Scalable Mesoporous Platinum Diselenide Nanosheet Synthesis in Water.

Newly discovered two-dimensional (2D) atomic crystals (nanosheet) of platinum diselenide (PtSe2) have progressively attracted attention due to their expected high performance in catalysis, sensing, electronics, and optoelectronics applications. Further extraordinary physicochemical properties are expected if these nanosheets of platinum diselenide can possess mesoporosity as this may enable a high range of molecular adsorption, enhancing their functionalities in catalysis, batteries, supercapacitors, and sensing. Here, we present for the first time a straightforward, aqueous-phase synthetic strategy for the preparation of scalable nanosheets of platinum diselenide with mesoporous structure via a surfactant-templated self-assembly followed by a thermal annealing phase-transformation process. We used hexamethylenetetramine as a hexagonal honeycomb (sp2-sp3 orbital) scaffold for assembling the Pt and Se organic complexes to form the nanosheet structure, which is stable, preserving the 2D structure and mesoporosity during a thermal annealing at 500 °C. Density functional theory analysis then indicated that the mesoporous nanosheets of platinum diselenide exhibit a high free-energy and large density of π electrons crossing the Fermi level, inferring a high-catalytic performance. This effortless strategy is currently being extended to the synthesis of other transition metal dichalcogenides, including the preparation of multi-metal atomic dichalcogenide nanosheets, for a wide variety of scientific and technological applications.

Introduction

Two-dimensional (2D) atomic crystals of trann class="Chemical">sition metal dichalcogenide (TMD) compounds have continued to be the focus of research in condensed-matter physics and the material chemistry synthetic field due to their rich electronic and optoelectronic properties that vary from insulator to semiconductor, metal and superconductor, depending on their metal and chalcogen atom pair. These features have enabled them to be actively implemented as a key agent in a variety of applications, such as electronic and optoelectronic devices, solar cells, sensing, batteries, and catalysis. A large range of new physical phenomena, such as enhancement of exciton formation and robustness,[1] in-plane plasmonic resonance and optical non-linearity,[2] strong light–matter coupling,[3] as well as ultra-fast energy-transfer at the surface,[4] have also been observed from nanosheets of TMDs so far. In particular, applications in which the surface properties play key roles, such as in sensing, dye-solar cells, storage in batteries, supercapacitors, and catalysis, nanosheets with mesoporosity offer high performance due to their excellent surface accessibility and enhanced surface interaction, bonding, and charge storage.[5] This mesoporosity also allows better selectivity as well as chemical species diffusion for efficient mass transportation, as a result of unusual pore surface chemistry due to adjustment of the quantum confinement effect, interlayer coupling, and symmetry elements.[6] In addition, it was also demonstrated that defect introduction, crystal twinning, etc., on the nanosheet structure effectively modified its electronic structure and surface chemistry, leading to enhanced performance in catalysis applications.[7] Combining such peculiar electronic properties with unusual mesoporosity in this ultimate 2D state promises a generation of materials with novel condensed-matter physics phenomena, enhancing their performance in existing applications.[8] Here, we present a facile chemical synthetic strategy to prepare large, scalable, mesoporous nanosheets of platinum diselenide (PtSe2), in an aqueous-phase reaction followed by a phase transformation using thermal annealing. Nanosheet PtSe2 is a new family member of the 2D crystal TMDs that has recently been discovered. With an energy gap in the range of semiconductor to semi-metal,[9,10] depending on the number of layers, it has a high electron mobility at room-temperature,[11] and is a potential candidate for high-speed electronics applications.[12] A recent theoretical study under circular polarization conditions revealed that PtSe2 nanosheets are also a promising alternative candidate for valleytronics applications.[10] Furthermore, the existence of the catalytically active noble metal Pt in the hexagonal lattice of the atomic sheet leads PtSe2 to exhibit excellent photo- and electro-catalytic and gas-sensing properties.[10,12,13] Regarding this point, the synthesis of nanosheet PtSe2 or other TMDs with mesoporous properties has not yet been achieved experimentally. The available synthetic methods for PtSe2 only realize mono- or polycrystalline nanosheets.[10,14] Our synthetic process is very straightforward and is able to produce PtSe2 nanosheets with scalable dimensions that can be realized from a simple two-step process, namely, the growth of nanosheets of PtSe complexes from an aqueous-phase reaction between a Pt salt and organoselenium compounds in the presence of hexamethylenetetramine (HMT) surfactant as the template and 2D directing agent, and a thermal annealing phase transformation. First-principle analysis has suggested that the mesoporous PtSe2 nanosheets should possess high-catalytic performance due to the existence of a high degree of system enthalpy. These large-scale and mesoporous PtSe2 nanosheets show significant promise with their novel physical and chemical properties providing high flexibility for device fabrication or design of proof of concept devices using mesoporous nanosheets.

