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Direct, one-step synthesis of molybdenum blue using an electrochemical method, and characterization studies
Synthetic Metals 233 (2017) 111–118
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Direct, one-step synthesis of molybdenum blue using an electrochemical method, and characterization studies Ozge Koyun, Semih Gorduk, Melih Besir Arvas, Yucel Sahin
MARK
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Department of Chemistry, Faculty of Arts & Science, Yildiz Technical University, TR34210 Istanbul, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyoxometalate Molybdenum blue Phosphomolybdenum blue Electrochemical synthesis Pencil graphite electrode
This article is the first to report the direct, one-step synthesis of molybdenum blue (MB) in solid state, via application of cyclic voltammetry. Electrochemical synthesis of MB was carried out in a three-electrode cell in 1.0 M sulfuric acid and at the appropriate amount of sodium molybdate dihydrate. Both a pencil graphite electrode and a platinum plate electrode were used as working electrodes. The structure of electrochemically synthesized MB was characterized using such techniques as Ultraviolet-visible spectroscopy, Fourier Transform Infrared Spectroscopy, Resonance Raman Spectroscopy, Scanning Electron Microscopy, Energy-Dispersive X-Ray Spectroscopy, X-Ray Powder Diffraction, X-ray Fluorescence Spectroscopy, and X-Ray Photoelectron Spectroscopy. In addition, the electrochemical behavior of the synthesized MB was examined via cyclic voltammetry. Finally, the determination of phosphate was conducted using a UV–vis spectrometer with electrochemically synthesized MB.
1. Introduction Polyoxometalates (POMs), are molecules formed by oxygen atoms with early transition metals such as V, Mo, and W in high oxidation values [1]. In addition, they may also contain various heteroatoms such as Si, Ge, and P, As. POMs are a group of molecular metal oxides which are definable from the majority of the metal oxides and may include a large number of metal atoms that achieve nuclearities as high as 368 in one single cluster structure creating nanoparticles [2]. POMs have unmatched structural variety and are derived from the assembly of great “virtual combinatorial libraries” of building blocks [3]. POMs that reflect this structural diversity exhibit a wide variety of features, derived from naturally redox-active and rigid anionic architectures, with numerous potential applications, from catalysis to medicine and materials science [3]. Berzelius reported the first synthesized POM in the early 19th century [1,4], after which Marignac victoriously synthesized 1:12 silicotungstic acid in 1864 [5]. A systematic review of the properties of POMs and their structural characterization began with Rosenheim in the early 20th century [6]. Many methods are used to characterize POMs, including ultraviolet visible spectroscopy, infrared spectroscopy, Raman spectroscopy, nuclear magnetic resonance, and electrochemical methods [7]. With regard to electrochemistry, Souchay [8] was the first person to apply polarography in the research solutions of POMs, which have recently received great attention, as a result of their versatile structure, their
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simple, synthesis, magnetic, and material features, and, especially, their catalytic activities [1,9]. One of the first structurally characterized POMs, the phosphomolybdate Keggin [10], has been the most comprehensive examined, primarily because of its impressive catalytic properties, for example, oxidation of aldehydes, CO, oxidative dehydrogenation, etc. [9]. Examples of other polyoxomolybdates include the molecular big spheres of the Mo132 type, the molecular big wheels of the Mo154/Mo176 type, and the structurally well-characterized, lemonshaped Mo368 cluster, which is the largest by far. There are several reasons for such versatile behavior, as follows: the simple change of coordination numbers on the side exchange of H2O ligands at Mo sites; the moderate strength of Mo-O-Mo type bonds, allowing “split and link”-type processes; the simple change, and particularly the increase, of electron densities without the strong propensity to form metal-metal bonds; and the existence of terminal Mo]O groups, which prevent, in principle, limitless growth to an extended structure [11]. “This is a substance or group of substances about which there has been much discussion” is an interpretation referred in an old inorganic chemistry textbook [12]. Refers to the blue material that was first mentioned by C. W. Scheele in 1778 and which is now known as “molybdenum blue” (MB) and is generally amorphous [13,14]. Since that time, this material has been subject to many publications, and generations of chemists and students have used the “MB test” as a rapid, straightforward and precise process in a qualitative determination of molybdenum. However, the structure of the compound has remained
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Sahin).
http://dx.doi.org/10.1016/j.synthmet.2017.09.009 Received 20 April 2017; Received in revised form 18 July 2017; Accepted 15 September 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.
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Advance) was used for XRD measurements. The XRD pattern was scanned in a 2θ range of 20–80°, and the sample was mounted on rotating sample holders (15 rpm). In order to determine the weight percentage of the elements present in the synthesized MB using the electrochemical method, a Rigaku ZSX Primus model XRF instrument was used. For XPS measurements, an Al-Kα micro-focused monochromator and a Thermo Scientific K-Alpha X-ray photoelectron spectrometer, equipped with a hemispherical electron analyzer were used. The morphology of the MB sample was examined by SEM, using an ESEM-FEG/ EDAX Philips XL-30 instrument, with dialyzed samples dropped on copper grids. In addition, the surface composition of MB was qualitatively and semi quantitatively determined by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX), using a ZEISS EVO® LS 10/EDAX Element SDD instrument.
