Abstract

Platinum diselenide (PtSe₂) represents a significant transition metal dichalcogenide compound with the chemical formula PtSe₂. This inorganic compound crystallizes in the cadmium iodide structure type with space group P3m1 (No. 164) and lattice parameters a = 3.728 Å and c = 5.031 Å. The material exhibits a density of 9.54 g/cm³ and appears as an opaque metallic yellowish-white solid. Platinum diselenide demonstrates unique thickness-dependent electronic properties, transitioning from semimetallic behavior in bulk form to semiconducting characteristics in monolayer configurations with band gaps ranging from 0 eV in bulk to 1.3 eV in monolayer form. The compound displays octahedral coordination geometry and shows promising applications in photodetection, catalysis, and electronic devices due to its exceptional electrical conductivity of 620,000 S/m and distinctive layer-dependent properties.

Introduction

Platinum diselenide belongs to the class of transition metal dichalcogenides (TMDCs), a family of layered materials that have attracted significant scientific interest due to their unique electronic and structural properties. This inorganic compound, with the systematic name platinum(IV) selenide, was first synthesized by Minozzi in 1909 through direct combination of the elements. The compound occurs naturally as the mineral sudovikovite, named after Russian petrologist N.G. Sudovikov (1903-1966), discovered in the Srednyaya Padma mine in Karelia Republic, Russia. Platinum diselenide has gained considerable research attention due to its thickness-dependent band structure, high carrier mobility, and potential applications in next-generation electronic and optoelectronic devices. The compound's ability to exhibit both metallic and semiconducting behavior depending on layer number makes it particularly valuable for fundamental studies of two-dimensional materials and their implementation in advanced technological applications.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Platinum diselenide adopts a hexagonal crystal structure isomorphous with cadmium iodide, belonging to space group P3m1 (No. 164). The unit cell parameters measure a = 3.728 Å and c = 5.031 Å, with each platinum atom occupying octahedral coordination sites between two selenium layers. The structure consists of Se-Pt-Se trilayers where platinum atoms form a hexagonal close-packed layer sandwiched between two selenium layers. This arrangement creates a 1T polytype structure characterized by octahedral coordination of platinum atoms. The interlayer bonding arises primarily from weak van der Waals forces, enabling mechanical exfoliation down to monolayer thickness. The electronic configuration involves platinum in the +4 oxidation state with electron configuration [Xe]4f¹⁴5d⁶, while selenium adopts the -2 oxidation state with configuration [Ar]3d¹⁰4s²4p⁶. The bonding exhibits predominantly covalent character with partial ionic contribution due to the electronegativity difference between platinum (2.28) and selenium (2.55).

Chemical Bonding and Intermolecular Forces

The chemical bonding in platinum diselenide involves significant covalent character with Pt-Se bond lengths of approximately 2.54 Å. The bonding arises from overlap of platinum 5d orbitals with selenium 4p orbitals, forming σ-bonds through sp³d² hybridization of platinum. The interlayer interactions consist primarily of weak van der Waals forces with a binding energy of approximately 0.19 eV per atom, significantly weaker than the intralayer covalent bonds. This anisotropic bonding results in highly directional properties with strong in-plane bonding and weak out-of-plane interactions. The compound exhibits no permanent dipole moment due to its centrosymmetric structure, though local dipoles may form at defect sites or under applied strain. The work function measures approximately 5.2 eV, while the electron affinity ranges from 3.8 to 4.2 eV depending on layer thickness.

Physical Properties

Phase Behavior and Thermodynamic Properties

Platinum diselenide appears as an opaque metallic yellowish-white solid with a Mohs hardness of 2 to 2.5 in its natural mineral form (sudovikovite). The compound exhibits a density of 9.54 g/cm³ at 298 K and decomposes rather than melting at elevated temperatures. Thermal stability extends to approximately 673 K in air, with decomposition occurring through selenium evaporation. The specific heat capacity measures 0.35 J/g·K at room temperature, while the thermal conductivity reaches 85 W/m·K in the basal plane and 5 W/m·K perpendicular to the layers. The linear thermal expansion coefficient is 6.7 × 10⁻⁶ K⁻¹ along the a-axis and 3.2 × 10⁻⁶ K⁻¹ along the c-axis. The compound demonstrates n-type semiconducting behavior in thin layers with electron mobility exceeding 300 cm²/V·s at room temperature. The electrical conductivity of bulk material reaches 620,000 S/m, decreasing to approximately 10,000 S/m for monolayer samples.

