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Transition metal dichalcogenide (TMDC) monolayers exhibit great photocatalytic performances and tunabilities. By using first-principles simulations based on density functional theory, we demonstrate here that external electric bias can be employed to further improve the photocatalytic hydrogen production efficiencies of the six AB2 (A = Mo, W and B = S, Se, Te) TMDC monolayers since these are influenced by their piezoelectric response. In particular, when subjected to a proper amount of electrically induced tensile biaxial strain, most TMDC monolayers turn into potentially ideal photocatalysts toward the hydrogen evolution reaction (HER). The beneficial effects of introducing tensile biaxial strain on the TMDC monolayers are not limited to the reduction of the band gap and proper adjustment of the band edge positions; the concomitant hydrogen adsorption free energies are also modified in such a way that the HER reaction is noticeably favored.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Energy Letters, copyright © 2022 American Chemical Society, after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsenergylett.2c02040. Electronic doping is a promising approach to modulating the optoelectronic properties of semiconductors, but its effect on optoelectronic behaviors of halide perovskites remains controversial. Here, we comprehensively investigate the impact of Pb substitution in n-type CsPbIBr2 perovskite by utilizing monovalent Ag, divalent Zn, and trivalent Sb. Our findings reveal that the trap densities in doped CsPbIBr2 films are in the order of Ag < Zn < Sb. Compared with the pristine CsPbIBr2, the Ag-doped perovskite features a significantly reduced phase separation; however, the Sb doping accelerates halide segregation, and Zn exerts a negligible influence. The p-doping effect from monovalent Ag can shift the Fermi level of CsPbIBr2 toward the intrinsic midgap, which helps prevent the formation of ionic defects and reduce the migration of halide ions in the perovskite lattice. Through combing the density functional theory simulation, this study discloses the correlation between valence-controlled metal doping and phase segregation, providing a guideline for judiciously doping mixed-halide perovskites for optoelectronic applications.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials and Interfaces, copyright © 2022 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsami.2c02675. Metallic 1T-phase MoS2 exhibits superior hydrogen evolution reaction (HER) performance than natural 2H-phase MoS2 owing to its higher electrical conductivity and abundance of active sites. However, the reported 1T-MoS2 catalysts usually suffer from extreme instability, which results in quick phase transformation at ambient conditions. Herein, we present a facile approach to engineer the phase of MoS2 by introducing intercalated hydrazine. Interestingly, the as-synthesized 1T-dominant MoS2 sample demonstrates excellent ambient stability without noticeable degradation for 3 months. Additionally, the 1T-dominant MoS2 exhibits superior electrical conductivity (~700 times higher than that of 2H-MoS2) and improved electrochemical catalytic performance (current density ~12 times larger than that of 2H-MoS2 at an overpotential of 300 mV vs the reversible hydrogen electrode, RHE). Through experimental characterizations and density functional theory (DFT) calculation, we conclude that the stabilization of the metallic phase could be attributed to the electron donation from hydrazine molecules to the adjacent Mo atoms. The phase control strategy in this work provides a guideline to develop other highly efficient and stable two-dimensional (2D) electrocatalysts.

Organic electrochemical transistors (OECTs) for bioelectronic applications require the design of conjugated polymers that are stable in aqueous environments and afford high energy efficiency and good performance in OECTs. Polymers based on poly(ethylenedioxythiophene) (PEDOT) are promising in this area due to their low oxidation potential and reversible redox, but they often require cross-linking to prevent dissolution and yield OECTs operating in the less efficient depletion mode. In this work, a new conjugated polymer PEDOT-Phos is presented, which combines a conjugated poly(ethylenedioxythiophene) (PEDOT) backbone with alkyl-protected phosphonate groups. PEDOT-Phos exhibits a low oxidation onset potential (-0.157 V vs Ag/AgCl) and its nanoporous morphology affords it a high volumetric capacitance (282 ± 62 F cm–3). Without any cross-linking, additives, or post-treatment, PEDOT-Phos can be used in aqueous OECTs with efficient accumulation mode operation, long-term stability when immersed in aqueous media, low threshold voltages (-0.161 ± 0.005 V), good transconductances (9.3 ± 1.8 mS), and ON/OFF current ratios (618 ± 54) comparable to other PEDOT-based materials in OECTs. These results highlight the great promise of PEDOT-Phos as a stand-alone channel material for energy-efficient, bioelectronic devices.

