Niobium Pentoxide Nanoparticles and Their Self-Assembled in Building Blocks for Gas Sensors
Authors: Kammoun Hela , Yurtsever Aycan , Lanza Marcos Roberto Vasconcelos , Tavares Ana C 
Published: May 2020

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