Spectroscopic Characteristics

Raman spectroscopy of platinum diselenide reveals two primary vibrational modes: the A_1g mode at 205 cm⁻¹ involving out-of-plane vibrations of selenium atoms and the E_g mode at 175 cm⁻¹ corresponding to in-plane vibrations. The E_g mode exhibits significant red-shifting with increasing layer number, decreasing to 166 cm⁻¹ for bilayers and 155 cm⁻¹ for bulk material. The A_1g mode shows minimal thickness dependence. Infrared spectroscopy identifies the A_2u mode at 187 cm⁻¹ (Se vibrating out-of-plane opposite to Pt) and E_u mode at 162 cm⁻¹ (in-layer vibration with Se opposite to Pt). X-ray photoelectron spectroscopy shows Pt 4f core level peaks at 72.3 eV (4f_7/2) and 75.6 eV (4f_5/2), while Se 3d peaks appear at 54.39 eV (3d_5/2) and 55.19 eV (3d_3/2). UV-Vis spectroscopy demonstrates strong absorption across the visible spectrum with absorption coefficients exceeding 10⁵ cm⁻¹ at 550 nm.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Platinum diselenide exhibits remarkable chemical stability under ambient conditions, resisting oxidation in air up to 473 K. The compound demonstrates inertness toward most acids and bases, though prolonged exposure to strong oxidizing agents such as aqua regia or hydrogen peroxide leads to gradual decomposition. The surface shows physisorption of water molecules with adsorption energy of -0.19 eV and oxygen with -0.13 eV, without chemical reaction at room temperature. Thermal decomposition occurs above 773 K through selenium sublimation, following first-order kinetics with an activation energy of 1.8 eV. The material serves as an effective catalyst for various redox reactions, particularly oxygen reduction and hydrogen evolution reactions. Catalytic activity stems from platinum's d-band electronic structure and the compound's appropriate adsorption energies for reaction intermediates. The compound maintains structural integrity under electrochemical conditions between -1.0 and +1.0 V versus standard hydrogen electrode in aqueous electrolytes.

Acid-Base and Redox Properties

Platinum diselenide demonstrates amphoteric character with both acidic and basic surface sites. The point of zero charge occurs at pH 4.2, indicating slightly acidic surface properties. The compound exhibits redox stability within a window of -0.8 to +0.6 V versus SHE in neutral aqueous solutions. Standard reduction potentials for various surface redox couples range from -0.3 to +1.2 V versus SHE, depending on the specific reaction and surface termination. The material shows excellent stability in reducing environments but undergoes gradual oxidation under strongly oxidizing conditions. The Fermi level position varies with layer thickness, shifting from near the conduction band in bulk material to mid-gap in monolayer samples. Electrochemical impedance spectroscopy reveals charge transfer resistances from 10 to 100 Ω·cm² depending on electrode potential and electrolyte composition.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of platinum diselenide primarily employs direct combination of elements. The most common method involves heating platinum thin foils in selenium vapor at 400°C for several hours, resulting in epitaxial growth of PtSe₂ layers. This technique produces high-quality crystals with controlled thickness by varying reaction time and temperature. Alternative approaches include precipitation from aqueous solutions containing Pt(IV) precursors treated with hydrogen selenide gas, though this method often yields less crystalline material. Another synthetic route involves heating platinum tetrachloride with elemental selenium at 500°C under inert atmosphere, producing polycrystalline PtSe₂ powder. For monolayer preparation, mechanical exfoliation from bulk crystals using adhesive tapes provides the highest quality material, while chemical vapor deposition using platinum precursors and selenium sources enables large-area growth. The selenization of platinum (111) surfaces at 270°C produces well-ordered monolayer PtSe₂ with epitaxial registry to the substrate.

Industrial Production Methods

Industrial production of platinum diselenide utilizes scaled-up versions of laboratory methods, particularly direct elemental combination in sealed quartz ampoules at 600°C. This process employs stoichiometric mixtures of platinum black and selenium powder heated for 24-48 hours, yielding polycrystalline material suitable for most applications. For electronic-grade material, chemical vapor transport using iodine as transport agent produces large single crystals up to 1 cm² in size. Industrial processes emphasize careful control of selenium vapor pressure to prevent formation of platinum-rich phases such as Pt₅Se₄. Production costs remain relatively high due to platinum's price and the need for controlled atmosphere processing. Yield optimization focuses on selenium utilization efficiency, typically achieving 85-90% conversion of selenium to product. Waste management strategies involve selenium recovery from off-gases and recycling of platinum from process residues.

Analytical Methods and Characterization

Identification and Quantification

X-ray diffraction provides definitive identification of platinum diselenide through its characteristic pattern with strong (001) reflections at d-spacings of 5.03 Å, (100) at 3.23 Å, and (101) at 2.70 Å. Raman spectroscopy serves as a rapid identification method with the distinctive E_g and A_1g peaks whose relative intensities and positions indicate layer number and quality. Energy-dispersive X-ray spectroscopy confirms stoichiometry with Pt:Se ratio of 1:2 ± 0.05. X-ray photoelectron spectroscopy quantifies elemental composition and oxidation states through integration of Pt 4f and Se 3d peaks. Atomic force microscopy measures layer thickness with monolayer height of approximately 0.8 nm. Optical microscopy identifies monolayer regions through contrast differences on SiO₂/Si substrates. Quantitative analysis employs inductively coupled plasma mass spectrometry with detection limits of 0.1 μg/g for platinum and 0.5 μg/g for selenium. Thermogravimetric analysis monitors decomposition behavior and purity.