mysterious, and it is therefore not surprising that most of its extraordinary behaviors have not been explained [13]. The ring structure of Mo154 polyoxomolybdate and other polyoxomolybdate nanostructures have the deep-blue MB coloration; therefore, all of the aforementioned polyoxomolybdate clusters are of the MB type [15]. Solutions of MB are almost immediately provided by the reduction of MoVI-type species in acid solutions (pH ≤ 3). As Reducing agents, metals (Cu, Mo, Pb, Al, Hg, Cd, Zn), B2H6, NaBH4, N2H4, NH2OH, H2S, S2O42−, S2O32−, SO32−, SO2, SnCl2, MoCl5, MoOCl52−, Mohr’s salt, ascorbic acid, formic acid, tartaric acid, hydroquinone, D-glucose, sucrose, thiourea, ethanol, gaseous H2, or CO (under pressure) can be used [14]. Furthermore, spectrophotometric analysis of phosphate, molybdenum, silicon, and other constituents by the formation of MB is generally used in chemical analysis [15]. MB formation by chemical/ thermal reduction under acidic conditions has been standardized by some of the working groups mentioned [13,16,17]; another group has used photochemical reduction in the existence of an electron donor [12]. Almost all of the methods for the synthesis of MB that have been proposed in previous studies are time-consuming, require a high temperature, and have relatively low product purity [13,18]. In this work, for the first time, we demonstrated that MB can be synthesized directly in one-step and a solid state using a voltammetric method. The pure product was obtained without the use of a reducing agent, meaning that an additional purification method was not necessary. The structure of the electrochemically synthesized MB was subsequently characterized using such techniques as Ultraviolet-visible Spectroscopy (UV-vis), Fourier Transform Infrared (FTIR) Spectroscopy, Resonance Raman Spectroscopy, Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDX), X-Ray Fluorescence Spectroscopy (XRF), X-Ray Powder Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS). Furthermore, the electrochemical behavior of the synthesized MB was investigated using cyclic voltammetry (CV). Finally, the determination of phosphate was carried out using a UV–vis spectrometer with electrochemically synthesized MB.
2.3. Measurement procedure A special electrochemical cell with five necks was used in all experiments. Three of these necks belonged to the electrodes that were used as the working, reference, and counter electrodes. The threeelectrode system was used for the electrochemical synthesis of MB. The Ag/AgCl electrode was used as the reference electrode, Pt wire was used as the counter electrode, and pencil graphite electrode (PGE) and Pt plate electrodes were used as working electrodes. Stock solutions of 1.0 mol L−1 of H2SO4 were prepared by dissolving the specified amount in double-distilled water, and the required amount of the stock solution was added to the electrochemical cell. Voltammograms were then recorded using a voltammetric analyzer under parameters that had been optimized for an improved sulfuric acid and sodium molybdate dihydrate concentration. These optimized parameters were used throughout the experiments. The synthesis of MB was realized using the CV method, and all electroanalytical measurements were made at room temperature. 2.4. UV–vis spectroscopy measurement for phosphate determination
2. Experimental procedure Standard phosphate solutions prepared using KH2PO4 were used. The relationship between phosphate concentration and absorbance was evaluated using a UV spectrometer with electrochemically synthesized MB. The reaction mixture contained a constant concentration of phosphate. Recording began immediately after MB was added, at 1-min intervals for a total of 75 min. It was observed that the optimum reaction time for the complexation reaction between phosphate and MB was 40 min. Six different solutions were prepared at phosphate concentrations of 5.0, 10.0, 20.0, 30.0, 40.0, and 50.0 mg/L using the standard phosphate solution, and absorbance measurements were made 40 min after the addition of MB solution into all solutions at wavelength of 400–900 nm (using purified water as a blank solution). The absorbance values were recorded on a UV–vis spectrometer.
2.1. Reagents and apparatus Sodium molybdate dehydrate (≥99%), potassium phosphate monobasic (ACS reagent) and sulfuric acid (95–97%) were obtained from Sigma-Aldrich (UK), and sodium sulfate was purchased from Merck (DE). All solutions were prepared using ultra-purified water provided by a Milli-Q system (Millipore®). The electrochemical experiments, including CV, were recorded with an Autolab electrochemical analyzer (PGSTAT302N Potentiostat/ Galvanostat), and cyclic voltammetric measurements were carried out in a three-electrode cell, in 1.0 M H2SO4. In all electrochemical measurements, both pencil ends (1.0 cm in length and 0.5 mm in diameter, HB, Tombo, geometric area = 0.16 cm2) and platinum (Pt) plate (geometric area = 3.44 cm2) were used as working electrodes, Ag/AgCl (sat. KCl) was used as a reference electrode, and Pt wire was used as a counter electrode. All the potentials are given against the Ag/AgCl (sat. KCl) electrode, and all experiments were conducted at an ambient temperature of 25° C. The weighing process was conducted using Shimadzu analytical balances (ATX224, d = 0.1 mg).