Results

We grew the mesoporous n class="Chemical">PtSe2 nanosheets from an aqueous-phase reaction process under a mild reaction temperature, that is, 90 °C, followed by a thermal annealing phase transformation in ambient N2 at 500 °C (see Methods). In a typical process, using standard reaction conditions, that contains 0.68 mM Pt ions, 98.7 mM Se, and 22.73 mM HMT, the nanosheets develop very quickly and can coat the entire reaction container wall as well as the substrate surface that is in contact with the solution within only 15 min of reaction time. By simply controlling the growth time, few-layer mesoporous PtSe complex nanosheets can be obtained on the reaction container wall or substrate surface. The nanosheets of PtSe2 were obtained by transferring the PtSe complex films onto a SiO2/Si substrate, or any solid substrate, and annealing at 500 °C under ambient nitrogen. The as-prepared PtSe complex film is easily exfoliated from the reaction container surface when clean pure water is added (Figure S1a). Due to their large scale, the peeled off nanosheets are visible in solution. Although the PtSe complex nanosheets have a lateral dimension according to the container’s wall or substrate size, the peeled off nanosheets can be up to several hundred micrometers wide (Figure S1b). Nevertheless, we did not notice any formation of dispersed nanosheets in the solution phase. Meanwhile, the byproduct was solely Se nanoparticles (Figure S2). Typically, although Raman spectroscopy analysis of the as-prepared sample nevertheless indicated that it was PtSe (Figure S1c), the energy-dispersive X-ray (EDX) elemental analysis signaled that the concentration ratio between Pt and Se atoms in the nanosheet obeys PtSe2 stoichiometry, and interestingly, they are homogeneously distributed throughout the nanosheet (Figure S1d–g). We then carried out high-resolution transmission electron microscopy (HRTEM) analysis on the sample and we found that the as-prepared nanosheets apparently had a highly porous or perforated structure with pore diameters in the range of 20–50 nm, making them mesoporous in character (Figure S1h). However, the HRTEM results did not find any lattice fringes on the nanosheet samples, inferring that they are amorphous in nature. Such findings were further verified by the selected-area electron diffraction (SAED) analysis results (Figure S1i), which showed bright halos upon exposure to the electron beam. In addition, the HRTEM data indicated that the nanosheets are actually composed of several monolayer sheets that are stacked instead of a solid structure.

To obtain a PtSe2 crystalline phan class="Chemical">se, we carried out a thermal annealing process on the PtSe nanosheets deposited on an indium tin oxide (ITO) substrate at 500 °C under ambient nitrogen for 5 h. Optical microscope analysis results then showed that the structure and morphology of the nanosheets was retained and no structural defects could be observed on the nanosheet structure upon thermal annealing treatment (Figure a), indicating that the nanosheets possess the crucial high thermal stability. To verify that the PtSe2 phase was formed, we conducted Raman spectroscopy analysis on the sample. As is widely understood, PtSe2 adopts the CdI2 lattice that is grouped into D3 point-group symmetry and is characterized by two active Raman modes, namely, Eg and A1g, the in-plane and out-of-plane vibration modes of the Se atoms, respectively, and two infrared active modes, Eu and A2u. The Raman active modes are predicted to be at 180 and 210 cm–1 for the in-plane and out-of-plane vibrational modes, respectively. Our Raman results (Figure b) obeyed this condition, thus proving that the sample is pure PtSe2. As can be seen from the figure, the Raman result shows the existence of two finely separated Raman peaks that emerge at frequencies of approximately 178 and 210 cm–1, which are the in-plane (Eg) and out-of-plane (A1g) modes, respectively. This is in good agreement with the recently reported Raman analysis results for PtSe2 nanosheets.[15] As can be seen from the spectrum, the intensity of the Eg peak is much higher compared to that of the A1g peak. This result can only occur in relatively thin van der Waal crystals where the in-plane mode surpasses the out-of-plane mode, which indicates the nanosheet’s low thickness. As the spectrum also reveals, another significant strong peak is additionally noticeable at approximately 235 cm–1. This can be related to the longitudinal optical signal from overlapping of the in-plane (Eu) and out-of-plane (A2u) infrared active modes.[15] Meanwhile, small ripples at frequencies of 300 and 370 cm–1 are associated with Raman modes of the Pt–N vibration in the PtSe2 nanosheet. Thus, these findings strongly verify that nanosheets of PtSe2 have been successfully realized in this study. Whilst the Raman result confirms the phase purity of PtSe2 in the annealed nanosheets, the X-ray diffraction (XRD) and EDX analysis results (Figure c,f) both further support such phase purity and further, the EDX analysis reveals that the elemental composition in the nanosheets is maintained and follows the stoichiometry of PtSe2. Regarding the XRD analysis result, a typical XRD pattern shows the presence of a strong peak at a 2θ value of approximately 16.60° with the full-width at half-maximum value as low as 1.9043. This peak is consistent with the (001) Bragg plane diffraction of PtSe2, as indicated in the standard diffraction data (JCPDS file no. 88-2281) for PtSe2. No other peaks related to the PtSe complex can be observed in the pattern. For comparison, there are no definite diffraction peaks in the patterns of the as-prepared samples or PtSe complex, inferring their amorphous nature, which was also revealed earlier by the SAED analysis (Figure S1i).

