Anisotropic thermo-mechanical response of layered hexagonal boron nitride and black phosphorus: application as a simultaneous pressure and temperature sensor.

Hexagonal boron nitride (hBN) and black phosphorus (bP) are crystalline materials that can be seen as ordered stackings of two-dimensional layers, which lead to outstanding anisotropic physical properties. Knowledge of the thermal equations of state of hBN and bP is of great interest in the field of 2D materials for a better understanding of their anisotropic thermo-mechanical properties and exfoliation mechanism towards the preparation of important single-layer materials like hexagonal boron nitride nanosheets and phosphorene. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the thermoelastic parameters of hBN and bP. Here, we report accurate thermal expansion and compressibility measurements along the individual crystallographic axes, using in situ high-temperature and high-pressure high-resolution synchrotron X-ray diffraction. In particular, we have quantitatively determined the subtle variations of the in-plane and volumetric thermal expansion coefficients and compressibility parameters by subjecting these materials to hydrostatic conditions and by collecting a large number of data points in small pressure and temperature increments. In addition, based on the anisotropic behavior of bP, we propose the use of this material as a sensor for the simultaneous determination of pressure and temperature in the range of 0-5 GPa and 298-1700 K, respectively.

Stability, Chemical Bonding, and Electron Lone Pair Localization in AsN at High Pressure by Density Functional Theory Calculations.

The covalent bonding framework of crystalline single-bonded cubic AsN, recently synthesized under high pressure and high temperature conditions in a laser-heated diamond anvil cell, is here studied by means of density functional theory calculations and compared to single crystal X-ray diffraction data. The precise localization of the nonbonding electron lone pairs and the determination of their distances and orientations are related to the presence of characteristic structural motifs and space regions of the unit cell dominated by repulsive electronic interactions, with the relative orientation of the electron lone pairs playing a key role in minimizing the energy of the structure. We find that the vibrational modes associated with the expression of the lone pairs are strongly localized, an observation that may have implications for the thermal conductivity of the compound. The results indicate the thermodynamic stability of the experimentally observed structure of AsN above ∼17 GPa, provide a detailed insight into the nature of the chemical bonding network underlying the formation of this compound, and open new perspectives to the design and high pressure synthesis of new pnictogen-based advanced materials for potential applications of energetic and technological relevance.

High-pressure and high-temperature synthesis of crystalline Sb3 N5.

A chemical reaction between Sb and N2 was induced under high-pressure (32-35 GPa) and high-temperature (1600-2200 K) conditions, generated by a laser heated diamond anvil cell. The reaction product was identified by single crystal synchrotron X-ray diffraction at 35 GPa and room temperature as crystalline antimony nitride with Sb3 N5 stoichiometry and structure belonging to orthorhombic space group Cmc21 . Only Sb-N bonds are present in the covalent bonding framework, with two types of Sb atoms respectively forming SbN6 distorted octahedra and trigonal prisms and three types of N atoms forming NSb4 distorted tetrahedra and NSb3 trigonal pyramids. Taking into account two longer Sb-N distances, the SbN6 trigonal prisms can be depicted as SbN8 square antiprisms and the NSb3 trigonal pyramids as NSb4 distorted tetrahedra. The Sb3 N5 structure can be described as an ordered stacking in the bc plane of bi- layers of SbN6 octahedra alternated to monolayers of SbN6 trigonal prisms (SbN8 square antiprisms). The discovery of Sb3 N5 finally represents the long sought-after experimental evidence for Sb to form a crystalline nitride, providing new insights about fundamental aspects of pnictogens chemistry and opening new perspectives for the high-pressure chemistry of pnictogen nitrides and the synthesis of an entire class of new materials.

High temperature decomposition of polymeric carbon monoxide at pressures up to 120 GPa.

