Efficient photocatalytic hydrogen evolution and CO2 reduction by HfSe2/GaAs3 and ZrSe2/GaAs3 heterostructures with direct Z-schemes.

The elaborate configuration of the heterostructure is crucial and challenging to achieve high solar-to-hydrogen efficiency or CO2 reduction efficiency . Here, we predict two heterostructures composed of HfSe2, ZrSe2, and GaAs3 monolayers. The maximum of 42.71%/35.12% with the heterostructures can be reached with the perfect match between the bandgap and band edges. The configurations of the heterostructures are discovered from 12 possible stacking types of the three monolayers. The formation energy, potentials of band edges, carrier mobilities, and optical absorption were used to identify the feasibility of the CO2 reduction reaction (CO2RR), the hydrogen evolution reaction (HER), and the oxygen evolution reaction (OER). The and based on overpotentials and bandgaps and the Gibbs free energies (ΔGs) are evaluated to quantificationally access the photocatalytic performance of the constructed heterostructures. The results demonstrate that high can be obtained for the solar photocatalytic Z-schemes with the HfSe2/GaAs3 and ZrSe2/GaAs3 heterostructures, and these values can be further enhanced through strain engineering. Moreover, small changes in ΔGs were observed for HER, OER, and CO2RR. Therefore, the two heterostructures have excellent performance in photocatalytic hydrogen evolution and CO2 reduction. The results of the electronic properties revealed that the delicate matching of the projected band edges of the monolayers in the heterostructures is responsible for the high photocatalytic performance.

Electron-phonon coupling, bipolar effects, and thermoelectric performance of the CuSbS2 monolayer.

The thermoelectric performance of the CuSbS2 monolayer is determined using the relaxation times obtained from electron-phonon coupling calculations and the transport properties of phonons and electrons. Based on the fully relaxed structure, the lattice thermal conductivity and the electronic transport coefficients are evaluated by solving the Boltzmann transport equation for phonons and electrons under relaxation time approximation, respectively. The tendencies of the transport coefficients depending on the carrier concentrations and temperatures are studied to understand the thermoelectric performance. Based on the bipolar effect, the transport coefficients and intrinsic carrier concentrations, we determined the dimensionless figure of merit ZT in the 300-800 K range. The results demonstrate that the CuSbS2 monolayer should be an p-type semiconductor, and the maximum ZT of 1.36 is obtained, indicating that the monolayer is a good candidate for high-temperature thermoelectric devices. Substantial bipolar effects are observed, and the ones in the x-direction are stronger in comparison to those in the y-direction, which is responsible for the smaller ZT in the x-direction.

Strain-tunable electronic structure and anisotropic transport properties in Janus MoSSe and g-SiC van der Waals heterostructure.

The van der Waals heterostructures (vdWHs) create a multi-purpose platform to design unique structures for efficient photovoltaic and optoelectronic applications. In this study, on the basis of the first-principles calculations, we present a type-II semiconducting MoSSe/g-SiC vdWH with a moderate bandgap value of 1.31 eV. In particular, the large conduction band offset of 1.18 eV and valence band offset of 0.90 eV are distinguished, which can act as powerful driving forces to promote interlayer charge transfer. Moreover, MoSSe/g-SiC vdWH possesses high carrier mobilities and anisotropic transport properties with a larger transport current along the zigzag direction. More importantly, tensile strain can transform indirect into direct band gap and enhance the visible-light absorption while sustaining type-II band alignment. These results reveal the new physical nature of MoSSe/g-SiC vdWH and demonstrate its practical application potential in photovoltaics and optoelectronic nanodevices.

The high power conversion efficiency of a two-dimensional GeSe/AsP van der Waals heterostructure for solar energy cells.

Constructing a van der Waals heterostructure is a practical way to promote the conversion efficiency of solar energy. Here, we demonstrate the efficient performance of a GeSe/AsP heterostructure in solar energy cells based on the first-principles calculations. The electronic properties, optical absorption, and optoelectronic properties are calculated to evaluate the efficiency of the newly designed heterostructure. The results indicate that the GeSe/AsP heterostructure possesses a type-II band alignment with an indirect bandgap of 1.10 eV, which greatly promotes the effective separation of photogenerated carriers. Besides, an intrinsic electric field is formed in the direction from the AsP to GeSe monolayer, which is beneficial to prevent the recombination of the photogenerated electron-hole pair. Simultaneously, a strong optical absorption is observed in the visible light range. The predicted power conversion efficiency (PCE) of the GeSe/AsP heterostructure is 16.0% and can be promoted to 17.3% by applying 1% biaxial compression strain. The present results indicate that the GeSe/AsP heterostructure is a promising candidate material for high-performance solar cells.

