Confocal micro-Raman Research Articles

Investigation of Residual Strain in 4H-SiC/Si Heterostructures Fabricated By Surface Activated Bonding

Silicon carbide (SiC) is one of the most potential materials for the next generation power devices because of its advantageous material properties and the maturity of processing technology. SiC/Si heterojunction is a potentially useful system for realizing high-performance bipolar transistors with wide band gap emitters, switching devices, electroluminescence devices, and sensors. On the other hand, it is difficult to grow SiC on Si due to the large mismatch in the lattice constant and the thermal expansion coefficients between Si and SiC. We previously fabricated 4H-SiC/Si junctions using surface activated bonding (SAB) method and investigated their electrical characteristics. It was found that the electrical properties of 4H-SiC/Si junctions improved with increasing annealing temperature and the amorphous layer formed at the interface without annealing vanished after the interface annealing at 1000 °C. It is possible to fabricate 4H-SiC/Si junctions at room temperature due to the amorphous layer absorbed the residual stresses caused by the mismatch in the lattice constant. However, the stress state of the bonded interface with annealing at 1000 °C is not clear after the recrystallization of the amorphous layer. The knowledge of the stress state near the bonded interface is very important for better understanding the vibrational properties and improving the electrical properties of the devices fabricated by SAB.In this work, the residual stress near the 4H-SiC/Si bonded interface was investigated using confocal micro-Raman scattering technique. The effects of thermal annealing process on the residual stress of the bonded interface were also investigated. The magnitude of the residual stress near the bonded interface was calculated from the measured frequency shift with respect to the stress-free Raman peak position. The structural properties of the bonded interfaces were examined by transmission electron microscopy (TEM) observation.B–doped (100) p+-Si substrate and n-4H-SiC epitaxial substrates (6 um, 5 × 10E15 cm-3 epitaxial layer / 0.5 um, 1 × 10E18 cm-3 buffer layer / substrate 3 × 10E18 cm-3) were used for the bonding experiment. Si and 4H-SiC substrates were bonded to each other by SAB. And then, the bonded samples were annealed separately at 400, 700, and 1000 °C for 60 s in N2 gas ambient. The residual stress in 4H-SiC/Si heterostructures was systematically investigated by micro-Raman spectroscopy. Micro-Raman mapping measurement was performed for an area of 40 × 40 μm2 with a step of 2 μm. A Renishaw InVia confocal micro-Raman spectroscopy measured under backscattering geometry, an Ar+ laser of 488 nm, a diffraction grating of 2400 lines/mm, a 50× long working distance objective lens, and a laser sport size of about 1.5 μm was used. 4H-SiC and Si peak frequencies were determined using the curve-fitting option integrated in the operation software by performing a Lorentz fitting to the spectral. The accuracy of the peak frequency is within ±0.02 cm-1. The interfacial structures of the bonded samples were investigated using TEM (JEM-2200FS)The Raman peak shift mappings of 4H-SiC E2 (TO) and Si phonon mode in 4H-SiC/Si heterostructures without annealing are shown in Fig. 1(a) and 1(b), respectively. The averaged Raman peak shift position for SiC and Si is observed to be 776.57 and 519.86 cm-1, respectively. A uniform distribution of Raman peak position is obtained for both SiC and Si phonon modes. The stress state of SiC and Si can be considered nearly stress free in 4H-SiC/Si heterostructures with respect to their bulk counterpart values of 4H-SiC and Si substrate at 776.54 and 519.93 cm-1, respectively. This result indicates that stress free 4H-SiC/Si heterostructures was achieved by SAB method. Figure 1

Comparative study on the photoluminescence properties of monoclinic and cubic erbium oxide

As a heavy rare earth oxide, erbium oxide (Er2O3) has many attractive properties. Monoclinic Er2O3 has useful properties not found in stable cubic Er2O3, such as unique optical properties and high radiation damage tolerance. In this study, pure cubic and mixed phase of cubic and monoclinic Er2O3 coatings were prepared. Photoluminescence properties of these coatings were characterized by a confocal micro-Raman spectrometer equipped with 325, 473, 514, 532, 633 nm lasers, and the influence of microstructure on the fluorescence properties was analyzed in detail. The room temperature fluorescence peaks of cubic Er2O3 were assigned. Furthermore, a novel method for rapid phase identification of Er3+ doped cubic and monoclinic rare earth sesquioxides at room temperature was proposed.

