Fang-Xing Xiao

Layer-by-Layer Self-Assembly of CdS Quantum Dots/Graphene Nanosheets Hybrid Films for Photoelectrochemical and Photocatalytic Applications

In recent years, increasing interest has been devoted to synthesizing graphene-semiconductor nanocomposites as efficient photocatalysts for extensive applications. Unfortunately, it is still challenging to make uniform graphene-semiconductor composite films with controllable film thickness and architecture, which are of paramount importance to meet the application requirements. In this work, stable aqueous dispersion of polymer-modified graphene nanosheets (GNs) was prepared via in situ reduction of exfoliated graphite oxide in the presence of cationic poly(allylamine hydrochloride) (PAH). The resultant water-soluble PAH-modified GNs (GNs-PAH) in conjunction with tailor-made negatively charged CdS quantum dots (QDs) were utilized as nanobuilding blocks for sequential layer-by-layer (LbL) self-assembly of well-defined GNs-CdS QDs hybrid films, in which CdS QDs overspread evenly on the two-dimensional (2D) GNs. It was found that the alternating GNs-CdS QDs multilayered films showed significantly enhanced photoelectrochemical and photocatalytic activities under visible light irradiation as compared to pure CdS QDs and GNs films. The enhancement was attributed to the judicious integration of CdS QDs with GNs in an alternating manner, which maximizes the 2D structural advantage of GNs in GNs-CdS QDs composite films. In addition, photocatalytic and photoelectrochemical mechanisms of the GNs-CdS QDs multilayered films were also discussed. It is anticipated that our work may open new directions for the fabrication of uniform semiconductor/GNs hybrid films for a wide range of applications.

Hierarchical Ni-Mo-S nanosheets on carbon fiber cloth: A flexible electrode for efficient hydrogen generation in neutral electrolyte

A flexible cloth-like electrode, which can efficiently split water to produce H2 at neutral pH, is successfully demonstrated. A unique functional electrode made of hierarchal Ni-Mo-S nanosheets with abundant exposed edges anchored on conductive and flexible carbon fiber cloth, referred to as Ni-Mo-S/C, has been developed through a facile biomolecule-assisted hydrothermal method. The incorporation of Ni atoms in Mo-S plays a crucial role in tuning its intrinsic catalytic property by creating substantial defect sites as well as modifying the morphology of Ni-Mo-S network at atomic scale, resulting in an impressive enhancement in the catalytic activity. The Ni-Mo-S/C electrode exhibits a large cathodic current and a low onset potential for hydrogen evolution reaction in neutral electrolyte (pH ~7), for example, current density of 10 mA/cm2 at a very small overpotential of 200 mV. Furthermore, the Ni-Mo-S/C electrode has excellent electrocatalytic stability over an extended period, much better than those of MoS2/C and Pt plate electrodes. Scanning and transmission electron microscopy, Raman spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy were used to understand the formation process and electrocatalytic properties of Ni-Mo-S/C. The intuitive comparison test was designed to reveal the superior gas-evolving profile of Ni-Mo-S/C over that of MoS2/C, and a laboratory-scale hydrogen generator was further assembled to demonstrate its potential application in practical appliances.

Metal-cluster-decorated TiO2 nanotube arrays : a composite heterostructure toward versatile photocatalytic and photoelectrochemical applications

Recent years have witnessed increasing interest in the solution-phase synthesis of atomically precise thiolate-protected gold clusters (Aux ); nonetheless, research on the photocatalytic properties of Aux -semiconductor nanocomposites is still in its infancy. In this work, recently developed glutathione-capped gold clusters and highly ordered nanoporous layer-covered TiO2 nanotube arrays (NP-TNTAs) are employed as nanobuilding blocks for the construction of a well-defined Aux /NP-TNTA heterostructure via a facile electrostatic self-assembly strategy. Versatile photocatalytic performances of the Aux /NP-TNTA heterostructure which acts as a model catalyst, including photocatalytic oxidation of organic pollutant, photocatalytic reduction of aromatic nitro compounds and photoelectrochemical (PEC) water splitting under simulated solar light irradiation, are systematically exploited. It is found that synergistic interaction stemming from monodisperse coverage of Aux clusters on NP-TNTAs in combination with hierarchical nanostructure of NP-TNTAs reinforce light absorption of Aux /NP-TNTA heterostructure especially within visible region, hence contributing to the significantly enhanced photocatalytic and PEC water splitting performances. Moreover, photocatalytic and PEC mechanisms over Aux /NP-TNTA heterostructure are elucidated and corresponding reaction models were presented. It is anticipated that this work could boost new insight for photocatalytic properties of metal-cluster-sensitized semiconductor nanocomposites.

