Polona Umek

Selective etching of metallic single-wall carbon nanotubes with hydrogen plasma

We present Raman scattering and scanning tunnelling microscopy (STM) measurements on hydrogen plasma etched single-wall carbon nanotubes (SWNTs). Interestingly, both the STM and Raman spectroscopy show that the metallic SWNTs are dramatically altered and highly defected by the plasma treatment. In addition, structural characterizations show that metal catalysts are detached from the ends of the SWNT bundles. For semiconducting SWNTs we observe no feature of defects or etching along the nanotubes. Raman spectra in the radial breathing mode region of plasma-treated SWNT material show that most of the tubes are semiconducting. These results show that hydrogen plasma treatment favours etching of metallic nanotubes over semiconducting ones and therefore could be used to tailor the electronic properties of SWNT raw materials.

Air-stable monodispersed Mo6S3I6 nanowires

We report on the properties of a new air-stable nanowire material with the chemical formula Mo6S3I6 .T he distinguishing features of the material are rapid one-step synthesis, easy isolation and controllable dispersion into small-diameter wire bundles. Elemental analysis, x-ray diffraction, thermogravimetry, differential thermal analysis, Raman scattering and electron microscopy were used to characterize the material.

Synthesis and manipulation of carbon nanotubes

This paper reviews recent results in the field of carbon nanotube (CNT) research obtained at our institute at EPFL. We show in particular that CNTs can be synthesized by the catalytic vapour deposition (CVD) technique with high efficiency and purity. Furthermore, we present recent examples of advances in the large-scale production of CNTs as well as in the chemical and mechanical manipulation of CNTs. The chemical manipulation involves covalent and non-covalent sidewall functionalization of single-wall CNTs and preparation of inorganic coatings on CVD-grown nanotubes for the realization of fibres and CNT-reinforced composites. Mechanical manipulation aims at the application of CNTs as tips for scanning probe microscopy.

Aerosol-Assisted CVD-Grown WO3 Nanoneedles Decorated with Copper Oxide Nanoparticles for the Selective and Humidity-Resilient Detection of H2S

A gas-sensitive hybrid material consisting of Cu2O nanoparticle-decorated WO3 nanoneedles is successfully grown for the first time in a single step via aerosol-assisted chemical vapor deposition. Morphological, structural, and composition analyses show that our method is effective for growing single-crystalline, n-type WO3 nanoneedles decorated with p-type Cu2O nanoparticles at moderate temperatures (i.e., 380 °C), with cost effectiveness and short fabrication times, directly onto microhot plate transducer arrays with the view of obtaining gas sensors. The gas-sensing studies performed show that this hybrid nanomaterial has excellent sensitivity and selectivity to hydrogen sulfide (7-fold increase in response compared with that of pristine WO3 nanoneedles) and a low detection limit (below 300 ppb of H2S), together with unprecedented fast response times (2 s) and high immunity to changes in the background humidity. These superior properties arise because of the multiple p-n heterojunctions created at the nanoscale in our hybrid nanomaterial.

Aerosol-Assisted CVD-Grown PdO Nanoparticle-Decorated Tungsten Oxide Nanoneedles Extremely Sensitive and Selective to Hydrogen

We report for the first time the successful synthesis of palladium (Pd) nanoparticle (NP)-decorated tungsten trioxide (WO3) nanoneedles (NNs) via a two-step aerosol-assisted chemical vapor deposition approach. Morphological, structural, and elemental composition analysis revealed that a Pd(acac)2 precursor was very suitable to decorate WO3 NNs with uniform and well-dispersed PdO NPs. Gas-sensing results revealed that decoration with PdO NPs led to an ultrasensitive and selective hydrogen (H2) gas sensor (sensor response peaks at 1670 at 500 ppm of H2) with low operating temperature (150 °C). The response of decorated NNs is 755 times higher than that of bare WO3 NNs. Additionally, at a temperature near that of the ambient temperature (50 °C), the response of this sensor toward the same concentration of H2 was 23, which is higher than that of some promising sensors reported in the literature. Finally, humidity measurements showed that PdO/WO3 sensors displayed low-cross-sensitivity toward water vapor, compared to bare WO3 sensors. The addition of PdO NPs helps to minimize the effect of ambient humidity on the sensor response.

Microsensors based on Pt–nanoparticle functionalised tungsten oxide nanoneedles for monitoring hydrogen sulfide

Gas microsensors based on non-functionalised and functionalised tungsten oxide nanoneedles with platinum nanoparticles, synthesised and integrated directly onto transducing microsensor platforms via Aerosol Assisted Chemical Vapour Deposition, are fabricated and tested towards various concentrations of hydrogen sulfide and some interfering gases (CO, H2, NH3, EtOH). Results show improved sensing characteristics in functionalised microsensors as a result of the incorporation of platinum nanoparticles of reduced size (<4 nm) with even distribution onto highly crystalline tungsten oxide nanoneedles. An arrangement of both non-functionalised and functionalised sensing films in an array of sensors shows the potential of these devices to selectively monitor hydrogen sulfide.