Figure 1

Structure and properties of the nanosheets after being annealed at 500 °C for 5 h in a N2 atmosphere. (a) Optical micrograph of a nanosheet indicating the stability of the structure. (b) Raman spectrum of the nanosheets showing the PtSe2 phase purity. Inset in b shows the region on which the Raman characterization was performed. (c–e) EDX analysis result for the nanosheets, the Pt and Se elemental concentrations in the nanosheets follows PtSe2 stoichiometry and they are homogenously distributed over the nanosheets. (f) XRD patterns of the nanosheet sample after being annealed and the as-prepared nanosheets. (g, h) X-ray photoelectron spectroscopy (XPS) spectra for the sample, the growth time was 15 min.

Structure and properties of the nanosheets after being annealed at 500 °C for 5 h in a N2 atmosphere. (a) On class="Chemical">ptical micrograph of a nanosheet indicating the stability of the structure. (b) Raman spectrum of the nanosheets showing the PtSe2 phase purity. Inset in b shows the region on which the Raman characterization was performed. (c–e) EDX analysis result for the nanosheets, the Pt and Se elemental concentrations in the nanosheets follows PtSe2 stoichiometry and they are homogenously distributed over the nanosheets. (f) XRD patterns of the nanosheet sample after being annealed and the as-prepared nanosheets. (g, h) X-ray photoelectron spectroscopy (XPS) spectra for the sample, the growth time was 15 min.

To further verify that the phase of the nanosheets is n class="Chemical">PtSe2, we carried out XPS. It was found that both Pt and Se elements can be observed in the spectrum as well as other elements from the substrate, and the analysis data indicate that the PtSe2 phase definitely exists. For example, in the case of Pt, the high-resolution scan of its binding energy spectrum (4f7/2) can be well-fitted by two overlapping Gaussian–Lorentzian (GL) curves centered at 72.52 and 72.92 eV (Figure g). We associate the higher intensity peak but lower in energy peak at 72.52 eV with PtSe2 bonding in the honeycomb lattice, which is assumed to be due to the three- or four-coordinated sp2–sp3 trigonal–planar molecular backbone of the 2D hexagonal honeycomb crystal.[16,17] The percentage of this bonding type is approximately as high as 74.94%. The lower intensity peak can be associated with PtO species (72.92 eV). However, its ratio is relatively low, that is, approximately 25.06%. A similar case was observed for Se (Figure h). Two peaks that can be attributed to Se at bonding energies of 53.78 and 54.13 eV can be observed in the fitted spectrum of Se (3d5/2). The higher peak that is lower in energy is associated with the sp2–sp3-coordinated Se covalent bonding energy[18] or PtSe2, with its percentage existence as high as 39.57%. Meanwhile, the peak at 54.13 eV, with a percentage as high as 60.43%, is associated with the metallic Se. Its percentage is high because it might originate from the Se nanoparticle byproduct, which is attached to the PtSe2 nanosheet product.

