Template Synthesis of Single-Crystal-Like Porous SrTiO₃ Nanocube Assemblies and Their Enhanced Photocatalytic Hydrogen Evolution

Porous nanostructures of semiconductors are well-known for their ability to enhance the photocatalytic activity thanks to the large specific surface area and abundant active sites for the reactions, interfacial transport, and high utilization of light arising from multireflections in the pores. In this paper, we have successfully fabricated a special porous SrTiO₃ three-dimensional (3D) architecture through a facile hydrothermal reaction at 150 °C, using layered protonated titanate hierarchical spheres (LTHSs) of submicrometer size as a precursor template. The SrTiO₃ architecture is characterized by the 3D assembly of hundreds of highly oriented nanocubes of 60–80 nm by the partial sharing of (100) faces, thereby displaying porous but single-crystal-like features reminiscent of mesocrystals. Our experimental results have shown the key roles played by the template effect akin to that in topotactic transformation in crystallography and Ostwald-ripening-assisted oriented attachment in the formation of such nanocube assemblies. Compared to the solid SrTiO₃ photocatalysts previously synthesized by high-temperature solid-state methods, the as-synthesized porous SrTiO₃ nanocube assemblies have relatively large specific surface areas (up to 20.83 m²·g^–1), and thus they have exhibited enhanced photocatalytic activity in hydrogen evolution from water splitting. Expectantly, our synthetic strategy using LTHSs as the precursor template may be extended to the fabrication of other titanate photocatalysts with similar porous hierarchical structures by taking advantage of the diversity of the perovskite-type titanate

Self-Limiting Assembly of Two-Dimensional Domains from Graphene Oxide at the Air/Water Interface

Self-assembly is a powerful approach to making new superstructures and high-level hierarchical structures with unique physical/chemical properties from nanosized building blocks. As-prepared graphene oxides (GOs) are in general highly polydisperse not only in size but also in shape. Yet we have demonstrated that such GO sheets tend to assemble into two-dimensional, nearly monodisperse aggregate domains at the air/water interface in a self-limiting fashion, which can be controlled. It was further shown that the self-limiting assembly was driven by the competing interactions between electrostatic repulsion between the negatively charged GO sheets and attractive potentials. This finding provides a convenient platform to understand the forces involved in the 2D assembly and opens a new direction for creating novel materials and structures at the air/water interface

Significantly Enhanced Open Circuit Voltage and Fill Factor of Quantum Dot Sensitized Solar Cells by Linker Seeding Chemical Bath Deposition

We have significantly improved open circuit voltage and fill factor with a Pt counter electrode of quasi-solid state quantum dot sensitized solar cells (QDSSCs) by achieving compact coverage of QDs on a TiO₂ matrix through a linker seeding chemical bath deposition process, leading to 4.23% power conversion efficiency, nearly two times that with conventionally deposited control photoanode. The distinguishing characteristic of our linker seeding synthesis is that it does not rely on surface adsorption of precursor ions directly on TiO₂ (TiO₂∼Cd_<i>x</i>) but rather nucleates special ionic seeds on a compact linker layer (TiO₂-COORS-Cd_<i>x</i>), thereby resulting in a full and even coverage of QDs on the TiO₂ surface in large area. We have shown that the compact coverage not only helps to suppress recombination from electrolyte but also gives rise to better charge transport through the QD layer. This linker seeding chemical bath deposition method is general and expected to reinforce the hope of quasi-solid state QDSSCs as a strong competitor of dye-sensitized solar cells after further optimization and development

Secondary Branching and Nitrogen Doping of ZnO Nanotetrapods: Building a Highly Active Network for Photoelectrochemical Water Splitting

A photoanode based on ZnO nanotetrapods, which feature good vectorial electron transport and network forming ability, has been developed for efficient photoelectrochemical water splitting. Two strategies have been validated in significantly enhancing light harvesting. The first was demonstrated through a newly developed branch-growth method to achieve secondary and even higher generation branching of the nanotetrapods. Nitrogen-doping represents the second strategy. The pristine ZnO nanotetrapod anode yielded a photocurrent density higher than those of the corresponding nanowire devices reported so far. This photocurrent density was significantly increased for the new photoanode architecture based on the secondary branched ZnO nanotetrapods. After N-doping, the photocurrent density enjoyed an even more dramatic enhancement to 0.99 mA/cm² at +0.31 V vs Ag/AgCl. The photocurrent enhancement is attributed to the greatly increased roughness factor for boosting light harvesting associated with the ZnO nanotetrapod branching, and the increased visible light absorption due to the N-doping induced band gap narrowing of ZnO

Coordination Polyhedra: A Probable Basic Growth Unit in Solution for the Crystal Growth of Inorganic Nonmetallic Nanomaterials?

