A thin film with phosphotungstate nanoclusters: an

01A thin film with phosphotungstate nanoclusters: an alternative to materials scientists investigate the electroreduction of hexazinone

Victória de Oliveira MargaridoUFSCar

Pedro Henrique de Paulo OlívioUFSCar

Leonardo Aparecido CorreiaUFSCar

Julia Helena de PaulaUFSCar

Kelly Roberta FranciscoUFSCar

Adriano Lopes de SouzaUFSCar

10.37885/210805624

https://dx.doi.org/10.37885/210805624

Ciência e Engenharia de Materiais: conceitos, fundamentos e aplicação

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Keywords: sol-gel Process, Ormosil, Hexazinone, Hybrid Film, Phosphotungstic Acid.

ABSTRACT

In the work reported here, we investigated the surface topography and the electrocataly-tical ability toward hexazinone herbicide of a hybrid organically modified silicate (ormosil) film using (3-aminopropyl)triethoxysilane and phosphotungstic acid (HPW), a polyoxo-metalate (POM) nanocluster. The morphology of the film on gold electrode was studied with a recent surface analysis technique based on atomic force microscopy coupled with infrared absorption spectroscopy. This hybrid ormosil film showed nanopores distributed throughout the surface and the interaction between HPW clusters and amine groups was electrostatic. The electrocatalytic reduction of hexazinone occurred at −0.20 V vs. Ag/AgCl in acidic medium pH 0.80 as shown by cyclic voltammetry analyses. Solid state 29Si nuclear magnetic resonance analyses showed that the distribution of silicate groups in the ormosil is formed by Q3 and Q4 species. Since a sensor for hexazinone was not aimed here, this system studied can contribute to the improvement of useful materials in environmental science research.


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INTRODUCTION

The sol gel process is a versatile method to produce hybrid organic-inorganic materials, the applications of which have gained prominence in the field of molecular nanotechnology (SANCHEZ et al., 2013; YUAN et al., 2016). With respect to the production of thin films, the types of deposition commonly used to successfully obtain hybrid nanocomposite mate-rials are layer-by-layer, spray drying, spin coating and dip coating (SANCHEZ et al., 2013). Alkoxysilanes can be used as an alternative in the sol gel route to prepare hybrid materials through two steps. The first step involves a hydrolysis reaction of the alkoxysilane to produce silanol species, which subsequently react with each other to create siloxane derivatives in a silica or silicate network. When an alkoxysilane with a specific organic functionality is used as a precursor in the sol gel route, a porous modified silica network called ormosil (organically modified silicate) is produced. Transition metal oxides, organic molecules and coordination compounds can be immobilized within the pores of an ormosil network to obtain hybrid mate-rials with applications in sensors, biomaterials and photochromic systems (CALDARA et al., 2016; CATAURO; CIPRIOTI, 2021; PARDO; ZAYAT; LEVY, 2011).

Hybrid nanostructures have also been built using polyoxometalates (POMs), which are metal oxide clusters formed by V, Mo or W, employed in applications such as catalysis, photo- and electrochromic devices, batteries, and magnetism (HORN et al., 2021; MA et al., 2021; SONG; TSUNASHIMA, 2012; WALSH et al., 2016). POMs are able to undergo electron transfer reactions while keeping their structures (HISKIA; MYLONAS; PAPACONSTANTINOU, 2001). This feature makes them effective electrocatalysts toward molecules such as hydrogen peroxide, nitrite and bromate (WANG et al., 2020).

Although the electrocatalytic ability of hybrid systems containing POMs has been explo-red extensively, there are few studies available in the literature reporting the electrocatalytic role of these systems toward herbicides (OLÍVIO et al., 2018; SOUZA et al., 2014), which are employed in agriculture to control weeds (BAILLIE et al., 2015). The main consequence of using herbicides is the contamination of watercourses as observed in the use of hexazinone herbicide (ZULKIFLI; RAHIM; LAU, 2018). Hexazinone has been quantified using high per-formance liquid chromatography with photodiode array detector (SCHEEL; TARLEY, 2017). Triazinic herbicides have been detected using molecularly imprinted polymers (ZHOU et al., 2016). Figure 1 shows the chemical structure of hexazinone.


