9 research outputs found

    Elucidating the Competitive Adsorption of H<sub>2</sub>O and CO<sub>2</sub> in CALF-20: New Insights for Enhanced Carbon Capture Metal–Organic Frameworks

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    In light of the pressing need for efficient carbon capture solutions, our study investigates the simultaneous adsorption of water (H2O) and carbon dioxide (CO2) as a function of relative humidity in CALF-20, a highly scalable and stable metal–organic framework (MOF). Advanced computer simulations reveal that due to their similar interactions with the framework, H2O and CO2 molecules compete for the same binding sites, occupying similar void regions within the CALF-20 pores. This competition results in distinct thermodynamic and dynamical behaviors of H2O and CO2 molecules, depending on whether one or both guest species are present. Notably, the presence of CO2 molecules forces the H2O molecules to form more connected hydrogen-bond networks within smaller regions, slowing water reorientation dynamics and decreasing water entropy. Conversely, the presence of water speeds up the reorientation of CO2 molecules, decreases the CO2 entropy, and increases the propensity for CO2 to be adsorbed within the framework due to stronger water-mediated interactions. Due to the competition for the same void spaces, both H2O and CO2 molecules exhibit slower diffusion when molecules of the other guest species are present. These findings offer valuable strategies and insights into enhancing the differential affinity of H2O and CO2 for MOFs specifically designed for carbon capture applications

    The Study of Near-Band-Edge Property in Oxygen-Incorporated ZnS for Acting as an Efficient Crystal Photocatalyst

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    A wide gap semiconductor material has attracted attention as a heterophotocatalyst because of its light harvesting nature to be used in alternative energy production for the next generation. We, herein, grow and synthesize ZnS<sub>(1–<i>x</i>)</sub>O<sub><i>x</i></sub> series compounds using the chemical vapor transport (CVT) method with I<sub>2</sub> serving as the transport agent. Different crystals, such as undoped ZnS and oxygen-doped ZnS<sub>0.94</sub>O<sub>0.06</sub> and ZnS<sub>0.88</sub>O<sub>0.12</sub>, revealed different bright palette emissions that were presented in photoluminescence spectra in our previous report. To study the electron–hole pair interaction of this sample series, the near-band-edge transitions of the sample series were characterized in detail by photoconductivity (PC) experiments. Additional results from surface photovoltage (SPV) spectra also detected the surface and defect-edge transitions from the higher oxygen-doped ZnS crystals. PC measurement results showed a red-shift in the bandgap with increasing incorporation of oxygen on ZnS. Consequently, the samples were subjected to photoirradiation by xenon lamp for the degradation of methylene blue (MNB) by acting as heterophotocatalysts. Undoped ZnS emerged as the best photocatalyst candidate with the fastest rate constant value of 0.0277 min<sup>–1</sup>. In cubic {111} ZnS [{111} c-ZnS], the polarized Zn<sup>+</sup> → S<sup>–</sup> ions may play a vital role as a photocatalyst because of their strong electron–hole polarization, which leads to the mechanism for degradation of the MNB solution

    In-Plane Axially Enhanced Photocatalysis by Re<sub>4</sub> Diamond Chains in Layered ReS<sub>2</sub>

