1 Introduction

Owing to the special and outstanding material properties (e.g., tunable band gap [1], high carrier mobility [2], excellent mechanic [3], and tunable carrier density and polarity [4]), two-dimensional (2D) materials have received significant interest in recent decades. Preparation of large-scale and high-quality two-dimensional crystals is foundation for its further application in next-generation electronic devices [5, 6], while transition metal dichalcogenide (TMD) materials are also shown for excellent catalytic activity for many important electrochemical reactions due to unsaturated bond at the edge of crystals, such as hydrogen evolution reaction (HER). Selenide platinum (PtSe2) is a typical 1T phase hexagonal lattice structure with P-3m1 space group [7]. Owing to layer-dependent band gap evolution, the band gap of PtSe2 will fall down to zero when the layer number is more than three layers [8, 9], which is an alternative material for infrared photodetection [8], large-area thin-film devices, mid-infrared polarizers and polarization sensors. Several works have reported the chemical vapor deposition growth of PtSe2 crystals on various substrates, such as molecular beam epitaxy [10, 11], chemical vapor transport [12, 13] and chemical vapor deposition [14,15,16]. Most of the feasible approach is thermally assisted selenation of Pt films to obtain large-area PtSe2 film [17,18,19,20,21,22,23,24].

Nanowall, a vertically distributed nano-structure, has shown high-aspect ratio with extensive exposure of edge sites. Owing to the unsaturated bond at edge of monolayer structure, nanowall has shown huge advantage in HER, hydrogen storage devices, supercapacitors hydrodesulfurization catalysis, and biological applications. For now, the carbon nanowalls were prepared by microwave plasma chemical vapor deposition (MPCVD) for its applications in field emission displays and energy storage [25], the MoS2@rGO nanowall has been synthesized as anode materials [26], and vertical and uniform ReS2 nanowalls were directly grown on a Si/SiO2 substrate by chemical vapor deposition (CVD) with prominent PHE efficiency under visible light irradiation [27]. However, the mechanism and controlled selenation of the ultra-chemical stable and high-melt point of Pt atoms to obtain PtSe2 are still unclear.

In this work, we reported the thickness and morphology-controlled growth of continuous large-area PtSe2 nanowall films by thermally assisted selenation of pre-deposited metal Pt films. The uniformity, composition and morphology of PtSe2 nanowall films are characterized by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and high-resolution transmission electron microscopy (HRTEM). The dynamic and thermodynamics of growth process are investigated by tuning growth temperature, heating rate, and flow of carrier gas. The PtSe2 nanowall films are transferred on Au film as the working electrodes, and the HER performances of PtSe2 nanowall films are checked with a three-electrode system. The results show low over-potential of 0.3 V at –10 mA·cm−2, and small Tafel slope of 52 mV·dec−1 by the well-ordered PtSe2 nanowall films, suggesting the excellent electrocatalytic properties of PtSe2 film for enhanced HER.

2 Experimental

2.1 Synthesis of PtSe2 film

The PtSe2 films were synthesized in a homemade furnace by a CVD method. Pt films were firstly sputtered on the SiO2/Si substrate by molecular beam epitaxy (MBE). The substrate was placed in the ceramic boat with Pt layer faced up. Se powders (Alfa Aesar, 99.5%, 100 mg) were as sources placed in the boat. The distance between Se and Pt was about 10 cm. Mixed hydrogen (H2, 1 ml·min−1) and argon (Ar, 30 ml·min−1) were both used as carrier gases. The samples were synthesized in a quartz tube at a temperature ranging from 400 to 800 °C with a growth time of 1 h. Finally, the furnace cooled down to room temperature naturally.

2.2 Crystal structure characterization

Optical images were taken on an Olympus BX51 microscope. AFM characterization was carried out with Bruker Multimode 8 system. The crystal structures images were acquired by transmission electron microscopy (TEM, FEI Tecnai G2 F30). The chemical state and composition studies were performed using X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD) with a basic chamber pressure of 1 × 10–8 Pa and with an Al anode as the X-ray source. The Raman spectrum was collected using a Bruker RFS 100/S spectrometer (laser wavelength of 532 nm).

