Tuning the Optical Property and Photocatalytic Performance of Titanate Nanotube toward Selective Oxidation of Alcohols under Ambient Conditions

Titanate nanotube (TNT) represents one class of novel one-dimensional semiconducting nanomaterials that can be used as photocatalyst for given applications. However, TNT is only UV-light photoactive because of its intrinsic limitation of light absorption in the UV region. Here, we report a facile approach to tune the optical property and photocatalytic performance of TNT by doping various metal ions (Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, and Mn²⁺) via an ion-exchange method in an aqueous phase. The optical properties of TNT can be finely tuned by incorporating different kinds of metal ions into its tubular framework. In particular, the incorporation of metal ions into the matrix of TNT is able to extend its light absorption to the visible-light region, thus making TNT have the visible-light photoactivity. Activity testing on photocatalytic selective oxidation of a variety of benzylic and allylic alcohols under mild conditions demonstrates that these metal-ion-doped TNTs exhibit markedly enhanced catalytic performance as compared to the undoped TNTs under both the irradiation of UV light and visible light. Such an enhancement of photocatalytic activity with regard to metal-ion-doped TNT is primarily attributed to the prolonged lifetime of photogenerated electron–hole pairs in comparison with that of undoped TNT. Our current research work demonstrates the tunable optical property of TNT by doping metal ions and, more significantly, opens promising prospects of one-dimensional nanotubular TNT or TNT-based materials as visible-light-driven photocatalyst in the area of selective transformation using molecular oxygen as benign oxidant under ambient conditions

Graphene Transforms Wide Band Gap ZnS to a Visible Light Photocatalyst. The New Role of Graphene as a Macromolecular Photosensitizer

We report the assembly of nanosized ZnS particles on the 2D platform of a graphene oxide (GO) sheet by a facile two-step wet chemistry process, during which the reduced graphene oxide (RGO, also called GR) and the intimate interfacial contact between ZnS nanoparticles and the GR sheet are achieved simultaneously. The ZnS–GR nanocomposites exhibit visible light photoactivity toward aerobic selective oxidation of alcohols and epoxidation of alkenes under ambient conditions. In terms of structure–photoactivity correlation analysis, we for the first time propose a new photocatalytic mechanism where the role of GR in the ZnS–GR nanocomposites acts as an organic dye-like macromolecular “photosensitizer” for ZnS instead of an electron reservoir. This novel photocatalytic mechanism is distinctly different from all previous research on GR–semiconductor photocatalysts, for which GR is claimed to behave as an electron reservoir to capture/shuttle the electrons photogenerated from the semiconductor. This new concept of the reaction mechanism in graphene–semiconductor photocatalysts could provide a new train of thought on designing GR-based composite photocatalysts for targeting applications in solar energy conversion, promoting our in-depth thinking on the microscopic charge carrier transfer pathway connected to the interface between the GR and the semiconductor

Synthesis of One-Dimensional CdS@TiO₂ Core–Shell Nanocomposites Photocatalyst for Selective Redox: The Dual Role of TiO₂ Shell

One-dimensional (1D) CdS@TiO₂ core–shell nanocomposites (CSNs) have been successfully synthesized via a two-step solvothermal method. The structure and properties of 1D CdS@TiO₂ core–shell nanocomposites (CdS@TiO₂ CSNs) have been characterized by a series of techniques, including X-ray diffraction (XRD), ultraviolet–visible-light (UV-vis) diffuse reflectance spectra (DRS), field-emission scanning electron microscopy (FESEM), photoluminescence spectra (PL), and electron spin resonance (ESR) spectroscopy. The results demonstrate that 1D core–shell structure is formed by coating TiO₂ onto the substrate of CdS nanowires (NWs). The visible-light-driven photocatalytic activities of the as-prepared 1D CdS@TiO₂ CSNs are evaluated by selective oxidation of alcohols to aldehydes under mild conditions. Compared to bare CdS NWs, an obvious enhancement of both conversion and yield is achieved over 1D CdS@TiO₂ CSNs, which is ascribed to the prolonged lifetime of photogenerated charge carriers over 1D CdS@TiO₂ CSNs under visible-light irradiation. Furthermore, it is disclosed that the photogenerated holes from CdS core can be stuck by the TiO₂ shell, as evidenced by controlled radical scavenger experiments and efficiently selective reduction of heavy-metal ions, Cr­(VI), over 1D CdS@TiO₂ CSNs, which consequently leads to the fact that the reaction mechanism of photocatalytic oxidation of alcohols over 1D CdS@TiO₂ CSNs is apparently different from that over 1D CdS NWs under visible-light irradiation. It is hoped that our work could not only offer useful information on the fabrication of various specific 1D core–shell nanostructures, but also open a new doorway of such 1D core–shell semiconductors as visible-light photocatalysts in the promising field of selective transformations

