Single Crystalline ZnO Nanorods Research Articles

3D-branched hierarchical 3C-SiC/ZnO heterostructures for high-performance photodetectors.

The ultra-sensitive photodetection of different wavelengths holds promising applications in high-performance optoelectronic devices and it requires an efficient and suitable semiconductor unit. Herein, we demonstrated the designable synthesis of 3D-branched hierarchical 3C-SiC/ZnO heterostructures by a three-step process and their assembling into an ultrasensitive photodetector. Microstructure analyses using high-resolution transmission electron microscopy reveal that the hierarchical 3C-SiC/ZnO heterostructure is composed of single-crystal 3C-SiC nanowires as a central stem and numerous well-aligned single-crystalline ZnO nanorods as branch shells. Optoelectronic tests on the 3C-SiC/ZnO heterostructure photodetector verify the outstanding photo-detection performance with an ultrahigh EQE (1.69 × 108%), a superior photoresponsivity (4.8 × 105 A W-1), a very fast response time (a rise time of 40 ms and a decay time of 60 ms), a high photo-dark current ratio of 187.8 and an excellent photocurrent stability and reproducibility, which is significantly advantageous or comparable to those of ZnO and other inorganic semiconductor nanostructure based photodetectors. To understand the excellent photodetection of hierarchical 3C-SiC/ZnO heterostructures, a band-gap energy diagram describing the photogenerated electron transport process is plotted and the corresponding mechanism is discussed. The strategy proposed in the present work will open up more opportunities for the design and boost of ultra-sensitive photodetectors based on semiconductor heterostructures.

Effects of etching parameters on ZnO nanotubes evolved from hydrothermally synthesized ZnO nanorods

This paper reports fabrication and characterization of ZnO nanotubes on Au cylindrical spirals. Highly oriented single-crystalline ZnO nanorods were hydrothermally synthesized on Au cylindrical spirals and were etched in NaOH solution to form nanotubes. Effects of etching parameters on the lengths, wall thicknesses and morphologies of the ZnO nanotubes were studied with the etching period, etching temperature and NaOH concentration regulated respectively. It turns out that the etching parameters affect significantly the morphologies of the ZnO nanotubes but negligibly the lengths and wall thicknesses. The surfaces of the ZnO nanotube-coated Au cylindrical spirals tend to be Gaussian rough ones as the etching parameters become larger. Adjusting the etching temperature readily gives rise to perfect ZnO nanotubes. The results are beneficial for large-scale preparation of ZnO nanotubes with controllable sizes and morphologies in nanodevices.

K doping effect on structural and optical properties of ZnO nanorods grown on semipolar (112-2) GaN films using a hydrothermal growth method

In this work, we investigated the potassium (K) doping effect on structural and optical properties of ZnO nanorods grown on semipolar (112-2) GaN films using a hydrothermal growth method. The X-ray diffraction θ-2θ scan of the ZnO nanorods without any seeding step showed that most of the single-crystalline ZnO nanorods were highly inclined to the c-axis [0001] direction. In the case of hydrothermally-grown K-doped ZnO nanorods, growth occurred not only in the normal [112-2] direction, but also along the c-axis [0001] and m-axis [101-0] directions. As the growth proceeded even further, micron-sized ZnO pyramids having nonpolar and semipolar planes were formed on the ZnO nanorods. The optical properties of K-doped ZnO nanorods and pyramids were studied using cathodoluminescence measurements. While the near band edge peak was centered at 382 nm for pure c-plane ZnO nanorods, a significant peak shift to 418 nm for the K-doped ZnO nanorods was observed with relatively broad emissions. It is believed that the violet emissions centered at 418 nm are mainly associated with both K interstitial and Zn interstitial defects. Thus, the electron transitions from K interstitial levels to the valence band can lead to relatively strong violet emissions at 418 nm for K-doped ZnO nanorods and pyramids.