(Invited) Interplay between Physicochemical Features and Electrochemical Performance in the ORR of “Platinum-Free” Elecrocatalysts Based on Hierarchical Graphene Supports

In this study, a new family of electrocatalysts (ECs) for the oxygen reduction reaction (ORR) is presented. The ECs, that exhibit a “core-shell” morphology, include: (i) a hierarchical graphene-based support (H-GR) “core”, that is covered by: (ii) a carbon nitride “shell” stabilizing the active sites in “coordination nests” [1]. The H-GR comprise two main components, namely: (i) ZnO nanoparticles supporting highly defected graphene nanoplatelets [2]; and (ii) carbon black nanoparticles; the latter improve the charge and mass transport phenomena associated to EC operation. The active sites of the ECs do not include platinum-group metals (PGMs); instead, they are based on Fe (playing the role of “active metal”) and also contain Sn as the “co-catalyst” [1]. The synthetic route that is pursued to obtain the proposed ECs consists in the following main steps: (i) preparation of a precursor comprising the H-GR support; and (ii) multi-step pyrolysis process yielding the carbon nitride “shell” and the active sites [1]. In this study, other treatments are introduced in the synthetic route (e.g., chemical etching; electrochemical cycling; and/or further pyrolysis steps), with the purpose of fine-tuning the morphology, pore structure, and chemical composition of the ECs and thus maximize the performance in the ORR. The ECs obtained in this study undergo an extensive characterization campaign, with the aim of elucidating the details of the interplay between the parameters of the synthetic route, the physicochemical features and the electrochemical performance and durability. In particular, the role of the various components included in the H-GR support (i.e., the highly defected graphene nanoplatelets, the ZnO and the carbon black nanoparticles) is studied accurately, striving to discern the impact of each in the properties of the final ECs. The bulk chemical composition of the samples is determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and CHNOS microanalysis. X-ray photoelectron spectroscopy is adopted to probe the chemical composition and the oxidation states of the elements on the surface of the ECs. The structure of the samples is studied by means of vibrational spectroscopies (e.g., confocal micro-Raman) and wide-angle X-ray diffraction (WAXD). The morphology and pore structure of the samples is investigated by means of high-resolution transmission electron microscopy (HR-TEM) and nitrogen physisorption techniques. The performance, reaction mechanism and durability of the ECs in the ORR is studied through the CV-TF-RRDE technique (cyclic voltammetry with the thin-film rotating ring-disk electrode). Finally, the interplay between the ORR kinetics and the chemical composition of the active sites is clarified by studying the ORR mechanism at different pH values. Acknowledgements This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 696656. The authors wish to thank the Strategic Project of the University of Padova “Materials for Membrane-Electrode Assemblies to Electric Energy Conversion and Storage Devices (MAESTRA)” for funding. V.D.N. thanks the University Carlo III of Madrid for granting him the “Catedra de Excelentia” (Chair of Excellence).

Probing the Nanoscale Heterogeneity of SEI on Silicon Anode Using Tip Enhanced Raman Spectroscopy (TERS)

The composition, morphology and thickness of solid electrolyte interphase (SEI) on silicon anodes are dictated by multiple factors, ranging from the chemistry of silicon surfaces, electrolyte solution structure and chemical passivation on Si surface. In addition, the SEI constantly evolves, as it is electrochemically lithiated (delithiated) due to the enormous volume changes. SEI on silicon typically is reported to be in the range of tens of nanometers and can be compositionally heterogeneous. Normal vibrational spectroscopy techniques such as confocal micro-Raman and FT-IR are useful, but they only provide chemical information at a relatively bulk scale, typically in micron-distance from the solid surface. This makes it extremely hard for getting information from the real SEI, which can be only on the order of 10 nm. Tip Enhanced Raman Spectroscopy (TERS) combines surface plasmon-enhanced Raman spectroscopy with atomic force microscopy, capable of providing the chemical vibrational information and topography of the sample in the nanometer spatial resolution simultaneously. In this report we demonstrate the first TERS study of SEI on cycled amorphous silicon. Amorphous silicon anode (20 nm in thickness on copper current collector) was cycled in a commercial lithium-ion battery electrolyte (1M LiPF6 in EC:DEC, 1:1 vol). TERS analysis on cycled amorphous silicon sample indicates that the nanometer scale SEI “islands” are unevenly distributed on the Si anode surface. Even for the same SEI “island”, the composition is different from point to point with inter-point distance smaller than 2 nm. Acknowledgment This work is funded by Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy

Transformation of hydrazinium azide to molecular N8 at 40 GPa.

