Single-crystalline Nanorods Research Articles

Synthesis of uniformly ordered single-crystalline TiO2 nanorods on flexible Ti substrate

With high conductivity and ionic mobility, one-dimensional (1D) titanium dioxide (TiO2) nanoarrays have attracted great interest in the photoanode of solar cells and the cathode of lithium-ion/sodium-ion batteries. However, the synthesis of TiO2 nanoarrays often requires highly toxic or corrosive solution circumstances. Meanwhile, the case that most TiO2 nanoarrays are grown on rigid conductive substrate can’t satisfy the demand for application in the space of complex shape. In this work, we developed a simple method to prepare highly ordered single crystalline rutile TiO2 nanorod arrays with high density on flexible metal Ti substrate in relatively weak acid solution, and the orientation of the nanorods were more uniform than those by other means ever before. Based on detailed analysis, an ion-assisted and concentration-limited epitaxial growth mechanism by H+ and is proposed for this uniformly ordered rutile TiO2 nanorod array. This work can promote the research and application of flexible solar cells and lithium-ion batteries, especially their integration.

Solution-processed Sb2Se3 nanorod array and its photovoltaic performance

A heterogeneous nucleation strategy is developed to prepare solution-processed Sb2Se3 single-crystalline nanorod array. With tiny Sb2S3 crystals as heterogeneous nuclei, [001]-oriented Sb2Se3 nanorod array is vertically grown on polycrystalline TiO2 nanoparticle film, offering a new materials system of Sb2Se3/TiO2-based nanoarray heterojunction. With a polymeric hole transporting layer, the Sb2Se3 nanorod array solar cell is successfully obtained on transparent substrate.

Insights in the two-step synthesis of single crystalline Ag2Te nanorods

Pure monoclinic single crystalline Ag2Te nanorods were synthesized by a two-step method. The first step concerns the electrodeposition of self-standing Te nanorods in an ionic liquid medium, taking advantage of its templating properties. These obtained nanorods are single crystalline with growth direction along the [001] axis in the hexagonal lattice. The average diameter of the as-deposited Te nanorods is 60 ± 13 nm with less than 300 nm long. Afterwards, the resulting nanostructured Te film is immerged in a dedicated silver-based bath in order to be transformed into Ag2Te binary compounds by a combined mechanism of chemical cementation and topotactic transformation. The chemical analysis reveals that the final nanostructures exhibit a slight excess of silver and TEM-SAED analysis shows that they are still single crystalline with an increase of the initial diameter, less than 30%. HR-TEM highlights the presence of an intermediate stützite phase and the hessite phase α-Ag2Te is obtained at the end of the synthesis process.

Vectorial probing of electric and magnetic transitions in variable optical environments and vice-versa

We use europium doped single crystalline NaYF4 nanorods for probing the electric and magnetic contributions to the local density of optical states (LDOS). Reciprocically, we determine intrinsic properties of the emitters (oscillator strength, quantum yield) by comparing their measured and simulated optical responses in front of a mirror. We first experimentally determine the specifications of the nanoprobe (orientation and oscillator strength of the electric and magnetic dipoles moments) and show significant orientation sensitivity of the branching ratios associated with electric and magnetic transitions. In a second part, we measure the modification of the LDOS in front of a gold mirror in a Drexhage’s experiment. We discuss the role of the electric and magnetic LDOS on the basis of numerical simulations, taking into account the orientation of the dipolar emitters. We demonstrate that they behave like degenerated dipoles sensitive to polarized partial LDOS.

A Friendly Environmental Route for the Fabrication of Spinel Co3O4 Nanorods, Using Inorganic Precursor Salt and Aqueous Extracts of Moringa oleifera Leaves

Nowadays many researchers are focused on the sytnhesis of nanoscale tricobalt tetraoxide (Co3O4) particles, owing to their unique properties. These particles have indeed many potential technological applications. The present investigation deals with the fabrication of this spinel oxide: it was successfully synthesized through a friendly environmental method, using cobalt (II) chloride as cobalt precursor and aqueous extract of Moringa oleifera leaves. The latter contains alkaloids as a base source while flavonoids present in the leaves acted as capping agent to prevent the particles agglomeration. Alkaloids present in the leaves were hydrolyzed in water and consequently, hydroxilated Co2+ leads to the formation of Co3O4 powder via calcination. The electronic transmission microscopy has revealed the single crystalline nanorods morphology of the synthetised materials. These nanorods are about several hundred nanometers long and several tens of nanometers in diameter. The bulk Co3O4 is known to be antiferromagnetic, the vibrating sample magnetometer data (at room temperature) of the prepared powder exhibited the lower coercivity and remanence, signaling that the produced spinel was pure and was constituted of superparamagnetic particles made of Co3O4. The UV–visible spectrum exhibited a photoluminescence peak in the visible light range positioned at about 538nm, suggesting this spinel as a visible light emitting material and as photocatalyst under visible light.