3. Results and discussion 3.1. Electrochemical synthesis of MB MB was synthesized by electrochemical reduction of sodium molybdate dehydrate under aqueous acidic conditions by applying an appropriate cathodic potential. A 1.0 mol L−1 H2SO4 solution was used as a supporting electrolyte. Experiments to optimize pH were conducted, and pH 1 was the most widely used [13,14]. There are a variety of parameters for electrochemical synthesis of MB in the solid state, such as the type of working electrode and size of its surface area, type and amount of supporting electrolyte, the amount of sodium molybdate dihydrate salt, pH level, scan rate and number of cycle. All these parameters are highly effective in increasing the quantity of MB on the surface of the working electrode. Realization of synthesis of MB in the cathodic potential is important because the formation of MB that has
2.2. Characterization of materials Resonance Raman spectra of MB were recorded in the region of 4.000–50 cm−1 with a Bruker Senterra Dispersive Raman Microscope using the 1064 nm excitation from a 3 B diode laser with 3 cm−1 resolution. Absorption spectra in the UV–vis were obtained with a Shimadzu 2001 UV spectrophotometer. IR spectra were recorded on a Perkin Elmer Spectrum One FTIR (ATR sampling accessory) spectrophotometer, and an X-ray powder diffractometer (BRUKER D8 112
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Fig. 1. Synthesis of molybdenum blue by cyclic voltammetry for 10 cycles with a) pencil graphite electrode and b) platinum plate electrode (potential range 1.0 V and −0.5 V, scan rate 100 mV/s, the number of stop crossings 20, step potential 0.00224 V, vs. Ag/AgCl).
accumulated in the solid state on the electrode surface is only applied when the cathodic potential. When an anodic potential was attempted, MB did not accumulate on the electrode surface in the solid state, and instead flowed into the supporting electrolyte solution. Therefore, it cannot be obtained in the solid state. To investigate the effect of the scan rate on the electrochemical synthesis of MB efficiency, electrochemical synthesis of MB was performed with scan rates of 25, 50, 75 and 100 mV/s. During the electrochemical synthesis of MB in sodium molybdate dihydrate solution which was carried out a potential cycling between −0.50 V and +1.00 V (vs Ag/AgCl) MB was not adhered to the either PGE or Pt electrodes surfaces at low scan rates. Instead, the generation of blue colored material was observed near the electrode which diffused away from the surface into the bulk solution, indicating the formation of soluble low molecular weight structures. The best results were obtained at a scan rate of 100 mV/s. No further improvements were observed at higher scan rates. To record cyclic voltammograms the following instrumental parameters were used: a potential range of between 1.0 V and −0.5 V, a scan rate of 100 mV/s, 20 stop crossings, and a step potential of 0.00224 V. Electrodeposition was performed by 10 multisweep cyclic voltammograms were taken between +1.00 V and −0.50 V (vs. Ag/AgCl) at a scan rate of 100 mV/s and finished at +1.0 V. We synthesized MB with sodium molybdate dihydrate salt in different concentrations (0.1, 0.2, 0.3, 0.4, 0.5 and 1.0 mol L−1), and the optimal concentration was different for the two
types of electrodes; if the working electrode was PGE, then 0.5 mol L−1 sodium molybdate dihydrate was used, and if the working electrode was Pt plate electrode, 1.0 mol L−1 molybdate salts was used. After the synthesis of the compound has been carried out with optimized experiment condition, the electrode was washed with deionized water (the washing operation was performed by manually dipping five times) to remove the ions coming from the supporting electrolyte solution from the electrode surface. The product stripped from the electrode surface with a spatula. The resulting dark-blue compound was dried at room temperature and used for the characterization measurements. The synthesis of MB with PGE has superior properties; it is cheap, abundant and environmentally friendly. It also has the potential to be a good material for sensor technology. Fig. 1 shows the synthesis of MB by CV for 10 cycles with both PGE and Pt plate electrode. All parameters used had been optimized. The current has been expressed as current density in order to normalize the measurement (mA/cm2). When all of these procedures had been completed, the MB deposited on the electrode surface was in solid state and ready for use.
Fig. 2. Fourier transform infrared spectroscopy spectrum of molybdenum blue in the solid state.
Fig. 3. Resonance Raman spectrum (with λe = 1064 nm) of molybdenum blue in the solid state.