2.3 Electrochemical measurements

All measurements were performed in a homemade three-chamber electrochemical cell using a CHI660E electrochemical workstation in 0.5 mol·L−1 H2SO4 solution. The PtSe2 films with different growth temperatures and thicknesses were transferred onto Au films, which served as the working electrodes with a platinum (Pt) counter electrode and an Ag/AgCl (saturated KCl) reference electrode (Hach). The electrochemically active surface area (ECSA) was determined using CV by sweeping the potential between 0.12 and 0.32 V (vs. RHE) at different scan rates is as follows: 10, 15, 20, 25, 30, 35, 40, 45 and 50 mV·s−1. The potentials in this work were referenced to RHE after iR correction using the following equation: ERHE = EAg/AgCl + 0.059 pH + EoAg/AgCl, where EoAg/AgCl is the potential of the Ag/AgCl electrode at 25 °C.

3 Results and discussion

A homemade one-inch tube furnace is used to provide a confined space for increasing the concentration of gaseous Se atoms to accelerate the selenation reaction in Fig. 1a. By controlling growth temperature and thickness of Pt film, a series of continuous PtSe2 nanowall films are obtained. The photograph of as-grown large-area PtSe2 nanowall films on SiO2/Si substrate with a size of > 4 cm2 is shown in Fig. 1b, which is prepared by selenation of Pt film with the thickness of about 1 nm. Figure 1c shows the transferred PtSe2 nanowall film on SiO2/Si substrate with a continuous large-area film. The AFM image is shown in Fig. 1d with a thickness of 2.9 nm, the zoom in image is also shown in Fig. S1, demonstrating the nanowall structure of as-obtained PtSe2 film. The Raman spectrum is shown in Fig. 1e, and the peaks are located at 178 and 210 cm−1, assigned to the Eg and A1g Raman vibration modes of PtSe2 by mechanical exfoliation. XPS spectra show only one couple peak of Pt and Se with the position at around 73.38 and 76.73 eV, and 54.83 and 55.66 eV, which respectively belong to Pt 4f7/2 and Pt 4f5/2, and Se 3d5/2 and Se 3d3/2, as shown in Fig. 1f, g [24]. The results demonstrated that the valence states of Se and Pt are assigned to –2 and + 4, and the chemical reaction of selenation process is completely finished.

Fig. 1
figure 1

Controlled growth of PtSe2 films: a schematic diagram of modified chemical vapor deposition method to control growth of PtSe2 films; b photograph and c optical microscopy image of as-obtained PtSe2 films on SiO2/Si substrates at growth temperature of 650 °C and heating rate of 4 °C·min−1; d AFM image of as-grown PtSe2 films; e Raman spectrum of as-obtained PtSe2 films; XPS spectra of f Se and g Pt for as-grown PtSe2 films

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The dynamic and thermodynamics of growth process are investigated by tuning growth temperature, heating rate, and thickness of Pt film. As shown in Fig. 2a, the AFM image is of PtSe2 nanowall films prepared at 400 °C, and a layer of nanoparticles is uniformly distributed on the surface of PtSe2 films. Those particles may be formed due to harder surface migration of PtSe2 clusters at low growth temperature. In Fig. 2b, the continuous film of PtSe2 crystals with high surface roughness is formed at growth temperature of 500 °C but without the nanoparticles. The results demonstrated that the vapor concentration of selenium is excess for selenation reaction, and Pt atoms are fully reacted finally. With the growth temperature up to 600 °C, as shown in Fig. 2c, the Pt clusters are well migrated at the surface; with an increase in the growth temperature, the surface roughness is decreased, and continuous films are formed finally. In Fig. 2d, continuous flat PtSe2 films are obtained with growth temperature of 650 °C, and the morphology has been significantly improved with better crystallinity, higher quality and more uniformity. As the growth temperature is higher than 700 °C, the surface migration process of Pt clusters is further increased, and some Pt atoms will agglomerate into islands and some of as-deposited Pt atoms are also vapor at such high temperature, and the individual PtSe2 nanoflakes are obtained finally in Fig. 2e, f. Figure 2g shows related Raman spectra of as-obtained PtSe2 films with different growth temperatures in Fig. 2a–f. Both the Eg and A1g peaks are redshift with temperature increasing from 400 to 800 °C (Fig. S2).