Hydrazinium azide (HA) has been investigated at high pressures to 68 GPa using confocal micro-Raman spectroscopy and synchrotron powder x-ray diffraction. The results show that HA undergoes structural phase transitions from solid HA-I to HA-II at 13 GPa, associated with the strengthening of hydrogen bonding, and then to N8 at 40 GPa. The transformation of HA to recently predicted N8 (N≡N+-N--N=N--N-+N≡N) is evident by the emergence of new peaks at 2384 cm-1, 1665 cm-1, and 1165 cm-1, arising from the terminal N≡N stretching, the central N=N stretching, and the N-N stretching, respectively. However, upon decompression, N8 decomposes to ε-N2 below 25 GPa, but the remnant can be seen as low as 3 GPa.

A robust approach to fabricate CZTSSe absorber layer for solar cells via a self-selenizations process conducted by concentrated selenium solution

It was reported that most of copper zinc tin sulfoselenide (CZTSSe) had been prepared via the conventional rapid thermal selenization of copper zinc tin sulfide (CZTS) using external selenium source. This approach often shows low utilization efficiency and high waste of selenium source, which will hamper the development of CZTSSe in solar cells. In this work, CZTSSe thin films with high crystallinity were designed by self-selenizing CZTS nanocrystalline thin films via direct rapid thermal processes (RTP) conducted by superficial selenium solution. The transformation processes of selenizing CZTS thin films into CZTSSe were systematically investigated by field emission scanning electron microscope, x-ray diffraction, and confocal micro-Raman spectroscopy. The results showed that CZTS nanocrystals only partially transformed into CZTSSe nanocrystals during RTP in selenium solution, while they can further be self-selenized by the residual elemental Se in a post-RTP process. More interesting, the energy dispersive x-ray spectroscopy indicated that selenium elements can be highly dispersed into CZTSSe thin films. Furthermore, the assembled solar cell using the obtained CZTSSe thin films can output power conversion efficiency of 1.96% for a total area of 0.5 cm2 (with an active area of 0.456 cm2) under standard AM 1.5 illumination. This research would give a new insight for preparation of high quality CZTSSe thin films.

Confocal mapping of myelin figures with micro-Raman spectroscopy

We employ confocal micro-Raman spectroscopy (CMRS) with submicron spatial resolution to study the myelin structures (cylindrical lamellae) composed of nested surfactant C12E3 or lipid DMPC bilayers. The CMRS mapping indicates that for a straight C12E3 myelin, the surfactant concentration increases with the myelin width and is higher in the center region than in the peripheral region. For a curved C12E3 myelin, the convex side has a higher surfactant concentration than the corresponding concave side. The spectrum of DMPC myelins undergoes a qualitative change as the temperature increases above 60 °C, suggesting that the surfactant molecules may be damaged. Our work demonstrates the utility of CMRS in bio-soft material research.

Effects of particle size and water content during cooking on the physicochemical properties and in vitro starch digestibility of milled durum wheat grains

The effect of water content and particle size on physicochemical properties and in vitro starch digestibility of three milled wheat fractions was investigated. The crystallinity of three milled fractions of particle sizes <0.15 mm, 0.15–0.25 mm and 0.25–0.5 mm decreased from 36% to 25% after heating at 27.2% moisture, and was almost undetectable after heating with higher water contents. The short-range molecular order, as determined by Fourier transform infra-red and laser confocal micro-Raman spectroscopy, was reduced gradually with increasing water content during cooking. The enthalpy changes of the three milled fractions after heating with 27.2% water was decreased by about 45%, and no endothermic transition was observed after heating with higher water contents. The pasting viscosities first decreased and then increased as water content was increased during cooking. Heating at low water content increased greatly the in vitro enzymatic digestibility of starch in three milled fractions, and there were no further increase as the water content increased up to 72.7%. From this study, we conclude that water content plays a more important role in the disruption of ordered structure of starch, whereas particle size influences more greatly the in vitro starch digestibility of milled fractions.

