEM image of the flake. (e) Low-magnification STEM image of a transferred 1 ML 1T-PtSe2 flake. (f) Atomic-resolution HAADF-STEM image of the flake addressing its high crystalline quality. Inset is the corresponding FFT pattern. (g) Atomic-resolution HAADF-STEM image on the flake edge (with the overlaid structural model revealing the edge type). (h) Zoom-in atomic-resolution HAADF-STEM image of a 1 ML 1T-PtSe2 flake. The corresponding intensity profile along the white rectangle direction confirms its 1T phase structure. Representative atomic-resolution HAADF-STEM images were also captured to present the possible defect types, including Se vacancy (i), Se antisite at Pt site (j), Pt antisite at Se site (k), and the coexistence of Se vacancy and Se antisite at Pt site (l). Insets are corresponding intensity line profiles along the green rectangles. Bottom panels show the corresponding defect configurations.

A1g modes, corresponding to in-plane and out-of-plane vibrations, respectively. Interestingly, a blue shift of the Eg mode from 175.3 to 178.7 cm−1 is obviously observed with decreasing the layer thickness from bulk to 1 ML. However, the A1g mode is pinned at 205.7 cm−1, regardless of the

Si and quartz are then utilized to explore the bandgap variations, with the corresponding optical microscopy (OM) images shown in Figure 3a−d. Notably, four representative Raman spectra are collected from 1 ML, 2 ML, 3 ML, and bulk 1T-PtSe2 (Figure 3e). The two typical peaks arise from Eg and E

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Figure 5. Ultrahigh electrocatalytic HER performance of CVD-synthesized 1 ML 1T-PtSe2 on Au foils. (a) Schematic illustration of the HER process. (b) Theoretical calculations of the hydrogen adsorption energies at the 50-edge and basal-plane of 1 ML 1T-PtSe2, respectively. (c) ΔGH* diagram of different H adsorption states on 1T-PtSe2 and 2H-MoS2. (d, e) Coverage-dependent polarization curves and corresponding Tafel plots of 1 ML 1T-PtSe2, with the values from Au foil and commercial Pt electrode as references. (f) Comparison of the HER performances of CVD-derived 1 ML 1T-PtSe2 with other MX2-based catalysts.5,39,40 (g) Statistical relation of exchange current density with the edge length of 1 ML 1T-PtSe2. (h) Durability test for the 1 ML 1T-PtSe2/Au catalyst (∼70% coverage).

reconfirm the large-area thickness uniformity and more significantly the semimetal to semiconductor transition from 3 to 1 ML on the CVD-derived 1T-PtSe2 layers. A low-magnification transmission electron microscopy (TEM) image in Figure 4a shows a highly transparent property of a 1 ML 1T-PtSe2 flake. The high-resolution TEM (HRTEM) image captured from the folded edge presents a single dark line to address its 1 ML feature (Figure 4b). Moreover, the representative selected area electron diffraction (SAED) pattern in Figure 4c reveals only one set of hexagonally arranged diffraction spots, highly suggesting its single crystalline feature. Furthermore, the atomic-resolution TEM image in Figure 4d presents a honeycomb structure with an interatomic distance of ∼0.37 nm (Figure S13), in good agreement with that of 1 ML 1T-PtSe2.16 All of these data indicate the achievement of high-quality single crystalline 1TPtSe2. Furthermore, the energy-dispersive X-ray spectroscopy (EDS) elemental mapping confirms the chemical fingerprints and their distributions (Figure S14). To further determine the phase structure of CVD-derived PtSe2 and distinguish the occupancy and coordination states of Pt and Se atoms, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) characterizations were then pursued on the transferred PtSe2 layer. A low-magnification STEM image for a 1 ML PtSe2 flake is presented in Figure 4e. Notably, some wrinkles are clearly observed (indicated by the white arrow) which is probably derived from the transfer process. The atomic-resolution STEM images and the corresponding FFT pattern in Figure 4f show its high crystalline quality. From the atomic-resolution STEM image focused on the flake edge (Figure 4g), the edge type is clearly defined. In the zoom-in atomic-resolution STEM image, the corresponding intensity line profile along the Pt− Se−Se direction clearly reveals the 1T phase structure of 1 ML PtSe2 (Figure 4h). The SAED patterns of 2 ML and 3 ML