While polymeric carbon monoxide (pCO) has been experimentally found to remain amorphous and undecomposed at room temperature up to 50 GPa, the question of whether crystalline counterparts of it can be obtained naturally raises. From different computational studies, it can be inferred that either the crystallization of amorphous pCO (a-pCO) or its decomposition into a mixture of CxOy suboxides (x > y) or carbon and CO2 may occur. In this study, we report experimental investigations of the high temperature (700-4000 K) transformation of a-pCO in the 47-120 GPa pressure range, conducted by x-ray diffraction in laser heated diamond anvil cells. Our results show the formation of no crystalline phases other than CO2 phase V, thus indicating the decomposition of the pristine a-pCO into CO2 and, likely, a mixture of amorphous CxOy suboxides and amorphous carbon hardly detectable at extreme conditions. These results support the theoretical picture of the pCO decomposition. We also show that the pressure-temperature kinetic border for this decomposition is very steep, thus indicating a strongly pressure-dependent kinetic barrier.

High-Pressure and High-Temperature Chemistry of Phosphorus and Nitrogen: Synthesis and Characterization of α- and γ-P3N5.

The direct chemical reactivity between phosphorus and nitrogen was induced under high-pressure and high-temperature conditions (9.1 GPa and 2000-2500 K), generated by a laser-heated diamond anvil cell and studied by synchrotron X-ray diffraction, Raman spectroscopy, and DFT calculations. α-P3N5 and γ-P3N5 were identified as reaction products. The structural parameters and vibrational frequencies of γ-P3N5 were characterized as a function of pressure during room-temperature compression and decompression to ambient conditions, determining the equation of state of the material up to 32.6 GPa and providing insight about the lattice dynamics of the unit cell during compression, which essentially proceeds through the rotation of the PN5 square pyramids and the distortion of the PN4 tetrahedra. Although the identification of α-P3N5 demonstrates for the first time the direct synthesis of this compound from the elements, its detection in the outer regions of the laser-heated area suggests α-P3N5 as an intermediate step in the progressive nitridation of phosphorus toward the formation of γ-P3N5 with increasing coordination number of P by N from 4 to 5. No evidence of a higher-pressure phase transition was observed, excluding the existence of predicted structures containing octahedrally hexacoordinated P atoms in the investigated pressure range.

Single-Bonded Cubic AsN from High-Pressure and High-Temperature Chemical Reactivity of Arsenic and Nitrogen.

Chemical reactivity between As and N2 , leading to the synthesis of crystalline arsenic nitride, is here reported under high pressure and high temperature conditions generated by laser heating in a diamond anvil cell. Single-crystal synchrotron X-ray diffraction at different pressures between 30 and 40 GPa provides evidence for the synthesis of a covalent compound of AsN stoichiometry, crystallizing in a cubic P21 3 space group, in which each of the two elements is single-bonded to three atoms of the other and hosts an electron lone pair, in a tetrahedral anisotropic coordination. The identification of characteristic structural motifs highlights the key role played by the directional repulsive interactions between non-bonding electron lone pairs in the formation of the AsN structure. Additional data indicate the existence of AsN at room temperature from 9.8 up to 50 GPa. Implications concern fundamental aspects of pnictogens chemistry and the synthesis of innovative advanced materials.

High pressure synthesis of phosphine from the elements and the discovery of the missing (PH3)2H2 tile.

High pressure reactivity of phosphorus and hydrogen is relevant to fundamental chemistry, energy conversion and storage, and materials science. Here we report the synthesis of (PH3)2H2, a crystalline van der Waals (vdW) compound (I4cm) made of PH3 and H2 molecules, in a Diamond Anvil Cell by direct catalyst-free high pressure (1.2 GPa) and high temperature (T â‰² 1000 K) chemical reaction of black phosphorus and liquid hydrogen, followed by room T compression above 3.5 GPa. Group 15 elements were previously not known to form H2-containing vdW compounds of their molecular hydrides. The observation of (PH3)2H2, identified by synchrotron X-ray diffraction and vibrational spectroscopy (FTIR, Raman), therefore represents the discovery of a previously missing tile, specifically corresponding to P for pnictogens, in the ability of non-metallic elements to form such compounds. Significant chemical implications encompass reactivity of the elements under extreme conditions, with the observation of the P analogue of the Haber-Bosch reaction for N, fundamental bond theory, and predicted high pressure superconductivity in P-H systems.

Anisotropic thermal expansion of black phosphorus from nanoscale dynamics of phosphorene layers.

