Fabrication of black phosphorus/CdS heterostructure with enhancement photocatalytic degradation activity for tetrabromobisphenol A and toxicity prediction of intermediates

Black phosphorus nanosheets (BPNs)/CdS heterostructure was successfully synthesized via hydrothermal method. The experimental results indicated that BPNs modified the surface of CdS nanoparticles uniformly. Meanwhile, the BPNs/CdS heterostructure exhibited a distinguished high rate of photocatalytic activity for Tetrabromobisphenol A (TBBPA) degradation under visible light irradiation (λ > 420 nm), the kinetic constant of TBBPA degradation reached 0.0261 min-1 was approximately 5.68 and 9.67 times higher than that of CdS and P25, respectively. Moreover, superoxide radical (•O2-) is the main active component in the degradation process of TBBPA (the relative contribution is 91.57%). The photocatalytic mechanism and intermediates of the TBBPA was clarified, and a suitable model and pathway for the degradation of TBBPA were proposed. The results indicated that the toxicities of some intermediates were higher than the parent pollutant. This research provided an efficient approach by a novel photocatalyst for the removal of TBBPA from wastewater, and the appraisal methods for the latent risks from the intermediates were reported in this paper.

Tailoring Advanced CdS Anisotropy-Driven Charge Spatial Vectorial Separation and Migration via In Situ Dual Co-Catalyst Synergistic Layout

Tailoring advanced anisotropy-driven efficient separation and migration of photogenerated carriers is a pivotal stride toward enhancing photocatalytic activity. Here, CdS-MoS2 binary photocatalysts are tailored into a dumbbell shape by leveraging the rod-shaped morphology of CdS and employing an in situ tip-induction strategy. To further enhance the photocatalytic activity, an in situ photo-deposition strategy is incorporated to cultivate MnOx particles on the dumbbell-shaped CdS-MoS2. The in situ deposition of MnOx effectively isolated the oxidatively active sites on the CdS surface, emphasizing the reductively active crystalline face of CdS, specifically the (002) face. Benefiting from its robust activity as a reduction active site, MoS2 adeptly captures photogenerated electrons, facilitating the reduction of H+ to produce hydrogen. The anisotropically driven separation of CdS photogenerated carriers markedly mitigates the Coulomb force or binding force of the photogenerated electrons, thus promoting a smoother migration toward the active site for photocatalytic hydrogen evolution. The hydrogen evolution rate of 35MnOx-CdS-MoS2-3 surpasses that of CdS by nearly an order of magnitude, achieving a quantum efficiency of 22.30% at 450 nm. Under simulated solar irradiation, it attains a rate of 42.86 mmol g-1 h-1. This work imparts valuable insights for the design of dual co-catalysts, anisotropy-driven spatial vectorial charge separation and migration, and the analysis of migration pathways of photogenerated carriers.

Single Atom Ru Doped Ni2P/Fe3P Heterostructure for Boosting Hydrogen Evolution for Water Splitting

Modulating the chemical composition and structure has been considered as one of the most promising strategies for developing high-efficient water splitting catalysts. Here, a single-atom Ru doped Ni2P/Fe3P catalyst is synthesized by introducing the dispersed Ru atoms to adjust Ni2P/Fe3P heterostructure. Single atom Ru provides effective hydrogen evolution reaction (HER) active sites for boosting catalytic activities. The catalyst with only 0.2 wt.% content of Ru exhibits an overpotential of 19.3 mV at 10 mA cm-2, which is obviously lower than 146.1 mV of Ni2P/Fe3P. Notably, an alkaline overall water electrolyzer based on Ru-Ni2P/Fe3P catalysts achieves a cell voltage of 1.47 V and operates over 600 h at 10 mA cm-2, which is superior to that of benchmark RuO2//Pt/C (1.61 V). The theoretical calculations further confirm that Ru single atom doping can effectively optimize the hydrogen/water adsorption free energy of the active site and therefore improve the HER activity of heterostructure. This work provides a valuable reference to design high-activity and durability catalyst for water splitting through the double modulation of interface-effect and atomic doping.

Photocatalytic Degradation of Malachite Green by Titanium Dioxide/Covalent Organic Framework Composite: Characterization, Performance and Mechanism

In this paper, a titanium dioxide/covalent organic framework (TiO2/COF) composite was prepared and its photocatalytic removal of dye was investigated. Using tetrabutyl titanate as a titanium source, TiO2 nanomaterial was prepared by sol-gel method. In the presence of TiO2, TiO2/COF core-shell composite was prepared by solvothermal synthesis using melamine and 1,4-phthalaldehyde as ligands. The prepared materials are characterized by SEM, TEM, XPS, XRD, TG, FTIR, BET, EPR, PL, and UV-Vis-DRS techniques. Using malachite green as a model of dye wastewater, the photocatalytic degradation performance of TiO2/COF composites was investigated under the irradiation of ultraviolet light. The results show that the modification of COF significantly improves the photocatalytic efficiency of TiO2, the degradation rate increases from 69.77 % to 93.64 %, and the reaction rate constant of the first-order kinetic equation is increased from 0.0078 min-1 to 0.0192 min-1. Based on the free radical capture experiment, the photocatalytic degradation mechanism of TiO2/COF was discussed, and the feasibility of its photocatalytic degradation of malachite green was theoretically clarified. Accordingly, a simple and practical method for photocatalytic degradation of malachite green was constructed, which has potential application value in the degradation of dye wastewater.

