The light absorption properties of semiconductor-based photocatalysts to a large extent determine the relevant catalytic performance. Traditional strategies in broadening the light absorption range are usually accompanied with unfavorable changes in redox ability and dynamics of photoinduced species that would confuse the comprehensive optimization. In this work, we propose a nontrivial excitonic transition regulation strategy for gaining sub-bandgap light absorption in low-dimensional semiconductor-based photocatalysts. Using bismuth oxybromide (BiOBr) as a model system, we highlight that the light absorption cut-off edge could be effectively extended up to 500 nm by introducing Bi vacancies. On the basis of theoretical simulations and spectroscopic analyses, we attributed the broadening of light absorption to the promotion of excitonic transition that is generally forbidden in pristine BiOBr system, associated with Bi-vacancy-induced excited-state symmetry breaking. In addition, Bi vacancy was demonstrated to implement negligible effects on other photoexcitation properties like excited-state energy-level profiles and kinetics. Benefiting from these features, the defective sample exhibits a notable advantage in gaining visible-light-driven photocatalytic reactions.

Photocatalytic water oxidation is a crucial step in water splitting, but is generally restricted by the slow kinetics. Therefore, it is necessary to develop high-performance water oxidation photocatalysts. Herein, the Fe-doped Bi₂WO₆ nanosheets with oxygen vacancies (OVs) were synthesized for enhanced photocatalytic water oxidation efficiency, showing a synergistic effect between Fe dopants and OVs. When a molar fraction of 2% Fe was doped into the Bi₂WO₆ nanosheets, the visible-light-driven photocatalytic oxygen evolution rate was increased up to 131.3 µmol·h⁻¹·g_cat⁻¹ under ambient conditions, which was more than 3 times that of pure Bi₂WO₆ nanosheets. The proper doping concentration of Fe could promote the formation of OVs and at the same time modulate the band structure of catalysts, especially the position of the valence band maximum (VBM), leading to effective visible-light absorption and enhanced oxidizing ability of photogenerated holes. With ameliorated localized electron distribution, fast charge transfer channel emerged between the OVs and adjacent metal atoms, which accelerated the charge carrier transfer and promoted the separation of photoexcited electrons and holes. This work provides feasible approaches for designing efficient two-dimensional semiconductor water oxidation photocatalysts that could utilize visible-light, which will make more use of solar energy.

Photooxidation provides a promising strategy for removing the dominant indoor pollutant of HCHO, while the underlying photooxidation mechanism is still unclear, especially the exact role of H₂O molecules. Herein, we utilize in-situ spectral techniques to unveil the H₂O-mediated HCHO photooxidation mechanism. As an example, the synthetic defective Bi₂WO₆ ultrathin sheets realize high-rate HCHO photooxidation with the assistance of H₂O at room temperature. In-situ electron paramagnetic resonance spectroscopy demonstrates the existence of •OH radicals, possibly stemmed from H₂O oxidation by the photoexcited holes. Synchrotron-radiation vacuum ultraviolet photoionization mass spectroscopy and H₂¹⁸O isotope-labeling experiment directly evidence the formed •OH radicals as the source of oxygen atoms, trigger HCHO photooxidation to produce CO₂, while in-situ Fourier transform infrared spectroscopy discloses the HCOO* radical is the main photooxidation intermediate. Density-functional-theory calculations further reveal the •OH formation process is the rate-limiting step, strongly verifying the critical role of H₂O in promoting HCHO photooxidation. This work first clearly uncovers the H₂O-mediated HCHO photooxidation mechanism, holding promise for high-efficiency indoor HCHO removal at ambient conditions.

