Zuju Ma

Cadmium-rare earth oxyborates Cd4ReO(BO3)3 (Re = Y, Gd, Lu): congruently melting compounds with large SHG responses

A new series of cadmium–rare earth (Re) oxyborates Cd4ReO(BO3)3 (Re = Y, Gd, Lu) have been synthesized through high-temperature solid-state reactions in platinum crucibles. In their crystal structures, ReO6 (Re = Y, Gd, Lu) and CdOn (n = 6, 8) are distorted polyhedra interconnected via shared edges and corners in a 3D framework with tunnels along the c-axis, where all B atoms are located. BO3 planar triangles lie approximately parallel to the (001) plane. The combination of triangular BO3 groups with π-conjugated systems and the d10 Cd2+ cation with polar displacement produces large SHG responses. Second harmonic generation (SHG) measurements indicate that Cd4YO(BO3)3, Cd4GdO(BO3)3, and Cd4LuO(BO3)3 feature large SHG responses that are approximately 5.2, 5.0, and 5.3 × KH2PO4 (KDP), respectively. They are all phase matchable nonlinear optical materials that have wide transparent regions ranging from UV to near IR. These title compounds melt congruently, suggesting that the Cd4ReO(BO3)3 (Re = Y, Gd, Lu) complexes are very promising NLO materials. Their electronic structures and optical properties were analyzed by using the Vienna ab initio theoretical method.

Anisotropic thermoelectric properties of layered compounds in SnX2 (X = S, Se): a promising thermoelectric material

Thermoelectrics interconvert heat to electricity and are of great interest in waste heat recovery, solid-state cooling and so on. Here we assessed the potential of SnS2 and SnSe2 as thermoelectric materials at the temperature gradient from 300 to 800 K. Reflecting the crystal structure, the transport coefficients are highly anisotropic between a and c directions, in particular for the electrical conductivity. The preferred direction for both materials is the a direction in TE application. Most strikingly, when 800 K is reached, SnS2 can show a peak power factor (PF) of 15.50 μW cm(-1) K(-2) along the a direction, while a relatively low value (11.72 μW cm(-1) K(-2)) is obtained in the same direction of SnSe2. These values are comparable to those observed in thermoelectrics such as SnSe and SnS. At 300 K, the minimum lattice thermal conductivity (κmin) along the a direction is estimated to be about 0.67 and 0.55 W m(-1) K(-1) for SnS2 and SnSe2, respectively, even lower than the measured lattice thermal conductivity of Bi2Te3 (1.28 W m(-1) K(-1) at 300 K). The reasonable PF and κmin suggest that both SnS2 and SnSe2 are potential thermoelectric materials. Indeed, the estimated peak ZT can approach 0.88 for SnSe2 and a higher value of 0.96 for SnS2 along the a direction at a carrier concentration of 1.94 × 10(19) (SnSe2) vs. 2.87 × 10(19) cm(-3) (SnS2). The best ZT values in SnX2 (X = S, Se) are comparable to that in Bi2Te3 (0.8), a typical thermoelectric material. We hope that this theoretical investigation will provide useful information for further experimental and theoretical studies on optimizing the thermoelectric properties of SnX2 materials.

Porous FeNi oxide nanosheets as advanced electrochemical catalysts for sustained water oxidation

Iron-nickel oxide porous nanosheets were synthesized via a controlled transformation from LDH precursors, which exhibit advanced OER performance with a low overpotential of 213 mV at 10 mA cm−2, small Tafel slope (32 mV dec−1) and long-term stability, due to their high specific surface area, abundant active sites, small charge transfer resistance, and suitable adsorption energy for intermediates.

Enhanced thermoelectric performance of layered SnS crystals: the synergetic effect of temperature and carrier concentration

We present a detailed theoretical study of the SnS compound, which has not been investigated in depth to date, concerning its crystal structure, electronic structure and thermoelectric property. The results of this study show that pure SnS is not a good thermoelectric material but that its ZT can be increased by adjusting both the temperature and carrier concentration. Further, the optimal temperatures and carrier concentrations for producing the peak ZT are identified. The peak ZT is always below unity in the low-temperature Pnma phase; conversely, when the crystal undergoes a displacive phase transition at 878 K, the peak ZT is enhanced to 1.61 ± 0.02 at 1080 K. Additionally, the average ZT in the Cmcm phase (e.g., approximately 1.3) is significantly higher than that in the Pnma phase (e.g., 0.31 ± 0.05). Therefore, the optimally doped SnS material may be highly efficient in its thermal-to-electrical energy conversion at high temperatures. We attribute the remarkable high ZT of doped SnS to the high sensitivity of the electrical conductivity to the carrier concentration. The results of this study describe a simple and viable strategy to optimize the ZT value of the SnS compound using the synergetic tuning of temperature and carrier concentration.

An effective strategy to achieve deeper coherent light for LiB3O5

LiB3O5 (LBO), although with high nonlinearity, is angular non-phase matched in deep ultraviolet second harmonic generation (SHG) processes due to its small birefringence. The structural configuration distortion is a promising way to improve the birefringence. By means of first principles computations, the effect of strain along the a, b and c-axes on the birefringence of LBO is systematically predicted in this work. We find that the birefringence can be effectively and considerably enhanced (to 0.075 at 266 nm) by applying a rather weak strain (0.42 GPa) along the c-axis. Such a strain-induced increase in the birefringence of LBO is favourable for extending the shortest wavelength that can be produced by means of the SHG process. The results indicate that strain-engineering is an effective strategy for enhancing birefringence, which behaves like a switch to control the generation of deeper coherent light output for some nonlinear optical crystals.