Electrostatic Self-Assembly Synthesis of Pd/In2O3 Nanocatalysts with Improved Performance Toward CO2 Hydrogenation to Methanol

CO2 hydrogenation to methanol has emerged as a promising strategy for achieving carbon neutrality and mitigating global warming, in which the supported Pd/In2O3 catalysts are attracting great attention due to their high selectivity. Nonetheless, conventional impregnation methods induce strong metal-support interaction (SMSI) between Pd and In2O3, which leads to the excessive reduction of In2O3 and the formation of undesirable PdIn alloy in hydrogen-rich atmospheres. Herein, we innovatively synthesized Pd/In2O3 nanocatalysts by the electrostatic self-assembly process between surface-modified composite precursors with opposite charges. And the organic ligands concurrently serve as Pd nanoparticle protective agents. The resultant Pd/In2O3 nanocatalyst demonstrates the homogeneous distribution of Pd nanoparticles with controllable sizes on In2O3 supports and the limited formation of PdIn alloy. As a result, it exhibits superior selectivity and stability compared to the counterparts synthesized by the conventional impregnation procedure. Typically, it attains a maximum methanol space-time yield of 0.54 gMeOH h-1gcat -1 (300 °C, 3.5 MPa, 21,000 mL gcat -1 h-1). Notably, the correlation characterization results reveal the significant effect of small-size, highly dispersed Pd nanoparticles in mitigating MSI. These results provide an alternative strategy for synthesizing highly efficient Pd/In2O3 catalysts and offer a new insight into the strong metal-support interaction.

Platinum-Tellurium Heterojunction Nanosheet Assemblies for Efficient Direct Formic Acid Electrooxidation Catalysis

Two-dimensional (2D) heterojunction nanomaterials offer exceptional physicochemical and catalytic properties, thanks to their special spatial electronic structure. However, synthesizing morphologically uniform 2D platinum (Pt)-based metallic nanomaterials with diverse crystalline phases remains a formidable challenge. In this study, we have achieved the successful synthesis of advanced 2D platinum-tellurium heterojunction nanosheet assemblies (Ptx-PtTe2 HJNSAs, x = 0, 1, 2), seamlessly integrating both trigonal PtTe2 (t-PtTe2) and cubic Pt (c-Pt) phases. By enabling efficient electron transport and leveraging the specific electron density present at the heterojunction, the Pt2-PtTe2 HJNSAs/C demonstrated exceptional formic acid oxidation reaction (FAOR) activity and stability. Specifically, the specific and mass activities reached 8.4 mA cm-2 and 6.1 A mgPt-1, which are 46.7 and 50.8 times higher than those of commercial Pt/C, respectively. Impressively, aberration-corrected high-angle annular dark field scanning transmission electron microscopy (AC-HAADF-STEM) revealed a closely packed arrangement of atomic layers and a coherent intergrowth heterogeneous structure. Density functional theory (DFT) calculations further indicated that rearrangement of electronic structure occurred on the surface of Pt2-PtTe2 HJNSAs resulting in a more favorable dehydrogenation pathway and excellent CO tolerance, beneficial for performance improvement. This work inspires the targeted exploration of Pt-based nanomaterials through 2D heterostructure design, leading to an important impact on fuel cell catalysis and beyond.

Selective Cocatalyst Decoration of Narrow-Bandgap Broken-Gap Heterojunction for Directional Charge Transfer and High Photocatalytic Properties

Narrow-bandgap semiconductors are promising photocatalysts facing the challenges of low photoredox potentials and high carrier recombination. Here, a broken-gap heterojunction Bi/Bi2 S3 /Bi/MnO2 /MnOx , composed of narrow-bandgap semiconductors, is selectively decorated by Bi, MnOx nanodots (NDs) to achieve robust photoredox ability. The Bi NDs insertion at the Bi2 S3 /MnO2 interface induces a vertical carrier migration to realize sufficient photoredox potentials to produce O2 •- and • OH active species. The surface decoration of Bi2 S3 /Bi/MnO2 by Bi and MnOx cocatalysts drives electrons and holes in opposite directions for optimal photogenerated charge separation. The selective cocatalysts decoration realizes synergistic surface and bulk phase carrier separation. Density functional theory (DFT) calculation suggests that Bi and MnOx NDs act as active sites enhancing the absorption and reactants activation. The decorated broken-gap heterojunction demonstrates excellent performance for full-light driving organic pollution degradation with great commercial application potential.

Constructing Spatially Separated Cage-Like Z-scheme Heterojunction Photocatalyst for Enhancing Photocatalytic H2 Evolution

Heterojunctions coupled into micro-mesoscopic structures is an attractive strategy to optimize the light harvesting and carrier separation of semiconductor photocatalysts. A self-templating method of ion exchange is reported to synthesize an exquisite hollow cage-structured Ag₂ S@CdS/ZnS that direct Z-scheme heterojunction photocatalyst. On the ultrathin shell of the cage, Ag₂ S, CdS, and ZnS with Zn-vacancies (V_Zn ) are arranged sequentially from outside to inside. Among them, the photogenerated electrons are excited by ZnS to the V_Zn energy level and then recombine with the photogenerated holes that are generated by CdS, while the electrons remained in the CdS conduction band are further transferred to Ag₂ S. The ingenious cooperation of the Z-scheme heterojunction with the hollow structure optimizes the photogenerated charges transport channel, spatially separated the oxidation and reduction half-reactions, decreases the charge recombination probability, and simultaneously improves the light harvesting efficiency. As a result, the photocatalytic hydrogen evolution activity of the optimal sample is 136.6 and 17.3 times higher than that of cage-like ZnS with V_Zn and CdS by, respectively. This unique strategy demonstrates the tremendous potential of the incorporation of heterojunction construction to morphology design of photocatalytic materials, and also provided a reasonable route for designing other efficient synergistic photocatalytic reactions.