We carried out HRTEM analysis on the annealed sample to obtain its detailed structural properties (Figure ). As the analyn class="Chemical">sis results show, the nanosheet structure, including its mesoporosity or perforated characteristic, is maintained during the high-temperature thermal annealing (Figure a,b), reflecting its high structural stability. Further, it was also discovered that the layered nature of the nanosheet structure is also preserved and unchanged, which indirectly confirms its excellent thermal stability. According to the high-resolution image, it can be seen that the nanosheet is polycrystalline in nature with crystallites in the form of trigonal prismatic (2H) and octahedral (1T) phases (Figure c). Such polycrystallinity was further confirmed by the SAED analysis result, which presents a crystalline character with an arbitrary symmetry diffraction pattern (Figure d). Beside revealing the polycrystallinity property, the SAED result also unveils the nature of the monolayer stacking during the growth process, which is random and un-oriented. This is reflected by observed disorder in the Moire pattern of a typical nanosheet sample (Figure e). However, we believe that single-crystalline PtSe2 nanosheets could be synthesized if a suitable pre- or post-growth treatment, with specific reaction conditions (e.g., concentration and growth temperature) or annealing process conditions (e.g., temperature, pressure, etc.) are discovered. We are in the process of finding PtSe2 nanosheets with monocrystalline properties. Next, we carried out atomic force microscopy (AFM) analysis to find the surface profile and the thickness of the nanosheets (Figure f). It was found that the nanosheets have a thickness in the range of 11–25 nm (Figure S2) with an average surface roughness of as high as 12.20 nm. Nanosheets with such low thickness provide the possibility of growing a monolayer nanosheet of PtSe2 if suitable growth conditions are discovered. Considering the interlayer spacing of 7.25 Å in the PtSe2 system, obtained from the first-principle study (discussed later), each PtSe2 nanosheet should be composed of 15–35 monolayers. In addition, and in good agreement with the HRTEM analysis results, such high surface roughness further adds additional verification that the nanosheets are mesoporous with a large-area of potentially active sites available for sensing and catalysis applications.

Figure 2

Crystallinity properties of the PtSe2 nanosheets. (a, b) Low-resolution electron image of a nanosheet, indicating mesoporosity. (c) High-resolution electron image of a nanosheet, revealing its polycrystalline nature, which is further supported by the electron diffraction (SAED) data (d). It was also found that the nanosheets comprise two different crystalline phases, e.g., 1T and 2H. (e) Disorder in the Moire pattern as a result of the arbitrary stacking of the monolayer sheets that comprise the nanosheet. This is in agreement with the SAED data shown in (d). (f) AFM analysis of a nanosheet, revealing the thickness of the nanosheet to be as low as 11 nm. Inset in (f) is the optical image of the nanosheet used for AFM analysis.

Crystallinity properties of the PtSe2 nanosheets. (a, b) Low-resolution electron image of a nanosheet, indicating mesoporon class="Chemical">sity. (c) High-resolution electron image of a nanosheet, revealing its polycrystalline nature, which is further supported by the electron diffraction (SAED) data (d). It was also found that the nanosheets comprise two different crystalline phases, e.g., 1T and 2H. (e) Disorder in the Moire pattern as a result of the arbitrary stacking of the monolayer sheets that comprise the nanosheet. This is in agreement with the SAED data shown in (d). (f) AFM analysis of a nanosheet, revealing the thickness of the nanosheet to be as low as 11 nm. Inset in (f) is the optical image of the nanosheet used for AFM analysis.

Regarding their chemical stability, both the PtSe complex and n class="Chemical">PtSe2 nanosheets exhibit excellent stability and do not show any significant modification in their structure and properties when they are aged for more than 4 months. However, in the case of the PtSe complex, during the aging process in aqueous solution, flocculation or a decrease in the lateral size occurred due to oxidation or the annealing process. Nevertheless, it was found that they dispersed well and did not agglomerate with each other. Optical microscopy analysis indicated that the aged sample still had a lateral size that was higher than several hundreds of micrometers in edge-length. Therefore, further analysis and application using aged samples are highly possible without any doubt about the modification of their properties. In other words, following the thermal annealing phase transformation, PtSe2 nanosheets that have been stored will still have a structure and properties that are similar to the freshly prepared ones. At the same time, the PtSe2 nanosheets on Si or ITO substrate also do not exhibit any modification to their structure and properties after aging in air for more than 4 months. The optical and electron microscopy analysis confirmed such stability as no change in the PtSe2 nanosheet properties was observed. The results are shown in Figure S3.