3.2. Characterization of MB MB that had been synthesized with Pt plate electrode was used in all of the characterization studies. The MB used in all characterization studies was stripped from the surface of the Pt plate working electrode
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electrochemically prepared samples recorded in the range 2000–400 cm−1 with ATR is shown in Fig. 2. The bands belonging to the functional groups are given in sequence (ATR, 2.000–400 cm−1): 1614 (s, δ (H2O)), 1178 (w), 1120 (w), 1060 (all νas(SO4)), 959 (s), 911 (m) (both ν(Mo-Oterm)), 741 (νs), 690 (sh) (doubly coordinated oxygen ν(Mo-O2)), 631 (m), 552 (s), and 468 (w) (triply coordinated oxygen ν(Mo-O3)). Raman spectroscopy is a useful method for the characterization of MB-based materials. A simple and rapid definition of MB is possible by measuring the resonance Raman spectrum (Fig. 3). The main properties of the spectrum are located in the 800–1000 cm−1 region, because of MoeOeMo at lower frequencies and Mo]O at higher frequencies [19,20]. Because of the presence of a comprehensive network of hydrogen bonds and sodium-based salt bridges within the core of the structure was observed a little broadening of the bands. Similarities have previously been observed, wherein proton-induced polarizabilities contributed to resulting in a broader distribution of accessible energy levels [20]. The UV–vis spectrum of the MB solutions in water recorded at room temperature is shown in Fig. 4. MB has a typical UV–vis absorption spectrum. The authors of previous studies all described their products as “MB”, although the relevant properties showed that completely identical solutions were not always formed [13,14]. MB exhibits absorbance maxima at 748 nm in deionized water, which is due to the unpaired electrons of oxygen (Fig. 4) [14,20]. Furthermore, MB has a typical electronic absorption spectrum that gives two maxima at 748 and 1093 nm with UV–vis-near infrared spectroscopy [14]. X-ray fluorescence spectrometry supplies the element composition of materials; therefore, XRF spectrometry has been used for determination of molybdenum. The XRF measurements of MB revealed an abundance of Mo (62.33%) and Na (5.46%), and these data support the results obtained in other analyses (used during the measurement flux: Li2B4O7). MB (amorphous) synthesized by electrochemical method preparations was examined using X-ray diffraction, and Fig. 5 shows the X-ray diffraction spectrum for MB prepared at room temperature for a deposition time of 20 min. We observe that two-theta is ∼28 and ∼53° for MB (Fig. 5). The X-ray diffractograms of MB precipitates obtained from different synthetic routes had almost the same patterns. Except for the characteristic sharp reflections of areas with higher degrees of crystallinity, they exhibit scattered scatter maxima (two-tetra ∼ 26, 29.5 and 51°), as observed for non-crystalline compounds [13]. X-ray photoelectron spectroscopy analyses were further conducted with the aim of obtaining a more detailed information of the speciation
Fig. 4. UV–vis spectroscopy absorption spectrum of molybdenum blue solutions (in deionized water).
Fig. 5. X-Ray diffraction pattern of the molybdenum blue (prepared at room temperature for a deposition time of 20 min).
with a spatula. It is thought that MB is carbon-contaminated when stripped from the PGE surface, which would make characterization difficult, but this problem is solved by using the Pt plate electrode. Infrared spectroscopy was used in the characterization of MB, and satisfactory results were obtained [11,19]. The FT-IR spectrum of the
Fig. 6. Experimental and deconvoluted Mo3d X-ray Photoelectron spectroscopy bands corresponding to molybdenum blue.
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[13,14,23,24]. In the present study, samples for SEM characterization were prepared by putting a drop of a highly dilute aqueous solution of MB onto a copper grid and allowing the solution to vaporize from the surface of the copper grid. Fig. 8 shows SEM images obtained at different magnifications of the sample. In addition, as the images show, they are conglomerated to form chains, and a wheel-shaped structural model was observed on the entire surface. When examining the SEM images, so-called big wheel-like structures (containing a large number of molybdenum atoms) were observed. The surface composition of electrochemically synthesized MB was qualitatively and semi-quantitatively determined by SEM/EDX. EDX analysis was conducted in a selected area via SEM images. The MB synthesized in the solid state was brought to the appropriate sizes for measurement and distributed on the sample holder. The appropriate vacuum was waited before taking the images of the sample, which were obtained where the sample surface was perfectly flat to ensure satisfactory analysis, before which EDX analysis was performed for the selected area. This analysis showed that high-intensity Mo, O and Na peaks were achieved (Fig. 9). Fig. 9 also shows that the MB weight percentages of Mo, O, Na and C were 57.9%, 34.6%, 6.0% and 1.5% respectively. In addition, it was observed that the MB atomic percentages of Mo, O, Na and C were 19.1%, 68.6%, 8.3% and 4.0% respectively. Therefore, it can be assumed that the material synthesized electrochemically was MB, as both qualitatively and semi-quantitatively determined.
Fig. 7. X-ray photoelectron spectroscopy survey spectrum of molybdenum blue.
of Mo compounds in the MB. Therefore, the XPS measurements were performed to characterize the MB, primarily from the perspective of the oxidation state of the principal elements. Fig. 6 show the experimental and deconvoluted Mo3d core electronic transitions for the MB. The presence of Mo6+ and Mo5+ was confirmed. The XPS Mo3d region is shown in Fig. 6, and reveals that the Mo was characterized by a double peak of 232.98 eV for Mo3d5/2 and 236.08 eV for Mo3d3/2 at binding energy. The ratio between the areas of the peaks was 1.5, which is the typical theoretical value for a three-dimensional doublet [18,21,22]. The Mo3d, 01s and N1s regions, together with the C1s region, were recorded (Fig. 7). We used SEM imaging to examine the nanostructure of the MB that we synthesized electrochemically. The first supporting structural models of MB, published by Müller et al. In 1995, were mainly based on powder X-ray diffraction, and the results showed that the material was formed complex constructions of wheel-shaped mixed-valence polyv 14− oxomolybdate [Moıv anion clusters 126Mo28O462H14H2O70]
3.3. Investigation of the cyclic voltammetric behavior of MB The electrochemical response of a solution with homogeneous MB was determined by CV 100 mV s−1 under optimized parameters using PGE. Phosphate buffer solutions and Britton-Robinson buffer solutions at different pH levels were tested to examine the MB of the redox behavior (for the acidic and basic medium). However, this resulted in a complexation reaction between phosphate ion-containing solutions and MB. Therefore, the electrochemical behavior of MB was not efficiently Fig. 8. Scanning electron microscopy micrographs of the molybdenum blue.