A comparative study of heterostructured CuO/CuWO4 nanowires and thin films

A comparative study of heterostructured CuO/CuWO4 core/shell nanowires and double-layer thin films was performed through X-ray diffraction, confocal micro-Raman spectroscopy and electron (SEM and TEM) microscopies. The heterostructures were produced using a two-step process, starting from a deposition of amorphous WO3 layer on top of CuO nanowires and thin films by reactive DC magnetron sputtering and followed by annealing at 650 °C in air. The second step induced a solid-state reaction between CuO and WO3 oxides through a thermal diffusion process, revealed by SEM-EDX analysis. Morphology evolution of core/shell nanowires and double-layer thin films upon heating was studied by electron (SEM and TEM) microscopies. A formation of CuWO4 phase was confirmed by X-ray diffraction and confocal micro-Raman spectroscopy.

ORR Electrocatalysts with a Low Pt Loading Comprising Hierarchical Graphene-Based Supports

This report presents a new family of electrocatalysts (ECs) for the oxygen reduction reaction (ORR) that comprises hierarchical graphene-based supports (H-GRs). The ECs, which are intended for application at the cathode of proton-exchange membrane fuel cells (PEMFCs), exhibit a “core-shell” morphology. The “core” is the H-GR support, which is covered by a carbon nitride “shell”. The latter bears “coordination nests”, which stabilize the ORR active sites [1]. The H-GRs consist of two components, namely: (i) high defected graphene nanoplatelets, which are supported on ZnO nanoparticles (NPs) [2]; and (ii) carbon black, that acts as spacer and facilitates the charge and mass transport phenomena that take place during the operation of the ECs. Two groups of ECs are synthesized, I and II. I includes the pristine H-GR support; II comprises H-GR suitably treated to etch the ZnO NPs. Both I and II are characterized by a very low loading of platinum, on the order of ca. 3 wt%, that is the “active metal” in the ORR active sites [1]. Ni and Cu are introduced as “co-catalysts” to improve the ORR performance [1]. All the ECs undergo an extensive post-synthesis activation process (A), that significantly affects their chemical composition, structure and morphology. As a result, a remarkable ORR performance is achieved that is significantly improved in comparison with state-of-the-art Pt/C reference ECs. The proposed ECs are extensively characterized both before and after A to study the complex interplay between the synthetic parameters, the physicochemical properties, and the electrochemical ORR performance both “ex-situ” and in single PEMFC. In this regard, particular attention is dedicated to clarify the role played by the graphene nanoplatelets included in the H-GR support. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and CHNOS microanalyses are adopted to determine the bulk chemical composition of the ECs. The surface composition and oxidation states are probed with X-ray photoelectron spectroscopy (XPS), while the structure of the ECs is investigated through wide-angle X-ray diffraction (WAXD) and vibrational spectroscopies (e.g., confocal micro-Raman). High-resolution transmission electron microscopy (HR-TEM) is used to study the morphology. Cyclic voltammetry with the rotating ring-disk electrode (CV-TF-RRDE) allows the details of the ORR performance and mechanism to be elucidated. Finally, the ECs are used to fabricate membrane-electrode assemblies (MEAs) that are tested in single PEMFC in operating conditions. Acknowledgements This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 696656. The authors thank the Strategic Project “From materials for Membrane electrode Assemblies to electric Energy conversion and SToRAge devices” (MAESTRA) of the University of Padova for funding this research.

Quantitative Micro-Raman Spectroscopy for Partial Pressure Measurement in Small Volumes.

We demonstrate the quantitative capabilities of Raman confocal microscopy as a nondestructive method to measure the partial pressure of molecular gases in mm3 range sealed volume having an optical access. Thanks to a calibration procedure, we apply this technique for the characterization of the absolute nitrogen partial pressure inside buffered micro electromechanical system (MEMS) atomic vapor cells developed for atomic clocks. Our results are compared with measurements obtained by rubidium hyperfine frequency spectroscopy and a good agreement is demonstrated between the two methods, with a three-sigma detection limit below 10 mbar for a 1 h integration time, using a 33 mW 532 nm excitation laser. These results prove the potential of confocal micro-Raman spectroscopy as a simple and nondestructive method for small-scale pressure measurements.