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Figure 1. Chemical structure of hexazinone.

Source: The authors.

This paper describes the preparation and characterization of a hybrid ormosil film con-taining (3-aminopropyl)triethoxysilane (APTS) and phosphotungstic acid (HPW), H3PW12O40. nH2O, a Keggin-type POM. The Keggin structure of POMs is a tetrahedral with the formula XO4, where X can be PV, Si or AsV, surrounded by octahedrons arranged in four M3O13 groups where M can be V, Mo or W. Infrared absorption spectroscopy and nuclear magnetic resonan-ce spectroscopy were used to study this hybrid ormosil in the powder form. The morphological aspects of the hybrid ormosil film were investigated using atomic force microscopy coupled with infrared absorption spectroscopy, and its electrocatalytic action toward hexazinone was explored. The purpose of this study was not to develop a sensor for hexazinone, but to explore the electrocatalytical ability of the hybrid film toward this herbicide.

METHOD

Chemicals

Phosphotungstic acid hydrate, (3-aminopropyl)triethoxysilane, tetraethyl orthosilicate, potassium hexacyanoferrate(II) trihydrate, potassium chloride and hexazinone PESTANAL were purchased from Sigma-Aldrich. Acetone and sulfuric acid were purchased from Qhemis. All chemicals were used as received. Commercial nitrogen gas was purchased from White Martins. The deionized water used had a resistivity of 18.2 MΩ cm.

Ormosil and Film Preparations

This procedure was similar to described by (OLÍVIO et al., 2018). In a polypropylene beaker, 25 mL of acetone, 0.25 mmol (59 µL) of (3-aminopropyl)triethoxysilane (APTS), 0.26 mmol (58 µL) of tetraethyl orthosilicate (TEOS), 0.78 mmol (14 µL) of deionized water and 100 µL of an aqueous solution of sulfuric acid (pH 0.8) were mixed and stirred for 15 minutes. Afterwards, 0.040 mmol (115 mg) of phosphotungstic acid hydrate (HPW) dissolved in 25


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mL of acetone was added to the mixture. The stirring was maintained for an additional 15 minutes and the final solution was used to prepare the ormosil film.

Slides of glass coated with gold 3 cm × 1 cm in size were used as substrates. Ormosil films were prepared using a dip coater elevator from Nadetech Innovations. The slide was immersed and removed from the ormosil solution at a speed of 150 mm min–1. When the slide was fully immersed in the solution, it was immediately withdrawn resulting in an immersion time near to zero. This cycle was repeated 20 times and after each withdrawal the slide was kept in the air for 60 s to dry.

Spectroscopic and Morphological Analyses

Fourier transform infrared (FTIR) spectra of the hybrid ormosil powder sample were obtained using KBr pellets with a Bruker spectrometer. 29Si solid state nuclear magnetic reso-nance experiments were performed in a Bruker spectrometer with magnetic field of 9.4 T. The frequency used in the magic angle spinning assays was 15 kHz. Atomic force microscopy coupled with infrared absorption spectroscopy (AFM-IR) measurements were performed on a nanoIR2 system from Anasys Instruments in contact mode using a gold-plated silicon nitride probe, at CNPEM - LNNano (Brazil). IR spectra were obtained in the range of 1530−1850 cm–1 with a wavelength resolution of 4 cm–1. Data were analyzed using Gwyddion software. All samples were prepared on a glass surface coated in gold in order to increase the cantile-ver response to the expansion of the sample. All images were acquired at room temperature and at 20% relative humidity.