Strain-induced improvements on linear and nonlinear optical properties of SrB4O7 crystal

Although the high nonlinearity, strontium tetraborate crystal SrB4O7 is angular non-phasematched in UV SHG process due to its low birefringence. In this Letter, we revealed that its birefringence can be significantly enhanced by uniaxial strain based on the first principles computations. The birefringence is thirteen and sixteen times larger than those of unstrained crystal at -10% a-axial compressive and 10% a-axial tensile strain, respectively. The compressive strain also effectively improve the static second-order coefficients and shift the optical absorption edge towards the UV side, which would shed light on the modulations of UV/VUV nonlinear optical crystals by directionally external stress.

Band engineering of AgSb1−xBixO3 for photocatalytic water oxidation under visible light

Ag-based oxides, particularly AgSbO3, have attracted attention in photocatalytic O2 evolution from water splitting. Employing state-of-the-art DFT calculations and a statistical mechanical approach, we investigated the incorporation of Bi into AgSbO3 to form a AgSb1−xBixO3 solid-solution for tuning the band gap to the optimum (around 2.0 eV) for the photocatalysis of water splitting under sunlight. A phase transition from the pyrochlore to ilmenite phase was predicted at x ≈ 0.21. The band gap decreases almost linearly with increasing Bi concentration for each phase. The decreased band gap is attributed to the lower energy of the Bi s–O p antibonding orbitals than that of the Sb s–O p antibonding orbitals in the conduction bands. Excitingly, a band gap of around 2.0 eV was obtained at x = 0.1875. The combination of a strong oxidizing potential and an optimal band gap for solar light absorption makes the pyrochlore AgSb0.8125Bi0.1875O3 solid-solution a promising candidate for the production of oxygen in a Z-scheme water-splitting system.

Computation-predicted, stable, and inexpensive single-atom nanocatalyst Pt@Mo2C – an important advanced material for H2 production

The finding that transition metals on Mo2C-supported nanocatalysts are promising for water-gas shift (WGS) reactions at room temperature has generated much excitement. However, the progress achieved with computational chemistry in this area is far behind that of experimental studies. Accordingly, density functional theory (DFT) calculations have been used to design the catalytic activity center structure and study the stabilities and catalytic performances of transition metals doped on β-Mo2C(001) surfaces. A new catalyst that comprises atomically dispersed Pt over Mo2C was designed using DFT. The bimetallic Mo2C surfaces doped with single metal Pt exhibit catalytic activities similar to those of the Pt systems for WGS, while demonstrating the advantages of lower costs and higher thermal stabilities. Importantly, the Pt@Mo2C catalyst is more efficient than the pure Pt catalyst for H2 production under the same reaction conditions. Meanwhile, the density of active sites of Pt@Mo2C(001) for H2 production is considerably increased due to its highly dispersed Pt structure. Therefore, Mo and Pt can synergistically increase H2 production. These findings are significantly beneficial for establishing the relationship between the structure and characteristics of the catalyst, understanding the catalytic activities of single-atom catalysts, and gaining insight into the feasibility of developing substitutes for expensive noble metal catalysts.

Thermoelectric enhancement in sliver tantalate via strain-induced band modification and chemical bond softening

The improvement of the thermoelectric performance (determined by the ZT value) of materials is limited by the inter-correlation of the transport coefficients. By performing first principle calculations combined with semi-classical Boltzmann theory, we have systematically investigated the geometric, electronic and thermoelectric properties of AgTaO3 under external strain. It is demonstrated that the electronic structure and phonon transport can be modified independently. Compressive strain along the c-axis decreases the energy separation between the light and heavy valence bands and generates multiple valence band valleys near the Fermi level. Such band structure modification greatly enhances the Seebeck coefficient for the p-type doped compound, which is increased from 600 μV K−1 (δc = 0%) to 690 μV K−1 (δc = −5%). Similarly, tensile strain softens the chemical bonding, favoring the reduction of thermal conductivity. Therefore, the improvement in the thermoelectric performance of AgTaO3 under strain suggests that strain engineering is a generally applicable route to enhancing the Seebeck coefficient and ZT in thermoelectric materials.

Thickness-controlled electronic structure and thermoelectric performance of ultrathin SnS2 nanosheets

The thermoelectric conversion efficiency of a material relies on a dimensionless parameter (ZT = S2σT/κ). It is a great challenge in enhancing the ZT value basically due to that the related transport factors of most of the bulk materials are inter-conditioned to each other, making it very difficult to simultaneously optimize these parameters. In this report, the negative correlation between power factor and thermal conductivity of nano-scaled SnS2 multilayers is predicted by high-level first-principle computations combined with Boltzmann transport theory. By diminishing the thickness of SnS2 nanosheet to about 3 L, the S and σ along a direction simultaneously increase whereas κ decreases, achieving a high ZT value of 1.87 at 800 K. The microscopic mechanisms for this unusual negative correlation in nano-scaled two dimensional (2D) material are elucidated and attributed to the quantum confinement effect. The results may open a way to explore the high ZT thermoelectric nano-devices for the practical thermoelectric applications.