ZnGa_2–<i>x</i>In_<i>x</i>S₄ (0 ≤ <i>x</i> ≤ 0.4) and Zn_{1–2<i>y</i>}(CuGa)_<i>y</i>Ga_1.7In_0.3S₄ (0.1 ≤ <i>y</i> ≤ 0.2): Optimize Visible Light Photocatalytic H₂ Evolution by Fine Modulation of Band Structures

Band structure engineering is an efficient technique to develop desired semiconductor photocatalysts, which was usually carried out through isovalent or aliovalent ionic substitutions. Starting from a UV-activated catalyst ZnGa₂S₄, we successfully exploited good visible light photocatalysts for H₂ evolution by In³⁺-to-Ga³⁺ and (Cu⁺/Ga³⁺)-to-Zn²⁺ substitutions. First, the bandgap of ZnGa_2–<i>x</i>­In_<i>x</i>S₄ (0 ≤ <i>x</i> ≤ 0.4) decreased from 3.36 to 3.04 eV by lowering the conduction band position. Second, Zn_{1–2<i>y</i>}(CuGa)_<i>y</i>­Ga_1.7In_0.3S₄ (<i>y</i> = 0.1, 0.15, 0.2) provided a further and significant red-shift of the photon absorption to ∼500 nm by raising the valence band maximum and barely losing the overpotential to water reduction. Zn_0.7Cu_0.15­Ga_1.85In_0.3S₄ possessed the highest H₂ evolution rate under pure visible light irradiation using S^2– and SO₃^2– as sacrificial reagents (386 μmol/h/g for the noble-metal-free sample and 629 μmol/h/g for the one loaded with 0.5 wt % Ru), while the binary hosts ZnGa₂S₄ and ZnIn₂S₄ (synthesized using the same procedure) show 0 and 27.9 μmol/h/g, respectively. The optimal apparent quantum yield reached to 7.9% at 500 nm by tuning the composition to Zn_0.6Cu_0.2­Ga_1.9In_0.3S₄ (loaded with 0.5 wt % Ru)

Ga₄B₂O₉: An Efficient Borate Photocatalyst for Overall Water Splitting without Cocatalyst

Borates are well-known candidates for optical materials, but their potentials in photocatalysis are rarely studied. Ga³⁺-containing oxides or sulfides are good candidates for photocatalysis applications because the unoccupied 4s orbitals of Ga usually contribute to the bottom of the conducting band. It is therefore anticipated that Ga₄B₂O₉ might be a promising photocatalyst because of its high Ga/B ratio and three-dimensional network. Various synthetic methods, including hydrothermal (HY), sol–gel (SG), and high-temperature solid-state reaction (HTSSR), were employed to prepare crystalline Ga₄B₂O₉. The so-obtained HY-Ga₄B₂O₉ are micrometer single crystals but do not show any UV-light activity unless modified by Pt loading. The problem is the fast recombination of photoexcitons. Interestingly, the samples obtained by SG and HTSSR methods both possess a fine micromorphology composed of well-crystalline nanometer strips. Therefore, the excited e^– and h⁺ can move to the surface easily. Both samples exhibit excellent intrinsic UV-light activities for pure water splitting without the assistance of any cocatalyst (47 and 118 μmol/h/g for H₂ evolution and 22 and 58 μmol/h/g for O₂ evolution, respectively), while there is no detectable activity for P25 (nanoparticles of TiO₂ with a specific surface area of 69 m²/g) under the same conditions

Open-Framework Gallium Borate with Boric and Metaboric Acid Molecules inside Structural Channels Showing Photocatalysis to Water Splitting

An open-framework gallium borate with intrinsic photocatalytic activities to water splitting has been discovered. Small inorganic molecules, H₃BO₃ and H₃B₃O₆, are confined inside structural channels by multiple hydrogen bonds. It is the first example to experimentally show the structural template effect of boric acid in flux synthesis

Rational Modulation of Effective Mass Near Band Edge of Li₂SnO₃ to Increase Photogenerated Carrier Separation Ratio and Photocatalytic Performance

Photogenerated carrier separation is important in photocatalysis. Doping may offer control over the effective masses of the photogenerated electrons and holes. Herein, a doping strategy in Li2SnO3 enhanced photogenerated carrier separation, boosting photocatalysis. Substitution of Ge with Sn increased the effective mass of holes and reduced that of electrons; hence, the photogenerated electron/hole lifetime ratio in Li2Sn0.90Ge0.10O3 was approximately 2.8 times as great as that of Li2SnO3. Photocatalytic degradation by Li2Sn0.90Ge0.10O3 reached 100% within 12 min. However, the opposite effect was achieved upon doping with Pb. Theoretical calculations revealed that the low Ge-4p valence band orbital reduced hole mobility, while the Ge-4s orbital hybridized with O-2p near the conduction band minimum increased the electron mobility. Steady-state and time-resolved photoluminescence spectroscopy, electron spin resonance, and liquid chromatography–mass spectrometry were conducted to explore the photocatalytic mechanism. This study provides an understanding of structure–activity relationships to guide the design of high-performance photocatalysts

Cr₂Ge₂Te₆: High Thermoelectric Performance from Layered Structure with High Symmetry

Cr₂Ge₂Te₆: High Thermoelectric Performance from Layered Structure with High Symmetr

Dopant Induced Impurity Bands and Carrier Concentration Control for Thermoelectric Enhancement in p‑Type Cr₂Ge₂Te₆

Our previous work demonstrated that Cr₂Ge₂Te₆ based compounds with a layered structure and high symmetry are good candidates for thermoelectric application. However, the power factor of only ∼0.23 mW/mK² in undoped material is much lower than that of conventional thermoelectrics. This indicates the importance of an electronic performance optimization for further improvements. In this work, either Mn- or Fe-substitution on the Cr site is investigated, with expectations of both carrier concentration control and band structure engineering. First-principle calculations indicate that an orbital hybridization between d orbitals of the doping atom and the p orbital of Te significantly increases the density of states (DOS) around the Fermi level. In addition, it is found that Mn doping is more favorable to improve the electrical properties than Fe doping. By tuning the carrier concentration via Mn doping, the peak power factor rises rapidly from 0.23 mW/mK² to 0.57 mW/mK² at 830 K with <i>x</i> = 0.05. Combined with the intrinsic low thermal conductivity, Cr_1.9Mn_0.1Ge₂Te₆ displays a decent <i>zT</i> of 0.63 at 833 K, a 2-fold value as compared to that of the undoped sample at the same direction and temperature