Electrochemical Experiments

Electrochemical studies were performed in a three-electrode electrochemical cell con-trolled by an Autolab PGSTAT101 potentiostat. The reference electrode was Ag/AgCl, the working electrode was the glass/gold slide modified with the ormosil film and the counter electrode was a platinum plate of 1 cm2. The supporting electrolyte used was an aqueous solution of sulfuric acid at pH 0.8. This solution was previously purged using nitrogen gas for 10 minutes, and a nitrogen environment was then kept in the cell during the measure-ments. The acidic medium used here was necessary to avoid decomposition of the HPW (MCGARVEY; MOFFAT, 1991). All modified glass/gold slides were cycled 100 times in the potential window used (− 0.35 to + 0.30 V vs. Ag/AgCl) before the electrochemical studies. This step was necessary to ensure the stability of the current densities for the hybrid films. The solution of hexazinone was freshly prepared in an aqueous solution of sulfuric acid at pH 0.8 and used immediately.


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RESULTS AND DISCUSSION

Surface Characterization

Figures 2A, 2B and 2C show atomic force microscopy coupled with infrared absorption spectroscopy (AFM-IR) images related to the topology, the chemical map acquired at 1625 cm–1, and the IR spectra obtained at two regions on the surface with spatial resolution ≤ 50 nm, respectively. The topography image (Figure 2A) reveals micro- and nanopores dispersed on a flat surface, with an average value and root mean squared roughness (rms) of 72.63 nm and 4.02 nm, respectively. The resulting chemical image (Figure 2B) suggests a homogeneous covering surface parallel to the surface, in which amine groups are exposed to air. Figure 2C shows the IR spectra with a laser wavelength range of 1530 –1850 cm–1 in different positions on the film surface, regions near to (red line) and far from (black line) a pore, respectively.

The strong absorption around 1610 cm–1 (region near to a pore) and 1625 cm–1 (area far from a pore) is assigned to the asymmetric deformation of -NH3

+ groups. The shift in the amine bands to a lower wavenumber in areas near the pores can be related to interactions between the amine groups and HPW due to hydrogen bonding as well as electrostatic forces, as already found in the surfaces of similar hybrid systems with POMs and amine functionality (OLÍVIO et al., 2018; SOUZA et al., 2012, 2014). Furthermore, the shoulder at 1550 cm–1

observed for the region further from the pores is assigned to the symmetric deformation of -NH3

+ groups. Additionally, the absorption around 1700 cm–1 characteristic of carbonyl groups is probably due to residues of acetone, used as a solvent, present on the film surface.


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Figure 2. AFM-IR images for hybrid ormosil film: A) AFM-IR topography image, B) AFM-IR chemical mapping at 1625 cm–1, C) IR spectrum of regions near (red line) and for (black line) from a porous.


Electrochemical Investigation

Graphic 3 shows a cyclic voltammogram for the hybrid ormosil film containing HPW and APTS at a scan rate of 20 mV s–1. There are two redox processes in the potential win-dow used, with formal potentials (E0´) + 0.032 ± 0.0042 V and −0.18 ± 0.0075 V vs. Ag/AgCl. The E0´ values were obtained using the equation (Epa + Epc)/2, where Epa is the potential of the anodic peak and Epc is the potential of the cathodic peak. According to


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Keita and Nadjo (KEITA; NADJO, 1987), HPW in solution shows five reduction waves at −0.0200 V, −0.275 V, −0.580 V, −0.775 V and −0.850 V, using a saturated calomelane elec-trode (SCE) as the reference electrode and an aqueous solution of 1 mol L–1 perchloric acid as the supporting electrolyte. The first two waves are monoelectronic and the third involves two electrons. The number of electrons in the last two waves has not been obtained. Thus, the redox processes shown in graphic 3 are ascribed to monoelectronic reactions.

This potential window has been used to avoid the appearance of the hydrogen evolution reaction that started at potentials lower than 0.35 V vs. Ag/AgCl in the study reported here (data not shown). Keita and Nadjo (KEITA; NADJO, 1988) observed the hydrogen evolution reaction around −0.2 V vs. SCE in cyclic voltammograms recorded in 0.5 mol L–1 sulfuric acid solution for α1-P2W17Mo immobilized on a glassy carbon electrode. (SUN; CA; COX, 2005) also identified the hydrogen evolution reaction at − 0.4 V vs. Ag/AgCl for HPW and poly(amidoamine) dendrimer film with platinum nanoparticles on indium tin oxide electrodes by cyclic voltammetry analyses in sulfuric acid 0.5 mol L–1.

