Solar-Light-Driven Improved Photocatalytic Performance of Hierarchical ZnIn2S4 Architectures

Sodium Alginate-CuS Nanostructures Synthesized at the Gel-Liquid Interface: An Efficient Photocatalyst for Redox Reaction and Water Remediation

The use of visible light to propel chemical reactions is an exciting area of study that is crucial in the current socioeconomic environment. However, various photocatalysts have been developed to harness visible light, which consume high energy during synthesis. Thus, synthesizing photocatalysts at gel-liquid interfaces in ambient conditions is of scientific importance. Herein, we report an environmentally benign sodium alginate gel being used as a biopolymer template to synthesize copper sulfide (CuS) nanostructures at the gel-liquid interface. The driving force for the synthesis of CuS nanostructures is varied by changing the pH of the reaction medium (i.e., pH 7.4, 10, and 13) to tailor the morphology of CuS nanostructures. The CuS nanoflakes obtained at pH 7.4 transform into nanocubes when the pH is raised to 10, and the nanostructures deform at the pH of 13. Fourier transform infrared spectroscopy (FTIR) confirms all the characteristic stretching of sodium alginate, whereas the CuS nanostructures are crystallized in a hexagonal crystal system, as revealed by the powder X-ray diffraction analysis. The high-resolution X-ray photoelectron spectroscopy (XPS) spectra show the +2 and -2 oxidation states of copper (Cu) and sulfur (S) ions, respectively. The CuS nanoflakes physisorbed a higher concentration of greenhouse CO₂ gas. Owing to a lower band gap of CuS nanoflakes synthesized at a pH of 7.4, compared to other CuS nanostructures prepared at pH 10 and 13, CuS photocatalytically degrades 95% of crystal violet and 98% of methylene blue aqueous dye solutions in 60 and 90 min, respectively, under blue light illumination. Additionally, sodium alginate-copper sulfide (SA-CuS) nanostructures synthesized at a pH of 7.4 demonstrate excellent performance in photoredox reactions to convert ferricyanide to ferrocyanide. The current research opens the door to developing new photocatalytic pathways for a wide range of photochemical reactions involving nanoparticle-impregnated alginate composites prepared on gel interfaces.

A review on potential sulfide-based ternary chalcogenides for emerging photo-assisted water purification applications

Sulfide-based ternary chalcogenides have been recognized widely as exceptional photocatalysts, thanks to their narrow band gap enabling them to harvest solar energy to the maximum extent. They provide excellent optical, electrical, and catalytic performance and are of abundant use as a heterogeneous catalyst. Among sulfide-based ternary chalcogenides, compounds exhibiting AB2X4 structure form a new class of materials with excellent stability in photocatalytic performance. In the AB2X4 family of compounds, ZnIn2S4 is one of the top performing photocatalyst for energy and environmental applications. However, to date, only limited information is available on the mechanism behind the photo-induced migration of charge carriers in ternary sulfide chalcogenides. Ternary sulfide chalcogenides with their visible region activity and substantial chemical stability greatly depend on crystal structure, morphology, and optical characteristics for their photocatalytic activity. Hence, in this review, a comprehensive assessment of the reported strategies for enhancement of the photocatalytic efficiency of this compound is presented. In addition, a meticulous investigation of the applicability of ternary sulfide chalcogenide compound ZnIn2S4, in particular, has been delivered. Also, the photocatalytic behavior of other sulfide-based ternary chalcogenides for water remediation applications has also been briefed. Finally, we conclude with an insight into the challenges and future advancements in the exploration of ZnIn2S4-based chalcogenide as a photocatalyst for various photo-responsive applications. It is believed that this review could contribute to a better understanding of ternary chalcogenide semiconductor photocatalysts for solar-driven water treatment applications.

