Hydrothermal Growth Conditions Research Articles

Deep-Level Emission Tailoring in ZnO Nanostructures Grown via Hydrothermal Synthesis

Zinc oxide (ZnO) nanostructures are widely used in various fields of science and technology due to their properties and ease of fabrication. To achieve the desired characteristics for subsequent device application, it is necessary to develop growth methods allowing for control over the nanostructures' morphology and crystallinity governing their optical and electronic properties. In this work, we grow ZnO nanostructures via hydrothermal synthesis using surfactants that significantly affect the growth kinetics. Nanostructures with geometry from nanowires to hexapods are obtained and studied with photoluminescence (PL) spectroscopy. Analysis of the photoluminescence spectra demonstrates pronounced exciton on a neutral donor UV emission in all of the samples. Changing the growth medium chemical composition affects the emission characteristics sufficiently. Apart the UV emission, nanostructures synthesized without the surfactants demonstrate deep-level emission in the visible range with a peak near 620 nm. Structures synthesized with the use of sodium citrate exhibit emission peak near 520 nm, and those with polyethylenimine do not exhibit the deep-level emission. Thus, we demonstrate the correlation between the hydrothermal growth conditions and the obtained ZnO nanostructures' optical properties, opening up new possibilities for their precise control and application in nanophotonics, UV-Vis and white light sources.

Theoretical and Experimental Investigate for the Magnetic and Optical Properties of Mn-ZnO Nanowire Microspheres

A Mn-ZnO nanowire microsphere was prepared by using the hydrothermal method. The effects of Mn doping concentration and hydrothermal growth conditions on the crystal structures, morphologies, magnetic and optical properties of ZnO nanowire microsphere were studied. The characterization results showed Mn-ZnO nanowire microsphere with uniform and dense distributions along the [0001] direction with a hexagonal wurtzite structure. No impurity phases were detected in microsphere specimens. The room-temperature ferromagnetism of the Mn-ZnO nanowire microsphere was detected, with the saturation magnetization of 2.4 × 10−1 emu/g and a coercive field of 369 Oe. Furthermore, with the increase of Mn2+ ions doping concentration, the luminescence intensity of the sample decreases in both UV and visible regions, and slight blueshift in the visible light regions was observed. The theoretical results presented obvious spin polarization near the Fermi level, with strong Mn 3d and O 2p hybridization effects. The magnetic moments were mainly generated by Mn 3d and partial contribution of O 2p orbital electrons. Therefore, the Mn-ZnO nanowire microsphere can be used as a potential magneto-optical material.

Controlled growth of hexagonal nanocrystals Co and Gd co-doping ZnO by hydrothermal method

The control of nanostructured zinc oxide (ZnO) materials is essential for the development of smart and functional nanoscale devices. Using controlled hydrothermal growth conditions, cobalt (Co)- and gadolinium (Gd)-co-doped zinc oxide hexagonal nanocrystals were synthesized at a low temperature (70°C). The role of hexamethylenetetramine (HMTA; C6H12N4) in engineering pure and doped zinc oxide morphology was examined. The results indicated that the hexagonal plates are transformed into a hexagonal flowerlike structure by changing the molar ratio of HMTA/precursors. The possible growth mechanism of the formation of hexagonal nanocrystals (hexagonal plates and nanoflowerlike structure) is discussed. The magnetic properties exhibited enhanced ferromagnetism after the incorporation of gadolinium into the zinc oxide matrix. An enhancement in saturation magnetization is discussed mainly in correlation with the strain effect. These doped zinc oxide nanostructures are expected to drive improvements mainly in the performance of spintronic devices.

ResNet-Locust-BN Network-Based Automatic Identification of East Asian Migratory Locust Species and Instars from RGB Images.