Electrochemical Deposition of a Single-Crystalline Nanorod Polycyclic Aromatic Hydrocarbon Film with Efficient Charge and Exciton Transport.

Electrochemical deposition has emerged as an efficient technique for preparing conjugated polymer films on electrodes. However, this method encounters difficulties in synthesizing crystalline products and controlling their orientation on electrodes. Here we report electrochemical film deposition of a large polycyclic aromatic hydrocarbon. The film is composed of single‐crystalline nanorods, in which the molecules adopt a cofacial stacking arrangement along the π–π direction. Film thickness and crystal size can be controlled by electrochemical conditions such as scan rate and electrolyte species, while the choice of anode material determines crystal orientation. The film supports exceptionally efficient migration of both free carriers and excitons: the free carrier mobility reaches over 30 cm2 V−1 s−1, whereas the excitons are delocalized with a low binding energy of 118.5 meV and a remarkable exciton diffusion length of 45 nm.

Fast growth of pure V2O5 nanoparticles by low-cost hydrothermal process

Vanadium pentoxide (V2O5) nanorods were successfully grown through an easy, quick and clean hydrothermal method using vanadium oxide sols (V2O5.1,6H2O) as precursor. Structurals studies based on the X-ray diffraction and Raman spectroscopy revealed that the synthesized nanomaterials are pure divanadium pentoxide with an orthorhombic phase. The morphological properties and the particle size of the developed V2O5 nanoparticles were investigated by scanning electron microscopy (SEM) and high resolution transmission electron microscope (HRTEM). The results show that the as-grown samples consist of a large amount of one-dimensional V2O5 nanorods with the widths approximately 52 nm and the lengths are up to several hundred nanometers. The effects of different growth conditions, such as hydrothermal duration and stirring temperature of the precursor on the formation of the nanorods has been recorded. Moreover, the structural and morphological properties of the resulting nanopowders have been experimentally studied. Time-dependent experiments showed that V2O5.1,6H2O were dehydrated progressively and transformed into orthorhombic V2O5 single-crystalline nanorods. It has been established that high stirring temperature of the precursor is benefical for the formation of V2O5 nanorods and minimizes the duration time of the heat treatment. Hence the adjusted parameters (time and temperature) appears to be the key element to achieving the desired nanomaterial with defined forms.

Synthesis of Asymmetric One-Dimensional Pd on Au Bimetallic Nanostructures.

Nanostructures composed of a gold nanorod (AuNR) core and a Pd/Pt shell are of great interest due to their potential application as plasmon resonance-enhanced catalysts. However, the synthesis of well-defined one-dimensional bimetallic nanostructures with precise control over shell thickness and length remains a challenge. In this study, we report a detailed and systematic study on the chemical synthesis of a uniform Pd shell on single crystalline and pentahedrally twinned (PHT) AuNRs of various lengths. AuNRs were used as a template, and the slow and controlled reduction of Pd(II) ions on preformed AuNRs was carried out for the formation of rectangular-shaped Au@Pd bimetallic nanorods. The Pd shell thickness around the AuNRs was controlled by the supply of Pd(II) ions in the growth solution. We were able to grow a ∼20 nm uniform Pd shell around the AuNR, keeping the rod-like morphology intact without local nucleation to form irregular shapes and randomly overgrown nanostructures. The formation of bimetallic nanorods was also extended beyond typical single crystalline nanorods to PHT high aspect ratio gold nanorods and nanowires, using them as templates. To our surprise, unusually curved asymmetric nanorods were formed when the Pd deposition was carried out on AuNRs longer than ∼800 nm which could be possibly due to a Pd and Au lattice mismatch at the interface and higher flexibility of the nanorods when they exceeded certain lengths.