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Fig. 9. Scanning electron microscopy photograph and energy-dispersive X-ray spectrum of the surface of the molybdenum blue.
3.4. Phosphate determination The generally applied methods for orthophosphate determination by spectrophotometry are based on the formation of a stable phosphomolybdenum blue complex, which allows the application of the Beer-Lambert Law in its most basic form. This method was originally developed by Osmond [25] in 1887 and Taylor & Miller [26] in 1914, and the number of studies published in this regard has greatly expanded [24,27–30]. The relationship between wavelength and absorbance values was recorded on a UV–vis spectrometer. It was found that the best time for the formation of the phosphomolybdenum blue complex was 40 min after the addition of MB (Fig. 11). Therefore, measurements of all standard solutions were taken at this point. The UV–vis spectra of the phosphomolybdenum blue complexes formed by phosphate-containing solutions at different concentrations are shown in Fig. 12. Although a single band was observed between 500 and 800 nm in the Uv–vis spectrum of the MB, the second band of between 800 and 900 nm was observed with the addition of different amounts of phosphate [29,31]. As shown in Fig. 12 (inset shows calibration graph), the rise in phosphate concentration increased the absorbance of the band of the resulting phosphomolybdenum blue complex.
Fig. 10. A cyclic voltammogram of a homogeneous solution of 0.10 g molybdenum blue in 0.1 M sodium sulfate solution using pencil graphite electrode (potential range 0.2 V and −0.6 V, scan rate 100 mV/s, the number of stop crossings 2, step potential 0.00224 V, vs. Ag/AgCl).
examined at different pH levels. The MB of electrochemical behavior in a 0.1 M sodium sulfate solution, at a potential range of between 0.2 V and −0.6 V, was then examined with the PGE (as shown in Fig. 10). The CV measurement data are presented in Table 1. The anodic peak obtained for oxidation of MB was observed at −0.12 V, and the cathodic peak obtained for reduction of MB was observed at −0.35 V. The difference between the anodic peak potential and cathodic peak potential (ΔE) was calculated as 0.24. This indicates that the happening redox reaction is a quasi-reversible reaction. The reduction and oxidation peaks are shown in Fig. 10, and were the result of the redox behavior of molybdenum metal (reduction of Mo6+ to Mo5+ and oxidation of molybdenum Mo5+ to Mo6+ (Mo6+ + e− ⇋ Mo5+)).
4. Conclusion In summary, we have been the first to show the direct, one-step synthesis of MB in the solid state, via the use of an electrochemical method. The structure of electrochemically synthesized MB was illuminated by spectroscopic techniques. Subsequently, the electrochemical behavior of the synthesized MB was examined using CV. Electrochemically synthesized MB is very well dissolved in water and most organic solvents. In the method described in our study is much easier than ordinary synthesis methods, and the pure product was obtained without the use of a reducing agent, meaning that an additional purification method was not necessary. In addition, the most important feature of the electrochemical method is that it synthesizes MB in a very short period of time, without heating. Furthermore, this study proved that it is possible to determine phosphate by MB that has been electrochemically synthesized, according to the spectrometric results. This also contributed to the characterization of the synthesized product. Finally, the literature entry of the electrochemically synthesized of molybdenum blue modified electrodes is provided thanks to this work.
Table 1 Peak potentials, currents, and areas data of cyclic voltammogram. Oxidation peak potential (V)
Oxidation peak current (A)
Oxidation peak area (C/cm2)
Reduction peak potential (V)
Reduction peak current (A)
Reduction peak area (C/cm2)
−0.12
−0.00012
0.0034
−0.35
0.00017
0.0018
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Fig. 11. The absorption spectra of the phosphomolybdenum blue complexes. The reaction mixture contained a constant concentration of phosphate. Recording began immediately after MB was added, for a total of 75 min with 1-min intervals.
Fig. 12. The absorption spectra for phosphate solutions at 5.0, 10.0, 20.0, 30.0, 40.0, and 50.0 mg/L (inset: calibration graph).
Besides, the proposed synthesis method may be important for further development of such syntheses. When all these results are evaluated, it is considered that the present study will shed light on future works.
[3]
[4]
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Direct, one-step synthesis of molybdenum blue using an electrochemical method, and characterization studies Ozge Koyun, Semih Gorduk, Melih Besir Arvas, Yucel Sahin
MARK
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Department of Chemistry, Faculty of Arts & Science, Yildiz Technical University, TR34210 Istanbul, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Polyoxometalate Molybdenum blue Phosphomolybdenum blue Electrochemical synthesis Pencil graphite electrode
This article is the first to report the direct, one-step synthesis of molybdenum blue (MB) in solid state, via application of cyclic voltammetry. Electrochemical synthesis of MB was carried out in a three-electrode cell in 1.0 M sulfuric acid and at the appropriate amount of sodium molybdate dihydrate. Both a pencil graphite electrode and a platinum plate electrode were used as working electrodes. The structure of electrochemically synthesized MB was characterized using such techniques as Ultraviolet-visible spectroscopy, Fourier Transform Infrared Spectroscopy, Resonance Raman Spectroscopy, Scanning Electron Microscopy, Energy-Dispersive X-Ray Spectroscopy, X-Ray Powder Diffraction, X-ray Fluorescence Spectroscopy, and X-Ray Photoelectron Spectroscopy. In addition, the electrochemical behavior of the synthesized MB was examined via cyclic voltammetry. Finally, the determination of phosphate was conducted using a UV–vis spectrometer with electrochemically synthesized MB.