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    Two-dimensional (2D) semiconductors play a crucial role in high-efficiency photocatalysts because of their high surface-to-volume ratio. The surface property is a key part of photocatalysis. In this work, the enhanced photocatalytic behavior of the layered ReS<sub>2</sub> with optical polarization along the Re<sub>4</sub> nano-diamond-chain (DC) direction (<i>b</i> axis) has been demonstrated. The unpolarized photoconductivity (PC) response of ReS<sub>2</sub> with an applied bias along the <i>b</i> axis is approximately 1 order higher than that of the applied bias perpendicular to the <i>b</i> axis. The polarization-dependent PC spectra of <i>E</i> ∥ <i>b</i> also reveal a higher photoresponsivity with respect to those measured along the <i>E</i> ⊥ <i>b</i> polarization for the layered ReS<sub>2</sub>. This result indicates that stronger polarization dipoles as well as a larger amount of photogenerated carriers and surface states can contribute to the <i>C</i>-plane ReS<sub>2</sub> under the illumination of <i>E</i> ∥ <i>b</i> polarized photons. With the special axial effect, the layered ReS<sub>2</sub> 2D photocatalyst shows much faster degradation rates of 5.6 and 12.3 than the other transition-metal dichalcogenides of TaS<sub>2</sub> and MoS<sub>2</sub> for the degradation of methylene blue (MNB) solution. For the polarization-dependent photodegradation test, the degradation rate of illuminated <i>E</i> ∥ <i>b</i> polarized photons is also approximately 12 times faster than that of the illuminated <i>E</i> ⊥ <i>b</i> polarized light in a 25 μM MNB solution. The enhanced photocatalytic behavior of ReS<sub>2</sub> along the DC also shows a peak photoreponse of ∼25 μV detected in the polarized photovoltaic spectrum of the 0.5 μM MNB dye-sensitized solar cell positioned at ∼1.99 eV. The formation of a nano-DC and a one-layer trigonal crystalline phase is beneficial for the versatile energy applications of ReS<sub>2</sub>

    Ultraefficient Ultraviolet and Visible Light Sensing and Ohmic Contacts in High-Mobility InSe Nanoflake Photodetectors Fabricated by the Focused Ion Beam Technique

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    A photodetector using a two-dimensional (2D) low-direct band gap indium selenide (InSe) nanostructure fabricated by the focused ion beam (FIB) technique has been investigated. The FIB-fabricated InSe photodetectors with a low contact resistance exhibit record high responsivity and detectivity to the ultraviolet and visible lights. The optimal responsivity and detectivity up to 1.8 × 10<sup>7</sup> A W<sup>–1</sup> and 1.1 × 10<sup>15</sup> Jones, respectively, are much higher than those of the other 2D material-based photoconductors and phototransistors. Moreover, the inherent photoconductivity (PC) quantified by the value of normalized gain has also been discussed and compared. By excluding the contribution of artificial parameters, the InSe nanoflakes exhibit an ultrahigh normalized gain of 3.2 cm<sup>2</sup> V<sup>–1</sup>, which is several orders of magnitude higher than those of MoS<sub>2</sub>, GaS, and other layer material nanostructures. A high electron mobility at room temperature reaching 450 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> has been confirmed to be one of the major causes of the inherent superior PC in the InSe nanoflakes. The oxygen-sensitized PC mechanism that enhances carrier lifetime and carrier collection efficiency has also been proposed. This work demonstrates the devices fabricated by the FIB technique using InSe nanostructures for highly efficient broad-band optical sensing and light harvesting, which is critical for development of the 2D material-based ultrathin flexible optoelectronics

    Humidity-Responsive Polymorphism in CALF-20: A Resilient MOF Physisorbent for CO<sub>2</sub> Capture

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    CALF-20 is a zinc-triazole-oxalate-based metal–organic framework (MOF) that exhibits selective CO2 physisorption, which makes it attractive for potential applications in CO2 capture. However, to implement such applications, the CALF-20 structure must endure extended exposure to environmental conditions that may lead to changes in structure, chemistry, and, hence, performance. Here we find that the originally reported CALF-20 phase (now denoted α-CALF-20) undergoes a structure transformation following exposure to humid environments to generate a new polymorph, β-CALF-20, identified here through combined synchrotron powder X-ray diffraction and pair distribution function (PDF) analysis. This α-to-β transformation is fully reversible with α-CALF-20 being regenerated by treating β-CALF-20 under dry air or vacuum. The rapid reversion of β- to α-CALF-20 under conditions relevant to measuring the gas adsorption isotherm required the CO2 adsorption properties for β-CALF-20 to be evaluated computationally. Experimental evaluation of the adsorption behavior of β-CALF-20 is not practical. These analyses suggest that β-CALF-20 has a higher CO2 heat of adsorption than α-CALF-20, which may be advantageous to CO2 sorption selectivity at low partial pressures