Accelerating Surface Photoreactions Using MoS2-FeS2 Nanoadsorbents: Photoreduction of Cr(VI) to Cr(III) and Photodegradation of Methylene Blue

Nanocubic MoS₂-FeS₂, as a photocatalyst, was synthesized with high catalytic active edges and high specific surface areas with the capability of absorbing visible light. The results showed that the photocatalytic efficiency of nanocubic MoS₂-FeS₂ for adsorption/degradation of methylene blue (MB) as well as the reduction of Cr(VI) was high. The adsorption process was found to follow a kinetic model of a pseudo-second-order kind (Q_e.cal = 464 mg g^-1) along with an isotherm described by the Langmuir model with Q_e.cal = 340 mg g^-1. The photodegradation process was achieved by holes. It was found that the photodegradation rate constant of MB by MoS₂-FeS₂ (0.203 min^-1) was about 22 times higher than that of MoS₂ (0.0091 min^-1). The percent apparent quantum yield for photoreduction of Cr(VI) to Cr(III) using MoS₂-FeS₂ (5.7%) was about 33 times higher than utilizing MoS₂ (0.1709%). Therefore, the synergistically prolonged visible-light harvesting as well as the photocarrier diffusion length proved that MoS₂-FeS₂ nanotubes can effectively be utilized in environmental pollutant's remediation.

ZnIn2 S4 -Based Photocatalysts for Energy and Environmental Applications

As a fascinating visible-light-responsive photocatalyst, zinc indium sulfide (ZnIn₂ S₄ ) has attracted extensive interdisciplinary interest and is expected to become a new research hotspot in the near future, due to its nontoxicity, suitable band gap, high physicochemical stability and durability, ease of synthesis, and appealing catalytic activity. This review provides an overview on the recent advances in ZnIn₂ S₄ -based photocatalysts. First, the crystal structures and band structures of ZnIn₂ S₄ are briefly introduced. Then, various modulation strategies of ZnIn₂ S₄ are outlined for better photocatalytic performance, which includes morphology and structure engineering, vacancy engineering, doping engineering, hydrogenation engineering, and the construction of ZnIn₂ S₄ -based composites. Thereafter, the potential applications in the energy and environmental area of ZnIn₂ S₄ -based photocatalysts are summarized. Finally, some personal perspectives about the promises and prospects of this emerging material are provided.

One-Pot Hydrothermal Synthesis of La-Doped ZnIn2S4 Microspheres with Improved Visible-Light Photocatalytic Performance

Impurity element doping is extensively taken as one of the most efficient strategies to regulate the electronic structure as well as the rate of photogenerated charge separation of photocatalysts. Herein, a one-pot hydrothermal synthesis process was exploited to obtain La-doped ZnIn₂S₄ microspheres, aiming at gaining insight into the role that doping ions played in the improvement of pollutant photodegradation. Systematical characterization means, comprising of X–ray photoelectron spectroscopy (XPS), ultraviolet–visible (UV–vis) diffuse reflection spectroscopy and Raman spectra, combination with high-resolution transmission electron microscopy (HRTEM), were employed to in depth reveal the concomitancy of La ions and ZnIn₂S₄ crystal lattice. The results showed that the La-doped ZnIn₂S₄ samples exhibited a slightly wider and stronger spectral absorption than pristine ZnIn₂S₄; and the specific surface area of doped ZnIn₂S₄ samples was a bit larger. The La-doped ZnIn₂S₄ electrodes showed improved photocurrent response, and the photocurrent density reached a maximum value at La content of 1.5 wt%. As expected, La-doped ZnIn₂S₄ samples exhibited a remarkable enhancement of photocatalytic behaviour toward the photodegradation of tetracycline hydrochloride (TCH) and methyl orange (MO). The prominently enhanced photoactivity of doped ZnIn₂S₄ samples was due to the synergistic effect of the elevated visible-light absorption ability and effective photogenerated charge carriers’ separation.