Regulating the selectivity of CO₂ photoreduction is particularly challenging. Herein, we propose ideal models of atomic layers with/without element doping to investigate the effect of doping engineering to tune the selectivity of CO₂ photoreduction. Prototypical ZnCo₂O₄ atomic layers with/without Ni-doping were first synthesized. Density functional theory calculations reveal that introducing Ni atoms creates several new energy levels and increases the density-of-states at the conduction band minimum. Synchrotron radiation photoemission spectroscopy demonstrates that the band structures are suitable for CO₂ photoreduction, while the surface photovoltage spectra demonstrate that Ni doping increases the carrier separation efficiency. In situ diffuse reflectance Fourier transform infrared spectra disclose that the CO₂^·- radical is the main intermediate, while temperature-programed desorption curves reveal that the ZnCo₂O₄ atomic layers with/without Ni doping favor the respective CO and CH₄ desorption. The Ni-doped ZnCo₂O₄ atomic layers exhibit a 3.5-time higher CO selectivity than the ZnCo₂O₄ atomic layers. This work establishes a clear correlation between elemental doping and selectivity regulation for CO₂ photoreduction, opening new possibilities for tailoring solar-driven photocatalytic behaviors.

Designing efficient electrocatalysts for the hydrogen evolution reaction (HER) has attracted substantial attention owing to the urgent demand for clean energy to face the energy crisis and subsequent environmental issues in the near future. Among the large variety of HER catalysts, molybdenum disulfide (MoS₂) has been regarded as the most famous catalyst owing to its abundance, low price, high efficiency, and definite catalytic mechanism. In this study, defect-engineered MoS₂ nanowall (NW) catalysts with controllable thickness were fabricated and exhibited a significantly enhanced HER performance. Benefiting from the highly exposed active edge sites and the rough surface accompanied by the robust NW structure, the defect-rich MoS₂ NW catalyst with an optimized thickness showed an ultralow onset overpotential of 85 mV, a high current density of 310.6 mA·cm^-2 at η = 300 mV, and a low potential of 95 mV to drive a 10 mA·cm^-2 cathodic current. Additionally, excellent electrochemical stability was realized, making this freestanding NW catalyst a promising candidate for practical water splitting and hydrogen production.

The efficient catalytic oxidation of water to dioxygen is envisioned to play an important role in solar fuel production and artificial photosynthetic systems. Despite tremendous efforts, the development of oxygen evolution reaction (OER) catalysts with high activity and low cost under mild conditions remains a great challenge. In this work, we develop a hybrid consisting of Co₃O₄ nanocrystals supported on single-walled carbon nanotubes (SWNTs) via a simple self-assembly approach. A Co₃O₄/SWNTs hybrid electrode for the OER exhibits much enhanced catalytic activity as well as superior stability under neutral and alkaline conditions compared with bare Co₃O₄, which only performs well in alkaline solution. Moreover, the turnover frequency for the OER exhibited by Co₃O₄/SWNTs in neutral water is higher than for bare Co₃O₄ catalysts. Synergetic chemical coupling effects between Co₃O₄ nanocrystals and SWNTs, revealed by the synchrotron X-ray absorption near edge structure (XANES) technique, can be regarded as contributing to the activity, cycling stability and stable operation under neutral conditions. Use of the SWNTs as an immobilization matrix substantially increases the active electrode surface area, enhances the durability of catalysts under neutral conditions and improves the electronic coupling between Co redox-active sites of Co₃O₄ and the electrode surface.

Analysis of the atomic structure of monoclinic BiVO₄ reveals its fascinating structure-related dual response to visible light and temperature. Although there have been a few reported studies of its responses to visible light and temperature, an understanding of the effects of quantum size, particle shape or specific exposed facets on its dual responsive properties remains elusive; this is primarily due to the limited availability of high-quality monodisperse nanocrystals with extremely small sizes and specific exposed facets. Herein, we describe a novel assembly-fusion strategy for the synthesis of mesostructured monoclinic BiVO₄ quantum tubes with ultranarrow diameter of 5 nm, ultrathin wall thickness down to 1 nm and exposed {020} facets, via a convenient hydrothermal method at temperatures as low as 100 ℃. Notably, the resulting high-quality quantum tubes possess significantly superior dual-responsive properties compared with bulk BiVO₄ or even BiVO₄ nanoellipsoids, and thus, show high promise for applications as visible-light photocatalysts and temperature indicators offering improved environmental quality and safety. This mild and facile methodology should be capable of extension to the preparation of other mesostructured inorganic quantum tubes with similar characteristics, giving a range of materials with enhanced dual-responsive properties.