Molecular nitrogen in N-doped TiO2 nanoribbons

The nitrogen doping of TiO2 nanoribbons during the thermal transformation of hydrogen titanate nanoribbons (HTiNRs) between 400 and 650 °C in a dynamic ammonia atmosphere was investigated using X-ray photoelectron spectroscopy (XPS), transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (NEXAFS-TXM), X-ray diffraction (XRD) and electron paramagnetic resonance measurements (EPR). Comprehensive structural characterizations have revealed that for a calcination temperature of 400 °C, the HTiNRs transform into pure monoclinic TiO2 β-phase (TiO2-B) whereas at higher calcination temperatures (580 and 650 °C) a mixture of TiO2-B and anatase is obtained. XPS and EPR results clearly reveal the nitrogen doping of TiO2 nanoribbons and that, depending on the calcination temperature, nitrogen atoms occupy interstitial and substitutional sites. Moreover, in samples calcined at 580 and 650 °C the presence of N2-like species in the HTiNRs was detected by NEXAFS-TXM. These species are trapped in the HTiNRs structure. EPR measurements upon light illumination have disclosed the generation of photoexcited states which implies that nitrogen has an important effect on the electronic structure of N-doped TiO2.

Protein Corona Prevents TiO2 Phototoxicity.

Background & Aim TiO2 nanoparticles have generally low toxicity in the in vitro systems although some toxicity is expected to originate in the TiO2-associated photo-generated radical production, which can however be modulated by the radical trapping ability of the serum proteins. To explore the role of serum proteins in the phototoxicity of the TiO2 nanoparticles we measure viability of the exposed cells depending on the nanoparticle and serum protein concentrations. Methods & Results Fluorescence and spin trapping EPR spectroscopy reveal that the ratio between the nanoparticle and protein concentrations determines the amount of the nanoparticles’ surface which is not covered by the serum proteins and is proportional to the amount of photo-induced radicals. Phototoxicity thus becomes substantial only at the protein concentration being too low to completely coat the nanotubes’ surface. Conclusion These results imply that TiO2 nanoparticles should be applied with ligands such as proteins when phototoxic effects are not desired - for example in cosmetics industry. On the other hand, the nanoparticles should be used in serum free medium or any other ligand free medium, when phototoxic effects are desired – as for efficient photodynamic cancer therapy.

Synthesis of single crystalline In2O3 octahedra for the selective detection of NO2 and H2 at trace levels

Single crystalline indium oxide (In2O3) octahedra have been synthesized by means of a vapor phase transport method at high temperature. The resulting material has been characterized by FE-SEM, HR-TEM, XRD, XPS and PL. The gas sensing properties of this material against oxidizing and reducing gases have been examined and the conditions for selectively detecting such gases have been established. A high response towards NO2 has been obtained at a relatively low optimal operating temperature (i.e., 130 °C) and even at room temperature. The fact that the response of the nanomaterial is more than two orders of magnitude higher for NO2 than for H2, even in the presence of ambient moisture, makes it very promising for the selective detection of oxidizing species (at ppb levels) under real ambient conditions. The addition of noble metal nanoparticles (Pt and Pd) combined with an increase in the operating temperature (i.e., 250 °C) significantly increases H2 sensitivity and dramatically decreases the response to NO2. However, in this case, the presence of humidity negatively affects the response to H2. The sensing mechanisms are introduced and discussed.

Towards atomic resolution in sodium titanate nanotubes using near-edge X-ray-absorption fine-structure spectromicroscopy combined with multichannel multiple-scattering calculations

Summary Recent advances in near-edge X-ray-absorption fine-structure spectroscopy coupled with transmission X-ray microscopy (NEXAFS–TXM) allow large-area mapping investigations of individual nano-objects with spectral resolution up to E/ΔE = 104 and spatial resolution approaching 10 nm. While the state-of-the-art spatial resolution of X-ray microscopy is limited by nanostructuring process constrains of the objective zone plate, we show here that it is possible to overcome this through close coupling with high-level theoretical modelling. Taking the example of isolated bundles of hydrothermally prepared sodium titanate nanotubes ((Na,H)TiNTs) we are able to unravel the complex nanoscale structure from the NEXAFS–TXM data using multichannel multiple-scattering calculations, to the extent of being able to associate specific spectral features in the O K-edge and Ti L-edge with oxygen atoms in distinct sites within the lattice. These can even be distinguished from the contribution of different hydroxyl groups to the electronic structure of the (Na,H)TiNTs.