Molecular oxidation-reduction junctions for artificial photosynthetic overall reaction

Constructing redox semiconductor heterojunction photocatalysts is the most effective and important means to complete the artificial photosynthetic overall reaction (i.e., coupling CO₂ photoreduction and water photo-oxidation reactions). However, multiphase hybridization essence and inhomogeneous junction distribution in these catalysts extremely limit the diverse design and regulation of the modes of photogenerated charge separation and transfer pathways, which are crucial factors to improve photocatalytic performance. Here, we develop molecular oxidation-reduction (OR) junctions assembled with oxidative cluster (PMo₁₂, for water oxidation) and reductive cluster (Ni₅, for CO₂ reduction) in a direct (d-OR), alternant (a-OR), or symmetric (s-OR) manner, respectively, for artificial photosynthesis. Significantly, the transfer direction and path of photogenerated charges between traditional junctions are obviously reformed and enriched in these well-defined crystalline catalysts with monophase periodic distribution and thus improve the separation efficiency of the electrons and holes. In particular, the charge migration in s-OR shows a periodically and continuously opposite mode. It can inhibit the photogenerated charge recombination more effectively and enhance the photocatalytic performance largely when compared with the traditional heterojunction models. Structural analysis and density functional theory calculations disclose that, through adjusting the spatial arrangement of oxidation and reduction clusters, the energy level and population of the orbitals of these OR junctions can be regulated synchronously to further optimize photocatalytic performance. The establishment of molecular OR junctions is a pioneering important discovery for extremely improving the utilization efficiency of photogenerated charges in the artificial photosynthesis overall reaction.

Enabling High Loading in Single-Atom Catalysts on Bare Substrate with Chemical Scissors by Saturating the Anchoring Sites

Atomically dispersed metal catalysts often exhibit high catalytic performances, but the metal loading density must be kept low to avoid the formation of metal nanoparticles, making it difficult to improve the overall activity. Diverse strategies based on creating more anchoring sites (ASs) have been adopted to elevate the loading density. One problem of such traditional methods is that the single atoms always gather together before the saturation of all ASs. Here, a chemical scissors strategy is developed by selectively removing unwanted metallic materials after excessive loading. Different from traditional ways, the chemical scissors strategy places more emphasis on the accurate matching between the strength of etching agent and the bond energies of metal-metal/metal-substrate, thus enabling a higher loading up to 2.02 wt% even on bare substrate without any pre-treatment (the bare substrate without any pre-treatment generally only has a few ASs for single atom loading). It can be inferred that by combining with other traditional methods which can create more ASs, the loading could be further increased by saturating ASs. When used for CH₃ OH generation via photocatalytic CO₂ reduction, the as-made single-atom catalyst exhibits impressive catalytic activity of 597.8 ± 144.6 µmol h^-1 g^-1 and selectivity of 81.3 ± 3.8%.

Hydrogen Production as a Clean Energy Carrier through Heterojunction Semiconductors for Environmental Remediation

Today, as a result of the advancement of technology and increasing environmental problems, the need for clean energy has considerably increased. In this regard, hydrogen, which is a clean and sustainable energy carrier with high energy density, is among the well-regarded and effective means to deliver and store energy, and can also be used for environmental remediation purposes. Renewable hydrogen energy carriers can successfully substitute fossil fuels and decrease carbon dioxide (CO2) emissions and reduce the rate of global warming. Hydrogen generation from sustainable solar energy and water sources is an environmentally friendly resolution for growing global energy demands. Among various solar hydrogen production routes, semiconductor-based photocatalysis seems a promising scheme that is mainly performed using two kinds of homogeneous and heterogeneous methods, of which the latter is more advantageous. During semiconductor-based heterogeneous photocatalysis, a solid material is stimulated by exposure to light and generates an electron–hole pair that subsequently takes part in redox reactions leading to hydrogen production. This review paper tries to thoroughly introduce and discuss various semiconductor-based photocatalysis processes for environmental remediation with a specific focus on heterojunction semiconductors with the hope that it will pave the way for new designs with higher performance to protect the environment.

A Hybrid Assembly with Nickel Poly-Pyridine Polymer on CdS Quantum Dots for Photo-Reducing CO2 into Syngas with Controlled H2 /CO Ratios

A hybrid photocatalytic assembly with Ni poly-pyridine polymers binding on CdS quantum dots was developed via thiophene immobilization. The fabricated hybrid assembly facilitated efficient charge separation, and each component endowed great synergy. As a result, a high syngas production rate was achieved over 5500 μmol g_cat ^-1  h^-1 from photocatalytic CO₂ reduction under visible-light irradiation, accompanied by an adjustable H₂ /CO ratio ranging from 4 : 1 to 1 : 3. A novel hybrid assembly was described for syngas synthesis with boosted activity and controlled selectivity, which provides a profile to ingeniously understand molecular-level design for photocatalysts.

