ZnO-capped nanorod gas sensors

This paper reports on the synthesis of pristine α-Fe₂O₃ nanorods and Fe₂O₃–ZnO core–shell nanorods using a combination of thermal oxidation and atomic layer deposition (ALD) techniques; the completed nanorods were then used for ethanol sensing studies. The crystal structure and morphology of the synthesized nanostructures were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The sensing properties of the pristine and core–shell nanorods for gas-phase ethanol were examined using different concentrations of ethanol (5–200 ppm) at different temperatures (150–250 °C). The XRD and SEM revealed the excellent crystallinity of the Fe₂O₃–ZnO core–shell nanorods, as well as their uniformity in terms of shape and size. The Fe₂O₃–ZnO core–shell nanorod sensor showed a stronger response to ethanol than the pristine Fe₂O₃ nanorod sensor. The response (i.e., the relative change in electrical resistance R_a/R_g) of the core–shell nanorod sensor was 22.75 for 100 ppm ethanol at 200 °C whereas that of the pristine nanorod sensor was only 3.85 under the same conditions. Furthermore, under these conditions, the response time of the Fe₂O₃–ZnO core–shell nanorods was 15.96 s, which was shorter than that of the pristine nanorod sensor (22.73 s). The core–shell nanorod sensor showed excellent selectivity to ethanol over other VOC gases. The improved sensing response characteristics of the Fe₂O₃–ZnO core–shell nanorod sensor were attributed to modulation of the conduction channel width and the potential barrier height at the Fe₂O₃–ZnO interface accompanying the adsorption and desorption of ethanol gas as well as to preferential adsorption and diffusion of oxygen and ethanol molecules at the Fe₂O₃–ZnO interface.     

Fabrication of TiO2 nanotubes by using electrodeposited ZnO nanorod template and their application to hybrid solar cells

Vertically aligned TiO₂ nanotubes have been fabricated on the indium-doped tin oxide (ITO) by a simple and versatile technique using the electrochemically deposited ZnO nanorods, oriented along the c-axis, as a template in the spin-on based sol-gel reaction of a Ti precursor. The diameter, length, and shape of TiO₂ nanotubes were controlled by changing the initial ZnO nanorod template and the spin conditions during sol-gel process of a Ti precursor. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis, and X-ray diffraction (XRD) were used to confirm the successful formation of TiO₂ nanotubes and characterize their structure and morphology. Furthermore, as an application of the TiO₂ nanotubes, hybrid solar cells based on TiO₂ and poly[2-methoxy,5-(2′-ethyl-hexyloxy)1,4-phenylenevinylene] (MEH-PPV) were successfully fabricated.     

Controlled synthesis of carbon nanostructures using aligned ZnO nanorods as templates

By combining in situ X-ray photoemission spectroscopy, ex situ high resolution transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy, we show that chemical vapor deposition (CVD) on vertically aligned ZnO nanorods can synthesize different carbon nanostructures (CNs), whose morphology is driven by the ZnO nanorods and whose dimensions and structures change as a function of the process temperature. The CNs range from amorphous carbon cups, completely covering the nanorods, to high density one-dimensional carbon nano-dendrites (CNDs), which start to appear like short hairs on the ZnO nanorods. The nanorods are partially etched when the process is done at 630–800 °C, while they are completely etched at temperatures higher than 800 °C. In the latter case, CNDs emerge from a porous carbon sponge formed at the substrate interface but they are preferentially aligned along the location of the pristine ZnO nanorods. When used as a chemiresisitor the CND–ZnO structures have a higher sensitivity to ammonia compared to chemiresistors made by bare ZnO nanorods, to other one-dimensional CNs, like carbon nanotubes or other metal/metal-oxides hybrid CNs.     