This document is the Accepted Manuscript version of a Published Work that appeared in final form inACS Sustainable Chemistry and Engineering, copyright © 2023 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acssuschemeng.2c06573. Hydrogen is a promising fossil-fuel alternative fuel owing to its environmentally neutral emissions and high energy density. However, the need for purified water and external power are critical hindrances to the implementation of hydrogen production. The present work demonstrates the potential to overcome these shortcomings through piezo-photocatalysis of seawater using defective BaTiO3–x (BTO) nanoparticles. This material was made piezoelectrically active by a straightforward annealing process under different atmospheres, including O2, N2, Ar, or H2, the latter of which caused Ti4+ ¿ Ti(4–x)+ multiple reductions and structural distortions that stabilize piezoelectric tetragonal domains. A suite of experimental techniques was employed to reveal the effects of reduction on the energy band structure. A substantial piezoelectric effect and the presence of self-polarization were confirmed by piezoresponse force microscopy, while simulation work clarified the role of vibrations on band bending deriving from the self-polarization. The hydrogen evolution through photocatalysis, piezocatalysis, and piezo-photocatalysis over the defective BaTiO3–x nanoparticles was characterized with deionized (DI) water, simulated seawater, and natural seawater. A promising HER with a rate of 132.4 µmol/g/h was achieved using DI water through piezo-photocatalysis without a cocatalyst. In contrast, a substantial HER rate of 48.7 µmol/g/h was obtained for natural seawater, despite the deleterious impact of dissolved ions. The present work offers new perspectives for large-scale green H2 production using abundant natural resources with a conventional piezoelectric material that is readily available but still affected by the ions dissolved in seawater.

The role of local chemical environments in the electron energy loss spectra of complex multiferroic oxides was studied using computational and experimental techniques. The evolution of the O K-edge across an interface between bismuth ferrite (BFO) and lanthanum strontium manganate (LSMO) was considered through spectral averaging over crystallographically equivalent positions to capture the periodicity of the local O environments. Computational techniques were used to investigate the contribution of individual atomic environments to the overall spectrum, and the role of doping and strain was considered. Chemical variation, even at the low level, was found to have a major impact on the spectral features, whereas strain only induced a small chemical shift to the edge onset energy. Through a combination of these methods, it was possible to explain experimentally observed effects such as spectral flattening near the interface as the combination of spectral responses from multiple local atomic environments.

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of physical chemistry C, copyright © 2022 American Chemical Society, after peer review and technical editing by the publisher. To access the final edited and published work see 10.1021/acs.jpcc.2c04297. Multiferroic BiFe0.5Cr0.5O3 (BFCO) thin films are promising candidates for emerging optoelectronics and all-oxide solar absorbers. Yet, a thorough understanding of the structural evolution and associated changes in the functional properties of BFCO is lacking. Here, we explore thickness-dependent structural phase transitions in epitaxial BFCO films and ascertain the impact of the accompanying crystallographic distortions on their photoresponse. The results show that the strain imposed by the substrate changes the crystal symmetry, inducing a transition from a tetragonal-like to rhombohedral-like phase through a rather complex strain relaxation mechanism upon increasing film thickness. This change in crystallographic distortion also induces a shift of ~150 meV in the bandgap. Moreover, wavelength-resolved photocurrent measurements reveal that the absorption onset is redshifted for the tetragonal-like structure, implying light absorption up to wavelengths of 800 nm. First-principles calculations shed further light on the symmetry-induced changes in the electronic structure of the BFCO films, where the crystallographic symmetry is shown to be a decisive factor for modifying the characteristics of the valence band maximum and conduction band minimum in the perovskite oxides, revealing a new type of Mott multiferroic in BFCO system. This work provides a new strategy to further engineer the optoelectronic properties of the multiferroic oxide films through thickness-induced phase transitions.