A hierarchical SnS@ZnIn2S4 marigold flower-like 2D nano-heterostructure as an efficient photocatalyst for sunlight-driven hydrogen generation

Herein, we report the in situ single-step hydrothermal synthesis of hierarchical 2D SnS@ZnIn₂S₄ nano-heterostructures and the examination of their photocatalytic activity towards hydrogen generation from H₂S and water under sunlight. The photoactive sulfides rationally integrate via strong electrostatic interactions between ZnIn₂S₄ and SnS with two-dimensional ultrathin subunits, i.e. nanopetals. The morphological study of nano-heterostructures revealed that the hierarchical marigold flower-like structure is self-assembled via the nanopetals of ZnIn₂S₄ with few layers of SnS nanopetals. Surprisingly, it also showed that the SnS nanopetals with a thickness of ∼25 nm couple in situ with the nanopetals of ZnIn₂S₄ with a thickness of ∼25 nm to form a marigold flower-like assembly with intimate contact. Considering the unique band gap (2.0-2.4 eV) of this SnS@ZnIn₂S₄, photocatalytic hydrogen generation from water and H₂S was performed under sunlight. SnS@ZnIn₂S₄ exhibits enhanced hydrogen evolution, i.e. 650 μmol h^-1 g^-1 from water and 6429 μmol h^-1 g^-1 from H₂S, which is much higher compared to that of pure ZnIn₂S₄ and SnS. More significantly, the enhancement in hydrogen generation is 1.6-2 times more for H₂S splitting and 6 times more for water splitting. SnS@ZnIn₂S₄ forms type I band alignment, which accelerates charge separation during the surface reaction. Additionally, this has been provoked by the nanostructuring of the materials. Due to the nano-heterostructure with hierarchical morphology, the surface defects increased which ultimately suppresses the recombination of the electron-hole pair. The above-mentioned facts demonstrate a significant improvement in the interface electron transfer kinetics due to such a unique 2D nano-heterostructure semiconductor which is responsible for a higher photocatalytic activity.

Facile Synthesis of a Z-Scheme ZnIn2S4/MoO3 Heterojunction with Enhanced Photocatalytic Activity under Visible Light Irradiation

Employing a visible-light-driven direct Z-scheme photocatalytic system for the abatement of organic pollutants has become the key scientific approach in the area of photocatalysis. In this study, a highly efficient Z-scheme ZnIn2S4/MoO3 heterojunction was prepared through the hydrothermal method, followed by the impregnation technique that facilitates the formation of an interface between the two phases for efficient photocatalysis. The structural, optical, and surface elemental composition and morphology of the prepared samples were characterized in detail through X-ray diffraction, UV-vis diffuse reflectance spectra, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that the composite materials have a strong intimate contact between the two phases, which is beneficial for the effective separation of photoinduced charge carriers. The visible-light-mediated photocatalytic activity of the samples was tested by studying the degradation of methyl orange (MO), rhodamine B (RhB), and paracetamol in aqueous suspension. An optimum loading content of 40 wt % ZnIn2S4/MoO3 exhibits the best degradation efficiency toward the above pollutants compared to bare MoO3 and ZnIn2S4. The improved photocatalytic activity could be ascribed to the efficient light-harvesting property and prolonged charge separation ability of the Z-scheme ZnIn2S4/MoO3 catalyst. Based on reactive species determination results, the Z-scheme charge transfer mechanism of ZnIn2S4/MoO3 was discussed and proposed. This study paves the way toward the development of highly efficient direct Z-scheme structures for a multitude of applications.

Enhanced visible-light-driven photocatalytic H2 production and Cr(vi) reduction of a ZnIn2S4/MoS2 heterojunction synthesized by the biomolecule-assisted microwave heating method

Photocatalytic applications of flower-like ZnIn₂S₄/MoS₂ composite, synthesized by biomolecule-assisted microwave heating method, in H₂ evolution and Cr(vi) reduction reactions.

Selectively Photocatalytic Activity of an Open-Framework Chalcogenide Built from Corner-Sharing T4 Supertetrahedral Clusters