Green material: ecological importance of imperative and sensitive chemi-sensor based on Ag/Ag2O3/ZnO composite nanorods

In this report, we illustrate a simple, easy, and low-temperature growth of Ag/Ag₂O₃/ZnO composite nanorods with high purity and crystallinity. The composite nanorods were structurally characterized by field emission scanning electron microscopy, X-ray powder diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy which confirmed that synthesized product have rod-like morphology having an average cross section of approximately 300 nm. Nanorods are made of silver, silver oxide, and zinc oxide and are optically active having absorption band at 375 nm. The composite nanorods exhibited high sensitivity (1.5823 μA.cm⁻².mM⁻¹) and lower limit of detection (0.5 μM) when applied for the recognition of phenyl hydrazine utilizing I-V technique. Thus, Ag/Ag₂O₃/ZnO composite nanorods can be utilized as a redox mediator for the development of highly proficient phenyl hydrazine sensor.     

Utilization of silane functionalized carbon nanotubes‐silica hybrids as novel reinforcing fillers for solution styrene butadiene rubber

Carbon nanotubes‐silica (CNTs‐SiO₂) nanohybrid filler was fabricated by coating inorganic silica on multi‐wall CNTs through a sol–gel process. The CNTs‐SiO₂ nanohybrids were then functionalized by 3‐methacryloxypropyltrimethoxysilane (3‐MPTS) followed by compounding to solution styrene butadiene rubber (S‐SBR) through mechanical mixing. The Fourier‐transform infrared spectroscopy showed that the CNTs were coated by inorganic SiO₂, and grafted with 3‐MPTS successfully. The functionalized CNTs‐SiO₂ nanohybrids had a rough surface as revealed by transmission electron microscope images. After hybridization and grafting, the functionalized CNTs‐SiO₂ nanohybrids still maintained the crystal structure of CNTs, which was determined by X‐ray diffraction and Raman spectrum. The addition of nanohybrids accelerated the vulcanization process and improved the crosslinking degree of vulcanizates. With adding 10 phr (parts per hundred of rubber) functionalized CNTs‐SiO₂, the mechanical properties of S‐SBR vulcanizates were improved significantly. The tensile moduli at 100% elongation (M₁₀₀) and tensile strength had 54% and 28% increase, respectively. The incorporation of functionalized CNTs‐SiO₂ nanohybrids also largely enhanced the storage modulus, and slightly increased the thermal conductivity of vulcanizates. POLYM. COMPOS., 00:000–000, 2013. © 2013 Society of Plastics Engineers POLYM. COMPOS., 2013. © 2013 Society of Plastics Engineers     

Carbothermal Synthesis of the Nanostructures of Al2O3 and ZnO

Nanostructures of Al₂O₃ and ZnO have been synthesized by a carbothermal route involving the reaction of the metal or the metal oxide with carbon. In the case of Al₂O₃, nanowires and nanotubes are obtained starting with Al metal and active carbon or graphite. ZnO nanowires are obtained by the reaction of zinc oxalate or ZnO with active carbon or multiwalled carbon nanotubes. The Al₂O₃ and ZnO nanostructures obtained have been characterized by X-ray diffraction, electron microscopy and photoluminescence spectroscopy. These nanostructures are likely to be of use as catalyst supports and in other technological applications.     

Structural and photoluminescence studies on catalytic growth of silicon/zinc oxide heterostructure nanowires

Silicon/zinc oxide (Si/ZnO) core-shell nanowires (NWs) were prepared on a p-type Si(111) substrate using a two-step growth process. First, indium seed-coated Si NWs (In/Si NWs) were synthesized using a plasma-assisted hot-wire chemical vapor deposition technique. This was then followed by the growth of a ZnO nanostructure shell layer using a vapor transport and condensation method. By varying the ZnO growth time from 0.5 to 2 h, different morphologies of ZnO nanostructures, such as ZnO nanoparticles, ZnO shell layer, and ZnO nanorods were grown on the In/Si NWs. The In seeds were believed to act as centers to attract the ZnO molecule vapors, further inducing the lateral growth of ZnO nanorods from the Si/ZnO core-shell NWs via a vapor-liquid-solid mechanism. The ZnO nanorods had a tendency to grow in the direction of [0001] as indicated by X-ray diffraction and high resolution transmission electron microscopy analyses. We showed that the Si/ZnO core-shell NWs exhibit a broad visible emission ranging from 400 to 750 nm due to the combination of emissions from oxygen vacancies in ZnO and In₂O₃ structures and nanocrystallite Si on the Si NWs. The hierarchical growth of straight ZnO nanorods on the core-shell NWs eventually reduced the defect (green) emission and enhanced the near band edge (ultraviolet) emission of the ZnO.     