thickness variations. Such a blue shift of the Eg mode is attributed to the stacking-induced structural change and the long-range Coulombic interlayer interaction, as previously demonstrated in exfoliated few-layer MoS2.13 Additionally, the peak intensity ratio of A1g/Eg mode is decreased from bulk to 1 ML accordingly, possibly due to the suppressed van der Waals interlayer interaction with reducing layer thickness. For more details, the thickness-dependent intensity ratio and the frequency difference between A 1g and E g modes are summarized in Figure 3f, which can serve as straightforward indexes to determine the thickness of 2D 1T-PtSe2. To determine the thickness-dependent bandgap evolution, the UV−vis-NIR and ultraviolet photoemission spectroscopy (UPS) measurements were then conducted on the transferred samples. According to the Tauc plots of transferred bulk, 3 ML, 2 ML, and 1 ML 1T-PtSe2 on quartz, the bandgap values are calculated to be ∼0, ∼0, ∼0.23, and ∼1.28 eV (Figure 3g), respectively, highly suggestive of the complete transition from semimetal to semiconductor at the 2D limit. Furthermore, DFT calculations were also performed to achieve the bandgap of 1 ML, 2 ML, and 3 ML PtSe2 (Figure S11), showing variable values of ∼1.24, ∼0.15 and ∼0 eV, respectively. Notably, the bandgap values achieved from the UV−vis-NIR measurements are consistent with the DFT results, which suggests the relatively high crystalline quality and thickness uniformity of the CVD-synthesized atomically thin 1T-PtSe2. To uncover the thickness-dependent energy band structure, corresponding UPS analyses were then performed to reveal the work function values as ∼4.63, ∼4.64, ∼4.67, and ∼4.85 eV, respectively (Figure S12). Additionally, the differences between Fermi level (EF) and valence band maximum (VBM) are estimated to be ∼0, ∼0, ∼0.08, and ∼0.48 eV, respectively (Figure 3h). By adding VBM with bandgap, the conduction band minimum (CBM) of 1T-PtSe2 can also be obtained (Figure 3i). Briefly, the multiscale characterizations F

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By applying an extrapolation method to the Tafel plots, the exchange current densities (j0) were achieved to show values of 115−215 μA/cm2 in such 1 ML 1T-PtSe2, which surpass other MX2 catalysts5,39,40,48−53 (Figure S19). A comprehensive comparison of the HER performances of 1 ML 1T-PtSe2 and MX2-based catalysts is listed in Figure 5f and Table S1. Moreover, according to the statistical analysis (Figure 5g), the j0 value is correlated linearly with the edge length per area of 1T-PtSe2, accounting for the linear in-crease of the HER activities with increasing the edge sites. The excellent electrocatalytic activities are further confirmed by the electrochemical impedance spectra (EIS) characterizations (Figure S20), which show ultralow charge-transfer resistances (3.9−4.4 Ω), and thus ultrafast charge transfer at the interface of 1 ML 1T-PtSe2/Au. Meanwhile, the turn over frequency (TOF) of 1T-PtSe2/Au at 0 V is calculated as 0.41−0.84 s−1. This value is very close to that of Pt (∼0.9 s−1) and much larger than that of MoS2/Au(111) (∼0.02 s−1),5 strained MoS2 (0.05−0.16 s−1,)39 and 1T-WS2 (∼0.043 s−1).49 Moreover, a negligible difference in the polarization curve is achieved between initial and after 5000 cycles for a ∼ 70% coverage sample (Figure 5h), highly suggesting its robust electrocatalytic durability. Furthermore, the XPS data of 1 ML 1T-PtSe2 samples before and after HER measurements were collected (Figure S21). The negligible variation for Pt 4f and Se 3d signals again indicates the robust stability of 1 ML 1T-PtSe2 throughout the electrocatalytic process. In addition, considering the strong interfacial interaction of 1 ML 1T-PtSe2/Au and the sharply reduced bandgap to zero (in line with dramatically enhanced DOS near the Fermi level), the interfacial electron transfer is greatly accelerated. Such factors should induce the enhanced HER efficiency in 1 ML 1T-PtSe2/Au concurrently. The effect of 1T-PtSe2 thickness on the HER performance was also systematically explored based on DFT calculations and experimental results (Figure S22). Specifically, a negligible difference in the electrocatalytic activity is observed for 1TPtSe2 samples with different thicknesses (from 3 to 1 ML), as in sharp contrast with that of CVD-synthesized few-layer semiconducting MoS 2 , where the catalytic activity is dramatically suppressed with increasing thickness.54 Hereby, the most exciting result from this research is that few-layer 1TPtSe2 assembled on a well selected electrode can manifest comparable HER performance with that of thicker flakes. The loading of the catalysts can thus be well optimized toward lowcost, high-efficiency, and large-scale application explorations.