Metallic Copper-Containing Composite Photocatalysts: Fundamental, Materials Design, and Photoredox Applications

Semiconductor photocatalysis has long been regarded as a potential solution to tackle the energy and environmental challenges since the first discovery of water splitting by TiO₂ almost 50 years ago. The past few years have seen a tremendous flurry of research interest in the modification of semiconductors because of their shortcomings in the aspects of solar harvesting, electron-hole pairs separation, and utilization of photogenerated carriers. Among the various strategies, the introduction of metallic copper into the photocatalysis system can not only enhance the absorption of sunlight and the separation efficiency of photogenerated electrons and holes, but also increase the adsorption ability of substrate and the number of active sites, so as to realize the high solar to chemical energy conversion efficiency. This review focuses on the rational design of copper-based composites and their applications in photoredox catalysis. First, the preparation methods of metallic copper-containing composites are discussed. Then, the applications of different types of copper-based composites in the photocatalytic removal of pollutants, splitting of water to hydrogen production, reduction of carbon dioxide, and conversion of organic matter are introduced. Finally, the opportunities and challenges in the design and synthesis of copper-based composites and their applications in the photocatalysis are prospected.

Crystal Facet Engineering of TiO2 Nanostructures for Enhancing Photoelectrochemical Water Splitting with BiVO4 Nanodots

Although bismuth vanadate (BiVO₄) has been promising as photoanode material for photoelectrochemical water splitting, its charge recombination issue by short charge diffusion length has led to various studies about heterostructure photoanodes. As a hole blocking layer of BiVO₄, titanium dioxide (TiO₂) has been considered unsuitable because of its relatively positive valence band edge and low electrical conductivity. Herein, a crystal facet engineering of TiO₂ nanostructures is proposed to control band structures for the hole blocking layer of BiVO₄ nanodots. We design two types of TiO₂ nanostructures, which are nanorods (NRs) and nanoflowers (NFs) with different (001) and (110) crystal facets, respectively, and fabricate BiVO₄/TiO₂ heterostructure photoanodes. The BiVO₄/TiO₂ NFs showed 4.8 times higher photocurrent density than the BiVO₄/TiO₂ NRs. Transient decay time analysis and time-resolved photoluminescence reveal the enhancement is attributed to the reduced charge recombination, which is originated from the formation of type II band alignment between BiVO₄ nanodots and TiO₂ NFs. This work provides not only new insights into the interplay between crystal facets and band structures but also important steps for the design of highly efficient photoelectrodes.

Dual metal nanoparticles within multicompartmentalized mesoporous organosilicas for efficient sequential hydrogenation

Controlling localization of multiple metal nanoparticles on a single support is at the cutting edge of designing cascade catalysts, but is still a scientific and technological challenge because of the lack of nanostructured materials that can not only host metal nanoparticles in different sub-compartments but also enable efficient molecular transport between different metals. Herein we report a multicompartmentalized mesoporous organosilica with spatially separated sub-compartments that are connected by short nanochannels. Such a unique structure allows co-localization of Ru and Pd nanoparticles in a nanoscale proximal fashion. The so designed cascade catalyst exhibits an order of magnitude activity enhancement in the sequential hydrogenation of nitroarenes to cyclohexylamines compared with its mono/bi-metallic counterparts. Crucially, an interesting phenomenon of neighboring metal-assisted hydrogenation via hydrogen spillover is observed, contributing to the significant enhancement in catalytic efficiency. The multicompartmentalized architectures along with the revealed mechanism of accelerated hydrogenation provide vast opportunity for designing efficient cascade catalysts.

Cooperative Coupling of Oxidative Organic Synthesis and Hydrogen Production over Semiconductor-Based Photocatalysts

Merging hydrogen (H₂) evolution with oxidative organic synthesis in a semiconductor-mediated photoredox reaction is extremely attractive because the clean H₂ fuel and high-value chemicals can be coproduced under mild conditions using light as the sole energy input. Following this dual-functional photocatalytic strategy, a dreamlike reaction pathway for constructing C-C/C-X (X = C, N, O, S) bonds from abundant and readily available X-H bond-containing compounds with concomitant release of H₂ can be readily fulfilled without the need of external chemical reagents, thus offering a green and fascinating organic synthetic strategy. In this review, we begin by presenting a concise overview on the general background of traditional photocatalytic H₂ production and then focus on the fundamental principles of cooperative photoredox coupling of selective organic synthesis and H₂ production by simultaneous utilization of photoexcited electrons and holes over semiconductor-based catalysts to meet the economic and sustainability goal. Thereafter, we put dedicated emphasis on recent key progress of cooperative photoredox coupling of H₂ production and various selective organic transformations, including selective alcohol oxidation, selective methane conversion, amines oxidative coupling, oxidative cross-coupling, cyclic alkanes dehydrogenation, reforming of lignocellulosic biomass, and so on. Finally, the remaining challenges and future perspectives in this flourishing area have been critically discussed. It is anticipated that this review will provide enlightening guidance on the rational design of such dual-functional photoredox reaction system, thereby stimulating the development of economical and environmentally benign solar fuel generation and organic synthesis of value-added fine chemicals.