Doping dependent properties of Cr-doped ZnO nanostructures prepared by microwave irradiation.

In this work, undoped and Cr-doped single-crystalline ZnO nanorods were prepared by a facile microwave assisted solution method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that Cr-doped ZnO was comprised of single phase nature with hexagonal wurtzite structure up to 5% Cr doping, however, secondary phase ZnCr2O4 appeared upon further increasing the Cr dopant concentration. Field emission scanning electron microscopy (FESEM) and TEM micrographs suggested that the undoped nanorods with an average length of -~2 μm and a diameter in the range of 150-200 nm, respectively were observed. Interestingly, the size of nanorods decreased with the increase of Cr concentration in ZnO. Optical studies depicted that the energy bandgap was decreased with the increase of Cr concentration. Raman scattering spectra of Cr-doped ZnO revealed the lower frequency shift of E2(high) phonon mode with the increase in concentration of Cr dopant, suggested the successful doping of Cr into Zn site in ZnO. Magnetic studies showed that Cr-doped ZnO exhibited room temperature ferromagnetism (RTFM) and the value of magnetization was continuously decreased with the increase in Cr doping.

Controllable electrodeposition of ZnO nanorod arrays on flexible stainless steel mesh substrate for photocatalytic degradation of Rhodamine B

Well-aligned single-crystalline ZnO nanorod arrays (ZNRAs) were prepared on flexible stainless steel mesh (SSM) substrate in large-scale by using a direct electrodeposition method. The effects of electrochemical parameters, such as applied potential, applied nucleation potential time, substrate pretreatment, electrodeposition duration and times, on the orientation, morphology and density of ZNRAs were systematically studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and the selected area electron diffraction (SAED). The results showed that ZNRAs on SSM substrate with [001] preferred orientation and well crystallization were obtained by controlling the applied potential in the range of −0.9 to −1.1V. The density of ZNRAs could be increased obviously by applying a nucleation potential (−1.3V for more than 10s before deposition) or by means of substrate pretreatment (the SSM immersed in zinc acetate colloid for more than 10min before deposition), meanwhile, the deposited ZNRAs also had small average diameter (<46±4nm), narrow size distribution and good orientation. In addition, it was also found that the average diameter of ZNRAs could be increased from 89 to 201±5nm by extending the electrodeposition duration from 1800 to 7200s, and the length of rods was from 0.8 to 2.2±0.1μm when the times of the electrodeposition from one to six times. Furthermore, the band gap energy (Eg) of as-prepared ZNTAs was not closely related to the electrodeposition times (only changed from 3.30 to 3.32eV). The ZNRAs prepared with more electrodeposition times showed enhanced photocatalytic performance under the UV-lamp for degradation of Rhodamine B. The degradation efficiency of ZNRAs improved from 89.4% to 98.3% with the deposition times from one to six times.

Bifunctional Effects of CdSe Quantum Dots and Nb2O5 Interlayer for ZnO Nanorods-based Photoelectrochemical Water-Splitting Cells

Single crystalline ZnO nanorods (ZONRs) with a length of 3.6μm and a diameter of 100nm were prepared using a hydrothermal method. Based on photoactive ZONR films, the thin Nb2O5 layers were introduced using spin-coating, followed by the deposition of CdSe quantum dots (QDs) as an absorber. The Nb2O5 layers, denoted as Nb2O5(1) and Nb2O5(2), were spin-coated one and two times, respectively, to demonstrate the thickness effect of Nb2O5. The Nb2O5(1)-coated ZONR (ZONR/Nb2O5(1)) exhibited a higher photoelectrochemical (PEC) response and better stability than the bare ZONR, due to the surface-related defect states passivated by the Nb2O5. In contrast, the PEC response of the ZONR/Nb2O5(2) was reduced about 40% due to the insulating nature of the thicker Nb2O5. CdSe QD-sensitized ZONR and ZONR/Nb2O5 films were compared, revealing that the ZONR/Nb2O5(1)/CdSe film exhibited significantly enhanced PEC performance when compared with the other films. Herein, the CdSe QDs acted as the light absorber and, in addition, the QDs on the exposed ZONR passivated the surface-related defect states of the ZONR. The Nb2O5 layers also passivated the surface-related defect states of the ZONR; furthermore, the Nb2O5 located between the ZONR and the QDs led to the stepwise energy band edge alignment, resulting in the faster charge separation. These binfunctional effects contributed to the significantly enhanced PEC response of the ZONR/Nb2O5(1)/CdSe.

