We have shown from both simulations and experiments that zwitterion functionalized carbon nanotubes (CNTs) can be used to construct highly efficient desalination membranes. Our simulations predicted that zwitterion functional groups at the ends of CNTs allow a high flux of water, while rejecting essentially all ions. We have synthesized zwitterion functionalized CNT/polyamide nanocomposite membranes with varying loadings of CNTs and assessed these membranes for water desalination. The CNTs within the polyamide layer were partially aligned through a high-vacuum filtration step during membrane synthesis. Addition of zwitterion functionalized CNTs into a polyamide membrane increased both the flux of water and the salt rejection ratio. The flux of water was found to increase by more than a factor of 4, from 6.8 to 28.7 GFD (gallons per square foot per day), as the fraction of CNTs was increased from 0 to 20 wt %. Importantly, the ion rejection ratio increased slightly from 97.6% to 98.6%. Thus, the nanotubes imparted an additional transport mechanism to the polyamide membrane, having higher flow rate and the same or slightly better selectivity. Simulations show that when two zwitterions are attached to each end of CNTs having diameters of about 15 Å, the ion rejection ratio is essentially 100%. In contrast, the rejection ratio for nonfunctionalized CNTs is about 0%, and roughly 20% for CNTs having five carboxylic acid groups per end. The increase in ion rejection for the zwitterion functionalized CNTs is due to a combination of steric hindrance from the functional groups partially blocking the tube ends and electrostatic repulsion between functional groups and ions, with steric effects dominating. Theoretical predictions indicate that an ideal CNT/polymer membrane having a loading of 20 wt % CNTs would have a maximum flux of about 20000 GFD at the conditions of our experiments.
Tetrahedral coordination structures, e.g. crystalline Si, GaAs, CdTe, and octahedral coordination structures, e.g. perovskites, represent two classes of successful crystal structures hitherto for solar cell absorbers. Here, via first‐principles calculations and crystal symmetry analysis, the two classes of semiconductors are shown exhibiting complementary properties in terms of bond covalency/ionicity, optical property, defect tolerance, and stability, which are correlated with their respective coordination number. Therefore, a spinel structure is proposed, which combines tetrahedral and octahedral coordination into a single crystal structure, as an alternative to perovskite and conventional semiconductors for potential photovoltaic applications. The case studies of a class of 105 spinel AB2X4 systems identify five spinel compounds HgAl2Se4, HgIn2S4, CdIn2Se4, HgSc2S4, and HgY2S4 as promising solar cell absorbers. In particular, HgAl2Se4 has suitable bandgap (1.36 eV by GW0 calculation), small direct–indirect bandgap difference (24 meV), appropriate carrier effective mass (me = 0.08 m0, and mh = 0.69 m0), strong optical absorption, and high dynamic stability. This study suggests that crystal systems with mixed tetrahedral and octahedral coordination may open a viable route for emerging solar cell absorbers.
Organic–inorganic hybrid perovskite solar cells have recently been developed at an unprecedented rate as an emerging solar cell technology, with its certified power conversion efficiency (PCE) (23.7%) surpassing conventional thin-film contenders. However, the poor long-term stability and toxicity of Pb pose major setbacks to its commercialization. Theoretical calculations and experimental trail-and-error processes have recently aimed to find alternative perovskites, including inorganic halide perovskites (CsPbI3, CsPbIBr2, etc.), inorganic halide double perovskites (Cs2AgBiBr6, etc.), and chalcogenide single perovskites (BaZrS3, etc.). However, their material properties are inferior to hybrid perovskite in terms of cell performance and material toxicity. Here, a class of lead-free chalcogenide double perovskites A2M(III)M(V)X6 [A = Ca2+, Sr2+, Ba2+; M(III) = Bi3+ or Sb3+; M(V) = V5+, Nb5+, Ta5+; X = S2–, Se2–] are comprehensively investigated with respect to its stability and electronic and optical properties. First-principles calculations on bandgaps, effective masses, optical absorption, and ideal power conversion efficiencies led to the selection of nine stable double chalcogenide perovskites that exhibit superior optoelectronic properties, i.e., quasi-direct bandgaps, balanced electron and hole effective masses, and strong optical absorption owing to the strong antibonding character both at the valence band maximum (VBM) and conduction band minimum (CBM). Unfortunately, thermodynamic stability calculations on massive decomposition pathways show negative decomposition energies ranging from 0 (−0.37) to −66 meV/atom, indicating the difficulty for a thin-film phase. The most likely compound is Ba2BiNbS6, with its decomposition energies (0 and −22 meV/atom for P21/n and R3̅ phases, respectively) within the computational errors, which may be further stabilized by the confinement effect in nanocrystal form.
Chalcogenide perovskites ABX3 (A = Ca, Sr, or Ba; B = Ti, Zr, or Hf; and X = O, S, or Se) have been considered as promising candidates for overcoming the stability and toxic issues of halide perovskites. In this work, we unveil the disparity of the nature of the band gap between halide and chalcogenide perovskites. First-principles calculations show that the prototype cubic phase of chalcogenide perovskites exhibits indirect band gaps with the valence band maximum and the conduction band minimum located at R and Γ points, respectively, in the Brillion zone. Therefore, the optical transitions near band edges of chalcogenide perovskites differ from those of its halide counterparts, although its stable orthorhombic phase embodies a direct band gap. We have further found that the direct–indirect band gap difference of chalcogenide perovskites in the cubic phase demonstrates a linear correlation with t + μ, where t and μ are the tolerance and octahedral factor, respectively, thereby providing a viable way to search chalcogenide perovskites with a quasi-direct band gap.
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