Nanoarray heterojunction and its efficient solar cells without negative impact of photogenerated electric field

Efficient, stable and low-cost solar cells are being desired for the photovoltaic conversion of solar energy into electricity for sustainable energy production. Nanorod/nanowire arrays of narrow-bandgap semiconductors are the promising light-harvesters for photovoltaics because of their excellent optoelectrical properties. Here, the array of preferentially oriented antimony trisulfide (Sb2S3) single-crystalline nanorods is grown on polycrystalline titania (TiO2) film by a tiny-seed-assisted solution-processing strategy, offering an Sb2S3/TiO2 nanoarray heterojunction system on a large scale. It is demonstrated that the Sb2S3 nanorod growth follows a tiny-seed-governed orientation-competing-epitaxial nucleation/growth mechanism. Using a conjugated polymer hole transporting layer on the heterojunction, we achieve a power conversion efficiency of 5.70% in the stable hybrid solar cell with a preferred p-type/intrinsic/n-type architecture featuring effectively straightforward charge transport channels and no negative impact of photogenerated electric field on device performance. An architecture-dependent charge distribution model is proposed to understand the unique photovoltaic behavior.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Cubes to Cubes: Organization of MgO Particles into One-Dimensional and Two-Dimensional Nanostructures.

Developing simple, inexpensive, and environmentally benign approaches to integrate morphologically well-defined nanoscale building blocks into larger high surface area materials is a key challenge in materials design and processing. In this work, we investigate the fundamental surface phenomena between MgO and water (both adsorption and desorption) with particles prepared via a vapor-phase process (MgO nanocubes) and a modified aerogel process (MgO(111) nanosheets). Through these studies, we unravel a strategy to assemble individual MgO nanoparticles into extended faceted single-crystalline MgO nanosheets and nanorods with well-defined exposed surfaces and edges. This reorganization can be triggered by the presence of H2O vapor or bulk liquid water. Water adsorption and the progressive conversion of vapor-phase grown oxide particles into hydroxides give rise to either one-dimensional or two-dimensional (1D or 2D) structures of high dispersion and surface area. The resulting Mg(OH)2 lamella with a predominant (001) surface termination are well-suited precursor structures for their topotactic conversion into laterally extended and uniform MgO(111) grain surface configurations. To understand the potential of polar (111) surfaces for faceting and surface reconstruction effects associated with water desorption, we investigated the stability of MgO(111) nanosheets during vacuum annealing and electron beam exposure. The significant surface reconstruction of the MgO(111) surfaces observed shows that adsorbate-free (111)-terminated surfaces of unsupported MgO nanostructures reconstruct rather than remain as charged planes of either three-fold coordinated O2– ion or Mg2+ ions. Thus, here we demonstrate the role water can play in surface formation and reconstruction by bridging wet chemical and surface science inspired approaches.

Single-crystalline 2D BiOCl nanorods decorated with 2D MoS2 nanosheets for visible light-driven photocatalytic detoxification of organic and inorganic pollutants

Visible light-driven photocatalytic degradation is one of the promising ways to deal with the major problem of water pollution. This study involves one-step, fast, in-situ preparation of BiOCl/MoS2 (2D/2D) heterostructure via microwave irradiations. The photocatalytic performance of BiOCl/MoS2 composite with different weight ratios (1:1, 3:1, and 1:3) have been evaluated for degradation of organic (methylene blue (MB) dye, fipronil pesticide) and inorganic pollutants (Cr(VI)) along with real industrial wastewater under visible light irradiation. XRD, XPS, FESEM, HRTEM, and SAED analysis indicate the formation of single-crystalline nanorods with a high aspect ratio of 1:10. The composites possess high surface area (~40 m2/g) with lower charge recombination and high visible light absorption tendency due to lower band gap (2.30 eV) energy. The high synergistic interaction (~2.29) between BiOCl and MoS2 results in enhancement of degradation activity which shows 94%, 89%, and 90% removal of MB, fipronil, and Cr(VI) metal, respectively by BiOCl/MoS2 (1:3) catalyst. The optimum conditions to get maximum degradation efficiency were determined by varying different reaction parameters like pH, catalyst dose, and illumination area. Radical trapping experiments indicate that holes and hydroxyl radicals had a dominant role in the degradation process. The catalyst is highly stable and reusable as confirmed by the reusability studies. The photocatalytic treatment of industrial wastewater without any physico-chemical pretreatment showed 75% COD and 63% TOC removal under visible light, which indicates high practical efficiency of the catalyst. The current study validates that the BiOCl/MoS2 composites with superior characteristic properties can be efficiently used for wastewater treatment under natural light.

Impact of doping on the morphology, structure and optical properties of c-oriented GaN nanorods

We report here the influence of Mg and Si-doping during growth, on the morphology, structure, and optical properties of single-crystalline GaN nanorods (NRs) grown on Si substrates using plasma-assisted molecular beam epitaxy. Mg-doping is shown to enhance the lateral growth of the NRs, leading to a higher degree of coalescence. Si-doping during the nucleation stage of the growth enhances the mutual misorientation of the NRs. Strain profile measurements along the length of individual NR by transmission electron microscopy shows that the top regions are relaxed. Evaluation of carrier concentration by Raman spectroscopy reveals that Si-doping leads to an increase of carrier concentration from 1016 to 1017 cm−3, and the optimal Mg incorporation for the realisation of p-doping is confirmed by photoluminescence spectroscopy. These results will significantly help in understanding and tuning the structural and optical properties of GaN NRs through doping in the fabrication of NR based optoelectronic devices.

