Dendritic and epitaxial growths of ZnO particle-rod and rod-rod nanostructures with quasi-one-dimensional and two-dimensional configurations

Quasi-one-dimensional and two-dimensional ZnO nanostructures have been fabricated through thermal evaporation approach. The microstructures of the ZnO nanostructures have been studied using scanning electron microscopy and high-resolution electron microscopy. Quasi-one-dimensional ZnO nanostructures are formed by dendritic growths of ZnO nanoparticles from the stem nanorods surfaces, forming particle-rod nanostructures. While epitaxial growths of branch nanorods from the stem nanorods configure two-dimensional ZnO nanostructures. The epitaxial growth orientation relationship can be described as [2? 110]_R1 || [2? 110]_R2 and (0001) _R1 || (011?0)_R2. The growth mechanism of the quasi-one-dimensional and two-dimensional ZnO nanostructures has been discussed.     

Substrate engineering of LaAlO3 for non-polar ZnO growth

We demonstrate that growth of non-polar ZnO in a-plane and m-plane can be achieved through substrate engineering of LaAlO₃ with (001) and (112) surface. X-ray diffraction, reflection high energy electron diffraction and cross-sectional transmission electron microscopy with selected area diffraction reveal that a-plane ZnO on LaAlO₃ (001) consists of two types of domains perpendicular to each other with in-plane orientation relationships of [0001]_ZnO // [11?0]_LAO and [11?00]_ZnO // [11?0]_LAO. Single domain epitaxy of m-plane ZnO on LaAlO₃ (112) can be obtained with in-plane orientation relationships of (101?0)_ZnO//(112)_LAO, [0001]_ZnO // [1?10]_LAO and [12?10]_ZnO // [1?1?1]_LAO.     

Synthesis and photo-catalytic degradation property of nanostructured-ZnO with different morphology

ZnO nanostructures with different morphology have been successfully fabricated by a simple relative low temperature approach at 90 °C for 5 h without surfactant assistance. These structures can be easily tailed using varied concentrations of sodium hydroxide (NaOH) and different amounts of the hydrazine hydrate (N₂H₄·H₂O). X-ray diffraction (XRD) result proves the formation of ZnO with wurtzite structure. Microstructure as revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicates that the rod-like and chrysanthemum-like ZnO nanostructures contain many radial nanorods, which grow along the [0001] direction. Furthermore, the as-prepared ZnO nanomaterials exhibit high activity on the photo-catalytic degradation of typical persistent organic pollutants (POPs), indicating that they are promising as semiconductor photo-catalysts.     

Microstructure of a-plane ZnO grown on LaAlO3 (001)

The microstructure of a-plane ZnO grown on LaAlO₃ (LAO) (001) has been systematically investigated by employing X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Based on the results of XRD and TEM, only a-plane ZnO has been found to grow on LAO (001), and it consists of two types of domains perpendicular to each other. The crystal orientation relationships of a-plane ZnO domains with LAO have been verified to be [0001]_ZnO//[110]_LAO and [11?00]_ZnO//[11?0]_LAO. The domain boundaries in the a-plane ZnO are along the direction in a rotation angle of about 45° from the c-axes of ZnO. The surface morphology of ZnO films in SEM exhibits domain structure in stripe-like shape. The formation of two domains can be attributed to the cubic symmetry of the surface configuration of LAO (001).     

Electrodeposition and properties of nanocrystalline ZnO films prepared in the presence of anionic surfactant SDS and ionic liquid 1-butyl-3-methylimidazolium methylsulfate

ZnO thin films were potentiostatically electrodeposited on transparent tin oxide conducting glass substrates at ?1.0 V (vs. Ag/AgCl) in the presence of anionic surfactant, sodium dodecyl sulphate (SDS) and hydrophobic ionic liquid, 1-butyl-3-methylimidazolium methylsulfate [bmim][CH₃SO₄]. X-ray diffraction (XRD) investigation confirm the formation of nanocrystalline and stoichiometric ZnO thin films. Scanning electron microscopy (SEM) results reveal that the grain size of the deposited ZnO film is reduced from 85 to 28 nm when small amount of ionic liquid is added to the deposition bath. Photoluminescence study at room temperature shows a clear absorption edge in the ultra violet (UV) region confirming the high quality, nanocrystalline and stoichiometric nature of the deposited ZnO films.     