The E0´ values found here are slightly higher than those found by (KASEM; SCHULTZ, 1995) for HPW entrapped in a polymeric film of ruthenium (II) (vinyl)bipyridine, where the E0´values were −0.20 V and −0.42 V vs. Ag/AgCl. (LIU et al., 2011) found E0´ values of 0.062 V and −0.20 V vs. Ag/AgCl for HPW immobilized in carbon nanotubes grafted with poly(4-vinylpyridine) using a glassy carbon electrode as the working electrode. The inset in graphic 3 shows that the current density (J) increases linearly with the scan rate. This aspect indicates that the HPW is confined to the electrode surface (ROHLFING et al., 2005).

Graphic 3. Cyclic Voltammograms at different scan rates for hybrid ormosil film with HPW on gold electrodes performed in sulfuric acid pH 0.80. Inset: J values for the first redox process in function of scan rates.



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Graphic 4 shows cyclic voltammograms for a bare gold electrode and the hybrid or-mosil film with HPW using 6.0 mmol L–1 K4[Fe(CN)6] and 0.1 mol L–1 KCl as the supporting electrolyte. It can be seen that the permeability of the hybrid ormosil film to Fe(CN)6

4– ions is maintained, as the shape of the cyclic voltammogram for the modified electrode has not been significantly changed. This behavior is evidence that the diffusion of [Fe(CN)6]4– ions through the film was not blocked (ROHLFING et al., 2005). This finding could be related to the existence of pores in the surface of the hybrid film with HPW, as seen by AFM-IR analyses.

Graphic 5 shows cyclic voltammograms performed in sulfuric acid solution (pH 0.8) for a bare gold electrode and for the hybrid ormosil film with HPW on gold with and without 3.8 × 10–5 mol L–1 hexazinone at a scan rate of 20 mV s–1. It is illustrated that for the hybrid ormosil film, the anodic current density decreased while the cathodic current density increased in the presence of hexazinone. This behavior is assigned to the electroreduction of hexazinone showing a higher current density at −0.20 V vs. Ag/AgCl. Recently, hexazinone has been electrochemically detected at pH 2.5 and −0.5 V vs. Ag/AgCl using molecularly imprinted polymers with carbon pastes as working electrodes by differential pulse adsorptive catho-dic stripping voltammetry (TORO; MARESTONI; DEL PILAR TABOADA SOTOMAYOR, 2015). In that study, the potential found is much higher than in our result, indicating that the system studied here is thermodynamically more efficient. A mechanism of electroreduction of hexazinone has been proposed by Privman and Zuman (PRIVMAN; ZUMAN, 1998) over a broad pH range using polarography and cyclic voltammetry analyses combined with con-trolled potential electrolysis and UV-visible spectroscopy. In an acidic medium, the proposed mechanism involved the reduction of the protonated azomethine bond followed by the elimi-nation of an amine. The protonated product of this reaction is further reduced, resulting in a compound free from the double bonds in the ring.


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Graphic 4. Cyclic Voltammograms performed in aqueous solution of KCl 0.10 mol L–1 and K4[Fe(CN)6] 6.0 × 10–3 mol L–1 at scan rate of 20 mV s–1 for bare gold electrode and for hybrid ormosil film with HPW.


Graphic 5. Cyclic Voltammograms performed in sulfuric acid pH 0.80 at scan rate of 20 mV s–1 in the absence and in the presence of hexazinone solution 3.8 × 10–5 mol L–1 for bare gold electrode and for hybrid ormosil film with HPW.