One-dimensional ZnO nanostructure growth prepared by thermal evaporation on different substrates: Ultraviolet emission as a function of size and dimensionality

Large-scale uniform one-dimensional ZnO nanostructures were fabricated through thermal evaporation via the vapor solid mechanism on different substrates. The effects of Si (100), Si (111), SiO₂ and sapphire substrates with constant oxygen treatment on the morphology and diameter of ZnO nanostructures were investigated. It is found that the type of substrate has a great effect on the shape and diameter of the synthesized nanowires, nanorods, and nanotubes. It is noticed that the size and dimensionality were the most influential parameters on both structural and optical properties of the grown ZnO nanostructures. X-ray diffraction analysis confirms the stability of the wurtzite crystal structure for all grown ZnO nanostructures and the preferred orientation is substrate dependent. The crystallinity as well as the defects within the crystal lattice of the grown ZnO nanostructures was studied through Raman spectroscopy. The photoluminescence spectra of ZnO nanostructures grown on Si (100), Si (111), SiO₂ and sapphire substrates showed two peaks at a near-band-edge (NBE) emission in the ultraviolet region and a broad deep-level emission (DLE) around the green emission.     

Coating multiwalled carbon nanotubes with gold nanoparticles derived from gold salt precursors

Derived gold nanoparticles supported on carbon nanotubes were generated via local reduction of AuCl₄⁻ ions, which are ion-paired with 3-aminopropyltriethoxysilane that are chemically attached to carbon nanotubes. The controllable loading of gold nanoparticles on carbon nanotubes can be tuned by regenerative ion exchange and subsequent reduction. This method is not limited to gold, however, it may be used to assemble a variety of other metal nanoparticles on surfaces of the modified carbon nanotubes. Transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction are all used to characterize the as-prepared gold nanoparticle coated carbon nanotube hybrids. The hybrids show characteristic plasmon absorption of gold nanoparticles in the UV-visible spectrum and very good electrochemical properties. The results imply that these nanohybrids may have a good application potential in catalysis, sensor, and fuel cells.     

Tailoring UV emission from a regular array of ZnO nanotubes by the geometrical parameters of the array and Al2O3 coating

Self-aligned and equal-spaced zinc oxide (ZnO) nanotube arrays were fabricated with anodic aluminum oxide (AAO)-assisted growth and the ALD technique. The near band-edge (NBE) emission was strongly affected by the nanotube's geometrical parameters, such as a packing density and thickness of the nanotube walls. The NBE emission was further enhanced with Al₂O₃ coating. The effect was analyzed by X-ray photoelectron spectroscopy (XPS) and ascribed to the surface defect passivation and a ZnAl₂O₄ spinel formation. The NBE emission enhancement was greater in ZnO nanotubes with thicker walls. A smaller UV enhancement factor was explained by less uniform and integral Al₂O₃ coverage of the ZnO nanotubes with thinner walls; this, possibly induced a variation of the Al₂O₃ refractive index along the nanotubes. As a result, the optical conditions at the ZnO/Al₂O₃/air interfaces was changed and the light extraction efficiency was reduced in the latter samples.     