Fig. 2
figure 2

Thermodynamic investigation of Pt growth: growth of PtSe2 crystals at temperatures of a 400 °C, b 500 °C, c 600 °C, d 650 °C, e 700 °C, and f 800 °C with heating rate of 4 °C·min−1; g Raman spectra of as-obtained PtSe2 crystals with different growth temperatures

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High-resolution transmission electron microscopy (HRTEM) was used to investigate the morphology evolution of as-obtained PtSe2 crystals. Figure 3a, b shows low-magnification and HRTEM images of PtSe2 film with growth temperature of 400 °C and a heating rate of 4 °C·min−1, respectively. The results reveal typical nanowall morphology of as-obtained PtSe2 film with visible lattice fringes. When the growth temperature is up to 650 °C, few-layer PtSe2 continuous films with polycrystalline layered structure are obtained, as shown in Fig. 3c. The domain sizes of each single crystal are almost at 50–70 nm. The insets in Fig. 3b, c are selected area electron diffraction (SAED) patterns of as-obtained PtSe2 nanowall and continuous PtSe2 film, showing the typical crystal diffraction pattern of 1T PtSe2 with (002) of 0.25 nm, (100) of 0.32 nm, and (001) of 0.50 nm. Figure 3d clearly reveals the periodic atom arrangement of nanowall structure for PtSe2 nanowall film at a selected location, which exhibits lattice spacing in different vertical domains as marked by the blue, green, and origin frame. Figure 3e shows the intensity distribution along three color-dashed lines in Fig. 3d, showing that the spacing is 0.50, 0.27 and 0.51 nm, corresponding to (001), (101) and (001) crystal planes of typical 1T PtSe2, respectively.

Fig. 3
figure 3

Structural characterization of as-obtained PtSe2 crystals at different growth temperatures: a normal and HRTEM images of as-obtained PtSe2 films with growth temperature of b 400 °C and c 650 °C and heating rate of 4 °C·min−1 (inset images: FFT pattern of as-obtained PtSe2 films); d HRTEM image of PtSe2 nanowall films at zoom in area; e intensity profiles along three color-dashed lines in d

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The temperature-dependent growth behavior of PtSe2 nanowall films was further investigated by analyzing the angle-off nanowalls with different growth temperatures. As shown in Figs. S3–S5, the PtSe2 nanowall films are obtained between 450 and 550 °C. The dynamic of selenation process is also further investigated by tuning heating rate and thickness of Pt film, as shown in Figs. S6–S7. With the heating rate increasing, the real selenate reaction time is decreased at low temperature range, the island agglomeration process is dominative behavior, and the individual PtSe2 nanoflakes are obtained with the heating rate up to 4 °C·min−1.

In Fig. 4a–c, HRTEM images of as-obtained PtSe2 nanowall with temperature of 400, 450 and 550 °C show different angle-off distributions of nanowalls. As shown in Fig. 4d, for the sample grown at low temperature of 400 °C, the values of angle-off are randomly distributed in the range from 30° to 160°, and the values at around 60° and 120° are seemingly more preferred. The angle-off evolution becomes more obvious and preferred to specific values when the growth temperature increases from 450 to 550 °C, which are mostly around at 30°, 60°, 90°, 120° and 150°. As shown in Fig. S6, PtSe2 nanowall films with different thicknesses are grown with different-thickness Pt films. The supposed mechanism for the nanowall-flat evolution is shown in Fig. 4e. The supposed mechanism is that Pt atoms on the substrate gradually rearrange to well-ordered structure of Pt (111) surface when the growth temperature increases with such low heating rate, named entropy increasing principle. At the low growth temperature, the surface migration is hard, the selenation reaction of Pt atoms situ occurs, the PtSe2 clusters are in situ piled up under confined space, and PtSe2 nanowalls are formed finally. With the temperature increasing, the surface migration rate of Pt atoms is increased, and the migration barrier is decreased immediately. Owing to the increased surface migration rates of both Pt atoms and as-formed PtSe2 clusters and layered structure which has more space advantage and nanowall structure, it preferred to form flat PtSe2 films. Then, the well-ordered PtSe2 nanowall films are synthesized at high growth temperature of 550 °C and heating rate of 4 °C·min−1 finally.