Structural Characterization

Graphic 6A shows the FTIR spectrum, obtained in a KBr pellet, for the hybrid ormosil with HPW in the powder form. The band located at 843 cm–1 is attributed to an asymmetric stretch from the W-O-W bonds of the M3O13 groups. The vibrations located at 941 cm–1 and 1078 cm–1 are ascribed respectively to the asymmetric stretches of the W-Ot bond, where Ot is


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the terminal oxygen atom, and of the P-O bond. The signal at 1127 cm–1 is assigned to the asymmetric vibrations of the Si-O-C and Si-O-Si bonds. The symmetric and asymmetric de-formations of the –NH3

+ groups appear at 1500 cm–1 and 1620 cm–1, respectively (FERREIRA-NETO et al., 2015; ROCCHICCIOLI-DELTCHEFF et al., 1983). This finding shows that the interaction between the HPW and the ormosil network is electrostatic as shown by AFM-IR experiments. Graphic 6B presents the 29Si NMR spectrum for the hybrid ormosil with HPW in the powder form. Information about tetrahedral silicate species (Qn) can be extracted from this, where n is the number of Si-O-Si bonds per tetrahedron. There is a broad signal between −100 ppm to −125 ppm, which could be attributed to the presence of Q3 and Q4 species in the ormosil because these species typically appear at −101.5 ppm and −110.5 ppm, respec-tively (OLÍVIO et al., 2018). In addition, siloxane species containing Si-C bonds were also identified. (SiO)3Si*R and (SiO)2Si*OHR, named as T3 and T2 species, were found at −67 ppm and −58 ppm, respectively.

Graphic 6. Spectroscopic analyses of hybrid ormosil with HPW. A) FTIR spectrum, B) High resolution 29Si spectrum.



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In summary, our experiments demonstrate that this hybrid ormosil film containing HPW and APTS has a porous morphology as indicated through AFM-IR and cyclic voltammetry ex-periments. With regard to electrocatalytic property in acidic medium, a triazinic herbicide was reduced by this film on gold electrodes. Additionally, electrostatic interactions and hydrogen bonding have also been identified between HPW clusters and ormosil matrix. These findings obtained here are beneficial in materials science because can stimulate the advancement of this promising area as well as to improve the knowledge of the researchers.

CONCLUSION

In this study, the hexazinone herbicide was electroreduced at −0.20 V vs. Ag/AgCl using a hybrid ormosil film containing HPW, a Keggin POM cluster, deposited on gold elec-trodes. It is the first work reported on the electrocatalysis of this herbicide using a hybrid film obtained by the sol gel method and this process occurred at lower potential value than that found in the literature. AFM-IR study was capable of identifying the presence of nanopores distributed on the surface of the hybrid ormosil film, corroborating with cyclic voltammograms using [Fe(CN)6]4– ions, while also confirming that HPW clusters interact with amine groups through electrostatic forces. A 29Si NMR spectrum showed the presence of Si-O-Si and Si-C bonds while the FTIR spectrum showed specific vibration stretches for the HPW cluster in addition to the electrostatic forces and hydrogen bonding between the HPW and amine groups from the ormosil. Taken together, these results showed that the hybrid ormosil film with HPW is effective in the electroreduction of hexazinone, and that it could be improved to develop applications related to the environmental field and materials science.

ACKNOWLEDGEMENTS

This work was supported by the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (Grants Numbers: 2015/24136-4 and 2017/20006-4) and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (Grant Number: 427253/2016-0). L.A.C. thanks FAPESP for his scientific initiation fellowship (2016/21603-3), J.H.P. thanks FAPESP for her scientific initiation fellowship (2016/21616-8) and P.H.P.O. thanks ICT sem remuneração and CNPq for his scientific initiation fellowship PIBIC/CNPq/UFSCar. The authors thank the Microfabrication Laboratory (LMF) from the Brazilian Nanotechnology National Laboratory (LNNano) for the production of gold electrodes. We also thank Evandro Martin Lanzoni from LNNano for assistance and helpful discussions with AFM-IR analyses.


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A thin film with phosphotungstate nanoclusters: an