One-dimensional hybrid nanostructures for heterogeneous photocatalysis and photoelectrocatalysis

Semiconductor-based photocatalysis and photoelectrocatalysis have received considerable attention as alternative approaches for solar energy harvesting and storage. The photocatalytic or photoelectrocatalytic performance of a semiconductor is closely related to the design of the semiconductor at the nanoscale. Among various nanostructures, one-dimensional (1D) nanostructured photocatalysts and photoelectrodes have attracted increasing interest owing to their unique optical, structural, and electronic advantages. In this article, a comprehensive review of the current research efforts towards the development of 1D semiconductor nanomaterials for heterogeneous photocatalysis and photoelectrocatalysis is provided and, in particular, a discussion of how to overcome the challenges for achieving full potential of 1D nanostructures is presented. It is anticipated that this review will afford enriched information on the rational exploration of the structural and electronic properties of 1D semiconductor nanostructures for achieving more efficient 1D nanostructure-based photocatalysts and photoelectrodes for high-efficiency solar energy conversion.

Hierarchical α-MnO2 Nanowires@Ni1-xMnxOy Nanoflakes Core–Shell Nanostructures for Supercapacitors

A facile two-step solution-phase method has been developed for the preparation of hierarchical α-MnO2 nanowires@Ni1-x Mnx Oy nanoflakes core-shell nanostructures. Ultralong α-MnO2 nanowires were synthesized by a hydrothermal method in the first step. Subsequently, Ni1-x Mnx Oy nanoflakes were grown on α-MnO2 nanowires to form core-shell nanostructures using chemical bath deposition followed by thermal annealing. Both solution-phase methods can be easily scaled up for mass production. We have evaluated their application in supercapacitors. The ultralong one-dimensional (1D) α-MnO2 nanowires in hierarchical core-shell nanostructures offer a stable and efficient backbone for charge transport; while the two-dimensional (2D) Ni1-x Mnx Oy nanoflakes on α-MnO2 nanowires provide high accessible surface to ions in the electrolyte. These beneficial features enable the electrode with high capacitance and reliable stability. The capacitance of the core-shell α-MnO2 @Ni1-x Mnx Oy nanostructures (x = 0.75) is as high as 657 F g(-1) at a current density of 250 mA g(-1) , and stable charging-discharging cycling over 1000 times at a current density of 2000 mA g(-1) has been realized.

Self-assembly preparation of gold nanoparticles-TiO2 nanotube arrays binary hybrid nanocomposites for photocatalytic applications

Increasing interest has been devoted to preparing gold nanoparticle (GNP) functionalized TiO2 nanotube array (TNTA) nanocomposites (GNP/TNTA) for photocatalytic applications. Nonetheless, achieving accurate control of surface assemblies of GNPs tethered on the TNTA substrate is far from satisfactory. Thus, in our work, applying 3-mercaptopropionic acid (MPA) as an interconnecting ligand, tailor-made monodispersed GNPs are evenly deposited on the interior and exterior surfaces of TNTAs with significant monodispersity via a self-assembly approach. The intrinsic self-assembly mechanism leading to the GNP/TNTA hybrid nanostructure is highlighted and ascertained. The photocatalytic performances of GNP/TNTAs are systematically evaluated in the photodegradation of organic dye pollutants under UV light irradiation. The ensemble of results indicates that the hierarchical nanostructure of the GNP/TNTA nanocomposite obtained via this self-assembly approach exhibits remarkably enhanced photocatalytic performance compared to its counterparts of P25 particulate film, a flat anodic TiO2 layer (FTL), and GNP/FTL, in which well-dispersed GNPs and conducting titanium substrate are proposed to play crucial roles as efficient “electron traps” for the transfer of the photogenerated electrons and thus retard the recombination of photogenerated electron–hole pairs during the harvesting of photon-energy. In addition, the detection of increased amounts of highly active species (especially hydroxyl radicals) on the GNP/TNTAs by photoluminescence (PL) and electron spin resonance spectra (ESR) techniques further confirms the proposed mechanism. It is hoped that our knowledge regarding this simple self-assembly approach may cast new insight into the design and fabrication of noble-metal NPs/1-D nanotubular semiconductor hybrid nanomaterials for a wide range of photocatalytic applications.