Black phosphorus (bP) is a crystalline material which can be seen as an ordered stacking of two-dimensional layers, referred to as phosphorene. The knowledge of the linear thermal expansion coefficients (LTECs) of bP is of great interest in the field of 2D materials for a better understanding of the anisotropic thermal properties and exfoliation mechanism of this material. Despite several theoretical and experimental studies, important uncertainties remain in the determination of the LTECs of bP. Here, we report accurate thermal expansion measurements along the three crystallographic axes using in situ high temperature X-ray diffraction. From the progressive reduction of the diffracted intensities with temperature, we monitored the loss of the crystal structure of bP across the investigated temperature range, evidencing two thermal expansion regimes at temperature below and above 706 K. Below 706 K, a strong out-of-plane anisotropy can be observed, while at temperatures above 706 K a larger thermal expansion occurs along the a crystallographic direction. From our data and by taking advantage of ab initio optimization, we propose a detailed anisotropic thermal expansion mechanism of bP, which leads to an inter- and intra-layer destabilization. An interpretation of it, based on the high T perturbation of the stabilizing sp orbital mixing effect, is provided, consistent with the high pressure data.

A Perspective on Recent Advances in Phosphorene Functionalization and Its Applications in Devices.

Phosphorene, the 2D material derived from black phosphorus, has recently attracted a lot of interest for its properties, suitable for applications in materials science. The physical features and the prominent chemical reactivity on its surface render this nanolayered substrate particularly promising for electrical and optoelectronic applications. In addition, being a new potential ligand for metals, it opens the way for a new role of the inorganic chemistry in the 2D world, with special reference to the field of catalysis. The aim of this review is to summarize the state of the art in this subject and to present our most recent results in the preparation, functionalization, and use of phosphorene and its decorated derivatives. We discuss several key points, which are currently under investigation: the synthesis, the characterization by theoretical calculations, the high pressure behavior of black phosphorus, as well as its decoration with nanoparticles and encapsulation in polymers. Finally, device fabrication and electrical transport measurements are overviewed on the basis of recent literature and the new results collected in our laboratories.

The p-sc structure in phosphorus: bringing order to the high pressure phases of group 15 elements.

Black phosphorus was studied by state-of-the-art synchrotron X-ray diffraction in a Diamond Anvil Cell during room temperature compression in the presence of He, H2, N2 and Daphne Oil 7474. The data demonstrate that the existence of the pseudo simple-cubic (p-sc) structure above 10.5 GPa is an intrinsic feature of P independent from the pressure transmitting medium. In the case of He, the pressure evolution of the lattice parameters and unit cell volume of P across the A17, A7 and p-sc structures was obtained and the corresponding EOS derived, providing a deeper insight on the recently reported p-sc structure. The results presented in this letter highlight the key role of the s-p orbital mixing in the formation and stabilization of the p-sc structure up to ∼30 GPa, solving apparent contradictions emerging from previous literature and finally bringing order to the sequence of the high pressure A7 layered structure in group 15 elements.

Spray-loading: A cryogenic deposition method for diamond anvil cell.

An efficient loading technique has been developed for flammable, toxic, or explosive gases which can be condensed at liquid nitrogen temperature and ambient pressure in membrane diamond anvil cells (DACs). This cryogenic technique consists in a deposition of small quantities of the desired gas directly into the sample chamber. The deposition is performed using a capillary that reaches the space between the diamond anvils. The DAC is kept under inert gas overpressure during the whole process, in order to avoid contamination from atmospheric O2, CO2, and H2O. This technique provides significant advantages over standard cryo-loading and gas-loading when the condensation of dangerous samples at liquid nitrogen temperature raises safety concerns because it allows dealing with minimum quantities of condensed gases. The whole procedure is particularly fast and efficient. The "spray-loading" has been successfully used in our laboratory to load several samples including acetylene, ammonia, ethylene, and carbon dioxide/water or red phosphorus/NH3 mixtures.

Interlayer Bond Formation in Black Phosphorus at High Pressure.