    Humidity-Responsive Polymorphism in CALF-20: A Resilient MOF Physisorbent for CO<sub>2</sub> Capture

    No full text
    CALF-20 is a zinc-triazole-oxalate-based metal–organic framework (MOF) that exhibits selective CO2 physisorption, which makes it attractive for potential applications in CO2 capture. However, to implement such applications, the CALF-20 structure must endure extended exposure to environmental conditions that may lead to changes in structure, chemistry, and, hence, performance. Here we find that the originally reported CALF-20 phase (now denoted α-CALF-20) undergoes a structure transformation following exposure to humid environments to generate a new polymorph, β-CALF-20, identified here through combined synchrotron powder X-ray diffraction and pair distribution function (PDF) analysis. This α-to-β transformation is fully reversible with α-CALF-20 being regenerated by treating β-CALF-20 under dry air or vacuum. The rapid reversion of β- to α-CALF-20 under conditions relevant to measuring the gas adsorption isotherm required the CO2 adsorption properties for β-CALF-20 to be evaluated computationally. Experimental evaluation of the adsorption behavior of β-CALF-20 is not practical. These analyses suggest that β-CALF-20 has a higher CO2 heat of adsorption than α-CALF-20, which may be advantageous to CO2 sorption selectivity at low partial pressures

    Interplay Between Cr Dopants and Vacancy Clustering in the Structural and Optical Properties of WSe<sub>2</sub>

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    Here, we analyze the effect of Cr doping on WSe<sub>2</sub> crystals. The topology and the chemistry of the doped samples have been investigated by atom-resolved scanning transmission electron microscopy combined with electron energy loss spectroscopy. Cr (measured to have formal valence 3+) occupies W sites (formal valence 4+), indicating a possible hole doping. However, single or double Se vacancies cluster near Cr atoms, leading to an effective electron doping. These defects organization can be explained by the strong binding energy of the Cr<sub>W</sub>–V<sub>se</sub> complex obtained by density functional theory calculations. In highly Cr-doped samples, a local phase transition from the 2H to the to 1T phase is observed, which has been previously reported for other electron-doped transition-metal dichalcogenides. Cr-doped crystals suffer a compressive strain, resulting in an isotropic lattice contraction and an anisotropic optical bandgap energy shift (25 meV in-plane and 80 meV out-of-plane)

    Humidity-Responsive Polymorphism in CALF-20: A Resilient MOF Physisorbent for CO<sub>2</sub> Capture

    No full text
    CALF-20 is a zinc-triazole-oxalate-based metal–organic framework (MOF) that exhibits selective CO2 physisorption, which makes it attractive for potential applications in CO2 capture. However, to implement such applications, the CALF-20 structure must endure extended exposure to environmental conditions that may lead to changes in structure, chemistry, and, hence, performance. Here we find that the originally reported CALF-20 phase (now denoted α-CALF-20) undergoes a structure transformation following exposure to humid environments to generate a new polymorph, β-CALF-20, identified here through combined synchrotron powder X-ray diffraction and pair distribution function (PDF) analysis. This α-to-β transformation is fully reversible with α-CALF-20 being regenerated by treating β-CALF-20 under dry air or vacuum. The rapid reversion of β- to α-CALF-20 under conditions relevant to measuring the gas adsorption isotherm required the CO2 adsorption properties for β-CALF-20 to be evaluated computationally. Experimental evaluation of the adsorption behavior of β-CALF-20 is not practical. These analyses suggest that β-CALF-20 has a higher CO2 heat of adsorption than α-CALF-20, which may be advantageous to CO2 sorption selectivity at low partial pressures

    Single-Layer ReS<sub>2</sub>: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy

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    Rhenium disulfide (ReS<sub>2</sub>) and diselenide (ReSe<sub>2</sub>), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS<sub>2</sub> with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS<sub>2</sub> and ReSe<sub>2</sub> can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a nonstoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nanoelectronic circuits in 2D materials
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