Synthesis,characterization and growth mechanism of ZnO nanowires on NiCl<Subscript>2</Subscript>-coated Si substrates

Well-crystallized ZnO nanowires have been successfully synthesized on NiCl₂-coated Si substrates via a carbon thermal reduction deposition process. The pre-deposited Ni nanoparticles by dipping the substrates into NiCl₂ solution can promote the formation of ZnO nuclei. The as-synthesized nanowires were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectrum. The results demonstrate that the as-fabricated nanowires with about 60 nm in diameter and several tens of micrometers in length are preferentially arranged along [0001] direction with (0002) as the dominate surface. Room temperature PL spectrum illustrates that the ZnO nanowires exist a UV emission peak and a green emission peak, and the peak centers locate at 387 and 510 nm. Finally, the growth mechanism of the nanowires is briefly discussed.     

The role of ammonia hydroxide in the formation of ZnO hexagonal nanodisks using sol–gel technique and their photocatalytic study

ZnO nanomaterials with large surface area are desired particularly for the gas sensor, biosensor and photocatalyst applications. In this study, ZnO hexagonal nanodisks with thickness to diagonal aspect ratio (～1/80) were successfully synthesised via sol–gel approach. By using aluminium sulphate as a complexing agent and carefully controlling the amount of ammonia hydroxide, zinc oxide hexagonal nanodisks were produced. The ZnO nanodisks had perfect hexagonal shape with about 4 μm in diagonal and 50 nm in thickness. The growth of the nanodisks was favoured along the six symmetric directions of ±[1ī00], ±[01ī0] and ±[10ī0]. The growth mechanism of ZnO hexagonal nanodisks is proposed as follows. The formation of ZnO hexagonal nanodisks was mediated by the adsorption of aluminate ions, Al(OH)₄^?, on the polar surface of ZnO. The Al(OH)₄^? ions were produced as a result of reaction between Al₂(SO₄)₃ and NH₄OH. The Al(OH)₄^? ions were bonded to the positively charged Zn²⁺-terminated (0001) polar surface of ZnO. This suppressed the preferential growth of ZnO along [0001] direction but allowed the lateral growth of ZnO in <01ī0>. Eventually, ZnO hexagonal nanodisks with ±(0001) top/bottom surfaces and {1ī00} side surfaces were formed. The size of the ZnO hexagonal nanodisks could be adjusted via the synthesis duration and the amount of ammonia hydroxide. The photocatalytic study indicates that ZnO hexagonal nanodisks were a good photocatalyst for the degradation of Rhodamine B under ultraviolet light irradiation with a rate constant of 0.036 min^?1.     

Oriented growth of urchin-like zinc oxide micro/nano-scale structures in aqueous solution

The urchin-like ZnO microcrystals with high crystallinity decomposed from [Zn(OH)₄]^2? directly were obtained via a hydrothermal method. The morphology, particle size, crystalline structure and fluorescence of the as-prepared ZnO were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL) analyses. The results demonstrated that the urchin-like ZnO crystals with wurtzite structure had a narrow distribution in size, which could be adjusted in the range of 30–80 μm by varying reaction time. Broad visible light emission peak was also observed in the PL spectra of the synthesized ZnO products. A multistep growth process about how to form such a structure was proposed.     