Engineering Semiconductor Quantum Dots for Selectivity Switch on High-Performance Heterogeneous Coupling Photosynthesis

Semiconductor-based photoredox catalysis brings an innovative strategy for sustainable organic transformation (e.g., C–C/C–X bond formation), via radical coupling under mild conditions. However, since semiconductors interact with photogenerated radicals unselectively, the precise control of selectivity for such organic synthesis by steering radical conversion is extremely challenging. Here, by the judicious design of a structurally well-defined and atomically dispersed cocatalyst over semiconductor quantum dots, we demonstrate the precise selectivity switch on high-performance selective heterogeneous coupling photosynthesis of a C–C bond or a C–N bond along with hydrogen production over the Ni-oxo cluster and single Pd atom-decorated CdS quantum dots crafted onto the SiO2 support. Mechanistic studies unveil that the Ph(•CH)NH2 and PhCH2NH2•+ act as dominant radical intermediates for such divergent organic synthesis of C–C coupled vicinal diamines and C–N coupled imines, as respectively enabled by Ni-oxo clusters assisted radical–radical coupling and single Pd atom-assisted radical addition–elimination. This work overcomes the pervasive difficulties of selectivity regulation in semiconductor-based photochemical synthesis, highlighting a vista of utilizing atomically dispersed cocatalysts as active sites to maneuver unselective radical conversion by engineering quantum dots toward selective heterogeneous photosynthesis

Synthesis of Titanate Nanotube–CdS Nanocomposites with Enhanced Visible Light Photocatalytic Activity

CdS–1D titanate nanotubes (CdS/TNTs) nanocomposites have been synthesized via a facile one-step in situ hydrothermal method. The structure and properties of CdS/TNTs nanocomposites have been characterized by X-ray diffraction, UV–vis diffuse reflectance spectra, transmission electron microscopy, photoluminescence spectra, nitrogen adsorption–desorption, and electron spin resonance spectra. The results show that (i) as compared to blank-CdS, it is found that the morphology of CdS in the CdS/TNTs nanocomposites can be finely tuned by TNTs formed during the one-step in situ hydrothermal process; and (ii) the CdS/TNTs nanocomposites exhibit remarkably much higher visible light photocatalytic activity than both blank-CdS and blank-TNT toward aerobic selective oxidation of alcohols under mild conditions. Three integrative factors lead to such a drastic photoactivity enhancement for CdS/TNTs nanocomposites. The first one is the different morphology of CdS in the CdS/TNTs nanocomposites from blank-CdS. The second one is the prolonged lifetime of photogenerated electron–hole pairs from CdS in CdS/TNTs nanocomposites under visible light irradiation. The third one is the higher surface area and adsorption capacity of CdS/TNTs nanocomposites than blank-CdS. In addition, the possible reaction mechanism for photocatalytic selective oxidation of alcohols over CdS/TNTs nanocomposites has also been investigated using the radical scavengers technique. It is hoped that this work could promote further interest in fabrication of various 1D TNT-based composite materials and their application to visible-light-driven photocatalytic selective organic transformations

A Unique Silk Mat-Like Structured Pd/CeO₂ as an Efficient Visible Light Photocatalyst for Green Organic Transformation in Water

The charm embedded in nature is its inherent power to create a myriad of materials, for example, a spider web and lotus leaf, with ordinary composition but exhibiting fascinating functional property owing to their unique structures. Such intricate natural designs inspire immense research in synthesizing materials with controlled structure and morphology toward achieving novel or enhanced properties for target applications. Herein, we report a rotary vacuum evaporation and support-driven nanoassembly of tiny Pd noble metal particles on nanosized CeO₂, which features a remarkable unique silk “mat-like” morphology with significant anti-aggregation of Pd nanoparticles during a high temperature calcination process, whereas the obvious aggregation phenomenon of Pd nanoparticles occurs when using commercial CeO₂ as a support. This nanocomposite with unique structural and morphology composition is able to act as a highly selective and active visible light photocatalyst toward organic redox transformations in water, including aerobic oxidation of alcohols and anaerobic reduction of nitro-compounds under ambient conditions, representing a typical tenet of photocatalytic green chemistry

Graphene Oxide as a Surfactant and Support for In-Situ Synthesis of Au–Pd Nanoalloys with Improved Visible Light Photocatalytic Activity