Hierarchical Co0.85 Se-CdSe/MoSe2 /CdSe Sandwich-Like Heterostructured Cages for Efficient Photocatalytic CO2 Reduction

Fabricating efficient photocatalysts with rapid charge carrier separation and high visible light harvesting is an advisable strategy to improve CO₂ reduction performance. Herein, hierarchical Co_0.85 Se-CdSe/MoSe₂ /CdSe cages with sandwich-like heterostructure are prepared to act as efficient photocatalysts for CO₂ reduction. In this study, the structure and composition of the final products can be regulated through the cation-exchange reaction in the presence of ascorbic acid. In the Co_0.85 Se-CdSe/MoSe₂ /CdSe cages, MoSe₂ nanosheets function as a bridge to integrate Co_0.85 Se-CdSe and CdSe on both sides of the MoSe₂ nanosheet shell into a sandwich-like heterostructured catalyst system, which possesses multiple positive merits for photocatalysis, including accelerated transport and separation of photogenerated carriers, improved visible light utilization, and increased catalytic active sites. Thus, the optimized Co_0.85 Se-CdSe/MoSe₂ /CdSe cages exhibit remarkable visible-light photocatalytic performance and outstanding stability for CO₂ reduction with a high CO average yield of 15.04 µmol g^-1 h^-1 and 90.14% selectivity, which are much higher than those of other control samples including single-component catalysts and binary hybrid catalysts. This study provides a promising way for the design and fabrication of high-efficiency photocatalysts.

Improved charge separation and carbon dioxide photoreduction performance of surface oxygen vacancy-enriched zinc ferrite@titanium dioxide hollow nanospheres with spatially separated cocatalysts

Here, we describe the fabrication of surface oxygen vacancy-enriched ZnFe₂O₄@TiO₂ double-shell hollow heterostructure nanospheres (ZnFe₂O₄@H-TiO_2-x) coupled with spatially separated CoO_x and Au-Cu bimetallic cocatalysts. The ZnFe₂O₄@TiO₂ heterojunction and spatially separated dual cocatalysts can significantly promote the separation of photoinduced charge carriers. Combined with the unique hollow double-shell heterostructure characteristics and improved surface state properties, the hybrid nanospheres can efficiently adsorb and activate CO₂ molecules. These advantages cause the optimized catalyst to exhibit remarkably higher gas-phase photocatalytic CO₂ reduction activity than the control CoO_x/ZnFe₂O₄/Au-Cu and ZnFe₂O₄@H-TiO_2-x double-shell hollow nanospheres loaded with a single cocatalyst. Meanwhile, the Au-Cu bimetal effect boosts the CO₂ conversion rate and CH₄ selectivity. The optimized hybrid catalyst with a Au/Cu ratio of 1:1 provides a CH₄ yield of 21.39 μmol g^-1 h^-1 with 93.8% selectivity. This work provides a rational photocatalyst design to improve CO₂ conversion and CH₄ selectivity.

Spatially Separating Redox Centers and Photothermal Effect Synergistically Boosting the Photocatalytic Hydrogen Evolution of ZnIn2 S4 Nanosheets

Spatially separated loading of reductive and oxidative cocatalysts is a useful strategy for expediting charge separation and surface reaction kinetics, which are two key factors for determining the photocatalytic efficiency. However, loading the spatial separation of dual cocatalysts on a 2D photocatalyst is still a great challenge. Herein, decorating the spatial separation of oxidative and reductive cocatalysts on ZnIn₂ S₄ nanosheets is realized by designing a ternary Co₉ S₈ @ZnIn₂ S₄ @PdS (CS@ZIS@PS) hollow tubular core-shell structure. Particularly, Co₉ S₈ and PdS functionally serve as the reduction and oxidation cocatalysts, respectively. Experimental results confirm that the spatial separation of Co₉ S₈ and PdS cocatalysts not only efficiently improve charge separation and accelerate surface reduction-oxidation kinetics, but also generate a photothermal effect to further enhance charge transfer and surface reaction kinetics. As a result, the optimized CS@ZIS@PS yields a remarkable H₂ evolution rate of 11407 µmol g^-1 h^-1 , and the apparent quantum efficiency reaches 71.2% at 420 nm, which is one of the highest values among ZnIn₂ S₄ so far. The synergistic effect of spatially separated dual cocatalysts and photothermal effect may be applied to other 2D materials for efficient solar energy conversion.

Yolk-shell nanostructures: synthesis, photocatalysis and interfacial charge dynamics

Solar energy has long been regarded as a promising alternative and sustainable energy source. In this regard, photocatalysts emerge as a versatile paradigm that can practically transform solar energy into chemical energy. At present, unsatisfactory conversion efficiency is a major obstacle to the widespread deployment of photocatalysis technology. Many structural engineering strategies have been proposed to address the issue of insufficient activity for semiconductor photocatalysts. Among them, creation of yolk-shell nanostructures which possess many beneficial features, such as large surface area, efficient light harvesting, homogeneous catalytic environment and enhanced molecular diffusion kinetics, has attracted particular attention. This review summarizes the developments that have been made for the preparation and photocatalytic applications of yolk-shell nanostructures. Additional focus is placed on the realization of interfacial charge dynamics and the possibility of achieving spatial separation of charge carriers for this unique nanoarchitecture as charge transfer is the most critical factor determining the overall photocatalytic efficiency. A future perspective that can facilitate the advancement of using yolk-shell nanostructures in sophisticated photocatalytic systems is also presented.