Formation of a two-dimensional electron gas in ZnO/MgZnO single heterostructures and quantum wells

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (112?0) and ZnO (0001?) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance-voltage (C-V) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C-V measurements the sheet carrier density in a Zn_0.8 Mg_0.2O/ZnO/Zn_0.8 Mg_0.2O quantum well (QW) structure with a well width of about 5 nm is calculated to be only about 9.0 × 10¹⁰ cm^− 2. For the films deposited on sapphire 2D growth is observed in the Burton-Cabrera-Frank mode, as confirmed by atomic force microscopy. Step flow growth mode was achieved for the homoepitaxial thin films. Quantum confinement effects have been confirmed by photoluminescence (PL) measurements. Homoepitaxial QWs are more homogeneous (smaller inhomogeneous recombination broadening) than heteroepitaxial QWs.     

Microwave-assisted synthesis of hydroxyapatite hollow microspheres in aqueous solution

We report a template-free microwave-assisted hydrothermal method for the preparation of hydroxyapatite hollow microspheres constructed by the self-assembly of nanosheets using Ca(CH₃COO)₂, Na₂HPO₄, NaH₂PO₄ and sodium citrate in aqueous solution. X-ray powder diffraction (XRD) patterns indicated that the as-prepared samples consisted of hydroxyapatite. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) micrographs showed that the as-prepared products were composed of hollow microspheres assembled with nanosheets and had three-dimensional nanoporous nanostructured networks. The experimental parameters were varied to investigate their effects on the product, and a possible formation mechanism was proposed. The as-prepared hydroxyapatite hollow microspheres have a potential application in drug delivery.     

Microstructure of ZnO films synthesized on MgAl2O4 from low-temperature aqueous solution: growth and post-annealing

The microstructure of ZnO films synthesized from low-temperature (90 °C) aqueous solution on (111) MgAl₂O₄ single crystal substrates was characterized by X-ray diffraction, high-resolution scanning electron microscopy, conventional and high-resolution transmission electron microscopy. To examine the thermally activated microstructural evolution of the ZnO, both as-deposited and annealed films were characterized. The ZnO films were confirmed to have a ZnO $ [10\bar{1}0](0001)\left\| {{\text{MgAl}}_{ 2} {\text{O}}_{4} [011](1\bar{1}1)} \right. $ orientation relationship, with Zn polarity normal to the surface. Despite their highly oriented nature, the ZnO films have a columnar grain structure with low-angle (<2.5°) grain boundaries. In addition to lattice dislocations forming low-angle grain boundaries, threading dislocations were observed, emanating from the interface with the substrate. In annealed films, thermally generated voids were observed and appeared to preferentially form at grain boundaries and dislocations. Based on these characterization results, mechanisms are proposed for film growth and microstructural evolution. Finally, the diffusion coefficient of vacancies via dislocations at grain boundaries in the produced ZnO films was estimated.     

SnO2/α-Fe2O3 hierarchical nanostructure: Hydrothermal preparation and formation mechanism

In this paper, we reported a simple solution method to assemble SnO₂ nanorods hierarchically on the surface of α-Fe₂O₃ nanosheets using Fe₃O₄ nanosheets as precursor. The product was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM). Our experimental results show that the lattice mismatch at the interface of SnO₂ nanorods with α-Fe₂O₃ nanosheets played an important role in determining the growth direction of SnO₂ nanorods. The interface prefers to take the least lattice mismatch and thus the preferential growth direction of SnO₂ nanorods was along [1 0 1] direction. The result may have important impact on the understanding of the nucleation growth process in a heterogeneous system.     

A facile approach to synthesize silver nanorods capped with sodium tripolyphosphate

Silver nanorods have been successfully synthesized via a facile route using sodium tripolyphosphate (Na₅P₃O₁₀) as the capping agent. Silver nitrate and glucose served as the Ag⁺ source and green reducing agent in aqueous solution, respectively. The products were characterized by X-ray powder diffraction (XRD), UV–visible spectroscopy (UV–vis), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). The results showed that the obtained silver nanorods are highly crystallized with an average diameter of 30 nm and lengths up to 400 nm. The linear structure of the capping agent and slow reaction rate are considered to be key factors in the control of particle morphology. The possible formation mechanism has been discussed based on the experimental results.     