Black phosphorus was compressed at room temperature across the A17, A7 and simple-cubic phases up to 30 GPa, using a diamond anvil cell and He as pressure transmitting medium. Synchrotron X-ray diffraction showed the persistence of two previously unreported peaks related to the A7 structure in the pressure range of the simple-cubic phase. The Rietveld refinement of the data demonstrates the occurrence of a two-step mechanism for the A7 to simple-cubic phase transition, indicating the existence of an intermediate pseudo simple-cubic structure. From a chemical point of view this study represents a deep insight on the mechanism of interlayer bond formation during the transformation from the layered A7 to the non-layered simple-cubic phase of phosphorus, opening new perspectives for the design, synthesis and stabilization of phosphorene-based systems. As superconductivity is concerned, a new experimental evidence to explain the anomalous pressure behavior of Tc in phosphorus below 30 GPa is provided.

Pressure induced polymerization of fluid ethylene.

The spontaneous polymerization of fluid ethylene under high temperature and pressure conditions has been characterized by using FTIR absorption spectroscopy. The fluid has been isobarically heated at pressures ranging between 0.4 and 1.5 GPa by means of a resistively heated membrane diamond anvil cell. Besides tracing the instability boundary for spontaneous polymerization in the fluid, we have also measured the reaction kinetics at 1.5 GPa and temperatures ranging between 340 and 423 K. From the rate constants the activation energy of the overall reaction could be computed, information that joined to the molecularity of the initiation step provides some insight about the reaction mechanism. The polymers recovered from the different reactions have been characterized by FTIR, Raman, and X-ray diffraction revealing in all the cases a crystalline material of astonishing quality, likely related to the growth of the polymer in the hot fluid monomer.

Light-induced catalyst and solvent-free high pressure synthesis of high density polyethylene at ambient temperature.

The combined effect of high pressure and electronic photo-excitation has been proven to be very efficient in activating extremely selective polymerisations of small unsaturated hydrocarbons in diamond anvil cells (DAC). Here we report an ambient temperature, large volume synthesis of high density polyethylene based only on high pressure (0.4-0.5 GPa) and photo-excitation (~350 nm), without any solvent, catalyst or radical initiator. The reaction conditions are accessible to the current industrial technology and the laboratory scale pilot reactor can be scaled up to much larger dimensions for practical applications. FTIR and Raman spectroscopy, and X-ray diffraction, indicate that the synthesised material is of comparable quality with respect to the outstanding crystalline material obtained in the DAC. The polydispersity index is comparable to that of IV generation Ziegler-Natta catalysts. Moreover the crystalline quality of the synthesised material can be further enhanced by a thermal annealing at 373 K and ambient pressure.

"Click" on tubes: a versatile approach towards multimodal functionalization of SWCNTs.

Organic functionalization of carbon nanotube sidewalls is a tool of primary importance in material science and nanotechnology, equally from a fundamental and an applicative point of view. Here, an efficient and versatile approach for the organic/organometallic functionalization of single-walled carbon nanotubes (SWCNTs) capable of imparting multimodality to these fundamental nanostructures, is described. Our strategy takes advantage of well-established Cu-mediated acetylene-azide coupling (CuAAC) reactions applied to phenylazido-functionalized SWCNTs for their convenient homo-/heterodecoration with a number of organic/organometallic frameworks, or mixtures thereof, bearing terminal acetylene pendant arms. Phenylazido-decorated SWCNTs were prepared by chemoselective arylation of the CNT sidewalls with diazonium salts under mild conditions, and subsequently used for the copper-mediated cycloaddition protocol in the presence of terminal acetylenes. The latter reaction was performed in one step by using either single acetylene derivatives or equimolar mixtures of terminal alkynes bearing either similar functional groups (masked with orthogonally cleavable protecting groups) or easily distinguishable functionalities (on the basis of complementary analytical/spectroscopic techniques). All materials and intermediates were characterized with respect to their most relevant aspects/properties by TEM microscopy, thermogravimetric analysis coupled with MS analysis of volatiles (TG-MS), elemental analysis, cyclic voltammetry (CV), Raman and UV/Vis spectroscopy. The functional loading and related chemical grafting of both primary amino- and ferrocene-decorated SWCNTs were spectroscopically (UV/Vis, Kaiser test) and electrochemically (CV) determined, respectively.

