Mikhael Bechelany

Atomic Layer Deposition of Nanostructured Materials for Energy and Environmental Applications

Atomic layer deposition (ALD) is a thin film technology that in the past two decades rapidly developed from a niche technology to an established method. It proved to be a key technology for the surface modification and the fabrication of complex nanostructured materials. In this Progress Report, after a short introduction to ALD and its chemistry, the versatility of the technique for the fabrication of novel functional materials will be discussed. Selected examples, focused on its use for the engineering of nanostructures targeting applications in energy conversion and storage, and on environmental issues, will be discussed. Finally, the challenges that ALD is now facing in terms of materials fabrication and processing will be also tackled.

MOF-Based Membrane Encapsulated ZnO Nanowires for Enhanced Gas Sensor Selectivity

Gas sensors are of a great interest for applications including toxic or explosive gases detection in both in-house and industrial environments, air quality monitoring, medical diagnostics, or control of food/cosmetic properties. In the area of semiconductor metal oxides (SMOs)-based sensors, a lot of effort has been devoted to improve the sensing characteristics. In this work, we report on a general methodology for improving the selectivity of SMOx nanowires sensors, based on the coverage of ZnO nanowires with a thin ZIF-8 molecular sieve membrane. The optimized ZnO@ZIF-8-based nanocomposite sensor shows markedly selective response to H2 in comparison with the pristine ZnO nanowires sensor, while showing the negligible sensing response to C7H8 and C6H6. This original MOF-membrane encapsulation strategy applied to nanowires sensor architecture pave the way for other complex 3D architectures and various types of applications requiring either gas or ion selectivity, such as biosensors, photo(catalysts), and electrodes.

Simple synthetic route for SERS-active gold nanoparticles substrate with controlled shape and organization.

We report a simple synthetic route based on electroless deposition (galvanic displacement) and natural lithography to simultaneously control the shape and organization of Au nanoparticles (NPs). We show for the first time the formation of organized extended domains of Au nanoflowers and nanocrowns with single crystalline tips. The dimension and morphology of the desired nanostructures (NSs) can be tuned easily by controlling the deposition conditions at room temperature using saccharin as an organic additive. The exact role of saccharin on the crystal growth process of Au NPs is also discussed. A systematic surface enhancement Raman spectroscopy (SERS) study of large, ordered areas of organized gold nanoflowers using p-mercaptoaniline (pMA) as the probe molecule shows massive and reproducible enhancements of the Raman signal. By comparing the relative enhancement of the different vibrational modes as a function of the morphology, the specific charge-transfer (chemical effect) SERS mechanism can be distinguished from the general electromagnetic field enhancement (physical effect). A wide range of applications can be envisaged for these SERS substrates.

ZnO 1D nanostructures designed by combining atomic layer deposition and electrospinning for UV sensor applications

We explored for the first time the ability of a three-dimensional polyacrylonitrile/ZnO material prepared by a low-cost and scalable synthesis method based on the combination of electrospinning and atomic layer deposition (ALD) as a new material with a large surface area to enhance the performance of UV photodetection. The UV photoresponse current was enhanced by a factor of 250 compared to a flat electrode. In addition an increase by a factor of 1.3 of the recovery time has been observed which is negligible versus the huge amount of current enhancement. The greatly improved performance and the good stability of these nanostructured electrodes induce exciting materials for use in UV sensor applications.

Evolution of microstructure and related optical properties of ZnO grown by atomic layer deposition

Summary A study of transmittance and photoluminescence spectra on the growth of oxygen-rich ultra-thin ZnO films prepared by atomic layer deposition is reported. The structural transition from an amorphous to a polycrystalline state is observed upon increasing the thickness. The unusual behavior of the energy gap with thickness reflected by optical properties is attributed to the improvement of the crystalline structure resulting from a decreasing concentration of point defects at the growth of grains. The spectra of UV and visible photoluminescence emissions correspond to transitions near the band-edge and defect-related transitions. Additional emissions were observed from band-tail states near the edge. A high oxygen ratio and variable optical properties could be attractive for an application of atomic layer deposition (ALD) deposited ultrathin ZnO films in optical sensors and biosensors.

A highly active based graphene cathode for the electro-fenton reaction

Reduced Graphene Oxide (rGO) was coated on Carbon Felt (CF) in order to design a novel cathode applied in the Electro-Fenton (EF) reaction for decontamination of wastewater polluted with Persistent Organic Pollutants (POPs). The new cathode was fabricated by an electrophoretic deposition of Graphene Oxide (GO) followed by its electrochemical reduction at a current density of −1.5 mA cm−2 for 10 min. The modified electrode and GO were characterized by SEM, AFM, XRD and XPS, showing the presence of rGO after optimization of the electrochemical conditions of synthesis. Electrode modification has improved the CF electrochemical properties as proved by the decrease of the charge-transfer resistance (Rct) determined by electrochemical impedance spectroscopy (EIS) and the increase of the CV response (2.5 times) of the FeIII/FeII couple used as a redox probe. Degradation of Acid Orange 7 (AO7), a model pollutant molecule, was monitored by UV-Vis spectrophotometry at the selected single wavelength λ = 485 nm. The results show that the degradation kinetics were 2 times higher on the graphene modified cathode compared to raw carbon felt proving the efficiency of this modification process.