Control of epitaxial growth orientation in ZnO nanorods on c-plane sapphire substrates

This paper presents a study on low temperature hydrothermal growth of ZnO nanorods (NRs) on pre-seeded (0001) sapphire substrates. Prior to hydrothermal growth of ZnO NRs, epitaxial ZnO seeds were grown by metal-organic chemical vapour deposition under various process conditions. Findings show that the majority of ZnO NRs inclined at a specific angle of about 38° to the direction perpendicular to the substrate surface and exhibited a preferential in-plane alignment, besides other NRs growing vertically from the sapphire surface. X-ray diffraction φ-scan measurements reveal that the ZnO nanorods displayed two distinct epitaxial relationships with sapphire which were (0001)_ZnO//(0001)_sapphire and (0001)_ZnO//(101?4)_sapphire, respectively. Reduced lattice mismatch between ZnO and sapphire is responsible for the inclined ZnO NRs growth. The growth direction of ZnO NRs is remarkably dependent on the growth conditions of ZnO seeds and sapphire substrate pre-treatment. The epitaxial orientations of ZnO seeds grown on the sapphire substrate dominate the subsequent ZnO NRs growth and can be controlled through adjusting growth conditions.     

Fe3O4 on ZnO: A spectroscopic study of film and interface properties

Magnetite (Fe₃O₄) thin films have been grown epitaxially on zinc oxide (ZnO) substrates, using reactive molecular beam epitaxy. The film quality was found to be strongly dependent on the oxygen partial pressure during growth. For a uniform Fe₃O₄ film a certain pressure variation was needed during growth. Structural, electronic, and magnetic properties were analyzed utilizing low energy electron diffraction, Hard X-ray Photoelectron Spectroscopy (HAXPES), Magneto-Optical Kerr Effect (MOKE), and X-ray Magnetic Circular Dichroism (XMCD). Diffraction patterns show clear indication for growth of Fe₃O₄ in the [111] direction on ZnO(0001). Non-destructive depth profiling by HAXPES revealed uniform magnetite thin films. Both, MOKE and XMCD measurements show easy in-plane magnetization. The dichroic spectra clearly support the formation of Fe₃O₄.     

Graphene/MnO2 hybrid nanosheets as high performance electrode materials for supercapacitors

Graphene/MnO₂ hybrid nanosheets were prepared by incorporating graphene and MnO₂ nanosheets in ethylene glycol. Scanning electron microscopy and transmission electron microscopy analyses confirmed nanosheet morphology of the hybrid materials. Graphene/MnO₂ hybrid nanosheets with different ratios were investigated as electrode materials for supercapacitors by cyclic voltammetry (CV) and galvanostatic charge–discharge in 1 M Na₂SO₄ electrolyte. We found that the graphene/MnO₂ hybrid nanosheets with a weight ratio of 1:4 (graphene:MnO₂) delivered the highest specific capacitance of 320 F g⁻¹. Graphene/MnO₂ hybrid nanosheets also exhibited good capacitance retention on 2000 cycles.     

Graphite oxide-assisted sonochemical preparation of α-Bi2O3 nanosheets and their high-efficiency visible light photocatalytic activity

Novel α-Bi₂O₃ nanosheets have been successfully synthesized by graphite oxide (GO)-assisted sonochemical route and subsequent calcining treatment. The morphology of the final products can be controlled by tuning GO nanosheets in the synthesis process. Porous and nanosheet-covered α-Bi₂O₃ microrods have also been obtained, and the growth mechanism has been discussed. Scanning electron microscopy and transmission electron microscopy analyses reveal that the α-Bi₂O₃ nanosheets are 10–20 nm in thickness with suppressed growth along the [2 1 ?4] direction. Owing to the unique structure, the α-Bi₂O₃ nanosheets show broader optical absorption and narrower band gap (2.25 eV) as compared to other two forms of α-Bi₂O₃ (~2.8 eV). Photocatalytic experiments demonstrated that the α-Bi₂O₃ nanosheets exhibited excellent activity much higher than the porous and nanosheet-covered α-Bi₂O₃ microrods in degrading organic pollutants under visible light. This study opens up a new route for the preparation of α-Bi₂O₃ nanosheets with superior photocatalytic performance.     