Changing the dissociative character of the lowest excited state of ethanol by pressure.

Syntheses based on physical methods, such as pressure and light, are extremely attractive to prepare novel materials from pure molecular systems in condensed phases. The structural and electronic modifications induced by selective optical excitation can trigger unexpected chemical reactions by exploiting the high density conditions realized at high pressure. The identification of the microscopic mechanisms regulating this reactivity, mandatory to design synthetic environments appealing for practical applications, requires a careful characterization of both structural and electronic properties as a function of pressure. Here, we report a spectroscopic study, by FTIR and Raman techniques, of the ambient temperature photoinduced reactivity of liquid C(2)H(5)OD up to 1 GPa. The results have been interpreted by comparison with those relative to the fully hydrogenated isotopomer. The dissociation along the O-H (D) coordinate is the primary reactive channel, but the different reactivity of the two isotopomers with rising pressure highlights a dramatic pressure effect on the energy surface of the first electronic excited state. Dissociation along the O-H (D) coordinate becomes the reaction rate-limiting step due to an increase with pressure of the binding character along this coordinate.

High-pressure reactivity of L,L-lactide.

L,L-Lactide, a dimer of L-lactic acid, is the typical monomer used for the catalytic synthesis of poly(L-lactic acid) (PLLA). We studied its phase diagram and reactivity at high pressure and high temperature by means of a diamond anvil cell. FTIR and Raman spectroscopy were employed to probe the changes occurring in the sample. An increase of temperature at pressure higher than 0.1 GPa revealed a solid-solid phase transition before the melting. A reaction was observed immediately after the melting with the almost complete transformation of the starting reactant to an amorphous poly(lactic acid) (PLA). The increase of pressure was found to accelerate the process, suggesting the reaction rate to be limited in the diffusion step. A steeper acceleration, likely due to multiphoton absorption processes of the 647.1 nm laser light by PLA, was observed in the Raman experiments.

High-pressure reactivity of clathrate hydrates by two-photon dissociation of water.

It is shown that photoinduced reactions are observed at room temperature and pressure of few tenths of gigapascal in clathrate hydrates of CO and of model hydrocarbons under mild irradiation at 350 nm with power in the 50-610 mW range. The reactions are triggered by highly reactive OH radicals produced by two-photon excitation of the lowest electronic excited state of water having dissociative character. The formation of CO(2) is observed in all the reactions involving carbonaceous clathrate hydrates, and direct or indirect evidence for the formation of molecular hydrogen is obtained. The CO(2) produced in the reactions can be sequestered as a clathrate hydrate whose stability range seems to extend to room temperature at pressures of 0.5-0.6 GPa. Although the N(2) hydrate is stable up to 0.9 GPa under irradiation, a partial cleavage of the N-N triple bond is produced once the hydrate decomposes at 0.1 GPa.

Photoinduced reactivity of liquid ethanol at high pressure.

The room temperature photoinduced reactivity of liquid ethanol has been studied as a function of pressure up to 1.5 GPa by means of a diamond anvil cell. Exploiting the dissociative character of the lowest electronic excited states, reached through two-photon absorption of near-UV photons (350 nm), irreversible reactive processes have been triggered in the pure system. The active species are radicals forming along two main dissociation channels involving the split of C-O and O-H bonds. The characterization of the reaction products has been performed by in situ FTIR and Raman spectroscopy. At pressures of a few megapascals, molecular hydrogen is the main reaction product, an important issue in the framework of environmentally friendly synthesis of this energetic vector. In the gigapascal range, the main products are ethane, 2-butanol, 2,3-butanediol, 1,1-diethoxyethane, and some carbonylic compounds. The relative amount of these species changes with pressure reflecting the nature of the radicals formed in the photodissociation process. As the pressure increases, the processes requiring a greater molecularity are favored, whereas those requiring internal rearrangements are inhibited. Disproportion products like CH(4), H(2)O, and CO(2) increase when the amount of ethanol decreases due to the reaction, becoming the main products only when ethanol is exhausted.