Locusts are agricultural pests found in many parts of the world. Developing efficient and accurate locust information acquisition techniques helps in understanding the relation between locust distribution density and structural changes in locust communities. It also helps in understanding the hydrothermal and vegetation growth conditions that affect locusts in their habitats in various parts of the world as well as in providing rapid and accurate warnings on locust plague outbreak. This study is a preliminary attempt to explore whether the batch normalization-based convolutional neural network (CNN) model can be applied used to perform automatic classification of East Asian migratory locust (AM locust), Oxya chinensis (rice locusts), and cotton locusts. In this paper, we present a way of applying the CNN technique to identify species and instars of locusts using the proposed ResNet-Locust-BN model. This model is based on the ResNet architecture and involves introduction of a BatchNorm function before each convolution layer to improve the network’s stability, convergence speed, and classification accuracy. Subsequently, locust image data collected in the field were used as input to train the model. By performing comparison experiments of the activation function, initial learning rate, and batch size, we selected ReLU as the preferred activation function. The initial learning rate and batch size were set to 0.1 and 32, respectively. Experiments performed to evaluate the accuracy of the proposed ResNet-Locust-BN model show that the model can effectively distinguish AM locust from rice locusts (93.60% accuracy) and cotton locusts (97.80% accuracy). The model also performed well in identifying the growth status information of AM locusts (third-instar (77.20% accuracy), fifth-instar (88.40% accuracy), and adult (93.80% accuracy)) with an overall accuracy of 90.16%. This is higher than the accuracy scores obtained by using other typical models: AlexNet (73.68%), GoogLeNet (69.12%), ResNet 18 (67.60%), ResNet 50 (80.84%), and VggNet (81.70%). Further, the model has good robustness and fast convergence rate.

Effect of Randomly Grown Morphology of ZnO Nanorods in Inverted Organic Solar Cells

Here, we analyzed the photovoltaic properties of the inverted organic solar cells (IOSCs) by using randomly oriented medium density ZnO nanorods (ZnO-NR) synthesized hydrothermally at low temperature conditions to avoid morphological defects. The IOSC with ZnO-NR (length < 150 nm) of medium density and random orientation showed an improvement of 83% in power conversion efficiency compared to the cell with (length < 20 nm) hydrothermally grown ZnO-NR. The optimized hydrothermal growth conditions for ZnO-NR enhanced the photovoltaic performance indicators by reducing recombination rate evidenced by the photovoltaic data. The qualitative elemental analysis of the ZnO-NR based interface was performed by the EDX, which confirmed that the as-grown ZnO-NR contain the Zn and O elements.

Using an image segmentation and support vector machine method for identifying two locust species and instars

Locusts are agricultural pests around the world. To cognize how locust distribution density and community structure are related to the hydrothermal and vegetation growth conditions of their habitats and thereby providing rapid and accurate warning of locust invasions, it is important to develop efficient and accurate techniques for acquiring locust information. In this paper, by analyzing the differences between the morphological features of Locusta migratoria manilensis and Oedaleus decorus asiaticus, we proposed a semi-automatic locust species and instar information detection model based on locust image segmentation, locust feature variable extraction and support vector machine (SVM) classification. And we subsequently examined its applicability and accuracy based on sample image data acquired in the field. Locust image segmentation experiment showed that the proposed GrabCut-based interactive segmentation method can be used to rapidly extract images of various locust body parts and exhibits excellent operability. In a locust feature variable extraction experiment, the textural, color and morphological features of various locust body parts were calculated. Based on the results, eight feature variables were selected to identify locust species and instars using outlier detection, variable function calculation and principal component analysis. An SVM-based locust classification experiment achieved a semi-automatic detection accuracy of 96.16% when a polynomial kernel function with a penalty factor parameter c of 2 040 and a gamma parameter g of 0.5 was used. The proposed detection model exhibits advantages such as high applicability and accuracy when it is used to identify locust instars of L. migratoria manilensis and O. decorus asiaticus, and it can also be used to identify other species of locusts.