Purity Assessment and Quality Control

High-purity platinum diselenide exhibits electrical resistivity ratios (R_300K/R_4K) exceeding 50, indicating low impurity concentrations. Common impurities include oxygen (100-1000 ppm), carbon (50-500 ppm), and other transition metals (<100 ppm). X-ray diffraction full width at half maximum values for the (001) peak below 0.1° indicate high crystalline quality. Raman spectroscopy quality assessment uses the ratio of E_g to A_1g peak intensities, with values near 1.0 indicating optimal quality. Optical microscopy reveals domain sizes exceeding 10 μm for high-quality material. Electrical characterization shows carrier concentrations below 10¹⁷ cm⁻³ for undoped material. Industry standards require metallic impurity levels below 100 ppm and chalcogen-to-metal ratios between 1.95 and 2.05 for electronic applications. Stability testing under accelerated aging conditions (85°C/85% relative humidity) demonstrates no degradation over 1000 hours for encapsulated samples.

Applications and Uses

Industrial and Commercial Applications

Platinum diselenide finds application in broadband photodetectors operating across visible to mid-infrared wavelengths (400 nm to 10 μm), leveraging its thickness-dependent band gap and high carrier mobility. The material serves as an effective catalyst for oxygen reduction reactions in fuel cells and metal-air batteries, outperforming pure platinum in certain potential ranges. Field-effect transistors fabricated from few-layer PtSe₂ demonstrate ON/OFF ratios exceeding 10⁴ and carrier mobilities up to 300 cm²/V·s, suitable for high-frequency applications. Gas sensors utilizing PtSe₂ show parts-per-billion sensitivity to nitrogen dioxide with response times under 10 seconds and recovery times of approximately 60 seconds. Thermoelectric devices benefit from the compound's Seebeck coefficient of 40 μV/K and high electrical conductivity, achieving ZT values up to 0.2 at room temperature. Electrochemical sensors exploit the material's stable electrochemical properties and large surface area for detection of various analytes.

Research Applications and Emerging Uses

Research applications focus on fundamental studies of thickness-dependent electronic properties and phase transitions in two-dimensional materials. Platinum diselenide serves as a platform for investigating metal-insulator transitions and correlation effects in reduced dimensions. Spintronics research explores the material's predicted helical spin texture and Rashba effects for spin manipulation devices. Heterostructures combining PtSe₂ with other two-dimensional materials enable band engineering and novel device concepts including tunneling transistors and flexible electronics. Photocatalytic applications utilize PtSe₂-graphene composites for generation of reactive oxygen species capable of organic pollutant degradation. Quantum transport studies examine unusual magnetoresistance effects and quantum Hall phenomena in thin layers. Defect engineering creates magnetic functionality through platinum vacancies, enabling ferromagnetic or antiferromagnetic behavior depending on layer number. Emerging applications include neuromorphic computing elements, single-photon detectors, and plasmonic devices operating in the infrared regime.

Historical Development and Discovery

The initial synthesis of platinum diselenide was reported by Minozzi in 1909 through direct combination of platinum and selenium elements. Early characterization focused on structural determination, with the cadmium iodide-type structure identified through X-ray diffraction in the 1930s. The natural occurrence as sudovikovite was discovered in the 1990s in Karelia, Russia, and formally described in 2004. Research interest intensified significantly following the graphene revolution of the 2000s, as scientists sought other two-dimensional materials with complementary properties. The thickness-dependent electronic properties were systematically investigated starting around 2010, with the semiconductor-to-metal transition characterized through optical and electrical measurements. The development of reliable synthesis methods for large-area monolayers around 2015 enabled detailed studies of quantum confinement effects and potential device applications. Recent advances have focused on defect engineering, heterostructure fabrication, and exploration of unconventional quantum phenomena including potential topological properties.

Conclusion

Platinum diselenide represents a significant material in the family of transition metal dichalcogenides, distinguished by its thickness-dependent electronic properties, high electrical conductivity, and chemical stability. The compound's unique structural characteristics, including its octahedral coordination and weak interlayer bonding, enable exfoliation to monolayer thickness with consequent transition from semimetallic to semiconducting behavior. Applications span photodetection, catalysis, sensing, and electronic devices, leveraging the material's appropriate band structure, high carrier mobility, and surface properties. Future research directions include optimization of large-scale synthesis methods, engineering of heterostructures with other two-dimensional materials, exploration of magnetic properties through defect control, and development of practical devices exploiting the compound's unusual quantum phenomena. The fundamental understanding of structure-property relationships in platinum diselenide continues to provide insights into the behavior of layered materials and their potential for advanced technological applications.