Self-assembly of hierarchically ordered CdS quantum dots–TiO2 nanotube array heterostructures as efficient visible light photocatalysts for photoredox applications

In recent years, much attention has been focused on the preparation of CdS–TiO2 nanotube array (CdS–TNTA) hybrid nanomaterials. Nevertheless, it is still challenging to synthesize hierarchically ordered CdS–TNTA heterostructures as efficient photocatalysts via a facile and simple approach. In this work, tailor-made negatively charged CdS quantum dots (QDs) are evenly deposited on a hierarchical framework of TiO2 nanotube arrays, consisting of periodically ordered nanorings on the surface and nanotubes underneath, by modulating the surface charge properties of the constituents. It has been demonstrated that the CdS–nanoporous TiO2 nanotube array (CdS–NP-TNTA) hybrid nanostructures exhibit promising visible-light photoactivity towards the photooxidation of organic dye pollutants and the photocatalytic reduction of nitrophenol derivatives as a result of the monodisperse deposition of CdS QDs on the well-defined NP-TNTA scaffold. Photoelectrochemical investigations have shown the significantly enhanced separation efficiency of photogenerated electron–hole charge carriers in the CdS–NP-TNTA heterostructure under visible light irradiation. Furthermore, the self-assembled CdS–NP-TNTA heterostructure demonstrates improved photostability. Our work should provide a new paradigm to prepare hierarchically ordered narrow band-gap semiconductor/one-dimensional semiconductor nanomaterials for efficient visible light-driven photocatalysis.

Bridging the Gap: Electron Relay and Plasmonic Sensitization of Metal Nanocrystals for Metal Clusters.

In recent years, enormous attention has been paid to the construction of metal cluster-semiconductor nanocomposites because of the fascinating and unique properties of metal clusters; however, investigations on photoelectrochemical (PEC) and photocatalytic properties of metal cluster-semiconductor systems are still rare. Moreover, to date, intrinsic correlation between metal clusters and bulk metal nanocrystals has yet to be elucidated. In this work, a facile layer-by-layer (LbL) self-assembly strategy has been developed to judiciously and intimately integrate gold nanocrystals (Au) within the interface between gold clusters (Au(x)) and hierarchically ordered TiO2 nanotube arrays framework, by which imperative roles of Au nanocrystals as electron relay mediator and plasmonic sensitizer for Aux clusters were revealed. In addition, it was found that synergistic interaction between Au nanocrystals and Aux clusters contributed to promising visible-light-driven photocatalytical and PEC performances. It is anticipated that our work could provide a general way for rationally constructing metal and metal clusters codecorated semiconductor heterostructures and, more significantly, bridge the gap between metal clusters and metal nanocrystals for a diverse range of applications.

A green and facile self-assembly preparation of gold nanoparticles/ZnO nanocomposite for photocatalytic and photoelectrochemical applications

Gold nanoparticles (GNP) passivated by dithiolated diethylenetriaminepentaacetic (DTDTPA) linkers, GNP@DTDTPA, have been synthesized. The well-defined ZnO nanocomposite (GNP@DTDTPA/ZnO) functionalized by GNP@DTDTPA was prepared via a facile and green self-assembly approach. The specific interaction mechanism responsible for the self-assembly motif was elucidated by XPS, zeta potential and FTIR analysis. The self-assembly process was established primarily by a large amount of polar functional groups such as carboxyl (COOH), carbonyl (CO), and amide (NH–CO) groups in the DTDTPA profile, which impels GNP@DTDTPA to bind intrinsically with hydroxyl groups on the ZnO surface through hydrogen bonding interactions. On the other hand, the attractive electrostatic force between the negatively charged GNP@DTDTPA and the positively charged ZnO surface also contributes to the monodispersivity of GNP@DTDTPA on the ZnO support. The GNP/ZnO obtained after calcination of GNP@DTDTPA/ZnO retains the mono-distribution of GNP and exhibits more enhanced photocatalytic and photoelectrochemical performances compared to pure ZnO. We propose a possible mechanism that the well-distributed GNP could serve as an “electron reservoir” and improve the separation efficiency of photogenerated electron–hole pairs. This method could provide a simple and straightforward approach for achieving a uniform distribution of noble-metal nanoparticles on the surface of semiconductors for versatile photocatalytic and photoelectrochemical applications.

Self-assembly of aligned rutile@anatase TiO2 nanorod@CdS quantum dots ternary core–shell heterostructure: cascade electron transfer by interfacial design

A novel self-assembly approach based on electrostatic interactions has been developed for the synthesis of a rutile@anatase TiO2 nanorod (NR)@CdS quantum dots (QDs) ternary core–shell heterostructure, in which an in situ formed monodisperse anatase TiO2 layer was intimately sandwiched between rutile TiO2 NRs and CdS QDs. It has been demonstrated that the well-defined bilayer interface significantly improves the photocatalytic performance of the ternary heterostructure (i.e. rutile@anatase TiO2 NR@CdS QDs), owing predominantly to the appropriate band alignment of the constituent semiconductors, thus facilitating photogenerated electron–hole separation and charge collection under simulated solar light irradiation.