samples are supplied in Figures S15 and S16. An A-A stacking configuration is determined for 2 ML and 3 ML 1T-PtSe2, that is, upper-layer Pt atoms positioned right on the Pt atoms in the bottom layer, as presented by the atomic-resolution STEM images. The point defects in 2D TMDCs (e.g., vacancies, antisites, and adatoms, etc.) were ubiquitous and usually induced lower carrier mobility and degraded mechanical property in 2D materials.46 They were also used as the sources of single quantum emitters.47 For the samples prepared under the poorly optimized growth conditions, various defect types can be noticed on the CVD-derived 1 ML samples. In the atomicresolution STEM images (Figure 4i−l), four types of point defects including Se vacancy, Se antisite at Pt site, Pt antisite at Se site, and coexisting Se vacancy and Se antisite at Pt site are unambiguously observed. Notably, it is the first report about the antisite defect in 1T-PtSe2. Some physical phenomena are likely to be produced (e.g., magnetism and spin polarization) nearby these defects, as previously demonstrated in 1 ML MoS2.46 More intensive exploration regarding the effects of defects on the electronic and optical properties of 2D TMDCs as well as their HER performances should be very intriguing issues. Notably, the defects density of CVD-derived 1 ML 1TPtSe2 was calculated as ∼1.5 × 1011 cm−2, comparable with the mechanically exfoliated samples.21 Such low defects concentration suggested the high crystalline quality of CVD-derived 1T-PtSe2. In this work, the as-grown 1 ML 1T-PtSe2/Au samples are directly used as electrocatalysts, as schematically illustrated in Figure 5a. Before this, DFT calculations were performed on 1 ML 1T-PtSe2 to identify the catalytically active sites (Figure 5b,c and Figure S17). As a result, the Gibbs free energies (ΔGH*) of H adsorption at the 50-edge, 100-edge, and basalplane of 1 ML 1T-PtSe2 are calculated as 0.07, 0.50, and 1.07 eV, respectively (Figure 5c). The relatively low ΔGH* values of H adsorption at the edges indicate that the catalytically active sites mainly sit at the domain edges of 1T-PtSe2. Additionally, the ΔGH* values for H adsorption at the domain edge and basal-plane of 1T-PtSe2 are much lower than those of 2HMoS2 (0.09 eV for the Mo edge and 2.07 eV for the basalplane), respectively, highly suggestive the robust catalytic activity of 1T-PtSe2 even at a 1 ML thickness. A series of 1 ML 1T-PtSe2 samples with different coverages (from ∼20 to ∼90%) were then selected as the electrocatalysts, with their morphologies presented in Figure S18. The corresponding polarization curves are displayed in Figure 5d, with curves from Au foil and commercial Pt as references. Interestingly, the overpotentials at the cathodic current density of 10 mA/cm2 increase from ∼210, ∼223, ∼234, ∼245, ∼256 to ∼261 mV, with decreasing the coverage from ∼90, ∼70, ∼60, ∼50, ∼30 to ∼20%, tentatively indicating the decreased active sites at the edges. In addition, the linear portions of Tafel plots fitted to the Tafel equation (Figure 5e), corresponding to Tafel slopes, are calculated as 33−38, 31, and 110 mV/dec for 1 ML 1T-PtSe2/Au, Pt, and Au foil, respectively. Intriguingly, all of the Tafel slopes (33−38 mV/ dec) for 1 ML 1T-PtSe2/Au catalysts approach that of Pt and exceed all the reported MX2 catalysts.5,39,40,48−53 This result highly indicates the excellent HER performance of CVDderived 1 ML 1T-PtSe2 on Au foils. The Tafel slope values also address the Volmer−Tafel mechanism for the 1T-PtSe2 catalyst in HER.