Remarkable variation of visible light photocatalytic activities of M/Sn0.9Sb0.1O2/TiO2 (M=Au, Ag, Pt) heterostructures depending on the loaded metals

Sn_0.9Sb_0.1O₂/TiO₂ (ATO/TiO₂) heterostructure is a potential visible light photocatalysts that function via an inter-semiconductor hole-transport mechanism. Herein we selectively deposited Au, Ag, or Pt onto the ATO surface of ATO/TiO₂ to investigate charge-trapping behaviors of the noble metals and their effects on photocatalytic performance. We observed that Pt deposition greatly enhanced the photocatalytic activity whereas effects of Au or Ag depositions were not significant. The result of spectroscopic analysis also indicates that Pt is the most effective in scavenging the electrons from the ATO CB. Particularly, Pt/ATO/TiO₂ (ATO:TiO₂ = 15:85 in weight) produced CO₂ of 42 ppmv in 2 h, which is 16 times and 4.8 times that of bare ATO/TiO₂ and nitrogen-doped TiO₂, respectively. Pt deposition on the ATO seems to suppress two independent charge recombination pathways, that is, recombination of electron-hole pairs in ATO and electron transport from the ATO CB to TiO₂ VB.

Titania/chitosan–lignin nanocomposite as an efficient photocatalyst for the selective oxidation of benzyl alcohol under UV and visible light

Developing functional materials from biomass is a significant research subject due to its unique structure, abundant availability, biodegradability and low cost. A series of chitosan–lignin (CL) composites were prepared through a hydrothermal method by varying the weight ratio of chitosan and lignin. Subsequently, these CL composites were combined with titania (T) to form a nanocomposite (T/CL) using sol–gel and hydrothermal based methods. T/CL nanocomposites exhibited improved photocatalytic performance in comparison with sol–gel and hydrothermally prepared pristine titania (SGH-TiO₂), towards the selective oxidation of benzyl alcohol (BnOH) to benzaldehyde (Bnald) under UV (375 nm) and visible light (515 nm). More specifically, the 75T/CL(25 : 75) nanocomposite (a representative photocatalyst from the 75T/CL nanocomposite series) showed very high selectivity (94%) towards Bnald at 55% BnOH conversion under UV light. Whereas, SGH-TiO₂ titania exhibited much lower (68%) selectivity for Bnald at similar BnOH conversion. Moreover, the 75T/CL(25 : 75) nanocomposite also showed excellent Bnald selectivity (100%) at moderate BnOH conversion (19%) under visible light. Whereas, SGH-TiO₂ did not show any activity for BnOH oxidation under visible light. XPS studies suggest that the visible light activity of the 75T/CL(25 : 75) nanocomposite is possibly related to the doping of nitrogen into titania from chitosan. However, according to UV-visible-DRS results, no direct evidence pertaining to the decrease in band-gap energy of titania was found upon coupling with the CL composite and the visible light activity was attributed to N-doping of titania. Overall, it was found that T/CL nanocomposites enhanced the photocatalytic performance of titania via improved light harvesting and higher selectivity through mediation of active radical species.

Combining titania with chitosan–lignin composites results in an active and selective photocatalyst for the oxidation of benzyl alcohol to benzaldehyde under green light (515 nm).

Hierarchical Ag3PO4@ZnIn2S4 nanoscoparium: An innovative Z-scheme photocatalyst for highly efficient and predictable tetracycline degradation

Z-scheme photocatalyst preserved with superior oxidicability is an innovative photocatalyst system that can be used for efficient photocatalytic detoxification of antibiotics. In this study, Z-scheme Ag₃PO₄@ZnIn₂S₄ photocatalyst was constructed by decorating Ag₃PO₄ nanoparticles on ZnIn₂S₄ nanoscopariums. ZnIn₂S₄ nanoscopariums were prepared by self-templated strategy and given hierarchical structures. The hierarchical Ag₃PO₄@ZnIn₂S₄ provides more active sites for generating photogenerated carriers and large surface area for capturing tetracycline. The study results show that Ag₃PO₄@ZnIn₂S₄ performed excellently well in the photocatalytic degradation of tetracycline and also in protecting Ag₃PO₄ nanoparticles from photo-corrosion. The highest removal efficiency (up to 92.3%) was achieved from the optimal composites of Ag₃PO₄ and ZnIn₂S₄. In stability tests, Ag₃PO₄@ZnIn₂S₄ did not reduce the photocatalytic activity of degrading tetracycline after five successive runs. Active radical identification proves that the transfer behavior of electron and hole over Ag₃PO₄@ZnIn₂S₄ follows a direct Z-scheme mechanism. Furthermore, the transformation pathway for degrading tetracycline was proposed by combining the Fukui index prediction with Mass Spectra identification of intermediates. This work presents in-depth sights into a regulated degradation pathway from theoretical prediction and practical identification based on innovative Z-scheme photocatalyst.

