Harnessing solar energy to produce hydrogen through semiconductor-mediated photocatalytic water splitting is a promising avenue to address the challenges of energy scarcity and environmental degradation. Ever since Fujishima and Honda’s groundbreaking work in photocatalytic water splitting, titanium dioxide (TiO₂) has garnered significant interest as a semiconductor photocatalyst, prized for its non-toxicity, affordability, superior photocatalytic activity, and robust chemical stability. Nonetheless, the efficacy of solar energy conversion is hampered by TiO₂’s wide bandgap and the swift recombination of photogenerated carriers. In pursuit of enhancing TiO₂’s photocatalytic prowess, a panoply of modification techniques has been explored over recent years. This work provides an extensive review of the strategies employed to augment TiO₂’s performance in photocatalytic hydrogen production, with a special emphasis on foreign dopant incorporation. Firstly, we delve into metal doping as a key tactic to boost TiO₂’s capacity for efficient hydrogen generation via water splitting. We elaborate on the premise that metal doping introduces discrete energy states within TiO₂’s bandgap, thereby elevating its visible light photocatalytic activity. Following that, we evaluate the role of metal nanoparticles in modifying TiO₂, hailed as one of the most effective strategies. Metal nanoparticles, serving as both photosensitizers and co-catalysts, display a pronounced affinity for visible light absorption and enhance the segregation and conveyance of photogenerated charge carriers, leading to remarkable photocatalytic outcomes. Furthermore, we consolidate perspectives on the nonmetal doping of TiO₂, which tailors the material to harness visible light more efficiently and bolsters the separation and transfer of photogenerated carriers. The incorporation of various anions is summarized for their potential to propel TiO₂’s photocatalytic capabilities. This review aspires to compile contemporary insights on ion-doped TiO₂, propelling the efficacy of photocatalytic hydrogen evolution and anticipating forthcoming advancements. Our work aims to furnish an informative scaffold for crafting advanced TiO₂-based photocatalysts tailored for water-splitting applications.

A new binuclear Gd(III) complex, [Gd₂(L)₆(Phen)₂]·4H₂O, was synthesized via the reaction of gadolinium(III) nitrate hexahydrate, 4-acetylphenoxyacetic acid (HL), NaOH, and 1,10-phenanthroline (Phen) in a solution of water–ethanol (v:v = 1:1). The Gd(III) complex was characterized using IR, UV–vis, TG-DSC, fluorescence, and single-crystal X-ray diffraction analyses. The results showed that the Gd(III) complex crystallizes in the triclinic system, space group P-1, and each Gd(III) ion was coordinated with two nitrogen atoms (N1, N2, or N1a, and N2a) from two Phen ligands and seven oxygen atoms (O1, O2, O7a, O9, O8, O8a, O10a, or O1a, O2a, O7, O8, O8a, O9a, and O10) from six L ligands, respectively, forming a nine-coordinated coordination mode. The Gd(III) complex molecules formed a one-dimensional chained and three-dimensional network structure via benzenering π-π stacking. The Hirschfeld surface analysis and the calculations of the electron density distributions of the frontier molecular orbitals of the Gd(III) complex were performed. The catalytic activities of the photocatalytic CO₂ reduction and benzyl alcohol oxidation using the Gd(III) complex as a catalyst were performed. The results of the photocatalytic CO₂ reduction showed that the yield and the selectivity of CO reached 41.5 μmol/g and more than 99% after four hours, respectively. The results of the benzyl alcohol oxidation showed that the yield of benzaldehyde was 45.7% at 120 °C with THF as the solvent under 0.5 MPa O₂ within 2 h.

Spin-gapless semiconductor (SGS), a class of zero-gap materials with fully spin-polarized electrons and holes, offers significant potential for high-speed, low-energy consumption applications in spintronics, electronics, and optoelectronics. Our first-principles calculations revealed that the Pca21 C₄N₃ monolayer exhibits a ferromagnetic ground state. Its band structure displays SGS-like characteristics, with the energy gap between the valence and conduction bands near the Fermi level in the spin-down channel much smaller than the one in the other spin channel. To enhance its SGS properties, we introduced electrons into the Pca21 C₄N₃ monolayer by adsorbing the CO gas molecule on its surface. Stable gas adsorption (CO@C₄N₃) effectively narrowed the band gap in the spin-down channel without changing the band gap in the spin-up channel obviously. Moreover, injecting holes into the CO@C₄N₃ system could increase the net magnetic moments and induce an SGS-to-metallic phase transition, while injecting electrons into the CO@C₄N₃ system is able to lower the net magnetic moments and cause an SGS-to-half-metallic phase transition. Our findings not only underscore a new promising material for practical metal-free spintronics applications but also illustrate a viable pathway for designing SGSs.