The photocatalysis process with high selectivity is a very important research forefront for the semiconductor photocatalytic decomposition of organic pollutants. However, the rational design of efficient photocatalysts with high selectivity is still a challenge. Here, we present an open-framework chalcogenide (Heta)₈[In₁₄Sn₂Zn₄Se₃₃] (Heta = ethanolamine-H⁺) (compound 1) constructed from T4 supertetrahedral clusters [In₁₄Sn₂Zn₄Se₃₅]^12- with visible-light-driven selectively photocatalytic degradation activity. Single-crystal XRD analysis shows that compound 1 crystallizes in I4₁/acd (no. 142) space group, with a = b = 24.3462(2) Å, c = 45.0062(9) Å, V = 26676.9(7) ų, and Z = 8. Under visible-light irradiation, the selectively photocatalytic activities of 1 were evaluated by photodegradation of two kinds of cationic dye molecules, i.e., methylene blue (MB) and rhodamine B (RhB), against two anionic dyes, methyl orange (MO) and Kermes red (KR), with different sizes. We show that the adsorption capability and charge-matching between organic dyes and the supertetrahedral cluster together with a suitable band structure make it an excellent and selective photocatalyst. This is the first example of an open-framework chalcogenide based on supertetrahedral T4 for the selectively semiconductor photocatalytic decomposition of organic dyes.

Polyaniline/Reduced Graphene Oxide Composite-Enhanced Visible-Light-Driven Photocatalytic Activity for the Degradation of Organic Dyes

Creation of an innovative composite photocatalyst, to advance its performance, has attracted researchers to the field of photocatalysis. In this article, a new photocatalyst based on polyaniline/reduced graphene oxide (PANI/RGO) composites has been prepared via the in situ oxidative polymerization method employing RGO as a template. For thermoelectric applications, though a higher percentage (50 wt %) of RGO has been used, for photocatalytic activity, lesser percentages (2, 5, and 8 wt %) of RGO in the composite have given a significant outcome. Furthermore, photoluminescence (PL) spectra, time-resolved fluorescence spectra, and Brunauer-Emmett-Teller surface area analyses confirmed the improved photocatalytic mechanism. PANI/RGO composites under visible light irradiation exhibit amazingly improved activity toward the degradation of cationic and anionic dyes in comparison with pristine PANI or RGO. Here, a PANI/RGO composite, with 5 wt % RGO(PG2), has emerged as the best combination with the degradation percentages of 99.68, 99.35, and 98.73 for malachite green, rhodamine B, and congo red within 15, 30, and 40 min, respectively. Experimental findings show that the introduction of RGO can relieve the agglomeration of PANI nanoparticles and enhance the light absorption of the materials due to an increased surface area. Moreover, the PG2 composite also showed excellent photocatalytic activity to reduce noxious Cr(VI). The effective removal of Cr(VI) up to 94.7% at pH 2 was observed within only 15 min. With the help of the active species trapping experiment, a plausible mechanism for the photocatalytic degradation has been proposed. The heightened activity of the as-synthesized composite compared to that of neat PANI or RGO was generally because of high concentrations of ^•OH radicals and partly of ^•O₂ ^- and holes (h⁺) as concluded from the nitroblue tetrazolium probe test and photoluminescence experiment. It is hoped that the exceptional photocatalytic performance of our work makes the conducting polymer-based composite an effective alternative in wastewater treatment for industrial applications.

Transition-Metal Ion-Doped Flower-Like Titania Nanospheres as Nonlight-Driven Catalysts for Organic Dye Degradation with Enhanced Performances

Titania has recently been identified as a new and effective nonlight-driven catalyst for degradation of organic pollutant with the use of H₂O₂ as an oxidant; however, either relatively low surface area or lack of diversity in chemical composition largely limits its catalytic performance. In this work, a series of transition-metal ion (Mn²⁺, Co²⁺, Ni²⁺, and Cu²⁺)-doped titania nanomaterials with regular flower-like morphology, good crystallinity (anatase), and large specific surface areas (71.4-124.4 m² g^-1) were facilely synthesized and utilized as catalysts for methylene blue (MB) degradation in the presence of H₂O₂ without light irradiation. It was revealed that the doping of transition-metal ions (especially Mn²⁺) into titania could significantly improve the catalytic efficiency. At 30 °C, 10 mL of MB with a concentration of 50 mg L^-1 could be completely degraded within 60-100 min for these doped samples, whereas the removal rate was only 35.1% within 100 min with the use of pure flower-like titania. Temperature-dependent kinetic studies indicated that the presence of transition-metal ion dopants could markedly lower the activation energy and thus resulted in enhanced catalytic performances. Test of reusability exhibited that these doped catalysts could well keep their original catalytic activities after reuse for several cycles, indicating their excellent catalytic durability.