Improving photocatalytic activity by construction of immobilized Z-scheme CdS/Au/TiO2 nanobelt photocatalyst for eliminating norfloxacin from water

To improve the photocatalytic activity of TiO₂ NBs under irradiation of solar light, an immobilized Z-scheme composite photocatalyst CdS/Au/TiO₂ NBs has been constructed. For the unique architectures, the TiO₂ NBs provide more absorption and reaction sites, the CdS nanoparticles enhance overall light harvesting, and Au acts as the electron transfer mediator, promoting the interfacial charge transfer and efficient separation of electrons and holes. The morphology, elements, crystal structure, optical and photoelectrochemical properties, and photocatalytic activity of CdS/Au/TiO₂ NBs were characterized. Results showed that CdS/Au/TiO₂ NBs possesses higher photocatalytic activity toward the degradation of antibiotic norfloxacin under irradiation of simulated sunlight, which is attributed to the synergetic interaction of increased light absorption and separation of photogenerated electrons and holes. Besides, the degradation of norfloxacin was promoted by HCO₃^-, but inhibited by NO₃^- and Cl^-. The radicals trapping experiments proved that superoxide radicals (O₂^-) was the dominating active species during the photocatalysis process. The photocatalytic degradation products of norfloxacin was analyzed, and nine intermediates were identified. Moreover, the photocatalytic degradation mechanism and photostability of CdS/Au/TiO₂ NBs were analyzed in detail. The matched energy levels and unique ternary Z-scheme design are the key for improved photocatalytic activity. The deactivation of CdS/Au/TiO₂ NBs after recycles mainly due to the release of CdS by photocorrosion and the loss of deposited Au.

Modulating the Plasmon-Mediated Oxidation of p-Aminothiophenol with Asymmetrically Grafted Thiol Molecules

A surface plasmon can drive many photochemical reactions, in which effective charge separation and migration is a key. In analogy to the plasmon-semiconductor interface, the plasmon-molecule interface may also be used to improve the separation and migration of hot carriers. In this work, by using in situ Raman spectroscopy, molecular grafting on silver nanostructures is found essential for modulating the charge separation and p-aminothiophenol (PATP) oxidation reaction. When the LUMO of the grafted molecules match well the energy distribution of the plasmon-generated hot electrons, the PATP oxidation process accelerates significantly. Moreover, compared with symmetrical grafting, asymmetrical grafting is more effective in regulating the charge separation and plasmon-mediated chemical reaction. This work provides an effective strategy for deep understanding and fine modulation of plasmon-mediated photochemistry.

Double-shelled Cu2O/MnOx mesoporous hollow structure for CO2 photoreduction with enhanced stability and activity

Photocatalytic CO₂ reduction reaction (CRR) represents a prospective route for the clean utilization of greenhouse gas CO₂ and solar energy, and cuprous oxide (Cu₂O) is a favourable material for the CRR to avoid excess generation of hydrogen through the competitive hydrogen evolution reaction (HER). However, the application of Cu₂O-based photocatalysts is limited by their poor stability and low activity, which result from the self-corrosion by photogenerated holes. Here we construct a double-shelled Cu₂O/MnO_x mesoporous hollow structure (D-CMH) to enhance the stability and activity of Cu₂O-based photocatalysts. Because of the thin shells, which can shorten the diffusion distance of charge carriers, and the oxidation cocatalyst MnO_x, which can efficiently attract holes, the photogenerated holes can be immediately removed from Cu₂O and react with reactants. Additionally, the D-CMH can also provide enhanced efficiency of charge separation, prolonged path of light scattering and reflection and enlarged surface area for active sites, which result in the initial activity enhanced by 7.1 times and stability enhanced by 11.2 times compared with benchmark Cu₂O nanoparticles.

Spatially Separated Bifunctional Cocatalysts Decorated on Hollow-Structured TiO2 for Enhanced Photocatalytic Hydrogen Generation

Efficient charge separation can promote photocatalysis of semiconductors. Herein, a hollow-structured TiO₂ sphere decorated with spatially separated bifunctional cocatalysts was designed, which exhibited enhanced photocatalytic hydrogen generation. Ultrasmall-sized MO_x (M = Pd, Co, Ni, or Cu) nanoparticles (NPs) were first introduced into a zeolite via confinement synthesis, and then, hollow TiO₂ was fabricated by using the zeolite as a sacrificial template forming MO_x@TiO₂. Finally, Pt NPs were decorated on the outer shell, giving rise to MO_x@TiO₂@Pt, in which the MO_x NPs and Pt NPs acted as hole capturers and electron sinks, respectively. Thanks to the enhanced light harvesting of the hollow structure and improved charge separation induced by the smaller-sized cocatalysts as well as spatially separated bifunctional cocatalysts, the as-prepared PdO_x@TiO₂@Pt catalyst exhibited a superior photocatalytic hydrogen-generation property (0.45 mmol h^-1). This work demonstrates the advantage of the spatially separated bifunctional cocatalysts in enhancing the photocatalytic properties of semiconductors.

