Vacancy Engineering in 2D Transition Metal Chalcogenide Photocatalyst: Structure Modulation, Function and Synergy Application

Transition metal chalcogenides (TMCs) are widely used in photocatalytic fields such as hydrogen evolution, nitrogen fixation, and pollutant degradation due to their suitable bandgaps, tunable electronic and optical properties, and strong reducing ability. The unique 2D malleability structure provides a pre-designed platform for customizable structures. The introduction of vacancy engineering makes up for the shortcomings of photocorrosion and limited light response and provides the greatest support for TMCs in terms of kinetics and thermodynamics in photocatalysis. This work reviews the effect of vacancy engineering on photocatalytic performance based on 2D semiconductor TMCs. The characteristics of vacancy introduction strategies are summarized, and the development of photocatalysis of vacancy engineering TMCs materials in energy conversion, degradation, and biological applications is reviewed. The contribution of vacancies in the optical range and charge transfer kinetics is also discussed from the perspective of structure manipulation. Vacancy engineering not only controls and optimizes the structure of the TMCs, but also improves the optical properties, charge transfer, and surface properties. The synergies between TMCs vacancy engineering and atomic doping, other vacancies, and heterojunction composite techniques are discussed in detail, followed by a summary of current trends and potential for expansion.

Composition-dependent activity of ZnxCd1-xSe solid solution coupled with Ni2P nanosheets for visible-light-driven photocatalytic H2 generation

Metal selenide semiconductors have been rarely used for photocatalytic water splitting because of their poor stability and severe photocorrosion properties. Hence, designing stable metal selenides with suitable bandgap energies has considerable practical significance in photocatalytic H₂ evolution. In this work, a novel series of Zn_xCd_1-xSe (x = 0 ∼ 1) with tunable band structure were fabricated through a simple solvothermal method. Impressively, the ZnSe exhibited a maximum H₂ production rate of 1056 µmol g^-1h^-1, which was higher than that of CdSe and Zn_xCd_1-xSe solid solutions. Such visible-light photoactivity for water reduction to H₂ was attained even after 6 cycling photocatalytic experiments. Moreover, the two-dimensional (2D) Ni₂P nanosheets act as a high-efficiency cocatalyst integrated with Zn_xCd_1-xSe semiconductor to boost photocatalytic H₂ generation performance. The optimal 8% Ni₂P/ZnSe composites displayed excellent cycling stability and superior photocatalytic H₂ evolution performance (4336 µmol g^-1h^-1), which was about 4.1 times that of pure ZnSe under visible light irradiation. Photoelectrochemical (PEC), photoluminescence (PL), and time-resolved photoluminescence (TRPL) measurements reveal that the improved photoactivity Ni₂P/ZnSe photocatalysts were ascribed to the effective separation and migration of photoinduced carriers. The present work paves a pathway to explore the fabrication of Zn_xCd_1-xSe solid solutions and the hybridization of 2D transition metal phosphides nanosheets toward photocatalytic applications.

Cadmium-Free Nanostructured Multilayer Thin Films with Bright Blue Photoluminescence and Excellent Stability

Cadmium-based quantum dots (Cd-QDs) show decent performance for lighting applications due to good color saturation, an excellent high quantum yield, and a narrow full-width at half-maximum. However, the intrinsic toxicity of Cd is a major hindrance to related applications, especially in the biological field. ZnSe, with a band gap of 2.7 eV and lower toxicity than CdSe or CdS, is promising as a blue luminescent material. Herein, we mainly reported the preparation and luminescence properties of nanostructured ZnSe/ZnS multilayer thin films with bright blue photoluminescence. The photoluminescence spectrum contained two emission peaks, located at about 442 nm (near band-edge emission) and 550 nm (defect-related emission), respectively. More importantly, the photoluminescence performance and decay were explored in detail through low-temperature photoluminescence spectra. In addition, the nanostructured ZnSe/ZnS multilayer thin films showed favorable photostability.