Direct laser cooling schemes for the triatomic SOH and SeOH molecules based on ab initio electronic properties.

Direct laser cooling is a very promising method to obtain cold molecules for various applications. However, a molecule with satisfactory electronic and optical properties for the optical scheme is difficult to identify. By suggesting criteria for the qualified molecules, we develop a method to identify the suitable polyatomic molecules for direct laser cooling. The new criteria from the equilibrium geometrical structures and fundamental frequencies of the ground and low-lying excited states are used to replace the past ones based on Franck-Condon factors. The new method can rapidly identify the preferable one among many candidate polyatomic molecules based on ab initio calculations because the new criteria are free from the construction of potential energy surfaces. The method is testified by using triatomic molecules containing OH. All the reported and two new molecules suitable for direct laser cooling are identified by comparing 168 electronic states of 28 molecules with the new criteria. The newly found molecules have been confirmed using the Franck-Condon factors from the construction of potential energy surfaces. Finally, the optical schemes for the direct laser cooling of the SOH and SeOH molecules are established.

Ab initio studies on the spin-forbidden cooling transitions of the LiRb molecule.

The spin-forbidden cooling of the LiRb molecule is investigated based on ab initio quantum chemistry calculations. The multireference configuration interaction method is used to generate the potential energy curves (PECs) of the ground state X(1)Σ(+) and the low-lying excited states a(3)Σ(+), B(1)Π, and b(3)Π. The spin-orbit coupling effects for the PECs and the transition dipole moments (TDMs) between the X(1)Σ(+), b(3)Π and a(3)Σ(+) states are also calculated. The analytical functions for the PECs are deduced. The rovibrational energy levels, the spectroscopic parameters and the Franck-Condon factors (FCF) are determined by solving the Schrödinger equation of nuclear movement with the obtained analytical functions. The b(3)Π0 ↔ X(1)Σ(+) and b(3)Π1 ↔ X(1)Σ(+) transitions have highly diagonal distributed FCFs and non-zero TDMs, demonstrating that the LiRb molecule could be a very promising candidate for laser cooling. Therefore, a three-cycle laser cooling scheme for the molecule has been proposed based on these two spin-forbidden transitions. Using the radiative lifetime and linewidth calculated from the obtained TDM functions, we present further analysis of the cooling of LiRb and the corresponding KRb molecule. The transition b(3)Π0 ↔ X(1)Σ(+) is found to be a practical transition to cool the LiRb molecule, and a sub-microkelvin cool temperature could be reached for the KRb molecule using a similar laser cooling scheme.

Ab initio study on the ground and low-lying states of BAlk (Alk = Li, Na, K) molecules.

The potential energy curves (PECs) and dipole moment functions of (1)Π, (3)Π, (1)Σ(+), and (3)Σ(+) states of BAlk (Alk = Li, Na, K) are calculated using multireference configuration interaction method and large all-electron basis sets. The effects of inner-shell correlation electron for BAlk are considered. The ro-vibrational energy levels are obtained by solving the Schrödinger equation of nuclear motion based on the ab initio PECs. The spectroscopic parameters are determined from the ro-vibrational levels with Dunham expansion. The PECs are fitted into analytical potential energy functions using the Morse long-range potential function. The dipole moment functions for the states of BAlk are presented. The transition dipole moments for (1)Σ(+) → (1)Π and (3)Σ(+) → (3)Π states of BAlk are obtained. The interactions between the outermost electron of Alk and B 2p electrons for (1)Π, (3)Π, (1)Σ(+), and (3)Σ(+) states are also analyzed, respectively.

Quasi-classical trajectory study of the N(2D) + H2(X1ΣG+) → NH(X3Σ-) + H(2S) reaction based on an analytical potential energy surface.

