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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

Oxygen evolution (OER) and oxygen reduction (ORR) reactions are the fundamental processes occurring at the air electrode of metal-air batteries and fuel cells. Presently, large overpotentials are required for these reactions to occur at an appreciable rate. Therefore, development of catalysts capable of reducing the activation losses at the oxygen electrode is highly desirable. Perovskite oxides are one of the most interesting low-cost electrocatalysts for air electrodes, because they can adopt a wide variety of chemical compositions and crystal structures, thereby enabling correlative studies between the electrocatalytic activity and their electronic structure (1, 2). Recently, we reported a possible relation between the cell volume of La0.6Sr0.4Fe0.6Mn0.4O3-δ perovskites and their ORR activity and selectivity (2 e- vs ‘’2+2’’ e- pathway), with lower cell volumes corresponding to higher number of exchanged electrons (3). It was also reported by others that some structural features in the perovskite influence the activity of these mixed oxides for both ORR and OER (4-6). In this presentation, we will discuss our recent studies on the ORR and OER activity of a series of La0.5Sr0.5Co0.8Fe0.2O3- δ electrocatalysts prepared by solution combustion synthesis. The choice of diverse experimental conditions allowed us to tune oxygen deficiency, microstrain and other structural properties. As exemplified in Figure 1, for one of the investigated electrocatalysts, thermal annealing in argon reduces the bond angle between the oxygen atom and the two B-site metal cations, resulting in a higher electrocatalytic activity for both OER and ORR. In addition to structure–electrocatalytic activity correlations, we will complement our comparative studies with analytical scanning electron microscopy and spatially resolved elemental mapping of selected electrode surfaces subjected to accelerated stability tests. Figure 1. Crystal structures of La0.5Sr0.5Co0.8Fe0.2O3- δ prepared from solution combustion using citrate as fuel and thermal annealed in Ar (top right) and in Air (bottom left). Differences in the Co-O-Co bond angle are highlighted. Oxygen evolution (OER, top left) and oxygen reduction (ORR, bottom right) polarization curves recorded in 0.1 M KOH at 5 mVs-1 and 1600 rpm. References 1. J. Suntivich, H. A. Gasteiger, N. Yabuuchi, H. Nakanishi, J. B. Goodenough and Y. Shao-Horn, Nat Chem, 3, 546 (2011). 2. J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough and Y. Shao-Horn, Science, 334, 1383 (2011). 3. F. Deganello, D. N. Oko, M. L. Testa, V. La Parola, M. L. Tummino, C. O. Soares, J. G. Rivera, G. Orozco, D. Guay and A. C. Tavares, ACS Applied Energy Materials, 1, 2565 (2018). 4. Y. Sun, Z. Liu, W. Zhang, X. Chu, Y. Cong, K. Huang and S. Feng, Small, 0, 1803513. 5. X. Xu, Y. Chen, W. Zhou, Y. Zhong, D. Guan and Z. Shao, Advanced Materials Interfaces, 5, 1701693 (2018). 6. W. Zhou and J. Sunarso, The Journal of Physical Chemistry Letters, 4, 2982 (2013). Figure 1

With unique physical and chemical properties, and high potential for many important applications, nanomaterials have attracted extensive attention in the past two decades. For instance, due to their unique, size- and shape-tunable surface plasmon resonance, plasmonic nanostructures have recently been explored for increasing the efficiency of solar cells and photocatalysis. Essentially they enhance and/or broaden solar photon harvesting via improved light scattering, strong near field effect and/or hot electron injection. Combination of different nanomaterials into a single architecture leads to greatly enhanced properties, or even better, promising, multifunctional nanomaterials. In this talk, I will present our recent work on the synthesis of hybrid nanomaterials (the assemblies of different types of nanoscale materials) as well as their applications in smart windows, solar cells, solar fuel, photocatalysis, etc. [1-8]. Rational design of hybrid nanomaterials, which is the key to maximize the benefits from respective nano-components, is highlighted. References: 1. Am. Chem. Soc., 2013, 135, 9616; 2. Adv. Energy Mater. 2018, 1703658; 3. Adv. Funct. Mater. 2018, 1706235; 4. ACS Catalysis, 2017, 7, 6225; 5. Adv. Funct. Mater, 2015, 25, 2950; 6. Adv. Funct. Mater. 2015, 25, 6650; 7. Adv. Funct. Mater., 2012, 22, 3914; 8. Adv. Mater., 2012, 24, 6289.