An Experimental and Computational Study of β-AgVO3: Optical Properties and Formation of Ag Nanoparticles

This article aims to gather together in one place and for first time the formation process of Ag nanoparticles (NPs) on β-AgVO3 crystals, driven by an accelerated electron beam from an electronic microscope under high vacuum. Synthesis and optical properties of β-AgVO3 are reported, and the relationship between structural disorder and photoluminescence emissions is discussed. First principle calculations, within a QTAIM framework, have been carried out to provide a deeper insight and understanding of the observed nucleation and early evolution of Ag nanoparticles (NPs) on β-AgVO3 crystals. The Ag nucleation and formation is a result of structural and electronic changes of the [AgO5] and [AgO6] clusters, consistent with Ag metallic formation.


INTRODUCTION
In the past years, materials based on silver vanadium oxide, such as AgVO 3, have attracted much interest owing to their technological applications in areas such as sensors, electrical and antibacterial agents, implantable medical devices, and photocatalysts [1][2][3][4][5][6][7][8][9][10][11][12][13][14] . This material is present in two stable phases, namely, α-AgVO 3 and β-AgVO 3 1, 15, 16 , both having a monoclinic structure, and α-AgVO 3 can be irreversibly transformed to β-AgVO 3 at around 200 °C 16 . Our group are engaged in a research project devoted to the study of an unwanted realtime in situ nucleation and growth processes of Ag NPs on different silver based semiconductors such as α-Ag 2 WO 4 19 , β-Ag 2 WO 4 20 , β-Ag 2 MoO 4 21, 22 , and Ag 3 PO 4 23 , which were driven by accelerated electron beam irradiation from an electron microscope under high vacuum. The reasons for this phenomena have been discussed in recent publications 19,22,24,25 , and the production of Ag NPs on α-Ag 2 WO 4 19, 26-29 , β-Ag 2 MoO 4 30 , and Ag 3 PO 4 23 resulted in interesting applications as sensors, photoluminescent materials, visible-light photocatalysts, and bactericide materials. In this work, we will report, discuss and analyze, for first time, the nucleation process and early evolution of Ag nanoparticles on β-AgVO 3 crystals, provoked by an electron beam, by means of the joint use of an experimental and theoretical studies.
In this work, a combined theoretical and experimental study on β-AgVO 3 has been carried out. The powders have been synthesized by a precipitation method (PM) at 30, 60, and 90 °C and were characterized using X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) measurements. Ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectroscopy measurements at room temperature were carried out to verify the correlation between the optical properties and the structural order-disorder effects.
Calculations, based on density functional theory (DFT), were performed to understand the physical phenomena involved in the nucleation process and early stages of metallic Ag NPs formation on the surface of β-AgVO 3 , driven by an accelerated electron beam from an electronic microscope under high vacuum The paper is organized as follows: section 2 describes the experimental procedure (synthesis and characterization) and the theoretical method whereas section 3 consists of results and discussion on the structure and optical properties of β-AgVO 3 , as well as we discuss our results to understand the formation Ag NPs on β-AgVO 3 crystals. Finally, we summarize our main conclusions in section 4. Raman spectroscopy measurements were carried out using a T64000 spectrometer (Horiba Jobin-Yvon, Japan) coupled to a CCD Synapse detector and an argon-ion laser, operating at 514 nm with a maximum power of 7 mW. The spectra were measured in the 100 cm -1 -1100 cm -1 range. UV-vis spectra were obtained using a Varian  Table 1, in which the statistic fitting parameters (R wp and GOF) indicate the quality of structural refinement data is acceptable. Significant changes in the lattice parameters and unit cell density were not found in these samples with the syntheses temperature, which are in good agreement with those published in the literature 31 .
We performed geometric optimization of the crystal structure by using means of DFT calculations. Graphical representation of the β-AgVO 3 structure using polyhedra is presented in Figure 2a Table S1 in the supplementary information.
We could identify fifteen Raman-active modes experimentally.   (1) for any wavelength is described as: where F(R ∞ ) is the Kubelka-Munk function or absolute reflectance of the sample, R ∞ = R sample /R MgO (R ∞ is the reflectance when the sample is infinitely thick), k is the molar absorption coefficient and s is the scattering coefficient 47 . In a parabolic band structure, the optical band gap and absorption coefficient of semiconductor oxides can be calculated by the following equation (2): where α is the linear absorption coefficient of the material, hν is the photon energy, C 1 is a proportionality constant, E gap is the optical band gap and n is a constant associated with the different kinds of electronic transitions (n = 0.5 for a direct allowed, n = 2 for an indirect allowed, n = 1.5 for a direct forbidden and n = 3 for an indirect forbidden).
Based on this theoretical information, the E gap values of our metastable β-AgVO 3 microcrystals were calculated using n = 2 in equation (2). Finally, using the equation (1) and the term k = 2α and C 2 as proportionality constant, is obtained the modified Kubelka-Munk equation as indicated in equation (3): Therefore, finding the F(R ∞ ) value from equation (3) 49 . According to Rietveld refinement (see Table 1) the sample obtained at 90 °C has a degree of crystallinity higher than samples obtained at 30 and 60°C. It is well stablished that there is a dependence between the E g values with the percentage of amorphous phase 50 . An important feature of the amorphous semiconductor is the existence of defects, i.e. dangling bonds, which are responsible for the formation of some defects in the band structure 48,51 . Generally, when a material with defects is submitted to the heat treatment or at higher temperatures syntheses provokes the presence a crystals lattice more organized, due the reduction of structural defects, oxygen vacancies, decreasing the concentration of intermediary electronic states within band gap and therefore a decreasing the E g value 52,53 .
Besides that, it is known that quantum confinement in semiconductor NPs increase the bandgap energy 54 . An analysis of the results of Table 1  confirmed that the electrons beam promoted the random growth of the metallic Ag NPs, since regions with high intensity Ag peaks and no Ag peaks in the EDS spectra are present. Carbon and copper atoms are observed in all the EDS analyses, which could be arising from the 300 mesh Cu grids used in the TEM. We also measured the interplanar distance of an Ag particle grown on the surface of β-AgVO 3 from regions 3 and 4. The  Figure 10. Since silver vanadate is an n-type semiconductor, an n/p interface is formed in this region. This interface increases the polarization and consequently, electron/hole recombination becomes more difficult.

<Table 2>
There are two types of [AgO 5 ] clusters centered by Ag2 and Ag3 atoms (see Figure   2b), both exhibiting similar bond distances and Table 2   and [AgO 6 ] clusters, consistent with metallic Ag formation.

SUPPORTING INFORMATION DESCRIPTION
The Table S1, in the supplementary information, presents a comparison between the experimental and calculated values of the Raman-active modes and those reported in the literature. Figure S1 shows TEM images of β-AgVO 3 powders obtained before and after 5 min exposure to the electron beam (accelerated at 10 kV) for the samples synthesized at 30 and 60 °C by PM. To verify the growth of metallic Ag NPs on β-AgVO 3 , an EDS system coupled with a TEM microscope was used for analyzing the samples, enabling a local elemental analysis on each individual β-AgVO 3 microparticles, Distinct regions in the focused β-AgVO 3 microparticles were selected for examination.