Using the multireference configuration interaction method with the Davidson correction and a large orbital basis set (aug-cc-pV5Z), we obtain an energy grid that includes 17,500 points for the construction of a new analytical potential energy surface (APES) for the N((2)D) + H(2)(X(1)Σ(g)(+)) → NH(X(3)Σ(-)) + H((2)S) reaction. The APES, which contains 145 parameters and is represented with a many-body expansion and a new switch function, is fitted from the ab initio energies using an adaptive nonlinear least-squares algorithm. The geometric characteristics of the reported APES in the literature and those of our APES are also compared. On the basis of the APES that we obtained, reaction cross sections are computed by means of quasi-classical trajectory calculations and compared with the experimental and theoretical values available in the literature.

An ab initio study of the ground and low-lying excited states of KBe with the effect of inner-shell electrons.

The potential energy curves (PECs) of 1(2)Σ(+), 2(2)Σ(+), 1(2)Π, and 2(2)Π states of KBe are calculated using multireference configuration interaction method and large all-electron basis sets. Four sets of frozen core orbitals (FCOs) are considered to examine the effect of inner-shell correlation electrons on the molecular properties. The ro-vibrational energy levels are obtained by solving the Schrödinger equation of nuclear motion based on the ab initio PECs. The spectroscopic parameters are determined from the ro-vibrational levels with Dunham expansion. The PECs are fitted into analytical potential energy functions using the Morse long-range potential function. The dipole moment functions of the states for KBe calculated with different FCOs are presented. The transition dipole moments for KBe between 1(2)Σ(+) and 2(2)Σ(+) states, 1(2)Π and 1(2)Σ(+) states, and 2(2)Π and 1(2)Σ(+) states are also obtained.

Insights into Photogenerated Carrier Dynamics and Overall Water Splitting of the CrS3/GeSe Heterostructure.

Based on the nonadiabatic molecular dynamics (NAMD) simulations and the first-principles calculations, we explore the overall water-splitting schemes and the photogenerated carrier dynamics for two configurations (CG and CyG) of the CrS3/GeSe van der Waals heterostructures. The photocatalytic direct Z-schemes and carrier migration pathways for hydrogen and oxygen evolution reactions (HER/OER) are constructed based on the electronic properties. The solar-to-hydrogen efficiency (η'STH values) of the schemes can reach 10.60% and 10.17% and further rise under tensile strain. The NAMD results demonstrate similar transfer times of the electron/hole for HER/OER and more rapid electron-hole recombination in CG enables it to be superior to CyG in photocatalytic performance. Moreover, the Gibbs free energy indicates that both the HERs and OERs turn to spontaneously proceed with CG and CyG at pH = 0-12.37 and pH = 2.55-11.01, respectively. These facts reveal that the CrS3/GeSe heterostructure is promising in photocatalytic overall water splitting.

A new analytical potential energy surface for the singlet state of He2H+.

The analytic potential energy surface (APES) for the exchange reaction of HeH(+) (X(1)Σ(+)) + He at the lowest singlet state 1(1)A(∕) has been built. The APES is expressed as Aguado-Paniagua function based on the many-body expansion. Using the adaptive non-linear least-squares algorithm, the APES is fitted from 15 682 ab initio energy points calculated with the multireference configuration interaction calculation with a large d-aug-cc-pV5Z basis set. To testify the new APES, we calculate the integral cross sections for He + H(+)He (v = 0, 1, 2, j = 0) → HeH(+) + He by means of quasi-classical trajectory and compare them with the previous result in literature.

LiXO2(X = Co, Rh, Ir) and solar light photocatalytic water splitting for hydrogen generation.

Alkali metal transition oxide LiCoO2 has been successfully commercialized as a lithium-ion battery material, and some attention is paid to its homologous derivatives LiRhO2 and LiIrO2. However, the photocatalytic properties have not been explored yet for these compounds. Using the first-principles calculations, we carry out investigations on the electronic properties, light absorption, and mobility to understand the feasibility of LiXO2(X = Co, Rh, Ir) for solar light photocatalytic hydrogen generation from water-splitting. The results show that the band edges of LiCoO2 and LiRhO2 meet the redox potential requirements of the water-splitting hydrogen evolution reaction. In addition, the enhanced absorptions of LiXO2(X = Co, Rh, Ir) in the visible light range imply that they could well respond to solar light, while the significant difference in the mobilities of electrons or holes can strengthen spatial charge separation of the photoexcited electron-hole pairs. The solar-energy-to-hydrogen conversion efficiencies of LiCoO2 and LiRhO2 can reach 11.2% and 15.5%, respectively. The results support LiCoO2 and LiRhO2 as promising candidates for visible-light photocatalytic hydrogen production from water-splitting.