Morphological exploration of chemical vapor–deposited P-doped ZnO nanorods for efficient photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is beneficial and has received attractive attention due to a greater potential to generate hydrogen and oxygen from water by using plentiful solar light to solve the problem of energy crisis. Various active semiconductor materials are used in PEC water splitting applications. Nevertheless, in past decades, most of the researchers suggested that titanium oxide (TiO₂) is the best photoanode for this type of applications. Now, Zinc oxide (ZnO) is considered a perfect substitution to TiO₂ due to its comparable energy band structure and superior photogenerated electron transfer rate. In this study, bare and phosphorous-doped ZnO nanorods were successfully developed on fluorine-doped tin oxide-coated glass (FTO) substrate by chemical vapor deposition. X-ray diffraction (XRD) pattern authenticated hexagonal structure formation with strong diffraction peak of (101), which showed that ZnO nanorods were perfectly developed along c axis. The optical and morphological properties were analyzed by UV–Vis and scanning electron microscopy images. The energy-dispersive X-ray spectra demonstrated that doping agent phosphorous was present in ZnO nanorods. The PEC properties of the developed ZnO nanorods were further investigated and obtained results suggested that a small amount of phosphorous-doped ZnO nanorods enhances their PEC performance.     

Optimized properties of ZnO nanorod arrays grown on graphene oxide seed layer by combined chemical and electrochemical approach

Novel hybrid architectures based on ZnO nanostructures supported on reduced graphene oxide films are reported by a facile two-step aqueous electrochemical approach. Graphene oxide (GO) obtained by oxidizing graphite with either H₂SO₄:H₃PO₄ mix or H₂SO₄ was subjected to electrochemical reduction (ErGO) by constant potential and sweeping potential methods and further employed as seeding layer for electrodeposition of ZnO nanorods. The results showed the GO reduction rate was markedly influenced by oxygen content while Infrared spectroscopy indicated the sweeping potential was effective in removing the carbonyl and alkoxy groups but also as more defect-inducing reduction approach than constant potential, according to the Raman measurements. The subsequent electroreduction of ErGO during the electrodeposition of ZnO resulted in interfacial interaction between the seed layer and ZnO nanorods. H₂SO₄:H₃PO₄ mix induced a less disrupted graphitic plane in GO which proved beneficial for the growth of higher density ZnO nanorods with improved crystalline quality. By controlling the chemical introduction/electroreduction conditions for oxygen groups in GO materials, the interfacial interaction with ZnO nanorods and their properties could be tailored in order to obtain high performance nanoplatforms.     

Effects of graphene oxide sheets-zirconia spheres nanohybrids on mechanical,thermal and tribological performances of epoxy composites

In this work, graphene oxide sheets-zirconia spheres (ZrO₂-rGO) nanohybrids were fabricated by Schiff base or Michael addition reaction. Their structure was characterized by FT-IR spectroscopy, UV–vis absorption spectra, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and atomic force microscope in detail. The reaction process of PDA-capping on rGO and APTES treatment on ZrO₂ nanoparticles were verified and it was proved that the ZrO₂ nanoparticles were successfully adhered onto the wrinkled surface of the graphene oxide. As a new multifunctional nanofillers, the ZrO₂-rGO nanohybrids were introduced into epoxy matrix and the mechanical, thermal properties and tribological performances of the fabricated composites were also detailedly investigated. Compared with the neat EP composites, the tensile strength and elongation at break of 0.1?wt% ZrO₂-rGO/EP are improved by 33% and 40%, respectively. Besides, the propagation of decomposition reactions in the composites could be impeded by anchoring ZrO₂ nanoparticles on the lamellar skeleton of graphene oxide. Furthermore, the lubricating effect and strong interfacial interaction contributed by the ZrO₂-rGO nanohybrids result in efficient load transfer from the matrix to the hybrids, which enables the ZrO₂-rGO/EP composites to have a fairly high wear resistance performance. This novel and effective approach using ZrO₂-rGO nanohybrids as multifunctional nanofillers could be beneficial to promote the development of high performance composites.     

Core–shell structured Cu@m-SiO2 and Cu/ZnO@m-SiO2 catalysts for methanol synthesis from CO2 hydrogenation

The core–shell catalysts with Cu and Cu/ZnO nanoparticles coated by mesoporous silica shells are prepared for CO₂ hydrogenation to methanol. With the confined effect of silica shell, the size of Cu nanoparticles is only about 5.0 nm, which results in high activity for CO₂ conversion. The CH₃OH selectivity is enhanced significantly with the introduction of ZnO. The core–shell structured catalysts endow the Cu nanoparticles trapped inside with excellent anti-aggregation and no deactivation is observed with time-on-stream. Therefore, the core–shell Cu/ZnO@m-SiO₂ catalyst exhibits the maximum CH₃OH yield with high stability.     