Hydrothermal synthesis, characterizations and photoluminescence study of single crystalline hexagonal ZnO nanorods with three dimensional flowerlike microstructures

A simple, low-cost, and environmentally benign hydrothermal approach has been successfully developed to synthesize uniform, large-scale well-crystallized ZnO nanorods with different aspect ratios that were united together to form three dimensional (3D) flowerlike structures. The method involved direct growth of ZnO 3D microstructures using aqueous solution of Zn(CH3COO)2 as the precursor and NaOH to adjust the pH of resultant solution. Surfactants or templates were not used during the entire synthetic process. Moreover, the morphology evolution of the ZnO nanorods with reaction time suggests a recrystallization–dissolution–growth mechanism that continuously takes place for prolonged interval of time. The XRD pattern of the as-grown ZnO nanorods and relevant analyses confirm the well crystallized hexagonal structure of the ZnO microstructures and no evidence of any other impurity phases. SEM observations reveal that the ZnO product grew in the form of nanorods that were united together to form 3D flowerlike morphology. The high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) shows that the ZnO nanorods were single crystalline and grew along the c-axis of the crystal plane. PL measurements of the as-synthesized nanorods exhibit excellent excitation features and strong band-edge UV luminescence even at room temperature. The uniform single crystalline, defect free, and high aspect ratio nanorods may find promising applications in optoelectronics and photo-catalysts. The growth habit of ZnO crystal is also illustrated. This method is suitable for large-scale production of ZnO microstructures and could be extended for syntheses of other metal oxides.

Effect of Different Anions on ZnO Morphology

Zinc oxide exhibit highly anisotropic growth due to the lower surface energy associated with the highly polar (002) plane. ZnO thin films are deposited from a solution containing ammonia complexed Zn2+ ions at 85 ̊C temperature. Deposited films consist of vertically aligned single crystalline ZnO nanorods, where the morphology is found anion dependent. A detailed study of nature of anisotropic grown ZnO nanorods using various Zn-salt and organic anions is executed. A remarkable change in ZnO film morphology is observed with nitrate and sulfate ions, where tip shape changes from pencil like structure to flat hexagonal shape, probably due to the change in polarity.

Hydrothermal Synthesis of Vertically Aligned Cesium-Doped ZnO Nanorods for Solar Cell Applications

Single crystalline cesium-doped ZnO nanorods with homogeneous size and shape were hydrothermally grown on Cs–ZnO nucleated glass substrate. The effect of hydrothermal growth duration on the characteristics of Cs-doped ZnO nanorods was examined. The samples were analyzed by X-ray diffraction (XRD), energy dispersive X-ray analysis, scanning electron microscopy, and electrical conductivity, optical transmission and photoluminescence (PL) measurements. XRD analysis showed that Cs-doped ZnO nanorods are wurtzite single crystals and are grown preferentially along the c-axis. Elemental analysis confirmed the presence of 1 at.% of Cs, according to the composition of growth solution. Electrical conductivity of typical samples showed higher values for the 1 at.%-doped sample, which confirmed incorporation of the Cs dopant. The samples were optically transparent and showed two UV and visible PL peaks from which the former peak experienced a red shift and a pronounced increase in intensity with increasing growth time. The fabricated Cs-doped ZnO nanorods are suitable candidates for applications as excitonic solar cells due to their ease of fabrication, morphology control and optical properties.