At this stage, experimental evidence of the PtSe2 nanosheet’s performance in applications, particularly in catalyn class="Chemical">sis, has not yet been obtained. Nevertheless, we carried out a first-principles study to obtain their general electronic properties, and from them, their catalytic performance was estimated. In this study, to simulate the mesoporous and polycrystalline properties, we restacked two bilayers of PtSe2 nanosheets by shifting the bottom bilayer plane’s origin fractional coordinates to u = 0.5 and v = 0.75 Å relative to the top layer plane origin. The optimized structure and the electronic properties of the simulated polycrystalline and mesoporous (restacked) PtSe2 nanosheets are shown in Figure . The lattice parameter is a = b = 3.705 Å, which was expanded by 0.24% compared to that of the monocrystalline bilayer PtSe2 system. The modification of the initial lattice parameter upon restacking is normal and it is likely due to the increase in charge density wave[19] in the layered system, which induces lattice expansion. This is commonly observed in most restacked 2D layered structures.[20] In common processes, restacking also causes an increase in the metal–metal or metal–chalcogen bond length in the 2D lattice (Figure a,b). For example, the Pt–Se bond length in single-crystalline PtSe2 is 0.256 nm but it is elongated to 0.258 nm in the restacked system. Such a process becomes more intense when the deviation in the stacking orientation is higher.[21] It was also found that there is a distortion in the symmetry of the hexagonal lattice upon restacking due to the existence of a slight difference between neighboring Pt–Se bond lengths in a single unit of the hexagonal system, that is, 0.2580 and 0.2575 nm (Figure b). Such distortion of the lattice structure upon restacking or due to the polycrystalline arrangement might change the overall density of states of the system, augmenting the surface energy of the system. Thus, high-catalytic performance is predicted to be generated from the nanosheets. Such an assumption agrees well with the electronic property calculation results, as shown in Figure c,d. Although the electronic properties of the restacked PtSe2 are generally similar to those of the monocrystalline PtSe2 nanosheets, that is, metallic in nature, restacking can modify the shape of either the band structure or the density of states. For example, as the analysis results revealed, there exists a high amount of electron wave confinement over a small space in all levels of energy. Near the Fermi level, with a high-density of π electrons from the p and d orbitals, this confinement may generate many effects, particularly an enhancement of surface energy and system enthalpy. Our calculations found that the enthalpy of the system remarkably increases upon being restacked, for example, from −2476 eV in the monocrystalline PtSe2 phase to −4593 eV in the restacked system. This implies that the restacked PtSe2 system is chemically active and has potential for catalysis and sensing applications. The enthalpy would be much higher if defects due to the polycrystallinity, polymorphism, as well as the mesoporosity, are considered, further heightening the physicochemical properties. Hence, extraordinary performance in catalysis applications is anticipated. We are in the process of studying the effect of polycrystallinity and mesoporosity on the catalytic, optical, and electrical properties of mesoporous PtSe2 nanosheets.

Figure 3

First-principles analysis of the mesoporous, polycrystalline PtSe2 nanosheets. Optimized geometry of the restacked PtSe2 nanosheet from top (a) and side (b) views. (c, d) Band structure and density of states of the restacked system. (f–i) Optimized geometry, band structure, and the density of states of the monocrystalline PtSe2 nanosheet for comparison. (e) k-Path in the Brillouin zone.

First-principles analysis of the n class="Chemical">mesoporous, polycrystalline PtSe2 nanosheets. Optimized geometry of the restacked PtSe2 nanosheet from top (a) and side (b) views. (c, d) Band structure and density of states of the restacked system. (f–i) Optimized geometry, band structure, and the density of states of the monocrystalline PtSe2 nanosheet for comparison. (e) k-Path in the Brillouin zone.