Cu-doped ZnO nanorod arrays: the effects of copper precursor and concentration

Cu-doped ZnO nanorods have been grown at 90°C for 90 min onto a quartz substrate pre-coated with a ZnO seed layer using a hydrothermal method. The influence of copper (Cu) precursor and concentration on the structural, morphological, and optical properties of ZnO nanorods was investigated. X-ray diffraction analysis revealed that the nanorods grown are highly crystalline with a hexagonal wurtzite crystal structure grown along the c-axis. The lattice strain is found to be compressive for all samples, where a minimum compressive strain of −0.114% was obtained when 1 at.% Cu was added from Cu(NO₃)₂. Scanning electron microscopy was used to investigate morphologies and the diameters of the grown nanorods. The morphological properties of the Cu-doped ZnO nanorods were influenced significantly by the presence of Cu impurities. Near-band edge (NBE) and a broad blue-green emission bands at around 378 and 545 nm, respectively, were observed in the photoluminescence spectra for all samples. The transmittance characteristics showed a slight increase in the visible range, where the total transmittance increased from approximately 80% for the nanorods doped with Cu(CH₃COO)₂ to approximately 90% for the nanorods that were doped with Cu(NO₃)₂.     

Preparation, characterization and visible-light-driven photocatalytic activity of Fe-doped titania nanorods and first-principles study for electronic structures

Fe-doped TiO₂ (Fe-TiO₂) nanorods were prepared by an impregnating-calcination method using the hydrothermally prepared titanate nanotubes as precursors and Fe(NO₃)₃ as dopant. The as-prepared samples were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, N₂ adsorption–desorption isotherms and UV–vis spectroscopy. The photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air under visible-light irradiation. The results show that Fe-doping greatly enhance the visible-light photocatalytic activity of mesoporous TiO₂ nanorods, and when the atomic ratio of Fe/Ti (R_Fe) is in the range of 0.1–1.0%, the photocatalytic activity of the samples is higher than that of Degussa P25 and pure TiO₂ nanorods. At R_Fe = 0.5%, the photocatalytic activity of Fe-TiO₂ nanorods exceeds that of Degussa P25 by a factor of more than two times. This is ascribed to the fact that the one-dimensional nanostructure can enhance the transfer and transport of charge carrier, the Fe-doping induces the shift of the absorption edge into the visible-light range with the narrowing of the band gap and reduces the recombination of photo-generated electrons and holes. Furthermore, the first-principle density functional theory (DFT) calculation further confirms the red shift of absorption edges and the narrowing of band gap of Fe-TiO₂ nanorods.     

Ultrafine palladium nanoparticle-bonded to polyetheylenimine grafted reduced graphene oxide nanosheets: Highly active and recyclable catalyst for degradation of dyes and pigments

Much attention has been increasingly focused on the applications of noble metal nanoparticles (NPs) for the catalytic degradation of various dyes and pigments in industrial wastewater. We have demonstrated that Pd NPs/Fe₃O₄-PEI-RGO nanohybrids exhibit high catalytic activity and excellent durability in reductive degradation of MO, R6G, RB. Specific surface area was successfully prepared by simultaneous reduction of Pd(OAc)₂ chelating to PEI grafted graphene oxide nanosheets modified with Fe₃O₄. The as-prepared Pd NPs/Fe₃O₄-PEI-RGO nanohybrids were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, high-resolution TEM and energy dispersive X-ray spectroscopy, and UV-lambda 800 spectrophotometer, respectively. The catalytic activity of Pd NPs/Fe₃O₄-PEI-RGO nanohybrids to the degradation of MO, R6G, RB with NaBH4 was tracked by UV-visible spectroscopy. It was clearly demonstrated that Pd NPs/Fe₃O₄-PEI-RGO nanohybrids exhibited high catalytic activity toward the degradation of dyes and pigments, which could be relevant to the high surface areas of Pd NPs and synergistic effect on transfer of electrons between reduced graphene oxide (RGO), PEI and Pd NPs. Notably, Pd NPs/Fe₃O₄-PEI-RGO nanohybrids were easily separated and recycled thirteen times without obvious decrease in system. Convincingly, Pd NPs/Fe₃O₄-PEI-RGO nanohybrids would be a promising catalyst for treating industrial wastewater.     