An in situ assembled WO3-TiO2 vertical heterojunction for enhanced Z-scheme photocatalytic activity

The face-to-face contact of a vertical heterojunction is beneficial to charge interaction in photocatalysis. However, constructing a vertical heterojunction with uncompromised redox ability still remains a challenge. Herein, we report the successful synthesis of a WO3-TiO2 vertical heterojunction via establishing an internal electric field across the interface. Experimental investigation and computational simulations reveal that strong electric coupling occurs at the WO3-TiO2 interface forming an internal electric field. The internal electric field induces a Z-scheme charge-carrier transfer through the heterojunction under light irradiation, which leads to effective charge separation and maintains high reaction potentials of charge-carriers. The improved photocatalytic activity of the WO3-TiO2 heterojunction is proved by enhanced generation of reactive oxygen species and accelerated Escherichia coli (E. coli) disinfection. This study provides new insights into understanding and designing Z-scheme heterogeneous photocatalysts.

Spatially separating redox centers on 2D carbon nitride with cobalt single atom for photocatalytic H2O2 production

Photocatalysts frequently require simultaneous loading of oxidative and reductive cocatalysts to achieve both efficient half-reactions within a single material. Nevertheless, unregulated loading and distribution of two cocatalysts will result in direct contact between oxidation and reduction centers, leading to detrimental charge recombination. This research presents a center/edge approach to load two redox cocatalysts with controlled physical separation in atomistic scale using single-atom architecture. This spatial separation is critical for enhancing surface charge separation and achieving efficient H₂O₂ production. We report that redox cocatalysts are spatially separated on a two-dimensional (2D) photocatalyst, which opens an approach for achieving both efficient oxidation and reduction reactions on 2D photocatalysts.

Redox cocatalysts play crucial roles in photosynthetic reactions, yet simultaneous loading of oxidative and reductive cocatalysts often leads to enhanced charge recombination that is detrimental to photosynthesis. This study introduces an approach to simultaneously load two redox cocatalysts, atomically dispersed cobalt for improving oxidation activity and anthraquinone for improving reduction selectivity, onto graphitic carbon nitride (C₃N₄) nanosheets for photocatalytic H₂O₂ production. Spatial separation of oxidative and reductive cocatalysts was achieved on a two-dimensional (2D) photocatalyst, by coordinating cobalt single atom above the void center of C₃N₄ and anchoring anthraquinone at the edges of C₃N₄ nanosheets. Such spatial separation, experimentally confirmed and computationally simulated, was found to be critical for enhancing surface charge separation and achieving efficient H₂O₂ production. This center/edge strategy for spatial separation of cocatalysts may be applied on other 2D photocatalysts that are increasingly studied in photosynthetic reactions.

A novel hollow-sphere cyclodextrin nanoreactor for the enhanced removal of bisphenol A under visible irradiation

A novel hybrid nanoreactor with spatially separated co-catalysts (SH-CD-Au@CdS@MnO_x) was successfully synthesised to remove bisphenol-A (BPA) from water by visible light. The photooxidation intermediates, degradation pathway of BPA and the enhancement mechanism were investigated in particular. Gold (Au) nanoparticles modified with SH-β-cyclodextrin and MnO_x nanoparticles were selectively decorated on the interior and exterior surface of hollow CdS nanoreactors, respectively. The directed migration of photogenerated electrons and holes induced by spatially separated co-catalysts lead to high utilization of light, and SH-β-cyclodextrin modification makes catalytic active sites more accessible for oxidation intermediates. Compared with pristine CdS, the hybrid nanoreactor increased the BPA photooxidation reaction rate and the TOC removal efficiency by 5.6-fold and 3.6-fold, respectively. Moreover, the toxic intermediates, such as phenol, were further degraded by visible light. Molecular orbital calculation predicted that the sites on BPA molecule values of (FED²_HOMO + FED²_LUMO) can be easier attacked by the radical, whereas atoms with higher values of 2FED²_HOMO can easily be extracted into electrons. Thus, SH-CD-Au@CdS@MnO_x can provide a new strategy for the high-efficiency photodegradation of endocrine disrupter compounds in advanced water treatments.

Core–shell structured catalysts for thermocatalytic, photocatalytic, and electrocatalytic conversion of CO2

An in-depth assessment of properties of core–shell catalysts and their application in the thermocatalytic, photocatalytic, and electrocatalytic conversion of CO₂into synthesis gas and valuable hydrocarbons.

A redox interaction-engaged strategy for multicomponent nanomaterials

The review article focuses on the redox interaction-engaged strategy that offers a powerful way to construct multicomponent nanomaterials with precisely-controlled size, shape, composition and hybridization of nanostructures.

Three-Phase Photocatalysis for the Enhanced Selectivity and Activity of CO2 Reduction on a Hydrophobic Surface

The photocatalytic CO2 reduction reaction (CRR) represents a promising route for the clean utilization of stranded renewable resources, but poor selectivity resulting from the competing hydrogen evolution reaction (HER) in aqueous solution limits its practical applicability. In the present contribution a photocatalyst with hydrophobic surfaces was fabricated. It facilitates an efficient three-phase contact of CO2 (gas), H2 O (liquid), and catalyst (solid). Thus, concentrated CO2 molecules in the gas phase contact the catalyst surface directly, and can overcome the mass-transfer limitations of CO2 , inhibit the HER because of lowering proton contacts, and overall enhance the CRR. Even when loaded with platinum nanoparticles, one of the most efficient HER promotion cocatalysts, the three-phase photocatalyst maintains a selectivity of 87.9 %. Overall, three-phase photocatalysis provides a general and reliable method to enhance the competitiveness of the CRR.