CONCLUSIONS We have accomplished the controllable growth of large-scale, thickness-tunable, ultrastable 1T-PtSe2 on Au foils via a facile APCVD method. The high-quality 2D 1T-PtSe2 samples ranging from 3 to 1 ML have proven to be attractive platforms for exploring fundamental physical issues, for example, a complete transition from semimetal to semiconductor at the 2D limit. A perfect paradigm for addressing the quantum confinement effect in tuning the conductivity of a 2D layered material is thus definitely demonstrated. More importantly, we have uncovered that the CVD-derived 1T-PtSe2 on Au foils can serve as an ideal platform for exploring the electrocatalytic property of metallic TMDCs, affording so far the most decent HER performance in a 1 ML level. We believe that these findings will promote the fundamental property investigations related to the quantum confinement effect, the batch production of 2D metallic TMDCs, as well as for the G

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ASSOCIATED CONTENT

exploration of highly efficient catalysts for energy-related applications.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b04312.

MATERIALS AND METHODS Material Synthesis. The PtCl2 (Alfa Aesar, purity 99.5%) and Se (Alfa Aesar, purity 99.5%) were used as the precursors, with the Au foil (thickness ∼25 and ∼50 μm, Alfa Aesar, purity 99.99%) as the growth substrate. The 1T-PtSe2 samples were synthesized in a threezone furnace (Lindberg/Blue M HTF55347c) equipped with a 1 in. diameter quartz tube. The temperatures of Au foil, PtCl2, and Se powders were set to ∼850, ∼800−850, and ∼450 °C, respectively. Ar (85 sccm) and H2 (15 sccm) were used as the carrier gases. After the growth period, the furnace was opened, and the samples were cooled to room temperature under a flow of mixed Ar/H2 (85/15 sccm). For the transfer process, the 1T-PtSe2 layers synthesized on Au foils were transferred onto arbitrary substrates by a wet-chemical etching method, as had been reported for CVD-synthesized MoS 2 heterostructures on Au foils.27 In this process, the PtSe2/Au samples were first spin coated with poly(methyl methacrylate) (PMMA) and then baked at 185 °C for 10 min. The samples were then soaked in Au etchant (KI + I2 + H2O with the mass ratio of 8:2:80) for the removal of Au at 65 °C for 3 h. Finally, the PMMA-supported PtSe2 was rinsed with deionized (DI) water for five times, and a fresh SiO2/Si or other substrates were then used to “fish out” the PMMA-capped PtSe2 followed by rinsing with acetone for removing the PMMA. Material Characterization. All of the samples were characterized by OM (Olympus BX51), SEM (Hitachi S-4800, 2 kV), XPS/UPS (Kratos Analytical AXIS-Ultra fitted with a monochromatic Al Kα Xray source), XRD (Shimadzu Thin Film, using Cu Kα radiation in the 2θ range of 10−90°), Raman spectroscopy (Renishaw, Invia Reflex), TEM (JEOL JEM-2100F LaB6; acceleration voltage, 200 kV), and AFM (Dimension Icon, Bruker). The absorption spectra of transferred 1T-PtSe2 with different thicknesses on quartz were measured by a UV−vis-NIR spectrometer. High-resolution STEMHAADF images were obtained on an aberration-corrected TEM FEI Titan Cubed Themis G2 300 equipped with a cold field emission gun with an acceleration voltage of 60 kV. The Unisoku STM/STS systems were utilized for the atomic-scale structural characterization under a base pressure better than 10−10 mbar. The STS spectra were acquired by recording the output of a lock-in system with the manually disabled feedback loop. A modulation signal of 10 mV at 932 Hz was selected under the specific tunneling conditions. Electrochemical Measurement. All of the electrochemical measurements were performed in a three-electrode system on a CHI 760E electrochemical workstation (CH Instruments), using 1TPtSe2/Au foil as the working electrode, a carbon rod as the counter electrode, and a saturated Ag/AgCl as the reference electrode. All the potentials were calibrated by a reversible hydrogen electrode (RHE). Linear sweep voltammetry with a scan rate of ∼5 mV s−1, from +0.10 to −0.70 V vs RHE, was conducted in 0.5 M H2SO4 (sparged with N2, purity ∼99.999%). The Nyquist plots were obtained with frequencies ranging from 100 kHz to 0.1 Hz at the overpotential of 10 mV. The impedance data were fitted to a simplified Randles circuit to extract the series and charge-transfer resistances. Theoretical Calculation. All DFT calculations were carried out by Vienna ab initio simulation package (VASP)55 adopting the Perdew−Burke−Ernzerhof functional and projector-augmented wave potentials.56 A vacuum layer thicker than 10 Å was chosen to keep the interactions from the nearest neighbors negligible. A fully structural optimization was performed with the convergence criteria for force and energy set as 0.01 eV/Å and 10−5 eV/Å, respectively. The solvation effect on H, PtSe2 surfaces, and HER reaction barriers was calculated using VASPsol. The thermodynamic free energies were calculated as G = E0 + EZPE − TS, where E0, EZPE, and S represent DFT ground-state energy, zero-point vibrational energy, and entropy, respectively. Based on the computational hydrogen electrode model, the energy of a proton, G(H+), is taken as half of that of a hydrogen molecule, G(H2)/2, under the standard hydrogen electrode conditions.