Up to now, the explanation for the origin of enhanced photocatalytic activity of N doped TiO₂ (N-TiO₂) with H incorporation, which is observed in experiment, is still lacking. In our work, the effects of hydrogenation on the stability and electronic properties of N-TiO₂ have been systematically investigated by first-principles calculations. Our results of the study on stability demonstrate that, both full and part hydrogenation could stabilize N-TiO₂ by largely reducing the formation energy of N doping under Ti-rich conditions. Moreover, the calculated results on the electronic structure show that, for the completely hydrogenated N-TiO₂, band gap becomes slightly larger, which is caused by the full passivation for unpaired electron from N atom. However, for the partially hydrogenated N-TiO₂, due to the interaction between hydrogenated and unhydrogenated N atoms, its valence band maximum shifts to higher energy by 0.32 eV and the valence band states mix with the wide band-gap states, which results in a higher light absorption capacity and carrier separation. Our results not only explain the enhancement of visible light photocatalytic activity experimentally found in N-TiO₂ specimen with H incorporation, but also indicate that, tuning the hydrogenation degree is a hopeful routine to improve the photocatalytic performance of N-TiO₂.

For metal-free low-dimensional ferromagnetic materials, a hopeful candidate for next-generation spintronic devices, investigating their magnetic mechanisms and exploring effective ways to regulate their magnetic properties are crucial for advancing their applications. Our work systematically investigated the origin of magnetism of a graphitic carbon nitride (Pca21 C₄N₃) monolayer based on the analysis on the partial electronic density of states. The magnetic moment of the Pca21 C₄N₃ originates from the spin-split of the 2p_z orbit from special carbon (C) atoms and 2p orbit from N atoms around the Fermi energy, which was caused by the lone pair electrons in nitrogen (N) atoms. Notably, the magnetic moment of the Pca21 C₄N₃ monolayer could be effectively adjusted by adsorbing nitric oxide (NO) or oxygen (O₂) gas molecules. The single magnetic electron from the adsorbed NO pairs with the unpaired electron in the N atom from the substrate, forming a N_sub-N_ad bond, which reduces the system’s magnetic moment from 4.00 μ_B to 2.99 μ_B. Moreover, the NO adsorption decreases the both spin-down and spin-up bandgaps, causing an increase in photoelectrical response efficiency. As for the case of O₂ physisorption, it greatly enhances the magnetic moment of the Pca21 C₄N₃ monolayer from 4.00 μ_B to 6.00 μ_B through ferromagnetic coupling. This method of gas adsorption for tuning magnetic moments is reversible, simple, and cost-effective. Our findings reveal the magnetic mechanism of Pca21 C₄N₃ and its tunable magnetic performance realized by chemisorbing or physisorbing magnetic gas molecules, providing crucial theoretical foundations for the development and utilization of low-dimensional magnetic materials.

Electrochemical CO₂ reduction reaction through utilizing renewable electricity under mild conditions is a promising pathway toward achieving carbon neutrality. In this work, we designed a tubular graphyne functionalized with isolated Co single atom and lowered the activation energy barrier of its rate-determining step to as low as 0.46 eV. The catalytic performance was systematically evaluated through density functional theory calculations. Compared with the planar graphyne functionalized with isolated Co single atom, the tubular one not only significantly improves the utilization efficiency of Co single atoms by exposing them more thoroughly, but also increases the catalytic activity of Co single atom by enhancing electron density of states at the Fermi level, which causes a higher level of activation state for the adsorbed CO₂ molecules. Furthermore, it brought about the CO₂-to-CH₄ reduction reaction pathway, resulting in remarkable catalytic activity and high methane selectivity. Our study demonstrates the efficacy of curvature engineering in enhancing the intrinsic activity of single-atom catalysts, offering a novel strategy for designing advanced carbon cycle catalysts.