Citrate-assisted one-pot assembly of palladium nanoparticles onto ZnO nanorods for CO sensing application

A facile one-pot strategy was developed for functionalization of palladium nanoparticles (Pd NPs) onto single crystalline ZnO nanorods (NRs). ZnO NRs (150–200 nm in length and 20–25 nm diameters) were synthesized by the solvothermal method, whereas the Pd NPs (below 5 nm) were deposited on the surface of ZnO NRs by the solution growth method. Sensor fabricated by the Pd/ZnO NRs showed significantly enhanced sensing performance as compared to pristine ZnO NRs. The increase in response of ZnO NRs after Pd NPs loading was attributed to electronic as well as chemical sensitization of Pd NPs.

Tuning Physical and Optical Properties of ZnO Nanowire Arrays Grown on Cotton Fibers

This article reports the first systematic study on the quantitative relationship between the process parameters of solution concentration ratio, structure, and physical and optical properties of ZnO nanowires grown on cotton surfaces. To develop a fundamental understanding concerning the process-structure-activity relations, we grew a series of well-defined, radially oriented, highly dense, and uniform single-crystalline ZnO nanorods and nanoneedles on cotton surfaces by a simple and inexpensive two-step optimized hydrothermal process at a relatively low temperature. This process involves seed treatment of a cotton substrate with ZnO nanocrystals that will serve as the nucleation sites for subsequent anisotropic growth of single crystalline ZnO nanowires. All of the ZnO nanowires exhibit wurtzite crystal structure oriented along the c-axis. For investigating structure-controlled properties, seed-to-growth solutions concentrations ratio ([S]/[G]) of the synthesis process was varied over six different values. Superhydrophobicity was achieved for all morphologies after 1-dodecanethiol modification, which was highly durable after prolonged UV irradiation. Durability of the ZnO materials under laundry condition was also verified. Variation of the [S]/[G] ratio resulted in a morphological transform from nanorods to needle-like structures in conjunction with a drastic change in the physical and optical properties of the ZnO modified cotton surfaces. Higher [S]/[G] ratios yielded formation of ZnO nanoneedles with high degree of crystallinity and higher aspect ratio compared to nanorods. Increasing [S]/[G] ratio resulted in the amount of ZnO grown on the cotton surface to drop significantly, which also caused a decrease in the surface hydrophobicity and UV absorption. In addition, room temperature photoluminescence measurements revealed that the band gap of ZnO widened and the structural defects were reduced as the morphology changed from nanorods to nanoneedles. A similar trend was observed in the UV-vis absorption of nanorods and nanoneedles, the onset of the latter exhibiting a blue-shift that correlates with the widening of band gap with nanoneedle formation.

Enhanced selective photoelectrochemical oxidation for small organic molecules derived from molecularly imprinted single crystalline ZnO nanorods electrodes

To solve the problem of poor selectivity on photoelectrochemical (PEC) oxidation toward small organic molecules especially has the similar chemical structures, a novel method in this work was proposed for achieving a high selective PEC oxidation by constructing molecularly imprinted sites on a single crystalline (SC) ZnO surface (imp-ZnO). Though a crystal-seed-assisted hydrothermal method, the phenol (C6H5-OH) imp-ZnO and p-hydroxybenzoic acid (PHBA, HOOC-C6H4-OH) imp-ZnO electrodes were prepared, which were in the form of vertically aligned nanorods with maintained single crystalline. The results showed the high selective PEC oxidation capabilities were obtained on SC imp-ZnO electrodes toward target between phenol and PHBA, with selective factor R of 5.64 and 2.38 respectively. This was attributed to their excellent imprinted expression with high value of imprinting factor S (1.86 and 1.55), deriving from an obvious difference of photocurrent enhancement (5.5–8 times) on SC imp-ZnO between target and nontarget. Such kind of single crystalline ZnO substrate is essential to constructed high-quality molecularly imprinted sites with clear size and hard shape for a great recognition effect.