Synthesis, crystal structure and topological features of microporous “anti-zeolite” Yb3(BO3)(OH)6∙2.1H2O, a new cubic borate with isolated BO3 groups

Abstract A new cubic borate, Yb3[BO3](OH)6⋅2.1H2O was synthesized in a multi-component hydrothermal system Yb2(CO3)3: B2O3: P2O5 = 5 : 2: 1 (T = 280 °C; P = 90–100 atm; pH = 3). The crystal structure was studied using single crystal X-ray analysis. The structure has been refined in the space group I432, possesses the unit cell parameter a = 12.2808(15) A. A microporous positively charged framework is formed by predominant Yb cations coordinated by O2−, OH− as a tetragonal antiprism, which are linked together via common edges and vertices. The extra framework BO3 triangles center narrow channels, directed along 3-fold axis, are perpendicular to them and clamped between large Yb polyhedra. The large cavities with the effective cavity width 7.7 A are filled by disordered water molecules. The crystal chemical formula could be written as (Z = 8): |(BO3)(H2O)2.1|[Yb3(OH)6]. A topological analysis was performed using the ToposPro software. The framework is characterized by the following tile sequence: [34]6[324.46.68]; the calculated structural complexity parameters are: v = 48 atoms, IG = 1.500 bits/atom, and IG,total = 72.000 bits/unit cell; the framework density is 12.96 Yb/1000 A3. The Yb-borate is similar to the tetragonal compounds with the general formula ABa12(BO3)7F4 (A = Li, Na) and LixNa1-xBa12(BO3)7F4, which are characterized by co-called “anti-zeolite” frameworks, formed by Ba2+ cations. The excess of water in the structure of the new borate compound Yb3[BO3](OH)6⋅2.1H2O is determined by its zeolitic nature and by hydrothermal growth conditions.

Magnetic and optical properties of Co-doped ZnO nanorod arrays

In this study, Zn1−xCoxO nanorod arrays were deposited on Si substrates by magnetron sputtering followed by the hydrothermal method at 100 °C. The effects of doping concentration and hydrothermal growth conditions on the crystal structures, morphologies, magnetic and optical properties of the obtained Zn1−xCoxO nanorod arrays were studied. Surface characterization showed Zn1−xCoxO nanorod arrays with uniform and dense distributions along the [0001] direction with the hexagonal wurtzite structure. Besides, no impurity phases were detected in Zn1−xCoxO nanorod arrays. The room-temperature ferromagnetism of Zn1−xCoxO nanorod arrays was detected based upon the high-saturation magnetization of 4.4 × 10–4 emu/g, the residual magnetization of 1.1 × 10–4 emu/g and the coercive field of 309 Oe. Furthermore, the photoluminescence (PL) spectra exhibited by the Zn1−xCoxO nanorod arrays with the luminescence intensity in the ultraviolet region were nearly five times that of the pure ZnO nanorod arrays. With the increase in the Co2+ doping concentration, the redshift in the ultraviolet emission peaks was observed. The theoretical results presented obvious spin polarization near the Fermi level, with strong Co 3d and O 2p hybridization effects. The magnetic moments were mainly generated by Co 3d and partial contribution of O 2p orbital electrons. These results indicated that Zn1−xCoxO nanorod arrays can be used as potential magneto-optical materials.

Fabrication and Study on Magnetic-Optical Properties of Ni-Doped ZnO Nanorod Arrays.

Zn1-xNixO nanorod arrays were prepared on Si substrates by magnetron sputtering and hydrothermal methods at 100 °C. We studied the effects of doped concentration and hydrothermal growth conditions on the crystal structure, morphology, photoluminescence, and magnetic properties of Zn1-xNixO nanorod arrays. The research results show that the Zn1-xNixO nanorod have the hexagonal wurtzite structure without the appearance of the second phase, and all samples have a highly preferred orientation of a (002) crystal face. The Zn1-xNixO nanorod arrays exhibit obvious room temperature ferromagnetism with saturation magnetization at 4.2 × 10−4 emu/g, the residual magnetization is 1.3 × 10−4 emu/g and the coercive field is 502 Oe, and also excellent luminescent properties with seven times greater luminous intensity than that of ZnO nanorod arrays. The redshift of the ultraviolet emission peak was found by Ni2+ doping. We further explained the source and essence of the magnetic properties of Zn1-xNixO nanorod arrays and deemed that the magnetic moment mainly comes from the hybrid electron exchange of O 2p and Ni 3d state.