2D XBiSe3(X = Ga, In, Tl) monolayers with high carrier mobility and enhanced visible-light absorption.

The geometrical configurations of the XBiSe3 (X = Ga, In, Tl) monolayers are identified by employing the first-principles density functional theory calculations, and the stabilities are confirmed by phonon dispersion, formation energy, and ab initio molecular dynamics simulation, respectively. The bandgap and band edges, the density of states, the optical absorption, mobility, and effect of strain engineering are evaluated to understand the photoelectronic properties of the monolayers. The results show that the XBiSe3 monolayers have the indirect bandgaps of 1.14-1.69 (1.20-1.84) eV by HSE06(GW), leading to the enhanced optical absorption from the visible to near-ultraviolet region. The large mobility of the electron and hole are also observed, which is helpful for the separation and transfer of the photogenerated carrier pair. The band edges and bandgaps, as well as the optical absorptions, can effectively be tuned by strain engineering. It should be noted that the band edges of the InBiSe3 monolayer could satisfy the condition of redox potential for the hydrogen evolution reaction under the compressive strain heavier than -3%, implicating this monolayer can also be used for photocatalytic water splitting to produce hydrogen. Therefore, these monolayers have potential applications in photocatalytic materials or photoelectronic devices such as energy harvesters and visible-light sensors.

Crystallization of alkane melts induced by carbon nanotubes and graphene nanosheets: a molecular dynamics simulation study.

The crystallization of alkane melts on carbon nanotubes (CNT) and the surface of graphene nanosheets (GNS) is investigated using molecular dynamics (MD) simulations. The crystallization process of the alkane melts is analyzed in terms of the bond-orientational order parameter, atomic radial distribution for the CNT/alkane, atomic longitudinal distribution for the GNS/alkane, and diffusion properties. The dimensional effects of the different carbon-based nanostructures on the crystallization of alkane melts are shown. It is found that one-dimensional CNT has a stronger ability to induce the crystallization of the polymer than that of two-dimensional GNS, which provides a support at molecular level for the experimental observation [Li et al., J. Am. Chem. Soc., 2006, 128, 1692 and Xu et al., Macromolecules, 2010, 43, 5000]. From the MD simulations, we also find that the crystallization of alkane molecules has been completed with the highly cooperative processes of adsorption and orientation.

Quasi-classical trajectory study of the Ne + H2(+) → NeH(+) + H reaction based on global potential energy surface.

Using the multireference configuration interaction method with a Davidson correction and a large orbital basis set (aug-cc-pVQZ), we obtain an energy grid that includes 32 038 points for the construction of a new analytical potential energy surface (APES) for the Ne + H(2)(+) → NeH(+) + H reaction. The APES is represented as a many-body expansion containing 142 parameters, which are fitted from 31 000 ab initio energies using an adaptive nonlinear least-squares algorithm. The geometric characteristics of the reported APES and the one presented here are also compared. On the basis of the APES we obtained, reaction cross sections are computed by means of quasi-classical trajectory (QCT) calculations and compared with the experimental and theoretical data in the literature.

Interactions of M(z)-X complexes (M = Cu, Ag, and Au; X = He, Ne, and Ar; and z = ±1).

The coupled cluster singles and doubles method with perturbative treatment of triple excitations is applied to calculate the potentials of M(z)-X complexes (M = Cu, Ag, and Au; X = He, Ne, and Ar; and z = ±1). The bond functions and the basis set superposition errors are considered to obtain accurate interaction energies. The potential energy curves of all complexes are obtained. The vibrational energy levels and the spectroscopic parameters for these complexes are determined. The analytical potential energy functions are also fitted based on the potential energies.

Direct laser cooling the NH molecule with the pseudo-closed loop triplet-triplet transition including intervening electronic states.