Controllable Synthesis of [11−2−2] Faceted InN Nanopyramids on ZnO for Photoelectrochemical Water Splitting

Indium nitride (InN) is one of the promising narrow band gap semiconductors for utilizing solar energy in photoelectrochemical (PEC) water splitting. However, its widespread application is still hindered by the difficulties in growing high‐quality InN samples. Here, high‐quality InN nanopyramid arrays are synthesized via epitaxial growth on ZnO single‐crystals. The as‐prepared InN nanopyramids have well‐defined exposed facets of [0001], [11?2?2], [1?212], and [?2112], which provide a possible routine for understanding water oxidation processes on the different facets of nanostructures in nanoscale. First‐principles density functional calculations reveal that the nonpolar [11?2?2] face has the highest catalytic activity for water oxidation. PEC investigations demonstrate that the band positions of the InN nanopyramids are strongly altered by the ZnO substrate and a heterogeneous n–n junction is naturally formed at the InN/ZnO interface. The formation of the n–n junction and the built‐in electric field is ascribed to the efficient separation of the photogenerated electron–hole pairs and the good PEC performance of the InN/ZnO. The InN/ZnO shows good photostability and the hydrogen evolution is about 0.56 µmol cm^?2 h^?1, which is about 30 times higher than that of the ZnO substrate. This study demonstrates the potential application of the InN/ZnO photoanodes for PEC water splitting.     

A new soft template-oriented method for the preparation of hollow analcime microspheres with nanosheets-assembled shells

This paper highlights a controlled synthesis of two-dimensional analcime nanosheets templated by organic additives and an impressive strategy that hollow hierarchical analcime microspheres with layered shells can be assembled by taking advantage of the intrinsic growth law of material. Specifically, ultrathin analcime nanosheets were initially obtained by precisely manipulating the amounts of cetyltrimethylammonium cation (CTA⁺) and ethylenediaminetetraacetate (EDTA^4?) in the synthesis system. As building blocks, these nanosheets then self-assembled layer by layer from outside to inside driven by the reversed crystal growth mechanism of analcime, resulting in a hollow structure with lamellar shells and enhanced specific surface area of 722.3 m² g^?1. Series of experiments were carried out in order to explore the influence of CTA⁺ and EDTA^4? on the formation of analcime nanosheets. The results indicated that CTA⁺ was the micro-mesoporogen of hierarchical analcime and synergistically collaborated with EDTA^4? in directing analcime nanosheets. The effect of hydrothermal temperature was discussed and a surfactant packing parameter (g = V/a ₀ l) was cited to explain the behavior of organics. In addition, the investigation of hydrothermal process clearly revealed the crystallization and self-assembly process of hollow structure. And the UV Raman results unraveled that four-membered rings (4MRs) as the active building units for analcime framework were firstly formed in the synthesis gel, followed by reconstruction and self-assembly which lead to the formation of 6MRs and 8MRs.     

Chemistry and Kinetics of ZnO Growth from Alkaline Hydrothermal Solutions

Special features (polar growth and nonstoichiometry) of ZnO single crystals grown on seed plates of various crystallographic orientations in the ZnO–KOH–H₂O and ZnO–KOH–LiOH–H₂O hydrothermal systems are analyzed. The growth proceeds via the interaction of ZnO^2- ₂ anions with crystal surfaces, and its rate depends on the atomic structure and electric charge of the surface. The mechanism underlying the influence of Li⁺ ions on polar ZnO growth is considered. Partial replacement of Zn²⁺ by Li⁺ decreases the positive charge on the (0001) face and hinders the attachment of ZnO^2- ₂ anions. The incorporation of Li ions into the (0001¯) face decreases its negative charge and accelerates growth in the [0001¯] direction.