Unlocking synergy in bimetallic catalysts by core-shell design.

Extending the toolbox from mono- to bimetallic catalysts is key in realizing efficient chemical processes1. Traditionally, the performance of bimetallic catalysts featuring one active and one selective metal is optimized by varying the metal composition1-3, often resulting in a compromise between the catalytic properties of the two metals4-6. Here we show that by designing the atomic distribution of bimetallic Au-Pd nanocatalysts, we obtain a synergistic catalytic performance in the industrially relevant selective hydrogenation of butadiene. Our single-crystalline Au-core Pd-shell nanorods were up to 50 times more active than their alloyed and monometallic counterparts, while retaining high selectivity. We find a shell-thickness-dependent catalytic activity, indicating that not only the nature of the surface but also several subsurface layers play a crucial role in the catalytic performance, and rationalize this finding using density functional theory calculations. Our results open up an alternative avenue for the structural design of bimetallic catalysts.

Single crystalline rutile TiO2 nanorods synthesis by onestep catalyst-free vapor transport method

In this report, a novel one-step approach to grow highly crystalline rutile TiO2 nanorods on bare silicon substrate by a simple vapor transport technique without a catalyst is discussed. The crystal structure of the nanorod ensemble was examined by X-ray diffraction. The structure and morphology of the nanorods were analyzed by scanning electron microscopy and transmission electron microscopy and the one dimensional growth direction was observed to be along (110) plane. Chemical composition of the individual nanorods was examined by X-ray energy-dispersive spectrometry. A possible growth mechanism for the nanorods has been proposed. As the synthesis was catalyst-free, the resulting nanorods were of high purity. These nanorods are expected to have a variety of applications.

Morphology-controlled synthesis of CeO2 nanocrystals and their facet-dependent gas sensing properties

Abstract Recently, the influence of polar facets on the gas sensing performance of metal oxides has drawn much attention, yet the facet-dependent gas sensing properties of rare earth oxides have not been reported so far. In this work, taking CeO2 as an example, uniform single-crystalline nanopolyhedra, nanorods and nanocubes of cubic CeO2 were selectively prepared by a hydrothermal method at different temperatures and NaOH concentrations, using Ce(NO3)3 as the cerium source. According to X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analysis, these CeO2 nanocrystals possess different exposed crystal planes: {111} and {100} for nanopolyhedra, {110} and {100} for nanorods, and {100} for nanocubes. Abundant defects were confirmed on the surface of three CeO2 nanocrystals by X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR) and photoluminescence (PL). Notably, gas sensors based on these three CeO2 nanocrystals exhibited distinct response values towards dimethylamine vapor in the following order: nanocubes > nanorods > nanopolyhedra, while excluding the effects of characteristic dimension and specific surface area. The response order of CeO2 nanocrystals is primarily ascribed to the different content of surface defects and {100} polar facets instead of {111} and {110} nonpolar facets.

Pressure Induced Assembly and Coalescence of Lead Chalcogenide Nanocrystals.

We report here pressure induced nanocrystal coalescence of ordered lead chalcogenide nanocrystal arrays into one-dimensional (1D) and 2D nanostructures. In particular, atomic crystal phase transitions and mesoscale coalescence of PbS and PbSe nanocrystals have been observed and monitored in situ respectively by wide- and small-angle synchrotron X-ray scattering techniques. At the atomic scale, both nanocrystals underwent reversible structural transformations from cubic to orthorhombic at significantly higher pressures than those for the corresponding bulk materials. At the mesoscale, PbS nanocrystal arrays displayed a superlattice transformation from face-centered cubic to lamellar structures, while no clear mesoscale lattice transformation was observed for PbSe nanocrystal arrays. Intriguingly, transmission electron microscopy showed that the applied pressure forced both spherical nanocrystals to coalesce into single crystalline 2D nanosheets and 1D nanorods. Our results confirm that pressure can be used as a straightforward approach to manipulate the interparticle spacing and engineer nanostructures with specific morphologies and, therefore, provide insights into the design and functioning of new semiconductor nanocrystal structures under high-pressure conditions.