Control growth of single crystalline ZnO nanorod arrays and nanoflowers with enhanced photocatalytic activity

Single crystalline vertical nanorod arrays and nanoflowers of ZnO have been grown in situ on cheap zinc foils under hydrothermal conditions, by means of hexamethylenetetramine and ethanolamine, respectively. Their morphologies and crystal structures are characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The nanorods and flowers of ZnO grew along the $$ \{ 10\bar{1}1\} $$ and $$ \{ 0001\} $$ planes, respectively. Both types of ZnO display high photocatalytic ability toward the degradation of methylene orange under UV irradiation. The ZnO nanorods show better performance than that of the ZnO nanoflowers, and the $$ \{ 10\bar{1}1\} $$ facets of the ZnO nanorods have higher photoactivity than that of the $$ \{ 000\bar{1}\} $$ or $$ \{ 10\bar{1}0\} $$ crystal planes. This is because the weaker coordinated O atoms on the surface are more likely to be saturated by H atoms in aqueous solution, thereby releasing more free OH radicals. A facile method was developed for selective control synthesis of ZnO nanoflowers and nanorod arrays on Zinc foil, with the assistance of ethanolamine and the hexamethylenetetramine, respectively. The illustration shows the time evolution of the two ZnO structures.

Synthesis, Characterization and Optical Properties of Single-Crystalline ZnO Nanorods by Thermal Evaporation

Synthesis, Characterization and Optical Properties of Single-Crystalline ZnO Nanorods by Thermal Evaporation

Schottky nanocontact on single crystalline ZnO nanorod using conductive atomic force microscopy

This article reports the formation of Schottky nanocontacts on single crystalline ZnO nanorods (NR) using atomic force microscopy (AFM) with a PtIr-coated Si cantilever in a contact mode. ZnO NRs were synthesized by thermal evaporation of metallic zinc thin film followed by annealing. The NRs are [11 $$ \overline{2} $$ 0] directed (i.e., along a-axis) which is quite unusual for wurtzite ZnO. The appearance of an intense visible emission band in room-temperature photoluminescence indicates the presence of a high density of intrinsic defects confirming n-type ZnO. The PtIr tip/ZnO Schottky nanocontacts with an ultra fine effective contact radius ~0.5 nm on horizontally dispersed NRs show an ideality factor of ~7, turn on voltage of ~1.0 V, Schottky barrier height of ~0.65 eV, breakdown voltage of ~−4.7 V, and ON to OFF current ratio of ~500 at ±2 V. The junction corresponds to a nanoscale Schottky contact with satisfactory properties which is comparable to the other PtIr/ZnO or Pt/ZnO reports at higher loading forces. Single crystallinity and contact on the side faces of the horizontally dispersed NRs are primarily thought to be the key factors for higher device performances.

Radial sandwich hybrid nanorods by analogously inserting Au nanoparticles in ZnO nanorods

We describe the fabrication of radial ZnO/Au/ZnO hybrid nanorods by analogously inserting a Au nanoparticle interlayer in single-crystalline ZnO nanorods through a feasible secondary growth process. The construction features of the ZnO/Au/ZnO hybrid nanorods can be tuned by the extent of the nanoparticle coverage and the growth processes. Investigations on the photoluminescence of the hybrid nanorods have preliminarily indicated that the metal nanoparticles lead to enhanced near-band-edge (NBE) emission and suppressed deep level emission (DLE). Such alternative processes and hybrid nanostructures are potential candidates for application in plasmonics and advanced optical devices.

