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We report the design and fabrication of a heterojunction photoanode consisting of two-dimensional (2D) functionalized hexagonal boron nitride (F-h-BN) nanoflakes, introduced at the interface between SnO2 and quantum dots (QDs) (e.g. CdS and CdS–CdSe), used as a photoanode in a photoelectrochemical cell (PEC) for hydrogen (H2) generation. We highlight the effect of 2D F-h-BN nanoflakes on the carrier recombination and performance of the PEC system. In addition, the tailoring of SnO2 film thickness and incorporation of multi-walled carbon nanotubes (MWCNTs) were investigated for their effect on carrier dynamics and overall device performance. Our results show that a PEC device based on SnO2/F-h-BN/CdS QDs exhibits a 60% improvement in the saturated photocurrent density (at 1.0 V vs. RHE) compared to the control device, due to improved electron injection and reduced carrier recombination at the metal oxide/QDs/electrolyte interface. The highest saturated photocurrent density value reached 6.35 mA cm−2 (at 1.0 V vs. RHE), after thickness optimization of the SnO2 film, incorporation (0.015 wt%) of MWCNTs, and cascade CdS–CdSe QDs. In addition, the PEC system maintains 98% of the initial value of photocurrent density after four hours of continuous one sun illumination (AM 1.5G, 100 mW cm−2). Our results offer a simple yet cost-effective and efficient strategy to enhance the performance of QD based PEC H2 generation and potentially other optoelectronic devices.

Most of the electrochemical studies are performed with macroscopic, bulk electrodes. The materials of interest are placed on centimeter sized electrodes and the current originating from large macroscopic areas are measured. Although, this traditional approach produces strong signals that are perfect for quantitative analyses it lacks the capability to resolve individual entities or sub-micron structural heterogeneities, that contribute to the electrochemical signal. It averages-out different structures and hence, it becomes impossible to perform in depth studies of chemical reactions occurring at different sites. With modern developments in nanofabrication, simulations and computer aided design, as well as in state of the art potentiostats, the doors were open for the rapid development of a highly active field of research in the last years: The electrochemistry at the nanoscale, which has allowed to achieve electrochemical measurements from submicron entities [1], [2], and it’s leading the way to the development of single molecule electrochemistry. In this work we will present our approach to the study of electrochemical reactions on single entities, which includes the fabrication of very sharp conical tips to be used as working electrodes and to achieve high spatial resolution. Figure A shows a scanning electron microscope image of a tungsten tip prepared by electrochemical etching of a tungsten wire in KOH solution, with a radius of curvature of 100 nm. Two main oxides are formed during the anodic polarization of the tungsten electrodes, passivating their surface [3], [4]. However, cyclic voltammetry studies in acid and neutral electrolytes, revealed a potential window and scan rate where a conductive oxide is formed and thus suitable to be used as electrode material. As a result, the tungsten tips were further characterized electrochemically by cyclic voltammetry in presence of a redox probe and as a function of the exposed surface area in the electrolyte solution. An electrochemical response characteristic of ultramicroelectrodes was recorded for tip lengths below 70 µm, Figure B. It was also found that the electroactive area is higher than the geometric one [5], Figure C. The contribution of the roughness of the electrode, the meniscus effect and a possible field enhancement due to the geometry of the electrodes to the electrochemical surface area will be quantified. This work represents thus, a step ahead towards a better understanding of the electrochemical processes in increasingly smaller structures to eventually reach the ultimate goal of the use of nanometer-sized electrodes: single molecule electrochemistry. Acknowledgements To Consejo Nacional de Ciencia y Tecnología (CONACYT) for the financial support with the scholarship 739820 for graduate studies abroad. References [1] L. A. Baker, “Perspective and Prospectus on Single-Entity Electrochemistry,” J. Am. Chem. Soc., vol. 140, pp. 15549–15559, 2018. [2] Y. Wang, X. Shan, and N. Tao, “Emerging tools for studying single entity electrochemistry,” Faraday Discuss., vol. 193, pp. 9–39, 2016. [3] M. Anik and K. Osseo-Asare, “Effect of pH on the anodic behavior of tungsten,” J. Electrochem. Soc., vol. 149, no. 6, 2002. [4] M. Anik, “pH-dependent anodic reaction behavior of tungsten in acidic phosphate solutions,” Electrochim. Acta, vol. 54, no. 15, pp. 3943–3951, 2009. [5] C. G. Zoski and M. V. Mirkin, “Steady-state limiting currents at finite conical microelectrodes,” Anal. Chem., vol. 74, no. 9, pp. 1986–1992, 2002. Figure 1