Enabling stress determination on alkaline textured silicon using Raman spectroscopy

Confocal micro-Raman spectroscopy allows for spatially resolved measurements of the phonon energy in silicon, which is correlated to mechanical stress. Mechanical stress is a tensorial quantity. For the confocal measurement geometry and certain crystal orientations approximations have been derived in the past which correlate the shift of the Raman frequency to a scalar stress value. For optimization of mono-crystalline solar cell manufacturing steps the determination of induced mechanical stress from the top view perspective is desirable. However, this method is so far restricted to planar wafers or cross sections. we find that the anti-reflection surface texture strongly affects the measurement result. To enable quantitative stress determination of alkaline textured silicon a suited measurement procedure is investigated using 3-point bended solar cell segments. Preceding elasticity test revealed Young’s module and allowed a prediction of maximum stress values for any bending radius. We propose a measurement procedure which yields an observable shift of the Raman frequency proportional to the induced stress. We state the requirements for determination of a calibration factor to quantify stress on any alkaline textured solar cell. Adapting the conversion factors allows calibrated measurements for textures with different average pyramid heights. By varying the numerical aperture and the focus setting even changes of the induced stress within the texture pyramids are resolved.

Study of hemoglobin response to mid-ultraviolet (UVB) radiation using micro-Raman spectroscopy

Confocal micro-Raman spectroscopy is employed to monitor the damage to haemoglobin from mid-ultraviolet (UVB) radiation. We obtained the Raman spectra of an erythrocyte, which indicated that a peroxidation reaction occurs after UVB radiation. Further, the surface enhanced Raman scattering (SERS) spectra of isolated haemoglobin show that the intensities of the 1375 and 1399 cm−1 bands, which are markers of haem aggregation, obviously increase with prolonged UVB irradiation. This increase reveals that haem aggregation occurs in the peroxidation of erythrocytes. The UV–Vis spectra of isolated haemoglobin indicate that the Soret band, which is indicative of excitonic interactions in the aggregated haems, has a redshift (~12 nm) after 30 min of UVB irradiation of erythrocytes. It can be deduced that an excitonic interaction occurs in the aggregated haems, which is caused by haemoglobin denaturation following UVB irradiation. In addition, the changes of the Raman marker bands during aggregation primarily originate from excitonic interactions. Throughout the process, a higher UVB radiation dose causes greater damage to haemoglobin.

CO2 fluid inclusions in Jack Hills zircons

The discovery of Hadean to Paleoarchean zircons in a metaconglomerate from Jack Hills, Western Australia, has catalyzed intensive study of these zircons and their mineral inclusions, as they represent unique geochemical archives that can be used to unravel the geological evolution of early Earth. Here, we report the occurrence and physical properties of previously undetected CO2 inclusions that were identified in 3.36–3.47 Ga and 3.80–4.13 Ga zircon grains by confocal micro-Raman spectroscopy. Minimum P–T conditions of zircon formation were determined from the highest density of the inclusions, determined from the density-dependence of the Fermi diad splitting in the Raman spectrum and Ti-in-zircon thermometry. For both age periods, the CO2 densities and Ti-in-zircon temperatures correspond to high-grade metamorphic conditions (≥5 to ≥7 kbar/~670 to 770 °C) that are typical of mid-crustal regional metamorphism throughout Earth’s history. In addition, fully enclosed, highly disordered graphitic carbon inclusions were identified in two zircon grains from the older population that also contained CO2 inclusions. Transmission electron microscopy on one of these inclusions revealed that carbon forms a thin amorphous film on the inclusion wall, whereas the rest of the volume was probably occupied by CO2 prior to analysis. This indicates a close relationship between CO2 and the reduced carbon inclusions and, in particular that the carbon precipitated from a CO2-rich fluid, which is inconsistent with the recently proposed biogenic origin of carbon inclusions found in Hadean zircons from Jack Hills.