Effect of hydrothermal conditions on superconductivity and magnetism in [Li1−xFexOH]FeS

Recent reports of superconductivity and magnetism in single crystals of [Li1−xFexOH]FeS show unexplained variations in both superconducting and magnetic properties. We investigate the effect of hydrothermal growth conditions on these properties and find that increasing the growth temperature systematically increases the superconducting transition temperature (Tc), sharpens the magnetic transition, and decreases the scattering rate (Γ). The slow rate of Tc suppression with increasing Γ indicates a conventional s-wave superconducting state according to the Abrikosov-Gorkov expression. Samples with higher scattering rate show broader magnetic transitions and a stronger temperature dependence in the magnetic susceptibility. These results identify disorder, due to interstitial iron impurities, as the unique internal parameter responsible for the unexplained variations in Tc and magnetic ordering. We demonstrate the optimal hydrothermal growth conditions to minimize disorder and maximize Tc in [Li1−xFexOH]FeS crystals.

Selective Area Growth of High-Quality ZnO Nanosheets Assisted by Patternable AlN Seed Layer for Wafer-Level Integration

In this work, a novel patternable seed layer of aluminum nitride (AlN) is proposed as promoter of high aspect ratio zinc oxide (ZnO) laminar nanosheets (NSs) under mild hydrothermal growth conditions. High-quality NSs are selectively grown on the patterned AlN seed layer showing a great uniformity and a surface coverage higher than 90%. Additionally, compared to the standardized growth of ZnO nanowires (NWs) in analogous conditions, this method is demonstrated to be much faster and to have higher reproducibility. A growth mechanism based on a local pH gradient due to AlN hydrolysis is proposed to explain the role of the AlN layer in the growth of NSs in contrast to the typical NW shape commonly obtained by standard hydrothermal methods with other well-known seed layers (i.e., gold, gallium nitride, sapphire, ZnO, or zinc acetate). All the results indicate that the AlN buffer layer is an effective route to obtain high-quality ZnO NSs on a silicon substrate at a wafer level for very large scale integration....

Electronic structure of hydrothermally synthesized single crystal U0.22Th0.78O2

Single crystals of ThO2, UO2, and their solid solutions, UxTh1–xO2, have been obtained through various hydrothermal growth conditions. This technique offers the better of two other growth processes: (i) single crystal purity as by photochemical growth of nanocrystals; and (ii) large/bulk sizes as obtained by the arc melt method. The band gap of the UxTh1–xO2 single crystal solid solution, along with the luminescence transition, have been characterized. The occupied and unoccupied structures are determined using ultraviolet and inverse photoemission spectroscopy and the electronic band gap was measured to be 3–4 eV. The strain of incorporating U into the ThO2 is analyzed through Vegard's law. In this crystal there are defect and impurity sites, likely arising from the kinetic growth process, giving rise to a similar yet slightly different optical gap evident with cathodoluminescence spectroscopy. There is a major luminescence feature spanning the range from 3.18 to 4.96 eV (250–390 nm) with a maximum at 4.09 eV (303 nm), corresponding with the measured electronic band gap. In this paper, the electronic properties of a solid solution U0.22Th0.78O2 are measured and interpreted compared to the pure actinide oxides, ThO2 and UO2.

