Role of crystallinity on the optical properties of Na2V6O16·3H2O nanowires
Introduction
One-dimensional nanostructured compounds of alkali-metal vanadium oxide have attracted the interest of many researchers due to their several range of applications [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. More specifically, due to their structural and morphologic characteristics, the Na2V6O16·nH2O nanostructures can be potentially applied as anode materials [7], sensors [5], photocatalysts [8], among others [9], [10], [11], once they have a typical layered structure that permits hydrated sodium to be located at the interstices between V3O8 layers, which can then be called interstitial hydrated sodium (Na+) [7], [8]. For example, Zhang et al. [5] reported that gas sensing properties to alcohols and acetones present a better sensitivity when compared to V2O5 compounds. Feng et al. [8] showed that Na2V6O16·3H2O nanoribbons present a promising photocatalytic activity for renewable hydrocarbon fuel (CH4) in the presence of water vapor under visible-light irradiation.
Due to their interesting properties, these compounds have been obtained by several methods [1], [2], [4], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Nevertheless, our group previously reported a promising procedure to obtain Na2V6O16·3H2O nanowires using an environmentally-friendly, one-step low temperature hydrothermal route [18]. This procedure has been considered very attractive due to its relative simplicity and the absence of organic or inorganic additives acting as templates and/or catalysts to the reaction system, enabling it to be free from undesired impurities. However, one aspect often neglected in this synthetic method is the habilitation of crystallinity control, which is a key parameter for many properties – specifically for optical properties, this parameter is very influent, although it has not been fully studied for sodium vanadates, leading to a lack of information about this subject.
Therefore, in this present study we report a correlation between structural and optical properties for the as-obtained Na2V6O16·3H2O 1D nanostructures. The morphology and structure of the samples were examined by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray absorption near-edge spectroscopy (XANES), and electron paramagnetic resonance (EPR) spectroscopy. The optical properties of the samples were also studied by UV–Visible diffuse reflectance (DRS) and photoluminescence (PL) spectroscopies.
Section snippets
Materials and methods
The samples were prepared by hydrothermal conditions at different treatment temperatures previously described in detail, see Ref. [18]. As reported earlier, the samples obtained by hydrothermal treatment at 200 °C exhibit similar structures to those treated at 140 °C, both during 24hrs [18]. In this sense, only the results relative to the sample treated at 140 °C (called SAM01) and to the one at 200 °C (called SAM02), both under extreme conditions, were discussed.
The crystalline structure was
Results and discussions
In a previous work [18], we reported that the samples obtained at different temperature treatments reveal similar structural results, observed by XRD patterns and shown in Fig. 1. The XRD patterns for these samples, SAM01 and SAM02, confirm the formation of the Na2V6O16·3H2O monoclinic phase, with lattice parameters: a = 12.17 Å, b = 3.602 Å and c = 7.78 Å, with β = 95.03° (JCPDS file no 16-0601). Nevertheless, from Fig. 1 we can observe small peaks at around 20.9 and 27.40 (both peaks
Conclusions
In summary, this work aimed to study the structural, spectroscopic and electronic properties of the Na2V6O16·3H2O monoclinic phase composed of nanowires and synthesized by a simple, environmentally-friendly hydrothermal method. The formed Na2V6O16·3H2O at the highest temperature studied could emit intense light at room temperature from 2.4 eV to 3.2 eV, i.e., from 520 nm to 390 nm. According to the results, high PL intensities in the sample synthesized at the highest temperature are related to
Acknowledgements
The authors gratefully acknowledge the financial support of the Brazilian research funding agencies: FAPESP (2013/17639-4), FAPEG (201200555800717 and 201510267000184), CAPES (88881.121134/2016-01) and CNPq (310863/2014-7, 454438/2014-1, 407966/2013-7 and 460089/2014-5). XANES measurements facilities were provided by LNLS-Campinas, SP, Brazil (research proposal number 10972). HRTEM/TEM facilities were provided by LCE (Department of Materials Science, UFSCar, São Carlos, SP, Brazil).
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