Structural, optical, photocatalytic, and optoelectronic properties of Zn2SnO4 nanocrystals prepared by hydrothermal method

Zn₂SnO₄ (ZTO) nanocrystals are extensively studied in various fields. However, size-dependent ZTO nanocrystals are still challenging to understand their structural, optical, photocatalytic, and optoelectronic properties. ZTO nanocrystals are synthesized by a facile hydrothermal reaction method. The structural properties of the synthesized ZTO nanocrystals are studied by x-ray diffraction and transmission electron microscope. The sizes of the ZTO nanocrystals are controlled by the pH values of the precursor and the molar ratios of the Zn:Sn in the starting materials. ZTO nanocrystals with the small size of 6 nm and large size of 270 nm are obtained by our method. The Eu³⁺ ions are doped into ZTO nanocrystals to probe size-dependent Eu doping sites, which shows significant potential applications in light emitting diode phosphors. Moreover, the photocatalytic activity of ZTO nanocrystals on rhodamine (RhB) decoloration are investigated, and the results show that 6 nm ZTO nanocrystals show better performance in the photocatalytic decoloration of RhB compared to 270 nm nanocrystals. Most importantly, we design and fabricate optoelectronic devices to detect IR light based on our nanocrystals and a self-prepared NIR cyanine dye. The device based on small sized ZTO nanocrystals exhibits better device performance under 808 nm IR light compared to that of the large sized ZTO nanocrystals. We believe this work represents ZTO size-dependent properties in term of structural, optical, photocatalytic, and optoelectronic properties as a multifunctional material.

Controllable Conversion of CdNCN Nanoparticles into Various Chalcogenide Nanostructures for Photo-driven Applications

Semiconductor nanocrystals of tunable shell/core configurations have great potential in photo-driven applications such as photoluminescence and photocatalysis, but few strategies realize a controllable synthesis with respect to both the size of the core and the shell with high crystallinity. Here, a new synthetic method based on cadmium cyanamide (CdNCN) nanoparticle anion exchange reactions was developed to access solid or hollow CdSe nanocrystals with tunable size and CdNCN@CdS heterostructures with modulated shell/core thickness. The gradual shift and narrow width of photoluminescence features demonstrate the high crystallinity and monodispersity of the resulting CdSe nanocrystals. In the CdNCN@CdS heterostructures, synergistic effects of the photocarrier separation is observed between the CdS shell and CdNCN core, which leads to great improvement in photocatalysis with optimized shell/core ratio.

Morphology-controlled Fabrication of SnO2 /ZnO Nanocomposites with Enhanced Photocatalytic Performance

In this work, a series of novel SnO₂ /ZnO nanocomposites with different morphologies were fabricated via a facile hydrothermal technique followed by calcination in air. The morphological, structural and photocatalytic properties of the SnO₂ /ZnO nanocomposites were studied using different methods. The results showed that the synthesized nanocomposites possessed crystal phases of wurtzite hexagonal phase ZnO and tetragonal rutile phase SnO₂ . In addition, the morphologies of SnO₂ /ZnO nanocomposites strongly depended on the molar ratios of Sn and Zn. Compared with ZnO and SnO₂ , the SnO₂ /ZnO nanocomposites exhibited considerably higher degradation efficiency for the photodegradation of methylene blue and quinolone antibiotics under mercury lamp irradiation. The SZ-2 nanospheres exhibited the highest degradation efficiency of 95.81%, which was about 2.63 times higher than that of ZnO nanoparticles. Moreover, the trapping experiments confirmed that ˙OH played the dominant role in MB degradation. Finally, the charge carriers potential transfer pathway and photocatalytic degradation mechanism were put forward. This study provides an economical way to prepare hybrid nanocomposites with controlled morphology for practical applications in the photocatalytic degradation of organic dyes and residual antibiotics.

ZnSe Nanorods as Visible-Light Absorbers for Photocatalytic and Photoelectrochemical H2 Evolution in Water

A precious-metal- and Cd-free photocatalyst system for efficient H₂ evolution from aqueous protons with a performance comparable to Cd-based quantum dots is presented. Rod-shaped ZnSe nanocrystals (nanorods, NRs) with a Ni(BF₄ )₂ co-catalyst suspended in aqueous ascorbic acid evolve H₂ with an activity up to 54±2 mmol H 2  g_ZnSe ^-1  h^-1 and a quantum yield of 50±4 % (λ=400 nm) under visible light illumination (AM 1.5G, 100 mW cm^-2 , λ>400 nm). Under simulated full-spectrum solar irradiation (AM 1.5G, 100 mW cm^-2 ), up to 149±22 mmol H 2  g_ZnSe ^-1  h^-1 is generated. Significant photocorrosion was not noticeable within 40 h and activity was even observed without an added co-catalyst. The ZnSe NRs can also be used to construct an inexpensive delafossite CuCrO₂ photocathode, which does not rely on a sacrificial electron donor. Immobilized ZnSe NRs on CuCrO₂ generate photocurrents of around -10 μA cm^-2 in an aqueous electrolyte solution (pH 5.5) with a photocurrent onset potential of approximately +0.75 V vs. RHE. This work establishes ZnSe as a state-of-the-art light absorber for photocatalytic and photoelectrochemical H₂ generation.