Jianwei Miao

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.

Identification of catalytic sites for oxygen reduction and oxygen evolution in N-doped graphene materials: Development of highly efficient metal-free bifunctional electrocatalyst.

Doping of graphene with nitrogen imparted bifunctional electrocatalytic activities for efficient energy conversion and storage. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical to renewable energy conversion and storage technologies. Heteroatom-doped carbon nanomaterials have been reported to be efficient metal-free electrocatalysts for ORR in fuel cells for energy conversion, as well as ORR and OER in metal-air batteries for energy storage. We reported that metal-free three-dimensional (3D) graphene nanoribbon networks (N-GRW) doped with nitrogen exhibited superb bifunctional electrocatalytic activities for both ORR and OER, with an excellent stability in alkaline electrolytes (for example, KOH). For the first time, it was experimentally demonstrated that the electron-donating quaternary N sites were responsible for ORR, whereas the electron-withdrawing pyridinic N moieties in N-GRW served as active sites for OER. The unique 3D nanoarchitecture provided a high density of the ORR and OER active sites and facilitated the electrolyte and electron transports. As a result, the as-prepared N-GRW holds great potential as a low-cost, highly efficient air cathode in rechargeable metal-air batteries. Rechargeable zinc-air batteries with the N-GRW air electrode in a two-electrode configuration exhibited an open-circuit voltage of 1.46 V, a specific capacity of 873 mAh g−1, and a peak power density of 65 mW cm−2, which could be continuously charged and discharged with an excellent cycling stability. Our work should open up new avenues for the development of various carbon-based metal-free bifunctional electrocatalysts of practical significance.

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.

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.

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.

All inorganic semiconductor nanowire mesh for direct solar water splitting.

The generation of chemical fuels via direct solar-to-fuel conversion from a fully integrated artificial photosynthetic system is an attractive approach for clean and sustainable energy, but so far there has yet to be a system that would have the acceptable efficiency, durability and can be manufactured at a reasonable cost. Here, we show that a semiconductor mesh made from all inorganic nanowires can achieve unassisted solar-driven, overall water-splitting without using any electron mediators. Free-standing nanowire mesh networks could be made in large scales using solution synthesis and vacuum filtration, making this approach attractive for low cost implementation.

Thermodynamically Driven One-Dimensional Evolution of Anatase TiO2 Nanorods: One-Step Hydrothermal Synthesis for Emerging Intrinsic Superiority of Dimensionality

In photoelectrochemical cells, there exists a competition between transport of electrons through the porous semiconductor electrode toward the conducting substrate and back-reaction of electrons to recombine with oxidized species on the semiconductor-electrolyte interface, which determines the charge collection efficiency and is strongly influenced by the density and distribution of electronic states in band gap and architectures of the semiconductor electrodes. One-dimensional (1D) anatase TiO2 nanostructures are promising to improve charge transport in photoelectrochemical devices. However, the conventional preparation of 1D anatase nanostructures usually steps via a titanic acid intermediate (e.g., H2Ti3O7), which unavoidably introduces electronic defects into the host lattice, resulting in undesired shielding of the intrinsic role of dimensionality. Here, we manage to promote the 1D growth of anatase TiO2 nanostructures by adjusting the growth kinetics, which allows us to grow single-crystalline anatase TiO2 nanorods through a one-step hydrothermal reaction. The synthesized anatase nanorods possess a lower density of trap states and thus can simultaneously facilitate the diffusion-driven charge transport and suppress the electron recombination. Moreover, the electronically boundary free nanostructures significantly enhance the trap-free charge diffusion coefficient of the anatase nanorods, which enables the emergence of the intrinsic superiority of dimensionality. By virtue of these merits, the anatase nanorods synthesized in this work take obvious advantages over the conventional anatase counterparts in photoelectrochemical systems (e.g., dye-sensitized solar cells) by showing more efficient charge transport and collection and higher energy conversion efficiency.

Achieving stable and efficient water oxidation by incorporating NiFe layered double hydroxide nanoparticles into aligned carbon nanotubes

A facile and scalable co-precipitation method is developed to prepare stable colloidal NiFe-LDH nanoparticles at room temperature. We further scrolled NiFe-LDH nanoparticles into well-aligned multi-walled carbon nanotube (MWCNT) sheets to form binder-free hybrid microfiber electrodes, which showed excellent OER activity, reaching 180 mA cm-2 at a small overpotential of 255 mV with outstanding durability.