Highly Fluorescent and Stable Quantum Dot-Polymer-Layered Double Hydroxide Composites

We report a designed strategy for a synthesis of highly luminescent and photostable composites by incorporating quantum dots (QDs) into layered double hydroxide (LDH) matrices without deterioration of a photoluminescence (PL) efficiency of the fluorophores during the entire processes of composite formations. The QDs synthesized in an organic solvent are encapsulated by polymers, poly­(maleic acid-alt-octadecene) to transfer them into water without altering the initial surface ligands. The polymer-encapsulated QDs with negative zeta potentials (−29.5 ± 2.2 mV) were electrostatically assembled with positively charged (24.9 ± 0.6 mV) LDH nanosheets to form QD-polymer-LDH composites (PL quantum yield: 74.1%). QD-polymer-LDH composite films are fabricated by a drop-casting of the solution on substrates. The PL properties of the films preserve those of the organic QD solutions. We also demonstrate that the formation of the QD-polymer-LDH composites affords enhanced photostabilities through multiple protections of QD surface by polymers and LDH nanosheets from the environment

Highly Efficient and Stable Cadmium Chalcogenide Quantum Dot/ZnO Nanowires for Photoelectrochemical Hydrogen Generation

Although cadmium chalcogenide quantum dot-sensitized photoanode can utilize the whole visible region of the solar spectrum, its poor photochemical stability owing to hole-induced anodic corrosion remains a major problem for the application in photoelectrochemical hydrogen generation systems. Here, modification with IrO_<i>x</i>·<i>n</i>H₂O, a well-known water-oxidation catalyst substantially improves the photochemical stability of the quantum dot-sensitized photoanode. Moreover, it induces an increased photocurrent and a cathodic shift of the onset potential. This is the first example that an oxygen-evolution catalyst is employed on a quantum dot-sensitized electrode system, and it shows 13.9 mA cm^–2 (at 0.6 V) and −0.277 V vs the reversible hydrogen electrode (RHE), which are the highest photocurrent density and the lowest onset potential attained with a ZnO-based electrode, respectively. An average hydrogen evolution rate of 240 μmol h^–1 cm^–2 at 0.6 V vs RHE has been achieved on a IrO_<i>x</i>·<i>n</i>H₂O modified electrode, with almost 100% of faradaic efficiency

Self-Assembled Gold Nanoparticle–Mixed Metal Oxide Nanocomposites for Self-Sensitized Dye Degradation under Visible Light Irradiation

Gold nanoparticle (Au NP)–mixed metal oxide (MMO) nanocomposite photocatalysts for efficient self-sensitized dye degradations under visible light were prepared by an electrostatically driven self-assembly. Dihydrolipoic acid (DHLA)-capped Au NPs (building block I) were synthesized through a room temperature reaction. Their hydrodynamic size was determined as being around 4.9 nm by dynamic light scattering measurements. MMO nanoplates with lateral dimensions of 100–250 nm (building block II) were prepared by a calcination of zinc aluminum layered double hydroxides at 750 °C for 2 h in air. In a pH 7.0 aqueous solution, the DHLA-capped Au NPs had a negative zeta potential (−22 ± 3 mV); on the other hand, the MMO nanoplates had a positive zeta potential (15 ± 2 mV). Electrostatic self-assembly was achieved by stirring an aqueous solution (pH 7.0) containing DHLA-capped Au NPs and MMO nanoplates at room temperature for 1 h. The self-assembled and sequentially calcined nanocomposites exhibited the superior self-sensitized dye degradation efficiency under visible light to that of ZnO, TiO₂ (P25), or pure MMO nanoplates. The enhanced degradation efficiency could be attributed to strong coupling interactions of ZnO and ZnAl₂O₄ phases of the MMO and the role of Au as an electron sink and mediator for formations of reactive oxidation species and as a light concentrator

Strategy for Synthesizing Quantum Dot-Layered Double Hydroxide Nanocomposites and Their Enhanced Photoluminescence and Photostability

Layered double hydroxide-quantum dot (LDH-QD) composites are synthesized via a room temperature LDH formation reaction in the presence of QDs. InP/ZnS (core/shell) QD, a heavy metal free QD, is used as a model constituent. Interactions between QDs (with negative zeta potentials), decorated with dihydrolipoic acids, and inherently positively charged metal hydroxide layers of LDH during the LDH formations are induced to form the LDH-QD composites. The formation of the LDH-QD composites affords significantly enhanced photoluminescence quantum yields and thermal- and photostabilities compared to their QD counterparts. In addition, the fluorescence from the solid LDH-QD composite preserved the initial optical properties of the QD colloid solution without noticeable deteriorations such as red-shift or deep trap emission. Based on their advantageous optical properties, we also demonstrate the pseudo white light emitting diode, down-converted by the LDH-QD composites

Light-Induced Cleaning of CdS and ZnS Nanoparticles: Superiority to Annealing as a Postsynthetic Treatment of Functional Nanoparticles

The generation of clean CdS nanoparticles from as-synthesized ones was examined using visible light. Irradiating visible light onto the nanoparticles removes organic impurities from the synthesis of CdS, while preserving the crystalline phase and nanoscale structure of the as-synthesized semiconductor as well as creating mesopores. Compared with conventional thermal annealing, which causes oxidation and sintering of nanoparticles, the indigenous light-induced cleaning provides a better post-treatment procedure for photoactive semiconductor nanoparticles. A similar feature was also observed for the ZnS nanoparticle system. The water reduction activity (λ ≥ 420 nm) of light-treated CdS was 5 times higher than that of annealed CdS

Single-Crystalline Thin Films for Studying Intrinsic Properties of BiFeO₃–SrTiO₃ Solid Solution Photoelectrodes in Solar Energy Conversion

Solid solutions have been widely investigated for solar energy conversion because of the ease to control properties (e.g., band edge positions, charge carrier transport, and chemical stability). In this study, we introduce a new method to investigate intrinsic solar energy conversion properties of solid solutions through fabricating high-quality single-crystalline solid solution films by pulsed laser deposition. This method rules out external factors, such as morphology, crystalline grain size, orientation, density and distribution, surface area, and particle–particle or particle–conducting layer connection, that have plagued previous studies on solid solution photoelectrodes. Perovskite BiFeO₃ (BFO) and SrTiO₃ (STO) were chosen as “end” members of the solid solutions (i.e., (BFO)_<i>x</i>(STO)_1–<i>x</i> (0 ≤ <i>x</i> ≤ 1)). Optical and photoelectrochemical (PEC) properties of the solid solutions significantly varied with changing compositions. Among the six studied compositions, BFO:STO (3:1 molar ratio) exhibited the highest photocurrent density with the photovoltage of 1.08 V. The photoelectrode also produced stable photocurrent for 12 h. Faradaic efficiencies of H₂ and O₂ formation close to 100% were measured