Effect of scanning velocity on femtosecond laser-induced periodic surface structures on HgCdTe crystal

In this paper, a femtosecond laser line-scanning irradiation was used to induce the periodic surface microstructure on HgCdTe crystal. Low spatial frequency laser induced periodic surface structures of 650–770nm and high spatial frequency laser induced periodic surface structures of 152–246nm were respectively found with different scanning speeds. The evolution process from low spatial frequency laser induced periodic surface structures to high spatial frequency laser induced periodic surface structures is characterized by scanning electron microscope. Their spatial periods deduced by using a two-dimensional Fourier transformation partly agree with the predictions of the Sipe-Drude theory. Confocal micro-Raman spectral show that the atomic arrangement of induced low spatial frequency laser-induced structures are basically consistent with the crystal in the central area of laser-scanning line, however a new peak at 164cm−1 for the CdTe-like mode becomes evident due to the Hg vaporization when strong laser ablation happens. The obtained surface periodic ripples may have applications in fabricating advanced infrared detector.

The formation of α-phase SnS nanostructure from a hybrid, multi-layered S/Sn/S/Sn/S thin films: Phase stability, surface morphology and optical studies

Single phase of SnS thin film was fabricated from S/Sn/S/Sn/S multilayer prepared by using atmospheric pressure and vacuum thermal evaporation methods Glancing angle high vacuum thermal evaporation technique was employed to grow Sn nanorods which facilitated the sulphur diffusion in a faster manner to prepare SnS nanoparticles. The sulphur deposition temperature, sulphur deposition time and tin deposition time were successfully tailored in the synthesis process and stabilized α-phase SnS by probing through confocal micro-Raman spectrometer. X-ray diffraction confirms the formation of SnS crystal structure at sulphur deposition temperature 200°C. The mechanism of formation of highly porous SnS phase with flower like morphology is explained from the morphological analysis of post deposition annealed film. The complete absence of any oxidation state as evident from Raman as well as EDAX analysis confirms that the proposed sulphurization method could be a suitable, simple and cheap technique for the successful sulphurization of metal films. Band gap calculation from Tauc plot showed a direct band gap value of 1.5eV for films with single phase of SnS which can be used as a p-type absorber layer in thin film solar cells. Emission studies showed the energy transitions attributed to band edge transition and due to the presence of intrinsic defects.

Raman spectroscopy differentiates between sensitive and resistant multiple myeloma cell lines

Current methods for identifying neoplastic cells and discerning them from their normal counterparts are often nonspecific and biologically perturbing. Here, we show that single-cell micro-Raman spectroscopy can be used to discriminate between resistant and sensitive multiple myeloma cell lines based on their highly reproducible biomolecular spectral signatures. In order to demonstrate robustness of the proposed approach, we used two different cell lines of multiple myeloma, namely MM.1S and U266B1, and their counterparts MM.1R and U266/BTZ-R subtypes, resistant to dexamethasone and bortezomib, respectively. Then, micro-Raman spectroscopy provides an easily accurate and noninvasive method for cancer detection for both research and clinical environments. Characteristic peaks, mostly due to different DNA/RNA ratio, nucleic acids, lipids and protein concentrations, allow for discerning the sensitive and resistant subtypes. We also explored principal component analysis (PCA) for resistant cell identification and classification. Sensitive and resistant cells form distinct clusters that can be defined using just two principal components. The identification of drug-resistant cells by confocal micro-Raman spectroscopy is thus proposed as a clinical tool to assess the development of resistance to glucocorticoids and proteasome inhibitors in myeloma cells.

Optical-lattice-like waveguide structures in Ti:Sapphire by femtosecond laser inscription for beam splitting

In this work, we report on the fabrication of deeply embedded optical-lattice-like structures in a Ti:Sapphire crystal by applying femtosecond laser inscription (FLI) to implement two-dimensional (2D) one-to-two and three-dimensional (3D) one-to-four beam splitting. Such a family of photonic microstructures is characterized at near-infrared both experimentally and numerically, showing excellent capability of simultaneous light confinement and beam tailoring at two orthogonal polarizations. The confocal micro-Raman image of the obtained structure reveals that the optical properties of the substrate have been well-preserved in the waveguide’s active volumes. Our results pave a way to construct complex integrated waveguide splitters in Ti:Sapphire crystals by using FLI for photonic applications.