Discussion

The formation mechanism of the PtSe2 nanosheets un class="Chemical">sing the present method is not yet understood. However, the following facts can be considered: firstly, HMT is likely the key agent for nanosheet growth, providing a scaffold for the sp2–sp3 hybrid orbital-built structure. Therefore, it should be present during the growth process and its concentration was in the range of 0.3–1.0 M for large-scale nanosheet PtSe complex formation. The thickness of the nanosheet decreased with increasing HMT concentration, as can be judged from the color contrast on the reaction container wall. Due to their large dimensions, this condition caused the nanosheets to have a tendency to be folded or rolled up, as can be ascertained from the optical microscope images. At a relatively high-concentration, that is, higher than 1.0 M, limited nanosheet formation was observed. This could be due to the effect of steric hindrance of the precursor, the result of the high-concentration of HMT. When the concentration of HMT was below 0.1 M, limited nanosheet production was observed. However, they were thick, not lateral in dimension, and cracked. The dominant products at such low HMT concentrations were quasi-spherical Se nanoparticle aggregates. The results are shown in Figures S4 and S5. Secondly, the reaction temperature should be 90 °C. This is to maintain the dissolution of Se in the NaBH4 solution. Therefore, a lower reaction temperature is not applicable. Similarly, owing to the process being in an aqueous phase, the use of a reaction temperature higher than 90 °C may cause the reaction to boil, which would impede the formation of the nanosheet structure. Thus, we did not carry out a study of the effect of growth temperature below or above this temperature.

We then performed Raman and Fourier transform infrared (FTIR) analyses on the as-prepared n class="Chemical">PtSe complex nanosheets in order to determine the mechanism of their formation. From the results (Figure S6), we hypothesized that HMT with its tetrahedral cage structure may act as an sp2–sp3 structural template[22,23] in the beginning for the Pt or Se complexes via Pt–N or Se–H bonds. For the Pt–N bonding, the Pt salt precursor that dissolves to PtCl6–2 complexes will bind to amine sites of the HMT molecule forming Pt–N coordination. This process could create a negatively charged ligand on the HMT molecule structure. Such a Pt–N bond is detected by the Raman analysis, as indicated by the presence of two strong peaks at 303 and 358 cm–1, which are associated with Pt–N stretching and rocking vibration, respectively.[24] At the same time, the organoselenium compound (sodium hydrogen selenide) that is produced from the reaction of NaBH4 and Se,[25] which may be available in the form of HSe–, will bind to the HMT molecule either via Se–H or C=Se bonds with the hydrogen or carbon at the alkyl site of the HMT molecule, respectively, which also produces a negatively charged ligand. The existence of Se–H and C=Se bonds is confirmed both by the Raman and FTIR analysis results. For example, the Se–H bond is represented by the appearance of strong peaks at 789 and 904 cm–1 and a weak peak at 2927 cm–1 in the FTIR spectrum, and a peak at 820 cm–1 in the Raman spectrum.[24] Nevertheless, in view of the FTIR peaks which are much stronger than that in the Raman spectrum, we believe the Se–H bond dominates the bonding between Se complexes and HMT molecules. Considering the molecular structure and the nature of its coordination with Pt and Se, we hypothesized that each HMT molecule should coordinate with at least three Pt and six Se complexes at the same time to form the PtSe2 compounds.

Whilst the coordination of Pt and n class="Chemical">Se atoms to the HMT molecule template has been proven by the Raman and FTIR analysis results, the formation of the nanosheet morphology is assumed to be directed by the negatively charged ligand from the Pt and Se complexes. This feature may control the direction of nanostructure growth, and hence the thickness of the nanosheet. HMT, with its well-known special properties in promoting 1D and 2D nanostructures,[26−28] and containing Pt and Se complexes, will then crystallize, probably via hydrogen bonding, as occurs in the formations of most metal-organic framework systems,[29] with a negatively charged layer resulting from coordination of HMT with both Pt and Se complexes projecting a nanosheet configuration (2D crystallites). We also predict that the crystallization of HMT containing Pt and Se complexes may occur both in the solution phase and on a solid surface. Following the oriented-attachment process, small 2D crystallites containing Pt and Se complexes form large-scale 2D nanosheets on the solid surface. The attachment process of 2D crystallites in solution onto the surface occurs randomly and very quickly, promoting a mesoporous structure. The process will continue until the entire precursor is consumed. Nevertheless, a continuous nanosheet structure on the substrate or the reaction container is obtained from as early as 30 min and becomes thicker over time. This process is similar to that observed in the reaction of HMT with AgSCN producing 2D and 3D supramolecular isomers.[30] With the subsequent thermal annealing process at 500 °C and under ambient N2, the Pt and Se complexes in the nanosheet react, producing a PtSe2 nanosheet with mesoporosity. We assumed that HMT and the other complexes decomposed completely because of the absence or limited presence of any complexes as indicated by the Raman and FTIR analysis data. The mechanisms of the PtSe complex and PtSe2 nanosheet formation are described in Figure .