1. Introduction Polyoxometalates (POMs), are molecules formed by oxygen atoms with early transition metals such as V, Mo, and W in high oxidation values [1]. In addition, they may also contain various heteroatoms such as Si, Ge, and P, As. POMs are a group of molecular metal oxides which are definable from the majority of the metal oxides and may include a large number of metal atoms that achieve nuclearities as high as 368 in one single cluster structure creating nanoparticles [2]. POMs have unmatched structural variety and are derived from the assembly of great “virtual combinatorial libraries” of building blocks [3]. POMs that reflect this structural diversity exhibit a wide variety of features, derived from naturally redox-active and rigid anionic architectures, with numerous potential applications, from catalysis to medicine and materials science [3]. Berzelius reported the first synthesized POM in the early 19th century [1,4], after which Marignac victoriously synthesized 1:12 silicotungstic acid in 1864 [5]. A systematic review of the properties of POMs and their structural characterization began with Rosenheim in the early 20th century [6]. Many methods are used to characterize POMs, including ultraviolet visible spectroscopy, infrared spectroscopy, Raman spectroscopy, nuclear magnetic resonance, and electrochemical methods [7]. With regard to electrochemistry, Souchay [8] was the first person to apply polarography in the research solutions of POMs, which have recently received great attention, as a result of their versatile structure, their
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simple, synthesis, magnetic, and material features, and, especially, their catalytic activities [1,9]. One of the first structurally characterized POMs, the phosphomolybdate Keggin [10], has been the most comprehensive examined, primarily because of its impressive catalytic properties, for example, oxidation of aldehydes, CO, oxidative dehydrogenation, etc. [9]. Examples of other polyoxomolybdates include the molecular big spheres of the Mo132 type, the molecular big wheels of the Mo154/Mo176 type, and the structurally well-characterized, lemonshaped Mo368 cluster, which is the largest by far. There are several reasons for such versatile behavior, as follows: the simple change of coordination numbers on the side exchange of H2O ligands at Mo sites; the moderate strength of Mo-O-Mo type bonds, allowing “split and link”-type processes; the simple change, and particularly the increase, of electron densities without the strong propensity to form metal-metal bonds; and the existence of terminal Mo]O groups, which prevent, in principle, limitless growth to an extended structure [11]. “This is a substance or group of substances about which there has been much discussion” is an interpretation referred in an old inorganic chemistry textbook [12]. Refers to the blue material that was first mentioned by C. W. Scheele in 1778 and which is now known as “molybdenum blue” (MB) and is generally amorphous [13,14]. Since that time, this material has been subject to many publications, and generations of chemists and students have used the “MB test” as a rapid, straightforward and precise process in a qualitative determination of molybdenum. However, the structure of the compound has remained
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Sahin).
http://dx.doi.org/10.1016/j.synthmet.2017.09.009 Received 20 April 2017; Received in revised form 18 July 2017; Accepted 15 September 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.
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Advance) was used for XRD measurements. The XRD pattern was scanned in a 2θ range of 20–80°, and the sample was mounted on rotating sample holders (15 rpm). In order to determine the weight percentage of the elements present in the synthesized MB using the electrochemical method, a Rigaku ZSX Primus model XRF instrument was used. For XPS measurements, an Al-Kα micro-focused monochromator and a Thermo Scientific K-Alpha X-ray photoelectron spectrometer, equipped with a hemispherical electron analyzer were used. The morphology of the MB sample was examined by SEM, using an ESEM-FEG/ EDAX Philips XL-30 instrument, with dialyzed samples dropped on copper grids. In addition, the surface composition of MB was qualitatively and semi quantitatively determined by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX), using a ZEISS EVO® LS 10/EDAX Element SDD instrument.
mysterious, and it is therefore not surprising that most of its extraordinary behaviors have not been explained [13]. The ring structure of Mo154 polyoxomolybdate and other polyoxomolybdate nanostructures have the deep-blue MB coloration; therefore, all of the aforementioned polyoxomolybdate clusters are of the MB type [15]. Solutions of MB are almost immediately provided by the reduction of MoVI-type species in acid solutions (pH ≤ 3). As Reducing agents, metals (Cu, Mo, Pb, Al, Hg, Cd, Zn), B2H6, NaBH4, N2H4, NH2OH, H2S, S2O42−, S2O32−, SO32−, SO2, SnCl2, MoCl5, MoOCl52−, Mohr’s salt, ascorbic acid, formic acid, tartaric acid, hydroquinone, D-glucose, sucrose, thiourea, ethanol, gaseous H2, or CO (under pressure) can be used [14]. Furthermore, spectrophotometric analysis of phosphate, molybdenum, silicon, and other constituents by the formation of MB is generally used in chemical analysis [15]. MB formation by chemical/ thermal reduction under acidic conditions has been standardized by some of the working groups mentioned [13,16,17]; another group has used photochemical reduction in the existence of an electron donor [12]. Almost all of the methods for the synthesis of MB that have been proposed in previous studies are time-consuming, require a high temperature, and have relatively low product purity [13,18]. In this work, for the first time, we demonstrated that MB can be synthesized directly in one-step and a solid state using a voltammetric method. The pure product was obtained without the use of a reducing agent, meaning that an additional purification method was not necessary. The structure of the electrochemically synthesized MB was subsequently characterized using such techniques as Ultraviolet-visible Spectroscopy (UV-vis), Fourier Transform Infrared (FTIR) Spectroscopy, Resonance Raman Spectroscopy, Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray Spectroscopy (EDX), X-Ray Fluorescence Spectroscopy (XRF), X-Ray Powder Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS). Furthermore, the electrochemical behavior of the synthesized MB was investigated using cyclic voltammetry (CV). Finally, the determination of phosphate was carried out using a UV–vis spectrometer with electrochemically synthesized MB.