Direct laser cooling molecule is useful way to obtain the accurate molecular spectroscopy. However, most of the reported direct laser cooling schemes are only involved the molecules with a singlet or doublet ground state because the one with a triplet ground state is more complex, especially when the first-excited state is not suitable for the pseudo-closed loop transition. Using NH as the prototype of the simplest heteronuclear molecule with a triplet ground state, we focus on constructing the direct laser cooling scheme with a pseudo-closed loop triplet-triplet transition including intervening electronic states. The potential energy curves and transition dipole moments are calculated for the X3Σ-, a1Δ, b1Σ+, and A3Π states by using the multireference configuration interaction including spin-orbit coupling with the aug-cc-pV5Z basis sets. The rotational and vibrational energy levels of each electronic state are obtained by solving the Schrödinger equation of nuclear motion with the obtained potential energy curves. A two-color laser cooling scheme is established based on the 3Π1 â†’ X3Σ- transition because the highly diagonal Franck-Condon factors make the transition suitable for constructing the pseudo-closed loop transition. The radiative lifetimes, the Doppler temperature, and the recoil temperature are calculated to access the cooling effect of the optical scheme. The results demonstrate that the 3Π1 â†’ X3Σ- transition is much superior to the other transitions and the intervening a1Δ and b1Σ+ will not significantly impact the pseudo-closed loop transition of the laser cooling scheme. The accumulate FCF reach 0.99996 implies that about 25,000 scattering photons are available before leaking, which can cool the NH molecule to the Doppler temperature of 20.2 µK.

Two-dimensional SiMI4(M = Ge, Sn) monolayers as visible-light-driven photocatalyst of hydrogen production.

Two-dimensional (2D) materials of SiMI4(M = Ge, Sn) monolayers are identified as promising visible-light-driven photocatalyst for hydrogen evolution reaction by DFT calculations. The dynamical and thermal stabilities of the two monolayers are confirmed by the phonon dispersion calculations and ab initiomolecular dynamics (AIMD) simulations, respectively.The results show that the two 2D materials have indirect bandgaps of 2.45 and 2.43 eV, and the band edges can match the hydrogen evolution reaction conditions. Absorption spectra show that the monolayers respond tovisible light and can be tuned by different strains.Besides, the hole and electron mobilitiesare different, which is beneficial for photoelectronic performance. The mechanisms of the hydrogen evolution reaction and the direct water splitting process are also explored. The calculational results support the promising applications of SiMI4(M = Ge, Sn) monolayers asvisible-light-driven photocatalyst of hydrogen production.

First-principles investigation on the thermoelectric performance of half-Heusler compound CuLiX(X = Se, Te).

The remarkable thermoelectric performance is predicted for half-Heusler (HH) compounds of CuLiX (X = Se, Te) based on the first-principles calculation, the deformation potential (DP) theory, and semi-classical Boltzmann theory. The Slack model is employed to evaluate the lattice thermal conductivity and the result is in good agreement with the previously reported data. The results of mechanical properties demonstrate that CuLiSe is ductile but CuLiTe is brittle. The relaxation time and the carrier mobility are calculated with DP theory. The electrical and thermal conductivities are obtained by using the semi-classical Boltzmann theory based on the relaxation approximation. The Seebeck coefficient and power factor are obtained and their characters are analyzed. The dimensionless figure of merits (ZT) is obtained for the p- and n-type CuLiX. The maximum ZT of 2.65 can be achieved for n-type CuLiTe at the carrier concentration of 3.19 × 1019 cm-3 and 900 K, which indicates that this compound is a very promising candidate for the highly efficient thermoelectric materials.

Photocatalytic water splitting for hydrogen generation driven by tetragonal, trigonal, hexagonal and cubic LiCoO2 and visible light.

The photocatalytic properties of LiCoO2 are not explored up to date although its cubic and trigonal structures are explored experimentally. Here, we investigate the feasibility of photocatalytic hydrogen production from water splitting driven by the tetragonal, trigonal, hexagonal and cubic LiCoO2 with the irradiation of the visible light. The band structure, density of state, optical absorption and mobility are calculated by the first-principles density functional theory. The results show that the band edges of all the four structures of LiCoO2 match to the conditions of the redox potentials of water splitting reaction and the enhanced optical absorption in the visible light range is observed. The obvious difference between the mobilities of the hole and electron are identified, especially for the cubic LiCoO2. All the obtained results suggest that the considered structures of LiCoO2 are promising candidates for the photocatalytic water splitting to produce hydrogen with the irradiation of the visible light.