Fig. 4
figure 4

Investigation of PtSe2 nanowall angle distribution: angle-off analysis of PtSe2 nanowall in HRTEM images with heating rate of 4 °C·min−1 and different growth temperatures of a 400 °C, b 450 °C, and c 550 °C; d angle-off distribution of nanowall for different growth temperatures; e schematic diagram of Pt atoms rearrangement with growth temperature increasing with an entropy increasing process

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The electrocatalytic HER activity of PtSe2 films with different morphologies is evaluated in 0.5 mol·L−1 H2SO4 using a standard three-electrode system. Figure 5a illustrates the vertically arranged PtSe2 nanowall structure, which exposes more active sites and unsaturated bonds at the edges and participates in the following catalytic reaction: 2H+  + 2e− → H2, the hydrogen ion obtains electrons to form hydrogen gas. Figure 5b shows polarization curves of different-morphology PtSe2 films by different growth temperatures (from 400 to 650 °C). The well-ordered PtSe2 nanowall films show the smallest Eonset of –181 mV (HER activity in terms of onset potential) and the lowest overpotential of –413 mV at –10 mA·cm−2 (η10) (overpotential at 10 mA·cm−2) of the most outstanding HER activity. For a better understanding of HER activity, Tafel plots are used to analyze HER kinetics of as-prepared samples, as shown in Fig. 5c. The Tafel slope of well-ordered PtSe2 nanowall film is 51 mV·dec−1, which is much smaller than those of as-obtained samples with growth temperature of 400, 450, 550, 600 and 650 °C with the Tafel slopes of 119, 159, 51,217 and 230 mV·dec−1, respectively. The smaller Tafel slope indicates the faster HER kinetics with a Volmer–Heyrovsky mechanism [28, 29].

Fig. 5
figure 5

Electrocatalytic HER activity in 0.5 mol·L−1 H2SO4: a illustration of HER for PtSe2 nanowall films; b LSV curves of PtSe2 films with heating rate of 4 °C·min−1 and growth temperatures of 400, 450, 550, 600 and 650 °C at a scan rate of 5 mV·s−1; c Tafel plots corresponding to samples in b; d comparison of overpotential at a current density of 10 mA·cm−2 and Tafel slope of various samples; e comparison of overpotential and Tafel slope in reported work and this work; f CV curves of well-ordered PtSe2 nanowall films in region from + 0.12 to + 0.32 V with scan rates from 10 to 50 mV·s−1; g plots showing extraction of Cdl for estimation of ECSA of various samples; h polarization curves normalized by ECSA for various samples with different growth temperatures

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Figure 5d presents HER activities of the samples (η10 and Tafel slope), which vividly reveals that well-ordered PtSe2 nanowall films can effectively provide all-round engineering of catalysts for optimized electrocatalytic property [30, 31]. In Fig. 5e, the lowest η10 value of ∼63 mV and Tafel slope of ∼51 mV·dec−1 achieved are listed among the excellent values for widely explored 2D few layered TMDSs measured with a microcell setup similar to this work [32,33,34,35,36,37]. These results demonstrated that the nanowall structure of 2D TMDs uncovers significance of growth temperature for promoting HER kinetics. In addition, ECSA is also estimated by the double-layer capacitance (Cdl) of as-obtained samples. Cyclic voltammetry (CV) curves are swept in static solution across non-Faradaic region and Cdl is obtained from linear slope of average current density versus scan rate in Fig. 5f–g and Fig. S8. As shown in Fig. 5g, Cdl value of well-ordered PtSe2 nanowall films is 2.6 mF·cm−2, maintaining at a high level compared to those of rest samples, which is mainly attributed to abundant active sites at edge of nanowalls [31]. The polarization curves normalized by ECSA for various samples are shown in Fig. 5h, and the results indicate that the well-ordered PtSe2 nanowall films exhibit superior intrinsic HER activity.