Figure 4

Mechanism of the formation of a mesoporous PtSe2 nanosheet. (a) The circumstance at the beginning of the reaction where the Pt and Se complexes bind to the HMT molecules via Pt–N and Se–H coordination. (b) The subsequent process during growth in which the coordinated HMT with Pt and Se complexes crystallizes via “hydrogen bonding”, forming monolayer sheet crystallites. (c) The monolayer crystallites attach to each other randomly producing the mesoporous PtSe complex nanosheet. (d) Annealing the PtSe complex nanosheet produces the PtSe2 polycrystalline nanosheet.

Mechanism of the formation of a mesoporous n class="Chemical">PtSe2 nanosheet. (a) The circumstance at the beginning of the reaction where the Pt and Se complexes bind to the HMT molecules via Pt–N and Se–H coordination. (b) The subsequent process during growth in which the coordinated HMT with Pt and Se complexes crystallizes via “hydrogen bonding”, forming monolayer sheet crystallites. (c) The monolayer crystallites attach to each other randomly producing the mesoporous PtSe complex nanosheet. (d) Annealing the PtSe complex nanosheet produces the PtSe2 polycrystalline nanosheet.

In conclusion, large-scale, n class="Chemical">mesoporous nanosheets of PtSe2 can be realized on a solid substrate from an aqueous-phase solution via a surfactant template assembly and a thermal annealing phase transformation. The nanosheet is polycrystalline in nature containing both 1T and 2H phases of PtSe2. The nanosheets have mesopores arising from the random oriented-attachment of 2D nanocrystallites, promising exceptional performance for catalysis and battery applications. The present approach is applicable to the preparation of nanosheets of different blends of TMD. This new structure of nanosheet PtSe2 may find potential applications in currently existing applications and may provide a new dimension of research for the scientific and industrial applications of these materials.

Methods

PtSe2 2D Nanosheet Synthesis

PtSe2 nanosheets were prepared un class="Chemical">sing a two-step process, that is, the wet-chemical growth of nanosheets of PtSe complexes and a thermal annealing phase transformation. In the first step, we prepared the PtSe complex nanosheet via a simple aqueous-phase wet-chemical process under a mild temperature, that is, 90 °C. In a typical procedure, 0.5 mL of 0.015 M potassium hexachloro platanate (K2PtCl6, Fluka) was mixed with 0.5 mL of 0.5 M HMT (Sigma). The solution was shaken for 15 s to fully mix the reagents. This mixture was later called the Pt precursor for simplicity. At the same time, 0.78 mg of Se powder (Sigma-Aldrich) was added to 10 mL of ice-cooled 0.1 M NaBH4 (Sigma). The mixture was also shaken to disperse the Se powder well in the solution. The mixture was then transferred into a water bath (Julabo) and the water bath temperature was then set and fixed at 90 °C. Within 10–15 min the Se dissolved producing a dark-brown solution. This mixture was called the Se precursor. Under these conditions, the concentrations of K2PtCl6, HMT, and Se are 0.68, 22.73, and 98.7 mM, respectively. We used an excess of Se concentration compared to that of Pt in order to facilitate a facile achievement of PtSe2 stoichiometry during the reaction. Nevertheless, the effect of Se concentration on the structural growth of PtSe2 nanosheets was not evaluated in this study. We did, however, examine the effect of HMT concentration on the structural growth of PtSe2 nanosheets by using five different HMT concentrations during the growth process, namely, 4.55, 13.63, 22.73 (standard reaction), 31.82, and 45.45 mM. Other reactants were kept unchanged.

All chemicals were used as received without any further purification process. Pure n class="Chemical">water (∼18 MΩ) was used throughout the reaction, which was obtained from a Milli-Q water purification system.

In the reaction, the main components were the Pt precursor in the form of n class="Chemical">PtCl6– and the Se precursor in the form of NaHSe along with the HMT surfactant template system without the presence of any reducing agent. NaHSe is the main product that is produced from the reaction of Se powder with NaBH4 solution, which also then breaks up into HSe–.