Hierarchical ZnO Nanowire Growth with Tunable Orientations on Versatile Substrates Using Atomic Layer Deposition Seeding

The ability to synthesize semiconductor nanowires with deterministic and tunable control of orientation and morphology on a wide range of substrates, while high precision and repeatability are maintained, is a challenge currently faced for the development of many nanoscale material systems. Here we show that atomic layer deposition (ALD) presents a reliable method of surface and interfacial modification to guide nanowire orientation on a variety of substrate materials and geometries, including high-aspect-ratio, three-dimensional templates. We demonstrate control of the orientation and geometric properties of hydrothermally grown single crystalline ZnO nanowires via the deposition of a ZnO seed layer by ALD. The crystallographic texture and roughness of the seed layer result in tunable preferred nanowire orientations and densities for identical hydrothermal growth conditions. The structural and chemical relationship between the ALD layers and nanowires was investigated with synchrotron X-ray diffraction, ...

Comprehensive studies of the hydrothermal growth of ZnO nanocrystals on the surface of bamboo

To determine the hydrothermal growth conditions for ZnO nanocrystals on the surface of bamboo and to further develop new ZnO/bamboo hybrid materials, ZnO nanostructured materials with six morphologies, namely, spheroidal nanoparticles, nanoparticles, nanoneedles, nanograsses, nanowall networks, and nanoorchids, were successfully grown on the surface of bamboo via a low-temperature hydrothermal method based on sol–gel-prepared ZnO seed layers. The effects of temperature, time, molar ratio (zinc acetate/sodium hydroxide), and zinc salt precursor (zinc nitrate, zinc sulfate, zinc chloride, and zinc acetylacetonate) on the ZnO morphologies were comprehensively studied. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and energy dispersive spectrometry (EDS) were employed to characterize the morphology, chemical structure, and crystalline structure of the ZnO nanomaterials deposited on the bamboo surface. The ZnO nanostructure morphology was most sensitive to temperature and could be controlled by the reaction temperature. All of the as-prepared ZnO nanostructured materials were wurtzite. ZnO nanomaterials were bonded through the interaction between the hydroxyl moiety of the bamboo surface and the corresponding ZnO. A possible hydrothermal fabrication mechanism was also proposed. Further research is required to assess the improvements in the properties of bamboo from the hydrothermal deposition of nanomaterials at the bamboo surface.

Controlled side coupling of light to cladding mode of ZnO nanorod coated optical fibers and its implications for chemical vapor sensing

Controlled light coupling from surrounding to the cladding mode of zinc oxide (ZnO) nanorod coated multimode optical fiber induced by the light scattering properties of the nanorod coating and their applications of sensing are reported here. A dense and highly ordered array of ZnO nanorods is grown on the cladding of silica fibers by using low temperature hydrothermal process and the effect of the hydrothermal growth conditions of the nanorods on the light scattering and coupling to the optical fibers is experimentally investigated. The nanorod length and its number per unit area are found to be most crucial parameters for the optimum side coupling of light into the fibers. Maximum excitation of the cladding mode by side coupling of light is obtained with ZnO nanorods of length ∼2.2μm, demonstrating average coupling efficiency of ∼2.65%. Upon exposure to different concentrations of various chemical vapors, the nanorod coated fibers demonstrated significant enhancement in the side coupled light intensity, indicating the potential use of these ZnO nanorod coated fibers as simple, low cost and efficient optical sensors. The sensor responses to methanol, ethanol, toluene and benzene vapor were investigated and compared, while the effect of humidity in the sensing environment on the sensor performance was explored as well.

Demonstration of side coupling to cladding modes through zinc oxide nanorods grown on multimode optical fiber

A novel concept is introduced that utilizes the scattering properties of zinc oxide nanorods to control light guidance and leakage inside optical fibers coated with nanorods. The effect of the hydrothermal growth conditions of the nanorods on light scattering and coupling to optical fiber are experimentally investigated. At optimum conditions, 5% of the incident light is side coupled to the cladding modes. This coupling scheme could be used in different applications such as distributed sensors and light combing. Implementation of the nanorods on fiber provides low cost and controllable nonlithography-based solutions for free space to fiber coupling. Higher coupling efficiencies can be achieved with further optimization.