Conventional refrigeration methods based on compression-expansion cycles of greenhouse gases are environmentally threatening and cannot be miniaturized. Electrocaloric effects driven by electric fields are especially well suited for implementation of built-in cooling in portable electronic devices. However, most known electrocaloric materials present poor cooling performances near room temperature, contain toxic substances, and require high electric fields. Here, we show that lead-free ferroelectric thin-film bilayers composed of (Bi

We report a strategy for the surface engineering of electrode materials for water splitting by constructing a nickel–iron (Ni0.95Fe0.05) layered double hydroxide (LDH) nanosheets on silver nanowire...

Magnetic materials with large magnetic anisotropy are essential for workaday applications such as permanent magnets and magnetic data storage. There is widespread interest in finding efficient ways of controlling magnetic anisotropy, among which strain control has proven to be a very powerful technique. Here, we demonstrate the strain-mediated magnetic anisotropy in SrCoO3−δ thin film, a perovskite oxide that is metallic and adopts a cubic structure at δ ≤ 0.25. We find that the easy-magnetization axis in SrCoO3−δ can be rotated by 90° upon application of moderate epitaxial strains ranging from −1.2 to +1.8%. The magnetic anisotropy in compressive SrCoO3−δ thin films is huge, as shown by magnetic hysteresis loops rendering an anisotropy energy density of ∼106 erg/cm3. The local variance in magnetic force microscopy upon temperature and magnetic field reveals that the evolution of magnetic domains in the SCO thin film is strongly dependent on magnetic anisotropy.

We study the high-pressure–high-temperature phase diagram and superionicity of alkaline earth metal (AEM) difluorides (AF2, A = Ca, Sr, Ba) with first-principles simulation methods. We find that the superionic behavior of SrF2 and BaF2 at high pressures differ appreciably from that previously reported for CaF2 [Phys. Rev. Lett. 2014, 113, 235902]. Specifically, the critical superionic temperature of SrF2 and BaF2 in the low-pressure cubic fluorite phase is not reduced by effect of compression, and the corresponding high-pressure orthorhombic contunnite phases become superionic at elevated temperatures. We get valuable microscopic insights into the superionic features of AEM difluorides in both the cubic fluorite and orthorhombic contunnite phases by means of ab initio molecular dynamics simulations. We rationalize our findings on the structural and superionic behavior of AF2 compounds in terms of simple ionic radii arguments and generalize them across the whole series of AEM dihalides (AB2, A = Mg, Ca, Sr...

Finding solid-state electrolytes with high ionic conductivity near room temperature is an important prerequisite for developing all-solid-state electrochemical batteries. Here, we investigate the effects of point defects (vacancies) and biaxial stress on the superionic properties of fast-ion conductors (represented by the archetypal compounds CaF2, Li-rich antiperovskite Li3OCl, and AgI) by using classical molecular dynamics and first-principles simulation methods. We find that the critical superionic temperature of all analyzed families of fast-ion conductors can be reduced by several hundreds of degrees through the application of relatively small biaxial stresses (|σ| ≤ 1 GPa) on slightly defective samples (cv ∼ 1%). In AgI, we show that superionicity can be triggered at room temperature by applying a moderate compressive biaxial stress of ∼1 GPa. In this case, we reveal the existence of a σ-induced order–disorder phase transition involving sizable displacements of all the ions with respect to the equil...