The PtSe complex nanosheet was grown by injecting the n class="Chemical">Pt precursor into the hot Se precursor. The reaction container was then tightly closed and exposure to the ambient atmosphere was avoided. The reaction was then left undisturbed for 15 min. After finishing the reaction, the color of the solution changed to green-brown. Under these conditions, nanosheets of PtSe complexes were grown on the reaction container’s wall. Meanwhile in the solution, the main product was Se nanoparticles and these were easily separated from the PtSe complex nanosheets by simply pouring out the solution, leaving the PtSe complex nanosheets on the wall of the container. To remove the Se nanoparticles from the PtSe complex nanosheets, pure water was poured into the container and then removed gently using a micropipette. This process was repeated at least three times. Finally, the cleaned PtSe complex nanosheets on the container wall were preserved in pure water by filling the container with pure water. Within 30–60 min, the PtSe complex nanosheets peeled off from the container wall and dispersed in the solution.

To obtain the PtSe2 nanosheets, the n class="Chemical">PtSe complex nanosheets were transferred onto a substrate (ITO or Si) by simply dropping the solution onto the substrate surface and letting the aqueous solvent to dry at 100 °C on a hot-plate. After that, the sample was transferred into a tube furnace with a quartz tube chamber and annealed at 500 °C for 5 h. During the annealing, nitrogen gas with a flow rate of 25 sccm was purged into the quartz tube chamber. Using this process, PtSe2 nanosheets can be obtained.

Characterization

The morphology and crystallinity of the 2D nanosheet samples were examined un class="Chemical">sing a scanning TEM FEI Tecnai G2 F20. The instrument was also equipped with an EDX apparatus with an X-maxN 80T detector (Oxford instrument) for elemental analysis. The SAED analysis was carried out using a Hitachi HT7700 high-contrast digital TEM apparatus with 120 kV acceleration voltage. The sample thickness analysis was carried out using a scanning probe microscopy CPII/ VEECO that was operated at tapping mode. The crystalline phase of the samples was evaluated via Raman spectroscopy using a CRM 200 Witex Raman spectrometer with 532 nm excitation lasers, XRD spectroscopy using a Bruker D8 advance XRD spectrometer, and FTIR spectroscopy using a PerkinElmer Spectrum 400 FTIR/FTNIR system apparatus. The chemical state and bonding of the sample were characterized via XPS using a KRATOS XSAM-HS apparatus with Al Kα radiation. In the analysis, a Shirley-type background was used for the curve fitting process with a GL mixed function background (70 and 30% for the Gaussian and Lorentzian components, respectively) for the line shaping. The calibration curve for the core level spectra was referenced to C 1s with a binding energy of 285 eV.

First-Principles Study Analysis

The first-principles density functional theory study was carried out un class="Chemical">sing Materials Studio 8.0 with a generalized-gradient approximation Perdew–Burke–Ernzerhof functional as implemented in Castep code.[31] In the present study, the electronic properties analysis of the mesoporous PtSe2 nanosheets was only carried out on the 2H phase. The calculation was performed on restacked nanosheets of PtSe2 (constructed using a supercell model of 2 × 2 × 1 containing 96 atoms that was built using the P63/mmc (194) space group with initial lattice parameters of a = b = 3.724 Å and c = 5.031 Å) as a model for our PtSe2 sample structure. One stacked nanosheet contained two monolayers of PtSe2. Therefore, the restacked system consisted of four monolayers of PtSe2. Pt and Se atoms were located at Wickoff positions 2(c) and 4(f), respectively. A large value of vacuum slab (15 A) was used during the calculation to prevent false interaction with the periodic image. To model polycrystallinity in the restacked nanosheet, the second layer original coordinate was shifted to u = 0.5 and v = 0.75. For geometry optimization, Brillouin zone integration was carried out on a 9 × 9 × 1 Monkhorst–Pack grid. Meanwhile for band structure calculation, Brillouin zone integration was carried out on a 3 × 3 × 1 Monkhorst–Pack grid to simulate the two-dimensionality. To simplify the electron correlation problem, the Koelling–Harmon relativistic effect was introduced into the geometry optimization and energy calculation. The energy convergence was set to 0.001 eV/cell and the maximum force on each atom was set to 0.05 eV/Å. Discrete Fourier transform calculations were performed on Pt 5d9 6s1 and Se 4s2 4p4 with the core-electron model based on the ultrasoft pseudopotential plane wave function. The band energy tolerance used for both band structure and density of states calculations was 1.0–5 eV. The electronic properties of the nonstacked system were also simulated in this work and were compared to the restacked system in order to obtain the change in their properties.