2.3. Measurement procedure A special electrochemical cell with five necks was used in all experiments. Three of these necks belonged to the electrodes that were used as the working, reference, and counter electrodes. The threeelectrode system was used for the electrochemical synthesis of MB. The Ag/AgCl electrode was used as the reference electrode, Pt wire was used as the counter electrode, and pencil graphite electrode (PGE) and Pt plate electrodes were used as working electrodes. Stock solutions of 1.0 mol L−1 of H2SO4 were prepared by dissolving the specified amount in double-distilled water, and the required amount of the stock solution was added to the electrochemical cell. Voltammograms were then recorded using a voltammetric analyzer under parameters that had been optimized for an improved sulfuric acid and sodium molybdate dihydrate concentration. These optimized parameters were used throughout the experiments. The synthesis of MB was realized using the CV method, and all electroanalytical measurements were made at room temperature. 2.4. UV–vis spectroscopy measurement for phosphate determination
2. Experimental procedure Standard phosphate solutions prepared using KH2PO4 were used. The relationship between phosphate concentration and absorbance was evaluated using a UV spectrometer with electrochemically synthesized MB. The reaction mixture contained a constant concentration of phosphate. Recording began immediately after MB was added, at 1-min intervals for a total of 75 min. It was observed that the optimum reaction time for the complexation reaction between phosphate and MB was 40 min. Six different solutions were prepared at phosphate concentrations of 5.0, 10.0, 20.0, 30.0, 40.0, and 50.0 mg/L using the standard phosphate solution, and absorbance measurements were made 40 min after the addition of MB solution into all solutions at wavelength of 400–900 nm (using purified water as a blank solution). The absorbance values were recorded on a UV–vis spectrometer.
2.1. Reagents and apparatus Sodium molybdate dehydrate (≥99%), potassium phosphate monobasic (ACS reagent) and sulfuric acid (95–97%) were obtained from Sigma-Aldrich (UK), and sodium sulfate was purchased from Merck (DE). All solutions were prepared using ultra-purified water provided by a Milli-Q system (Millipore®). The electrochemical experiments, including CV, were recorded with an Autolab electrochemical analyzer (PGSTAT302N Potentiostat/ Galvanostat), and cyclic voltammetric measurements were carried out in a three-electrode cell, in 1.0 M H2SO4. In all electrochemical measurements, both pencil ends (1.0 cm in length and 0.5 mm in diameter, HB, Tombo, geometric area = 0.16 cm2) and platinum (Pt) plate (geometric area = 3.44 cm2) were used as working electrodes, Ag/AgCl (sat. KCl) was used as a reference electrode, and Pt wire was used as a counter electrode. All the potentials are given against the Ag/AgCl (sat. KCl) electrode, and all experiments were conducted at an ambient temperature of 25° C. The weighing process was conducted using Shimadzu analytical balances (ATX224, d = 0.1 mg).
3. Results and discussion 3.1. Electrochemical synthesis of MB MB was synthesized by electrochemical reduction of sodium molybdate dehydrate under aqueous acidic conditions by applying an appropriate cathodic potential. A 1.0 mol L−1 H2SO4 solution was used as a supporting electrolyte. Experiments to optimize pH were conducted, and pH 1 was the most widely used [13,14]. There are a variety of parameters for electrochemical synthesis of MB in the solid state, such as the type of working electrode and size of its surface area, type and amount of supporting electrolyte, the amount of sodium molybdate dihydrate salt, pH level, scan rate and number of cycle. All these parameters are highly effective in increasing the quantity of MB on the surface of the working electrode. Realization of synthesis of MB in the cathodic potential is important because the formation of MB that has
2.2. Characterization of materials Resonance Raman spectra of MB were recorded in the region of 4.000–50 cm−1 with a Bruker Senterra Dispersive Raman Microscope using the 1064 nm excitation from a 3 B diode laser with 3 cm−1 resolution. Absorption spectra in the UV–vis were obtained with a Shimadzu 2001 UV spectrophotometer. IR spectra were recorded on a Perkin Elmer Spectrum One FTIR (ATR sampling accessory) spectrophotometer, and an X-ray powder diffractometer (BRUKER D8 112
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Fig. 1. Synthesis of molybdenum blue by cyclic voltammetry for 10 cycles with a) pencil graphite electrode and b) platinum plate electrode (potential range 1.0 V and −0.5 V, scan rate 100 mV/s, the number of stop crossings 20, step potential 0.00224 V, vs. Ag/AgCl).