In this study, we report the effect of doping in morphotropic BiFeO3 (BFO) thin films on mechanical properties, revealing variations in the elasticity across the competing phases and their boundaries. Spectroscopic force–distance (F–D) curves and force mapping images by AFM are used to characterize the structure and elastic properties of three BFO thin-film candidates (pure-BFO, Ca-doped BFO, La-doped BFO). We show that softening behavior is observed in isovalent La-doped BFO, whereas hardening is seen in aliovalent Ca-doped BFO. Furthermore, quantitative F–D measurements are extended to show threshold strengths for phase transitions, revealing their dependence on doping in the system. First-principles simulation methods are also employed to understand the observed mechanical properties in pure and doped BFO thin films and to provide microscopic insight on them. These results provide key insight into doping as an effective control parameter to tune nanomechanical properties and suggest an alternative fram...

Oxide materials facilitating high-ion mobility and fast-ion transport are highly sought after for applications requiring intensive redox cycling. Some of these materials, in addition, may exhibit interesting multifunctional properties originating from electron correlation such as magnetism and metal–insulator transitions. SrCoO3-δ (SCO) is one such compound, which recently has attracted a lot of interest due to its ability of taking on and releasing oxygen fluently. Here we investigate thoroughly the dynamics of oxygen vacancies and redox cycles in SCO under broad epitaxial strain conditions (−1.2% ≤ η ≤ +3.9%). We show that the capacity of this material to act as an “oxygen sponge” depends strongly on the strain conditions, with moderate strains of ca. +2% providing the optimal conditions for reversible redox cycling. First-principles simulation methods are employed to understand the experimental trends observed for SCO reduction in vacuum, and to provide microscopic insight into the formation of oxygen ...

Mechanocaloric materials experience a change in temperature when a mechanical stress is applied on them adiabatically. Thus, far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, Ts, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterized by a large entropy increase of the order of 10(2) JK(-1) kg(-1), can be externally tuned with hydrostatic, biaxial, or uniaxial stresses. In particular, Ts can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF2 and PbF2, two archetypal fluorite-structured FIC, close to their critical points are of the order of 10(2) and 10(1) K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications.

We study the phase diagram of calcium fluoride (CaF2) under pressure using classical molecular dynamics simulations performed with a reliable pairwise interatomic potential of the Born−Mayer−Huggins form. Our results obtained under conditions 0 ≤ P ≲ 20 GPa and 0 ≤ T ≲ 4000 K reveal a rich variety of multiphase boundaries involving different crystal, superionic, and liquid phases, for all of which we provide an accurate parametrization. Interestingly, we predict the existence of three special triple points (i.e., solid−solid−superionic, solid−superionic−superionic, and superionic−superionic−liquid coexisting states) within a narrow and experimentally accessible thermodynamic range of 6 ≤ P ≤ 8 GPa and 1500 ≤ T ≤ 2750 K. In addition, we examine the role of short-ranged repulsive (SR) and long-ranged van der Waals attractive (LA) interactions in the prediction of melting lines with the finding that SR Ca−F and LA F−F contributions are most decisive.

We report a first-principles study of a CO2 gas-sorbent material consisting of calcium atoms and carbon-based nanostructures. In the low gas pressure regime, we find that Ca decoration of nanotubes and graphene possess unusually large CO2 uptake capacities (similar to 0.4-0.6 g CO2/g sorbent) as a result of their topology and a strong interaction between the metal dopants and CO2 molecules. Decomposition of the gas-loaded nanomaterials into CO gas and calcium oxide (CaO) is shown to be thermodynamically favorable; thus performance of the carbon capture process is further enhanced via formation of calcium carbonate (CaCO3). Gas adsorption CO2/N-2 selectivity issues have been also addressed with the finding that N-2 molecules bind to the metal-doped surfaces more weakly than CO2 molecules. The predicted molecular binding and accompanying gas selectivity features strongly suggest the potential of Ca-doped carbon materials for CO2 capture applications.