Traditional ways for the synthesis of bimetallic alloyed nanoparticles involve successive or simultaneous reduction of metallic precursors either in an organic solvent phase or in an aqueous phase. However, these two approaches generally require the use of surfactants or polymers, dendrimers, or ligands as protecting or capping agents in order to achieve stable colloidal bimetallic nanoalloys for potential use, for example, loading them onto supports as heterogeneous catalysts. Here, we report the direct synthesis of stabilizing-molecules-free bimetallic Au–Pd nanoalloys promoted by graphene oxide (GO) in an aqueous phase. Formation of Au–Pd nanoalloys and loading onto the reduced GO (denoted as GR) are accomplished simultaneously. Controlled experiments suggest that GO vividly acts as a unique “solution processable macromolecular surfactant” and 2D “flat-mat” support to promote formation and loading of alloyed Au–Pd bimetallic nanoparticles onto the GR sheet. The as-formed Au–Pd/GR exhibits higher photocatalytic activity than both monometallic Au/GR and Pd/GR, prepared by the same approach toward degradation of dye, Rhodamine B (RhB), which thus demonstrates the promising potential of bimetallic nanoalloys rather than the monometallic one in promoting visible light photocatalysis. It is anticipated that our work could boost further interest for harnessing the versatile soft materials features of GO in solution to synthesize other bimetallic alloy catalysts and exploring their applications in photocatalysis

Toward Improving the Graphene–Semiconductor Composite Photoactivity <i>via</i> the Addition of Metal Ions as Generic Interfacial Mediator

We report a simple and general approach to improve the transfer efficiency of photogenerated charge carriers across the interface between graphene (GR) and semiconductor CdS by introducing a small amount of metal ions (Ca²⁺, Cr³⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺) as “mediator” into their interfacial layer matrix, while the intimate interfacial contact between GR and CdS is maintained. This simple strategy can not only significantly improve the visible-light-driven photoactivity of GR–CdS semiconductor composites for targeting selective photoredox reaction, including aerobic oxidation of alcohol and anaerobic reduction of nitro compound, but also drive a balance between the positive effect of GR on retarding the recombination of electron–hole pairs photogenerated from semiconductor and the negative “shielding effect” of GR resulting from the high weight addition of GR. Our current work highlights that the significant issue on improving the photoactivity of GR–semiconductor composites <i>via</i> strengthening interfacial contact is not just a simple issue of tighter connection between GR and the semiconductor, but it is also the optimization of the atomic charge carrier transfer pathway across the interface between GR and the semiconductor

Noncovalently Functionalized Graphene-Directed Synthesis of Ultralarge Graphene-Based TiO₂ Nanosheet Composites: Tunable Morphology and Photocatalytic Applications

Ultralarge graphene-based TiO₂ nanosheet composites are successfully fabricated by a noncovalent functionalization approach with use of benzyl alcohol as the linking agent. In the synthetic procedure, the aromatic molecules of benzyl alcohol direct themselves onto graphene (GR) surface via π–π interaction. Therefore, the basal planes of GR nanosheets are uniformly functionalized with hydroxyl groups derived from benzyl alcohol, which not only improves the dispersion of GR in solution but also induces a finely homogeneous coating of TiO₂ nanocrystals onto the surface of GR nanosheets. The resulting GR@TiO₂ nanocomposites, which feature unique ultralarge 2D sheet-like morphology with the lateral size far larger than the original GR and densely interfacial contact, are able to act as highly active photocatalysts toward selective reduction of aromatic nitro compounds to amines in water under ambient conditions. The higher photoactivity of GR@TiO₂ than blank TiO₂ is attributed to the efficient charge carriers separation and transfer by the GR platform. It is hoped that the facile synthesis strategy in this work could contribute to fabricating other ultralarge functional GR-based 2D sheet-onto-sheet composites with tunable morphology toward target photocatalytic applications

Two-Dimensional MoS₂ Nanosheet-Coated Bi₂S₃ Discoids: Synthesis, Formation Mechanism, and Photocatalytic Application

Myriad materials with desirable functional property resulting from their unique structures ignite enormous interest in synthesizing materials with controlled structural morphology toward achieving novel or enhanced properties for target applications. Herein, the novel and unique two-dimensional (2D) MoS₂ nanosheet-coated Bi₂S₃ discoids composites, which feature a Bi₂S₃-core/MoS₂-shell structure, have been elaborated via a facile anion-exchange strategy. Using the MoS₂ nanosheets to coat the surface of Bi₂S₃ discoids boosts the light-harvesting efficiency and charge separation and promotes faster charge transport and collection, thus leading to the higher activity of the photocatalytic reduction of Cr­(VI) under visible light irradiation (λ > 400 nm). In particular, the phase evolution and possible formation mechanism of the MoS₂–Bi₂S₃ core–shell structure have been explored by virtue of temperature- and time-dependent experiments. It is anticipated that this work could promote further interest in adopting an anion-exchange strategy to fabricate semiconductor-based composite materials with controlled architectural morphology and enhanced photocatalytic performance toward diverse applications