High-pressure Behaviors Of N₂Confined In AFI Zeolite Nanomaterials

The study of structural and physical properties by means of nanotechnologycombined with high-prssure technology has been an important research field in highpressure, and it will reveal many attractive and novel phenomena, structures andmechanism. In this work, we study the high-pressure behaviors ofquasi-one-dimensional molecules confined in nanochannels system by taking thenitrogen confined inside the channel of zeolite single crystal as an example, and studythe finite nanosize effect on pressure-induced structure transition by taking Mn₃O₄nanoparticles as an example. The work will be of great significance for searching forstable high-pressure phase and revealing the new phase different from bulk material.Among the diatomic molecules, nitrogen has a special position since it has thegreatest binding energy and, except for H₂, the shortest bond length. Experimental andtheoretical work has revealed that there are several phase transitions occurred innitrogen. Among them, the cubic gauche phase releases a large amount of energyunder the transformation to the molecular state, much more than that of the mostpowerful energy materials. However, experimental study on the stability of confinednitrogen at ambient and high pressure is still a new subject. Besides, AlPO₄-5(AFI), atype of transparent microporous crystal containing one-dimensional channels, is alsoan ideal host template to study the confinement effect and has been used forintroducing some guest molecular species. The framework stability of AFI underpressure is an important topic, which, however has not been studied and discussed upto now. In this work, the porous zeolite AFI is used as the storage medium for nitrogen to study high pressure behavior of the confined system and reveal the confinementeffect on the structure transition of materials.1) The structural stability of AlPO₄-5zeolite (AFI) has been studied as a functionof pressure up to34.4GPa by using synchrotron X-ray diffraction. It is revealed thatthe phase transition sequence of AFI is hexagonal phase→the mixedhexagonal/orthorhombic phase→amorphous phase using the pressure transmissionmedium (PTM) with both silicone oil and a mixture of silicone oil and liquid nitrogen.It is found that AFI has a faster deformation in axial than in radial direction underpressure. The AFI structural stability can be enhanced significantly by using PTM withnitrogen. The crystalline-to-amorphous transition pressure for AFI increased from8.5GPa to15.9GPa. The results demonstrate that nitrogen molecules can be inserted intothe channels of porous zeolite AFI single crystals, exerting a supporting effect againstthe structure collapse of AFI and thus improving their structural stability. The differentpressure behaviors observed in several crystal planes have been discussed in term ofthe possible anisotropic interactions between the filled nitrogen and the host AFI. Thisresult may provid some information for deeply understanding the structure transitionof AFI and it has been shown that AFI is an ideal template to study the nanosizeconfinement of small molecules.2) High-pressure behavior of nitrogen confined inside the AFI channels isinvestigated by Raman spectroscopy up to44GPa. The Raman A1gmodeν of nitrogenwas split intoν andν under the pressure below14.5GPa. Compared with bulk nitrogen,the pressure coefficient of ν belowe5GPa was closed to that of the A1gmode of βphase, and remarkably higher than that of the liquid nitrogen. The result indicates thatnitrogen has been inserted into the channels of AFI. In accordance with the XRDinformation of AFI, the confined nitrogen on analysis is found to be a solid state, not the liquid state. At5GPa-14.5GPa, the pressure coefficient decreased noticeably, andwas closed to that of δ phase, which means the confined nitrogen is a more solid form.Above14.5GPa, the frequence of Raman mode from confined nitrogen decreasedsuddenly, and the full width half maximum increased. The pressure coefficientdecreased further, and was closed to that of ε phase. It shows that the solid nitrogen isstill confined inside the amorphous AFI.We also observed the frequence of A1gmode at2331cm^-1did not change underpressure from the intact AFI in the high pressure experiment. As another point of view,it proves that the nitrogen can be inserted into the channels of porous zeolite AFI. Dueto the low concentration of nitrogen molecules in AFI channels, the nitrogenmolecules have weak interactions between each other, and maintain the low-pressurephase under pressure. These high-pressure Raman spectra of confined nitrogen havenever been reported before, which shows the nitrogen confined in AFI gives a verydifferent results from the bulk nitrogen.3) Stability of nitrogen confined inside the nanosystem has been studied. The A1gmode of nitrogen has been first observed in the intact AFI after releasing pressure from33GPa, and the mode could be effectively stabilized for at least125days, whichelucidates that the confined nitrogen can be obtained at ambient pressure. Comparedwith the bulk nitrogen, data point of the nitrogen confined in AFI lies on the fittingline of β-N₂, and is far from that of liquid nitrogen, which probably means that thenitrogen confined in AFI maintains a solid state. Chemical and structural modificationsof the AFI structure can be made in order to modify the insertion conditions and allowuse in some energy medium (N₂) capture and storage technologies.Effect of grain size on pressure-induced structural transition in Mn₃O₄has alsobeen investigated. Mn₃O₄is a typical AB₂O₄-type spinel. Spinels have important technological applications, and can be used as magnetic materials, superhard materials,and high-temperature ceramics. Recently, Mao et al. compared the compressionalbehaviors of bulk and nanorod LiMn₂O₄. It is the first report of high-pressure researchon AB₂O₄-type spinel, which makes the nanomaterials possibly have enhancedapplication in battery and displays the striking effect of grain size on high pressurestudy. The high-pressure behaviors of Mn₃O₄nanoparticles are studied for the firsttime in this work, and the following are concluded:High-pressure behaviors of bulk and nano-material Mn₃O₄are investigated byXRD and Raman spectroscopy. Compared with the bulk, nanocrystalline Mn₃O₄shows an elevation of phase transition pressure and bulk modulus and different phasetransformation routines. Under pressure bulk Mn₃O₄transformed directly fromhausmannite phase to marokite phase, while nanocrystalline Mn₃O₄undergone twophase transitions: from hausmannite phase to a new high-pressure phase and then toorthorhombic marokite-like phase. The new high-pressure phase has been recognizedas an orthorhombic CaTi₂O₄-type structure. On decompression to ambient pressure,the marokite phase is quenchable in bulk Mn₃O₄. While for Mn₃O₄nanoparticles, thecoexistence of hausmannite and the marokite phase have been observed in therecovered samples. It is proposed that the unique atomic conformation in Mn₃O₄spinel structures, the cation distribution and the higher surface energy together withthe nanosize effect play crucial roles in the unusual high-pressure behavior of Mn₃O₄nanoparticles. As a representative spinel with excellent physical properties, thestructure stability of the nanocrystalline Mn₃O₄under pressure is of significance for anunderstanding of the structure transition of AB₂O₄-type spinels, which is relevant tomany research areas, including understanding the phase transition mechanism andengineering materials with enhanced properties.