Controllable electrochemical synthesis of La/ZnO hierarchical nanostructures and their optical and magnetic properties

Here we presented a facile electrochemical deposition route for the controllable preparation of La³⁺/ZnO hierarchical nanostructures, such as flower-like nanostructures consisted of nanorods, flower bundles, and hexagonal nanorods with nests at the top. These prepared La³⁺/ZnO deposits were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis spectrophotometer, and photoluminescence spectroscopy. The formation process of La³⁺-doped ZnO and the growth mechanisms of La³⁺/ZnO hierarchical nanostructures were discussed. The UV and PL spectra measurements show that the surface morphologies of La³⁺/ZnO deposits have an obvious effect on their optical properties and we can easily adjust their optical properties as well as La³⁺/ZnO nanostructures by changing electrochemical deposition parameters. In addition, the magnetic properties of La³⁺/ZnO deposits were also investigated.     

Regularly arranged ZnO/TiO2, HfO2, and ZrO2 core/shell hybrid nanostructures - towards selection of the optimal shell material for efficient ZnO-based UV light emitters

Luminescent properties of ZnO/TiO₂, ZnO/HfO₂, and ZnO/ZrO₂ core/shell hybrid nanotubes (NTs) with the shell thickness varying between 9 and 40 nm were studied. The hybrid nano-ceramics demonstrated distinct differences in their luminescence performance. The highest UV/VIS ratio and the longest fluorescence lifetime were observed for the ZnO/TiO₂ NTs. The behavior was ascribed to resonance energy/charge transfer between TiO₂ and ZnO owing to the similar position of conduction and valence band edges, and comparable bandgap energies (E_g) which allowed for a simultaneous excitation of electron-hole pairs in both semiconductors. The difference between the other two core/shell NTs was attributed to the larger bond energy of HfO₂ as compared to that of ZrO₂ and smaller refractive index of HfO₂ as compared to that of ZnO. The results obtained in this work strongly indicate that in the optimal core/shell heterostructure, not only the shell material should form a type-I heterojunction with the ZnO nanostructure but also the excitation energy should be comparable to or larger than the E_g of the coating material. Moreover, the shell material with a high negative formation enthalpy and lower refractive index than that of ZnO would assure an efficient surface passivation and better photon extraction from the emitter.     

XPS and optical studies of different morphologies of ZnO nanostructures prepared by microwave methods

Zinc oxide (ZnO) nanostructures of various morphologies were prepared using a microwave-assisted aqueous solution method. Herein, a comparative study between three different morphologies of ZnO nanostructures, namely nanoparticles (NPs), nanoflowers (NFs) and nanorods (NRs) has been reviewed and presented. The morphologies of the prepared powders have been studied using field effect scanning electron microscopy (FESEM). X-ray diffraction (XRD) results prove that ZnO nanorods have biggest crystallite size compared with nanoflowers and nanoparticles. The texture coefficient (T_c) of three morphologies has been calculated. The T_c changed with varying morphology. A comparative study of surfaces of NPs, NFs and NRs were investigated using X-ray photoelectron spectroscopy (XPS). The possible growth mechanisms of ZnO NPs, NFs and NRs have been described. The optical properties of the ZnO nanostructures of various morphologies have been investigated and showed that the biggest crystallite size of ZnO nanostructures has lowest band gap energy. The obtained results are in agreement with experimental and theoretical data of other researchers.