Further experimental and theoretical details, including XPS, XRD, OM, SEM, AFM, UPS, STM, HRTEM, EDS, STEM, DFT, and HER measurement results (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Runzhang Xu: 0000-0001-9880-3837 Zhepeng Zhang: 0000-0002-9870-0720 Xiaolong Zou: 0000-0002-3987-6865 Yanfeng Zhang: 0000-0003-1319-3270 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant nos. 51861135201, 51472008, and 51290272), the China Postdoctoral Science Foundation (grant nos. 2018M631252 and 2019T120016), and the Beijing Natural Science Foundation (grant no. 2192021). REFERENCES (1) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699− 712. (2) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (3) Kang, K.; Xie, S.; Huang, L. J.; Han, Y. M.; Huang, P. Y.; Mak, K. F.; Kim, C.-J.; Muller, D.; Park, J. High-Mobility Three-Atom-Thick Semiconducting Films with Wafer-Scale Homogeneity. Nature 2015, 520, 656−660. (4) Yu, W. J.; Li, Z.; Zhou, H. L.; Chen, Y.; Wang, Y.; Huang, Y.; Duan, X. F. Vertically Stacked Multi-Heterostructures of Layered Materials for Logic Transistors and Complementary Inverters. Nat. Mater. 2013, 12, 246−252. (5) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100−102. (6) Acerce, M.; Akdoğ an, E. K.; Chhowalla, M. Metallic Molybdenum Disulfide Nanosheet-Based Electrochemical Actuators. Nature 2017, 549, 370−373. (7) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (8) Radisavljevic, B.; Kis, A. Mobility Engineering and a MetalInsulator Transition in Monolayer MoS2. Nat. Mater. 2013, 12, 815− 820. (9) Zhang, Y.; Chang, T.-R.; Zhou, B.; Cui, Y.-T.; Yan, H.; Liu, Z. K.; Schmitt, F.; Lee, J.; Moore, R.; Chen, Y. L.; Lin, H.; Jeng, H.-T.; Mo, S.-K.; Hussain, Z.; Bansil, A.; Shen, Z.-X. Direct Observation of the Transition from Indirect to Direct Bandgap in Atomically Thin Epitaxial MoSe2. Nat. Nanotechnol. 2014, 9, 111−115. (10) Nayak, A. P.; Bhattacharyya, S.; Zhu, J.; Liu, J.; Wu, X.; Pandey, T.; Jin, C. Q.; Singh, A. K.; Akinwande, D.; Lin, J.-F. Pressure-Induced H

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