Application of ZnO Single Crystal Rods as Anti-Reflective Material for PV Cells

In this study, ZnO nanorods for antireflection coatings application to the silicon solar cells were epitaxially grown over the Si substrate using both thermal evaporation method and wet chemical method and their characteristics were investigated. A precursor for the thermal evaporation method was prepared by impregnating zinc acetate over the activated carbon at three different loading amounts: 10, 20, and 30 wt%. The temperature for the epitaxial growth of ZnO nanorods over the substrate was fixed to 850°C. The diameter of the synthesized single crystalline ZnO nanorods was about 1 μm. The shape and size of the ZnO nanorods varied with the content of the precursors loaded over the activated carbon. The length of the ZnO nanorods increased as the loading amount of the zinc was increased. Single crystalline ZnO nanorods were grown over the substrates by a hydrothermal synthesis in an autoclave reactor. Zinc nitrate was used for the precursor material in the wet chemical method. The length and diameter of the ZnO nanorods synthesized by the wet chemical method were about 200 nm and a few nm, respectively. The surface morphology of the ZnO nanorods prepared by two growth methods was observed by scanning electron microscope. Its crystal structure was analyzed by an X-ray diffraction spectrometer. The optical properties, such as reflectance, were measured by the UV-Visible spectrophotometer. It was confirmed that ZnO nanorods decrease the reflectance of UV and Visible lights. Therefore, it was concluded that ZnO nanorods can be used as the antireflective material for photo voltaic cells.

Surfactant Inducing Phase Change of ZnO Nanorods to Low Friction

Single crystalline ZnO nanorods have been synthesized via a low-temperature hydrothermal route on n-(111) Si substrates. After being modified with stearic acid, the friction coefficient of ZnO nanorods against the Si3N4 ball in dry sliding decreases from 0.33 to 0.15. Changing chain length or terminal linking group by replacing stearic acid with lauric acid, octadecyltrimethoxysilane, respectively, has no influence on the friction coefficient. Taking into account the field emission scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive spectrometer, and X-ray photoelectron spectroscopy data, the results clearly indicate that the low friction is ascribed to self-lubricious amorphous tribofilm formation and this tribofilm is composed of ZnO and the low-surface-energy surfactant. The low-surface-energy surfactants play a role in the decreasing of the phase change pressure of ZnO nanorods, assisting tribofilm formation.

Intrinsic defects, impurities and doping in ZnO nanorods grown at low temperature

We report on the defect properties of single-crystalline ZnO nanorods grown from solutions at temperatures below 90 °C. The nanorods can easily be doped by providing impurity precursors during growth. In the as-grown state the nanorods exhibit considerable lattice strain and distortions which compromise their electrical and optical properties. Upon annealing at moderate temperatures of <400 °C the lattice strain is converted into dislocation-type defects, and the dopant impurities become optically active. In the annealed state the near-bandgap photoluminescence quantum efficiency is improved more than 5 times and reaches ∼16 % at room temperature. Thus with moderate annealing, interesting device applications become feasible for nanorods grown at T<90 ∘C.

Structural Analysis of Vertically-Aligned Single Crystalline ZnO Nanorods Grown on Different Seed Layers with Chemical Solution Deposition

We report the structural properties of the vertically-oriented ZnO nanorods fabricated on various ZnO seed layers with chemical solution deposition (CSD) technique. The ZnO nanorods were prepared using an aqueous solution with Zinc nitrate (Zn(NO3)2 x 6H2O, Aldrich) and hexamethylenetetramine (HMT, Aldrich) in a convection oven. A-plane sapphire substrates with a deposited ZnO thin film were placed upside down in a quartz holder to avoid any micro-crystalline contamination. Especially, our hydro-thermal syntheses are automatically processed on precision pump drive systems (Masterflex) to accurately control the pH of the aqueous solution. The [002] crystal orientation of the ZnO seed layer was observed by the X-ray diffraction pattern. Structural features of ZnO nanorods were systematically analyzed by scanning electron microscopy and tunneling electron microscopy, together with selective area electron diffraction patterns. Experimental observations clearly demonstrated the dependence of the growth direction of the ZnO nanorods on the crystal structures of the ZnO seed layers.