High-yield synthesis of hollow boron nitride nano-polyhedrons

Hollow boron nitride nano-polyhedrons have been successfully prepared by annealing of boron nitride nanoparticles in a nitrogen atmosphere at 1800 °C without a catalyst. In our two-step process, we demonstrated that these boron nitride nanoparticles prepared by spray-pyrolysis of borazine are key precursors to grow these architectures. The samples were carefully analyzed using electron microscopies and Energy-Dispersive X-ray spectroscopy analysis. Based on such characterization tools, the growth mechanism of these architectures has been discussed and detailed. It is noteworthy that these nanostructures are generated via a solid-state transformation in relatively high yields without a metal catalyst and might open new opportunities in exploring chemical and physical properties.

A hierarchical CoFe-layered double hydroxide modified carbon-felt cathode for heterogeneous electro-Fenton process

Hierarchical CoFe-layered double hydroxide (CoFe–LDH) was grown on carbon felt (CF) as a heterogeneous catalyst by in situ solvothermal growth. The CoFe–LDH/CF serves as a cathode as well as a Fe2+ (catalyst) source in the electro-Fenton (EF) process. A combined structural and electrochemical characterization revealed highly ordered and well crystallized CoFe–LDH anisotropically grown on a CF substrate with highly dense urchin-like structures that were highly stable at circumneutral pH. EF experiments with the CoFe–LDH/CF cathode showed excellent mineralization of Acid Orange II (AO7) over a wide pH range (2–7.1). The mineralization of AO7 with CoFe–LDH/CF was by both homogeneous and surface-catalyzed processes at low acidic pH, whereas only the surface-catalyzed process occurs at circumneutral pH due to the stability of the LDH as well as precipitation of the catalyst. Higher mineralization was achieved with CoFe–LDH/CF compared to homogeneous EF with raw CF using the Fe2+/Co2+ catalyst at all pH values studied and the TOC removal with the CoFe–LDH/CF cathode was at least 1.7 and 3.5 times higher than that with the homogeneous system with Fe2+/Co2+ at pH 5.83 and 7.1, respectively. The enhanced performance observed with CoFe–LDH/CF was ascribed to (i) the surface-catalyzed reaction occurring at the surface of the cathode which can expand the working pH window, avoiding the precipitation of iron sludge as pH increases, (ii) enhanced generation of H2O2 due to the improved electroactive surface area of the cathode and (iii) the co-catalyst effect of the Co2+ in the LDH that can promote regeneration and additional production of Fe2+ and the hydroxyl radical, respectively. The CoFe–LDH/CF cathode exhibited relatively good reusability as the TOC removal after 2 h was still above 60% after 7 cycles of degradation, indicating that the prepared CoFe–LDH/CF is a promising cathode for the removal of organic pollutants by EF technology.

High Spatial Resolution Time-of-Flight Secondary Ion Mass Spectrometry for the Masses: A Novel Orthogonal ToF FIB-SIMS Instrument with In Situ AFM

We describe the design and performance of an orthogonal time-of-flight (TOF) secondary ion mass spectrometer that can be retrofitted to existing focused ion beam (FIB) instruments. In particular, a simple interface has been developed for FIB/SEM instruments from the manufacturer Tescan. Orthogonal extraction to the mass analyser obviates the need to pulse the primary ion beam and does not require the use of monoisotopic gallium to preserve mass resolution. The high-duty cycle and reasonable collection efficiency of the new instrument combined with the high spatial resolution of a gallium liquid metal ion source allow chemical observation of features smaller than 50 nm. We have also demonstrated the integration of a scanning probe microscope (SPM) operated as an atomic force microscope (AFM) within the FIB/SEM-SIMS chamber. This provides roughness information, and will also allow true three dimensional chemical images to be reconstructed from SIMS measurements.

Tuning of ZnO 1D nanostructures by atomic layer deposition and electrospinning for optical gas sensor applications

We explored for the first time the ability of a three-dimensional polyacrylonitrile/ZnO material-prepared by a combination of electrospinning and atomic layer deposition (ALD) as a new material with a large surface area-to enhance the performance of optical sensors for volatile organic compound (VOC) detection. The photoluminescence (PL) peak intensity of these one-dimensional nanostructures has been enhanced by a factor of 2000 compared to a flat Si substrate. In addition, a phase transition of the ZnO ALD coating from amorphous to crystalline has been observed due to the properties of a polyacrylonitrile nanofiber template: surface strain, roughness, and an increased number of nucleation sites in comparison with a flat Si substrate. The greatly improved PL performance of these nanostructured surfaces could produce exciting materials for implantation in VOC optical sensor applications.