Water mediated growth of oriented single crystalline SrCO3 nanorod arrays on strontium compounds

Morphology-controlled strontianite nanostructures have attracted interest in various fields, such as electrocatalyst and photocatalysts. Basic additives in aqueous strontium solutions is commonly used in controlling strontianite nanostructures. Here, we show that trace water also serves an important role in forming and structuring vertically oriented strontianite nanorod arrays on strontium compounds. Using in situ Raman spectroscopy, we monitored the structural evolution from hydrated strontium to strontianite nanorods, demonstrating the epitaxial growth by vapor–liquid–solid mechanism. Water molecules cause not only the exsolution of Sr liquid droplets on the surface but also the uptake of airborne CO2 followed by its ionization to CO32−. The existence of intermediate SrHO+–OCO22− phase indicates the interaction of CO32− with SrOH+ in Sr(OH)x(H2O)y cluster to orient strontianite crystals. X-ray diffraction simulation and transmission electron microscopy identify the preferred-orientation plane of the 1D nanostructures as the (002) plane, i.e., the growth along the c-axis. The anisotropic growth habit is found to be affected by the kinetics of carbonation. This study paves the way for designing and developing 1D architecture of alkaline earth metal carbonates by a simple method without external additives at room temperature.

Hydrothermal temperature-controlled size and distribution of CeO2 nanoparticles over TiO2 nanorods: Heterojunction characteristics and photoelectrochemical performance

The search for a suitable photoelectrode material remains the most challenging problem in photoelectrochemical applications. Metal oxides are favorable for their chemical stability under an aqueous environment. This work presents size and distribution controlled CeO2 nanoparticles on TiO2 nanorod arrays achieved through a systematic variation of the hydrothermal process temperature. In a two-step hydrothermal process, single crystalline TiO2 nanorods are first grown on fluorine doped tin oxide (FTO) coated glass substrate using titanium (IV) butoxide precursor followed by a treatment with cerium nitrate to obtain CeO2 nanoparticles over TiO2. Variation of the hydrothermal process temperature in the second step from 80 °C to 150 °C results in CeO2 nanoparticles with a systematic variation of size and distribution over TiO2 nanorods. We demonstrate that an effective heterojunction between the CeO2 nanoparticles and TiO2 nanorod forms at a process temperature of 120 °C, which is manifested by improved photoelectrochemical performance. The CeO2–TiO2 heterojunction photoanode shows a photocurrent density of 3.77 mA/cm2 (at 1.23 V vs. RHE) in 1 M KOH solution under one Sun (100 mW/cm2) illumination, which is approximately three times higher than that of bare TiO2 nanorod arrays. Further, Applied Bias Photon-to-current Efficiency (ABPE) is estimated to be 2.01%. Diffuse Reflectance Spectra (DRS) shows a redshift of ~0.1 eV in CeO2–TiO2 heterojunction that signifies the contribution of CeO2 towards visible light absorption. The Electrochemical Impedance Spectroscopy (EIS) shows a lower value for charge transfer resistance in samples processed at 120 °C. The superior photoelectrochemical performance of CeO2–TiO2 heterojunction is attributed to the collective contributions of visible light absorption and efficient charge transfer at the CeO2–TiO2 interface.

Single-crystalline rutile TiO2 nanorod array on flexible Ti substrates for efficient photoelectrocatalytic degradation of phenol in water

Single-crystalline nanorods are intrinsic highways for charge transfer and hence ideal nanofeatures for efficient photoelectrochemical (PEC) degradations of pollutants in wastewater. Photocatalytic thin films on flexible substrates avoid the nuisance catalysts-recovering procedure and also allow for the flexible design in photocatalytic apparatus. We therefore directly deposited single-crystalline rutile TiO2 nanorod arrays on flexible and conductive Ti substrates through a hydrothermal technique and optimized the synthetic parameters to achieve an extremely high PEC activity towards phenol removal in water under UV light illumination. In the presence of tetrabutyltitanate (TBT) and HCl reactants under a hydrothermal environment, a balance in the erosion of the Ti substrate and the precipitation of TiO2 nanorods is found to be the key to tune the nanofeatures of the TiO2 thin films and in turn the PEC efficiency. The decreasing TBT concentration achieved a more sparsely distributed nanorod array, which significantly contributes to the PEC activity; yet too little TBT resulted in total dissolution of Ti plates. The PEC activity increased with increasing hydrothermal duration because of more loaded TiO2. The maximum reaction rate constant is 2.74 h−1, which is 39 times that of a commercial P25 TiO2 nanoparticulate film. The photocurrent stabilized at 0.9 mA/cm2 under the illumination of AM1.5G simulated solar light, suggesting a high separation efficiency of photogenerated holes and electrons along the single-crystalline rutile TiO2 nanorod arrays.