Niobium pentoxide (Nb2O5) is characterized by an outstanding chemical stability, high corrosion resistance and color changing chemistry. Nb2O5 can adopt different crystal structures (pseudohexagonal, orthorhombic and monoclinic), depending on the temperature [1]. Still, it is one of the least studied metal oxides. [2], [3]. Thus, exploring new synthetic methods and the physicochemical properties of the resulting oxide nanoparticles (NPs) could open new opportunities in different applications such as electrocatalysis, electrochromic displays, Surface-enhanced Raman spectroscopy and gas, humidity, and biological/chemical sensing. For gas sensing applications, nanostructured individual NPs of Nb2O5 with uniform shape, narrow size distribution along with high surface area are required to facilitate the adsorption of the target gas molecules. [1], [3], [4], [5]. Hydrothermal and solvothermal methods are widely used to produce metal oxides NPs with various morphologies. [1] These methods are easily scaled up and produce materials with high purity. Due to the high autogenous pressure inside the reactor, the nanoparticles are usually crystalline; and depending on the temperature and the time of reaction, the crystal phase can be controlled. However, the control of the size and the shape of the NPs with hydrothermal synthesis is a challenge because after nucleation, the nuclei precipitate and agglomerate forming microstructures with ill-defined shapes. [3] ,[5] In this work, we report the synthesis of spherical-like niobium oxide nanoparticles by one-pot hydrothermal synthesis using as a precursor ammonium niobium oxalate aqueous solution, Figures 1.A and 1.B. The kinetics of the NPs growth was investigated by Dynamic Light Scattering (DLS), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). Figure 1.B. shows that the Nb2O5 NP grow according to the Oswald ripening mechanism, with the size of the nanoparticles increasing from 2 nm to 80 nm with the reaction time. However, the nanoparticles tend to coalesce forming larger nanostructures with uncontrollable size landing to unstable suspensions. A comparative study between different charged ligands, revealed citric acid as the best ligand to ensure the stability of the Nb2O5 suspensions and to control the nanoparticles’ size. Indeed, using citric acid the size of the size of the aggregates in the suspensions is reduced by the factor of 10, from 200 to around 20 nm of diameter. The control of the size and shape of the NPs was found to be critical to form 3D superlattices and to maximize the surface area once the nanoparticles form thin films, Figure 1.C. The kinetic of the NPs growth and the physicochemical properties of the NPs and films will be discussed in detail. Acknowledgements The authors would like to thank the funding from NSERC (Strategic partnership program, Canada). References [1] YD Wang, LF Yang, ZL Zhou, YF Li, XH Wu. Materials Letters, 49 (5), 277-281 (2001). [2] I. C. M. S. Santos, L. H. Loureiro, M. F. P. Silva and A. M. V. Cavaleiro, Polyhedron. 21,2009-42015 (2002). [3] J.F. Carneiro, M.J. Paulo, M. Siaj, A.C. Tavares, and M.R.V. Lanza, J. Catal. 332, 51–61 (2015). [4] A. K.Rosmalini, A. R. Rozina, M. M. Y. A. Alsaif, & al, ACS Appl. Mater. Interfaces, 7, 8, 4751-4758 (2015). [5] J. A. Darr, J. Zhang, N. M. Makwana, X. Weng, Chem. Rev., 117, 17, 11125-11238 (2017). Figure 1

Plasmon Probe When light of certain wavelengths strikes a metal surface, it sets the metal's electrons in motion along trajectories termed “plasmon modes.” Yurtsever et al. (p. 59 ; see the Perspective by Batson ) constructed a type of electron microscope that can probe the electric fields associated with this process by measuring the energy of a separate pulse of electrons that is bounced off the surface immediately (less than trillionths of a second) after the light strikes. The spatial resolution was sufficient to map intensities in distinct regions of a single silver nanoparticle.

Converging on Dynamics Electron diffraction is a versatile technique for discerning atomic-level structure, but the data emerge averaged over the micron–scale area sampled by the electrons, and so blur local distinctions in systems that aren't strictly periodic. A recent approach to minimizing this problem has been to focus the electron beam impinging on the sample. Yurtsever and Zewail (p. 708 ) have now applied convergent focusing to an ultrafast electron diffraction apparatus and were thus able to resolve picosecond structural dynamics in local regions tens of nanometers across. The technique was used to probe heterogeneous temperature changes in laser-heated silicon.

Characteristic energies of photonic modes are a sensitive function of a nanostructures’ geometrical parameters. In the case of translationally invariant planar waveguides, the eigen-energies reside in the infrared to ultraviolet parts of the optical spectrum and they sensitively depend on the thickness of the waveguide. Using swift electrons and the inherent Cherenkov radiation in dielectrics, the energies of such photonic states can be effectively probed via monochromated electron energy-loss spectroscopy (EELS). Here, by exploiting the strong photonic signals in EELS with 200 keV electrons, we correlate the energies of waveguide peaks in the 0.5–3.5 eV range with planar thicknesses of the samples. This procedure enables us to measure the thicknesses of cross-sectional transmission electron microscopy samples over a 1–500 nm range and with best-case accuracies below ±2%. The measurements are absolute with the only requirement being the optical dielectric function of the material. Furthermore, we provide empirical formulation for rapid and direct thickness estimations for a 50–500 nm range. We demonstrate the methodology for two semiconducting materials, silicon and gallium arsenide, and discuss how it can be applied to other dielectrics that produce strong optical fingerprints in EELS. The asymptotic form of the loss function for two-dimensional materials is also discussed.

We use a monochromated 200 keV electron beam as a nanometer-resolution probe of the photonic density of states in bulk and nanoparticle $\mathrm{Si}/{\mathrm{SiO}}_{2}$ systems, observing infrared to ultraviolet waveguided Cherenkov modes in Si slabs, but none in ${\mathrm{SiO}}_{2}$. While isolated Si nanoparticles are too small to support Cherenkov radiation, we find high densities of Si nanoparticles in ${\mathrm{SiO}}_{2}$ support a damped form of the radiation, with the modes determined by the effective medium of the $\mathrm{Si}/{\mathrm{SiO}}_{2}$ mixture. The guided nature of the radiation is confirmed by the presence of a thickness-dependent cutoff.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Silicon nanoparticles embedded in silica show promising optoelectronic properties, due to quantum confinement and/or radiative interface states that should correlate with the particles’ average size and shape. Here the authors report the combination of electron tomography with plasmon-filtered microscopy in order to reconstruct the three-dimensional morphology of silicon nanoparticles. They find that particles with complex morphologies and high surface to volume ratios are dominant, rather than the commonly assumed near-spherical structures. These results should affect quantum-confined excitons and the interface density of states. Their findings may help to explain the physical origin of the unusually broad photoluminescence bands and efficiencies.