Learning from the classical crystallization mode and the conventional oriented attachment mode, we demonstrate another understanding of the crystal growth of inorganic nonmetallic nanomaterials in solution from the perspective of coordination polyhedra. A family of β-Ni­(OH)₂ hourglass-like nanostructures is controllably synthesized and chosen to illustrate this understanding, in which the coordination polyhedra of Ni­(OH)₆^4– are supposed to serve as the basic growth unit to grow these crystals in solution. According to this “coordination polyhedra growth unit” mode, a probable crystal growth mechanism featuring two-stage oriented attachment is put forth. In addition, with this deliberate mode, a series of anisotropic features as well as interesting structural patterns of the as-prepared β-Ni­(OH)₂ nanocrystals have also been successfully explained. The nanocrystal growth mechanism proposed in this paper may be general; for example, it might reflect the actual circumstances of crystallization of certain inorganic nonmetallic nanocrystals in solution

Unveiling Two Electron-Transport Modes in Oxygen-Deficient TiO₂ Nanowires and Their Influence on Photoelectrochemical Operation

Introducing oxygen vacancies (V_O) into TiO₂ materials is one of the most promising ways to significantly enhance light-harvesting and photocatalytic efficiencies of photoelectrochemical (PEC) cells for water splitting among others. However, the nature of electron transport in V_O-TiO₂ nanostructures is not well understood, especially in an operating device. In this work, we use the intensity-modulated photocurrent spectroscopy technique to study the electron-transport property of V_O-TiO₂ nanowires (NWs). It is found that the electron transport in pristine TiO₂ NWs displays a single trap-limited mode, whereas two electron-transport modes were detected in V_O-TiO₂ NWs, a trap-free transport mode at the core, and a trap-limited transport mode near the surface. The considerably higher diffusion coefficient (<i>D</i>_n) of the trap-free transport mode grants a more rapid electron flow in V_O-TiO₂ NWs than that in pristine TiO₂ NWs. This electron-transport feature is expected to be common in other oxygen-deficient metal oxides, lending a general strategy for promoting the PEC device performance

Building High-Efficiency CdS/CdSe-Sensitized Solar Cells with a Hierarchically Branched Double-Layer Architecture

We report a double-layer architecture for a photoanode of quantum-dot-sensitized solar cells (QDSSCs), which consists of a ZnO nanorod array (NR) underlayer and a ZnO nanotetrapod (TP) top layer. Such double-layer and branching strategies have significantly increased the power conversion efficiency (PCE) to as high as 5.24%, nearly reaching the record PCE of QDSSCs based on TiO₂. Our systematic studies have shown that the double-layer strategy could significantly reduce charge recombination at the interface between the charge collection anode (FTO) and ZnO nanostructure because of the strong and compact adhesion of the NRs and enhance charge transport due to the partially interpenetrating contact between the NR and TP layers, leading to improved open-circuit voltage (<i>V</i>_oc) and short-circuit current density (<i>J</i>_sc). Also, when the double layer was subjected to further branching, a large increase in <i>J</i>_sc and, to a lesser extent, the fill factor (FF) has resulted from increases in quantum-dot loading, enhanced light scattering, and reduced series resistance

Enhancing Full Water-Splitting Performance of Transition Metal Bifunctional Electrocatalysts in Alkaline Solutions by Tailoring CeO₂–Transition Metal Oxides–Ni Nanointerfaces

Rational design of highly efficient bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is critical for sustainable energy conversion. Herein, motivated by the high activity of OER catalyst on water dissociation that is the rate-determining step of alkaline HER, a bifunctional catalyst of metallic nickel-decorated transition metal oxide nanosheets vertically grown on ceria film (ceria/Ni-TMO) is synthesized by composition controlling and surface engineering. Because of the idealized electronic structure of the active centers and the abundance of such sites, as well as a synergistic effect between the carbon cloth/ceria film and the in situ formed TMO/Ni nanoparticles, the as-synthesized ceria/Ni-TMO exhibited long-time stability and a low cell voltage of 1.58 V at 10 mA/cm² when applied as both the cathode and anode in alkaline solutions. Moreover, it is the first time that pH-independent four-proton-coupled-electron-transfer processes and multiple adsorption–desorption processes were found to occur at the interfaces of ceria/TMO and Ni/TMO in a single catalyst for catalyzing OER and HER, respectively

Design Hierarchical Electrodes with Highly Conductive NiCo₂S₄ Nanotube Arrays Grown on Carbon Fiber Paper for High-Performance Pseudocapacitors

We report on the development of highly conductive NiCo₂S₄ single crystalline nanotube arrays grown on a flexible carbon fiber paper (CFP), which can serve not only as a good pseudocapacitive material but also as a three-dimensional (3D) conductive scaffold for loading additional electroactive materials. The resulting pseudocapacitive electrode is found to be superior to that based on the sibling NiCo₂O₄ nanorod arrays, which are currently used in supercapacitor research due to the much higher electrical conductivity of NiCo₂S₄. A series of electroactive metal oxide materials, including Co_<i>x</i>Ni_1–<i>x</i>(OH)₂, MnO₂, and FeOOH, were deposited on the NiCo₂S₄ nanotube arrays by facile electrodeposition and their pseudocapacitive properties were explored. Remarkably, the as-formed Co_<i>x</i>Ni_1–<i>x</i>(OH)₂/NiCo₂S₄ nanotube array electrodes showed the highest discharge areal capacitance (2.86 F cm^–2 at 4 mA cm^–2), good rate capability (still 2.41 F cm^–2 at 20 mA cm^–2), and excellent cycling stability (∼4% loss after the repetitive 2000 cycles at a charge–discharge current density of 10 mA cm^–2)