Graphene-Based “Core-Shell” Hierarchical Nanostructured Low-Pt Electrocatalysts for Proton Exchange Membrane Fuel Cells

The operation of proton exchange fuel cells (PEMFCs) is bottlenecked by the sluggishness of the oxygen reduction reaction (ORR) [1]. Accordingly, the development of advanced electrocatalysts (ECs) capable to promote the ORR kinetics is one of the main goals of the research. It is further highlighted that, as of today, the only ORR ECs capable to provide PEMFCs with a performance level compatible with applications require a high loading of strategic elements such as platinum-group metals (PGMs), raising critical issues associated with supply shortages and high costs [1]. This work addresses the above points by the development of innovative ECs characterized by the following features: (i) a low loading of PGMs; (ii) an improved ORR activity in comparison with conventional state-of-the-art ECs [2]; (iii) a “core-shell” morphology. In the proposed ECs the “core” support exhibits a hierarchical structure including the following constituents: (i) graphene flakes, to exploit the benefits associated with the large specific surface area and high electron mobility of graphene [3-6]; (ii) carbon black nanoparticles, to further promote the mass and charge transfer processes of the ECs; and (iii) copper nanoparticles, which are introduced as a sacrificial component modulating the EC morphology and the chemical composition of ORR active sites. The hierarchical “core” support is covered by a carbon nitride “shell”, providing “coordination nests” that embed the ORR active sites [7]. The latter are based on a very low loading of Pt (ca. 3 wt% of the EC) and also include Ni and Cu as “co-catalysts”. The proposed L-PGM ECs are obtained customizing the synthetic protocol devised in our research group [7]. In this work, the final ECs are obtained after a post-synthesis activation process carried out by electrochemical cycling, that plays a crucial role to modulate the physicochemical properties and the morphology. Preliminary results indicate that the proposed approach is promising, as the proposed L-PGM ECs exhibit an improved specific and mass activity in comparison with the state of the art (see Figure). The assay of the metals in the L-PGM ECs is evaluated by inductively-coupled plasma atomic emission spectroscopy (ICP-AES). Vibrational spectroscopies (e.g., confocal micro-Raman) and wide-angle X-ray diffraction (WAXD) are adopted to probe the structure. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM), both conventional and at high resolution, are used to study the morphology. Cyclic voltammetry with the rotating ring-disk electrode method (CV-TF-RRDE) investigates the electrochemical performance and ORR reaction mechanism. Finally, the fuel cell performance in operating conditions is tested on PEMFC prototypes including the proposed L-PGM ECs at the cathode. Acknowledgements This work was funded by the Strategic Project of the University of Padova “From Materials for Membrane-Electrode Assemblies to Energy Conversion and Storage Devices – MAESTRA”. The research leading to these results has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement n°696656. REFERENCES [1] I. Katsounaros, S. Cherevko, A. R. Zeradjanin, K. J. J. Mayrhofer, Angew. Chem. Int. Ed., 53, 102 (2014). [2] J. Zhang, Front. Energy, 5, 137 (2011). [3] S. Sharma, B. G. Pollet, J. Power Sources, 208, 96 (2012). [4] M. Liu, R. Zhang, W. Chen, Chem. Rev., 114, 5117 (2014). [5] A. C. Ferrari, F. Bonaccorso, V. Fal’ko et al., Nanoscale, 7, 4587 (2015). [6] J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer, Nature Nanotech., 3, 206 (2008). [7] V. Di Noto, E. Negro, K. Vezzù, F. Bertasi, G. Nawn, The Electrochemical Society Interface, Summer 2015, 59 (2015). Figure 1

Bi2Te3 thin hexagonal nanoplatelets: Synthesis and its characterization studies

Solvothermal synthesis and optimization of pure Bismuth telluride (Bi2Te3) hexagonal nanoplatelets was carried out from Bismuth Oxide (Bi2O3) and Tellurium dioxide (TeO2). XRD measurements revealed a sensitive change in crystallization behaviour in correlation with variation in Te/Bi stoichiometry identified through the exchange in intensities between (1010̅) and (110) peaks. Further, Energy Dispersive X-ray (EDAX) analysis revealed the variation in Te/Bi ratio with respect to autoclave temperature. Field emission scanning electron Microscope (FESEM) and the high resolution transmission electron Microscope (HRTEM) studies show the complete growth of hexagonal nanoplatelets at 200°C. Confocal Micro-Raman measurements revealed the occurrence of symmetry breaking in the synthesized hexagonal nanoplatelets. The electrical conductivity and the activation energy were recorded as 6.01×10−3S/m and 0.042eV respectively. Highest maximum absolute value of Seebeck coefficient of –355μV/K was obtained for the hexagonal nanoplatelets.