Solution-Based Epitaxial Growth of ZnO Nanoneedles on Single-Crystalline Zn Plates

We report a method for growing aligned single-crystalline ZnO nanoneedles on single-crystalline Zn plates via a solution-phase reaction. An Al film deposited on a Si wafer was immersed and maintained in an aqueous ammonia solution containing Zn ions at 90 °C. After 30 min, the substrate surface was found to be covered with hierarchical ZnO nanoneedles/Zn heterostructures. ZnO nanoneedles evolved epitaxially from the surface of a hexagonal “mother” Zn disk. The mother Zn crystals were thin, hexagonal disks with lateral dimensions on the order of 2−8 μm. The nanoneedles had an average diameter of 100 nm and an average length of 700 nm. The mechanism of formation of the hierarchical ZnO/Zn structures was investigated by characterizing the crystalline nature, morphology, and composition of the synthesized materials as a function of the reaction time. In this way, the evolution of structures could be determined. The role of the Al layer in this method is also discussed. On the basis of the experimental results, we propose a mechanism for the spontaneous growth of such ZnO/Zn heterostructures

Morphology-Controlled Growth of ZnO Nanostructures Using Microwave Irradiation: from Basic to Complex Structures

Morphology-controlled growth of ZnO nano- and microstructures was achieved by microwave irradiation. Various basic ZnO structures, including nanorods, nanocandles, nanoneedles, nanodisks, nanonuts, microstars, microUFOs, and microballs were simply synthesized at a low temperature (90 °C) with low power microwave-assisted heating (about 50 W) and a subsequent aging process. These results could be obtained by changing the precursor chemicals, the capping agents, and the aging times. Even more complex ZnO structures, including ZnO bulky stars, cakes, and jellyfishes, were constructed by microwave irradiation to a mixture of the as-prepared basic ZnO structures and the solution I, IV, or V. This is a fast, simple, and reproducible method which does not require any template, catalyst, or surfactant but can control the morphology of ZnO crystals from simple to complex. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) were used to observe the morphology, crystallinity, and chemical composition of the ZnO structures. Growth mechanisms for shape-selective ZnO synthesis were proposed based on these results

Exposed Crystal Face Controlled Synthesis of 3D ZnO Superstructures

We report a method for synthesizing exposed crystal face controlled 3D ZnO superstructures under mild conditions (at room temperature or 90 °C under 1 atm) without organic additives. The exposed crystal faces of the building blocks of the 3D structures were controlled by varying the reactant concentrations and the reaction temperatures. On the basis of the experimental results, we speculated a possible mechanism for the formation of the four distinct 3D ZnO superstructures (structures I, II, III, and IV) under the different growth conditions. The optical properties of the 3D ZnO superstructures were probed by UV−vis diffuse reflectance spectroscopy. The spectra were shifted depending on the dimensions and sizes of the building blocks of the 3D superstructures. The photocatalytic activities of the 3D superstructures varied according to the exposed crystal faces, which could be controlled by this method (structure I > structure IV > structure III > structure II)

Facile and Fast Synthesis of Single-Crystalline Fractal Zinc Structures through a Solution Phase Reaction and Their Conversion to Zinc Oxide

We report a novel method for the synthesis of fractal Zn structures through a solution phase reaction. An Al-film-deposited substrate was immersed in an aqueous ammonia solution containing Zn ions and maintained at 95 °C for 5 min. After the reaction, the Al-deposited side of the substrate was found to be covered with fractal Zn structures. These Zn structures are highly oriented, and the Zn(002) planes are parallel to the substrate. They are single-crystalline and the average thickness of the plates is ∼50 nm. On the basis of our results, we propose a mechanism for the spontaneous growth of such fractal Zn structures. Single-crystalline fractal ZnO structures can also be obtained by calcination of the as-synthesized fractal Zn crystals at 500 °C for 5 h in air. These fractal ZnO structures are highly oriented and inherit their morphologies from the Zn structures

Simultaneous Synthesis of Al-Doped ZnO Nanoneedles and Zinc Aluminum Hydroxides through Use of a Seed Layer

Al-doped ZnO nanoneedles with a tip diameter of ∼10 nm and hexagonal zinc aluminum hydroxide were simultaneously synthesized and completely separated in one step. An aqueous solution containing metal ions (Zn2+ and Al3+), in which a Zn-deposited substrate was immersed, was irradiated with microwaves. Al-doped ZnO nanoneedles (average Zn/Al molar ratio of 55.5) grew on the substrate, whereas zinc aluminum layered double hydroxides (average Zn/Al molar ratio of 2.51) grew as homogeneous precipitates in the bulk solution. The 1D Al-doped ZnO nanoneedles had an average length of ∼1.3 μm. The length, diameter, and shape of the nanoneedles were uniform. The 2D hexagonal zinc aluminum hydroxides had lateral dimensions of 500 nm to 1.8 μm and an average thickness of ∼50 nm. Asymmetric partitioning of Zn and Al species was observed in the simultaneous synthesis of these nano/microcrystals

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

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

Three-Dimensional Type II ZnO/ZnSe Heterostructures and Their Visible Light Photocatalytic Activities

We report a method for synthesizing three distinct type II 3D ZnO/ZnSe heterostructures through simple solution-based surface modification reactions in which polycrystalline ZnSe nanoparticles formed on the surfaces of single-crystalline ZnO building blocks of 3D superstructures. The experimental results suggested a possible formation mechanism for these heterostructures. The formation of the ZnO/ZnSe heterostructures was assumed to result from a dissolution–recrystallization mechanism. The optical properties of the 3D ZnO/ZnSe heterostructures were probed by UV–vis diffuse reflectance spectroscopy. The 3D ZnO/ZnSe heterostructures exhibited absorption in the visible spectral region. The visible photocatalytic activities of 3D ZnO/ZnSe heterostructures were much higher than those of the 3D pure ZnO structures. The activities of the 3D ZnO/ZnSe heterostructures varied according to the structures under visible light. The morphologies and exposed crystal faces of pure ZnO building blocks prior to surface modification had a significant effect on the visible light photocatalytic processes of ZnO/ZnSe heterostructures after surface modification

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

Precursor Effects of Citric Acid and Citrates on ZnO Crystal Formation

We have studied the precursor effects of citric acid and various citratesincluding triethyl citrate, tripotassium citrate, trisodium citrate and triammonium citrateon the formation of ZnO crystals in alkaline solution. These citrate-related chemicals could be divided into three groups (group A, triethyl citrate; group B, tripotassium citrate and trisodium citrate; and group C, citric acid and triammonium citrate) based on their activity for modifying the ZnO growth direction and solution pH dependency on their concentration. We could obtain ZnO structures with various distinct morphologies by simply changing the concentration of citric acid or citrate additive dissolved in the alkaline reaction solution. On the basis of the results, we propose the growth mechanisms underlying the formation of the various ZnO structures in the absence and presence of citric acid or citrate additives