Facile growth of aligned WO3 nanorods on FTO substrate for enhanced photoanodic water oxidation activity

We demonstrate a facile hydrothermal method to grow oriented WO3 nanorods on a transparent conductive substrate without the assistance of any seed layer or structure directing agent for photocatalytic applications. The effects of hydrothermal growth conditions such as reaction time, precursor solution, and post annealing temperature on the crystalline phase and morphology of the WO3 on the FTO substrate are discussed. XRD studies reveal that the as-prepared orthorhombic WO3·0.33H2O nanorods are transformed to the hexagonal phase by post annealing at 400 °C. Moreover, post annealing above 500 °C converts them to monoclinic WO3 nanorods on the FTO substrate, which is the photoactive crystal phase of WO3 for water oxidation. The synthesized WO3 nanorods were revealed to have a single crystalline structure by HRTEM analysis. The photoelectrochemical water splitting properties of the annealed WO3 nanorod arrays were investigated in 0.5 M Na2SO4 under AM 1.5G illumination. The optimized WO3 nanorod arrays exhibit a photocurrent of 2.26 mA cm−2 at 1.23 V (versus RHE), and an incident photon-to-current conversion efficiency (IPCE) of as high as 35% at 400 nm for the photoelectrochemical oxidation of water. This simple hydrothermal method can allow the use of WO3 for photoanodic applications with high efficiency.

Effects of Hydrothermal Growth Conditions of ZnO Nanorods Arrays on Flexible Dye-sensitized Solar Cells

ZnO nanorod arrays(ZNRs) were prepared on the tin-doped indium oxide polyethylene terephthalate(ITO-PET) by hydrothermal method under different conditions.Morphology of ZNRs was characterized by some quantitative parameters such as the diameter and length of nanorod and the density of rods.All these parameters were adjusted by changing the reaction conditions.Two important reaction conditions including reaction time and precursor concentration are discussed separately.It is demonstrated that the precursor concentration has great influence on the aspect ratio of nanorod.Flexible substrates with ZNRs are used as the novel photoanode of flexible dye-sensitized solar cells(DSCs).The aspect ratio of nanorod has major influence on the performance of flexible DSCs.The performance of flexible DSCs is improved by controlling the reaction conditions.

Hydrothermal Growth of ZnO Nanostructures on Nylon Fabrics

ABSTRACTWe present a facile approach for growing radially oriented and dense ZnO nanorods and nanoneedles on the commercially available nylon fabrics by a simple, two-step wet chemical route. The samples were characterized by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), UV-Vis transmission spectroscopy, and wettability measurements. It was observed that the morphology of the resulting ZnO nanostructures strongly depended on the hydrothermal growth conditions. Excellent UV blocking activities were observed for ZnO nanorods containing nylon textiles in the wavelength region of 280-400 nm. Superhydrophobicity was achieved, for both ZnO nanorods and nanoneedles treated nylon fabric, upon 10mM 1-dodecanethiol treatment. ZnO nanostructures were durably attached to the nylon fabric after stirring 2 h in deionized water.

Anisotropic Wet Etched Silicon Substrates for Reoriented and Selective Growth of ZnO Nanowires and Enhanced Hydrophobicity

Herein we report the fabrication of ZnO nanowires on anisotropic wet etched silicon substrates by selective hydrothermal growth. <100> oriented silicon wafers were first patterned by anisotropic wet etch with a KOH solution, resulting in V-shaped stripes of different periods. Then, a thin layer of gold was deposited and annealed to promote the hydrothermal growth of ZnO nanowires. It was found that the growth rate of ZnO nanowires on <111> surfaces was much higher than that on <100> surfaces. As a first application of such micro- and nanostructured surfaces, we show enhanced wetting properties by measuring the contact angle of water droplets on the samples obtained under different patterning and growth conditions. Our results also demonstrated the possibility of tuning the contact angle of the sample in the range between 115° and 155°, by changing either the pattern of the silicon template or the hydrothermal growth conditions.