We further studied the thickness-dependent HER performance of well-ordered PtSe2 nanowall films. In Fig. 6a, the LSV curves of different samples indicate that the well-ordered PtSe2 nanowalls with thickness of 5.1 nm show the most outstanding HER activity. The corresponding Tafel plots are shown in Fig. 6b, and the PtSe2 nanowall films with thickness of 5.1 nm have the smallest Tafel slope and the fastest HER kinetics. When the thickness increases from 2.9 to 5.1 nm, more vertically PtSe2 nanowalls are formed, and more edges are exposed, resulting in great enhancement of HER activity. Further increasing the thicknesses from 7.8 to 20.3 nm, multilayer PtSe2 nanowall structures are formed, and the electron transfer pathway in electrode consists of the inside grain boundary of multilayer nanowalls and the interface of electrode. As the results, both the η10 value and Tafel slope of HER performance are decreased with the thickness of PtSe2 nanowall increasing, as shown in Fig. 6c. Besides, the intrinsic activities of PtSe2 nanowall films with different substrate thicknesses are also investigated. Cdl is obtained from linear slope of average current density versus scan rate, as shown in Fig. 6d and Fig S9. The values of Cdl show a trend of first increasing and then decreasing, reaching a maximum of 4.0 mF·cm−2 at thicknesses of 5.1 nm (Fig. 6e). As shown in Fig. 6f, the normalized LSV curves by ECSA also reveal that the well-ordered PtSe2 nanowall films with thicknesses of 5 nm exhibit the optimal HER activity, which is consistent with the results in Fig. 6a. Besides, we studied a continuous HER process to generate hydrogen through a current–time test at a current density of –10 mA·cm−2 by applying an overpotential of –0.4 V. The current density shows a plus or minus at range of 7.6% after a long period of 18 h (Fig. S10), indicating prominent stability for HER.

Fig. 6
figure 6

HER performance of well-ordered PtSe2 nanowall films with different thicknesses: a LSV curves of well-ordered PtSe2 films with heating rate of 4 °C·min−1 and thickness of 2.9, 5.1, 7.8, 10.2 and 20.3 nm at a scan rate of 5 mV·s−1; b Tafel plots corresponding to samples in a; c comparison of overpotential at a current density of 10 mA·cm−2 and Tafel slope of various samples; d CV curves of well-ordered PtSe2 nanowall films in region from + 0.12 to + 0.32 V with scan rates from 10 to 50 mV·s−1; e plots showing extraction of Cdl for estimation of ECSA of various samples; f polarization curves normalized by ECSA for various samples with different growth temperatures

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4 Conclusion

In conclusion, we have developed a modified CVD method to obtain high-quality PtSe2 nanowall films and continuous layered PtSe2 films with a large area of 4 cm2. The continuous PtSe2 nanowall films can be obtained with the growth temperature of below 600 °C, and the continuous layered PtSe2 polycrystalline film when the growth temperature is at around 650 °C. The individual PtSe2 islands will form when the growth temperature is higher than 700 °C. The thickness of PtSe2 nanowall films can be tuned by the thickness of as-deposited Pt films on substrate. The well-ordered PtSe2 nanowall films with growth temperature of 550 °C show the enhanced HER performance, which exposes more edge active sites at the nanowall structure. The highest HER performances are achieved with a η0 value of ∼61 mV and a Tafel slope of ∼52 mV·dec−1 by well-ordered PtSe2 nanowall films with the thickness of 5.1 nm. This work can encourage more works on fabricating 2D TMDs-based nanowall structure for enhanced HER performance.