accumulated in the solid state on the electrode surface is only applied when the cathodic potential. When an anodic potential was attempted, MB did not accumulate on the electrode surface in the solid state, and instead flowed into the supporting electrolyte solution. Therefore, it cannot be obtained in the solid state. To investigate the effect of the scan rate on the electrochemical synthesis of MB efficiency, electrochemical synthesis of MB was performed with scan rates of 25, 50, 75 and 100 mV/s. During the electrochemical synthesis of MB in sodium molybdate dihydrate solution which was carried out a potential cycling between −0.50 V and +1.00 V (vs Ag/AgCl) MB was not adhered to the either PGE or Pt electrodes surfaces at low scan rates. Instead, the generation of blue colored material was observed near the electrode which diffused away from the surface into the bulk solution, indicating the formation of soluble low molecular weight structures. The best results were obtained at a scan rate of 100 mV/s. No further improvements were observed at higher scan rates. To record cyclic voltammograms the following instrumental parameters were used: a potential range of between 1.0 V and −0.5 V, a scan rate of 100 mV/s, 20 stop crossings, and a step potential of 0.00224 V. Electrodeposition was performed by 10 multisweep cyclic voltammograms were taken between +1.00 V and −0.50 V (vs. Ag/AgCl) at a scan rate of 100 mV/s and finished at +1.0 V. We synthesized MB with sodium molybdate dihydrate salt in different concentrations (0.1, 0.2, 0.3, 0.4, 0.5 and 1.0 mol L−1), and the optimal concentration was different for the two
types of electrodes; if the working electrode was PGE, then 0.5 mol L−1 sodium molybdate dihydrate was used, and if the working electrode was Pt plate electrode, 1.0 mol L−1 molybdate salts was used. After the synthesis of the compound has been carried out with optimized experiment condition, the electrode was washed with deionized water (the washing operation was performed by manually dipping five times) to remove the ions coming from the supporting electrolyte solution from the electrode surface. The product stripped from the electrode surface with a spatula. The resulting dark-blue compound was dried at room temperature and used for the characterization measurements. The synthesis of MB with PGE has superior properties; it is cheap, abundant and environmentally friendly. It also has the potential to be a good material for sensor technology. Fig. 1 shows the synthesis of MB by CV for 10 cycles with both PGE and Pt plate electrode. All parameters used had been optimized. The current has been expressed as current density in order to normalize the measurement (mA/cm2). When all of these procedures had been completed, the MB deposited on the electrode surface was in solid state and ready for use.
Fig. 2. Fourier transform infrared spectroscopy spectrum of molybdenum blue in the solid state.
Fig. 3. Resonance Raman spectrum (with λe = 1064 nm) of molybdenum blue in the solid state.
3.2. Characterization of MB MB that had been synthesized with Pt plate electrode was used in all of the characterization studies. The MB used in all characterization studies was stripped from the surface of the Pt plate working electrode
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electrochemically prepared samples recorded in the range 2000–400 cm−1 with ATR is shown in Fig. 2. The bands belonging to the functional groups are given in sequence (ATR, 2.000–400 cm−1): 1614 (s, δ (H2O)), 1178 (w), 1120 (w), 1060 (all νas(SO4)), 959 (s), 911 (m) (both ν(Mo-Oterm)), 741 (νs), 690 (sh) (doubly coordinated oxygen ν(Mo-O2)), 631 (m), 552 (s), and 468 (w) (triply coordinated oxygen ν(Mo-O3)). Raman spectroscopy is a useful method for the characterization of MB-based materials. A simple and rapid definition of MB is possible by measuring the resonance Raman spectrum (Fig. 3). The main properties of the spectrum are located in the 800–1000 cm−1 region, because of MoeOeMo at lower frequencies and Mo]O at higher frequencies [19,20]. Because of the presence of a comprehensive network of hydrogen bonds and sodium-based salt bridges within the core of the structure was observed a little broadening of the bands. Similarities have previously been observed, wherein proton-induced polarizabilities contributed to resulting in a broader distribution of accessible energy levels [20]. The UV–vis spectrum of the MB solutions in water recorded at room temperature is shown in Fig. 4. MB has a typical UV–vis absorption spectrum. The authors of previous studies all described their products as “MB”, although the relevant properties showed that completely identical solutions were not always formed [13,14]. MB exhibits absorbance maxima at 748 nm in deionized water, which is due to the unpaired electrons of oxygen (Fig. 4) [14,20]. Furthermore, MB has a typical electronic absorption spectrum that gives two maxima at 748 and 1093 nm with UV–vis-near infrared spectroscopy [14]. X-ray fluorescence spectrometry supplies the element composition of materials; therefore, XRF spectrometry has been used for determination of molybdenum. The XRF measurements of MB revealed an abundance of Mo (62.33%) and Na (5.46%), and these data support the results obtained in other analyses (used during the measurement flux: Li2B4O7). MB (amorphous) synthesized by electrochemical method preparations was examined using X-ray diffraction, and Fig. 5 shows the X-ray diffraction spectrum for MB prepared at room temperature for a deposition time of 20 min. We observe that two-theta is ∼28 and ∼53° for MB (Fig. 5). The X-ray diffractograms of MB precipitates obtained from different synthetic routes had almost the same patterns. Except for the characteristic sharp reflections of areas with higher degrees of crystallinity, they exhibit scattered scatter maxima (two-tetra ∼ 26, 29.5 and 51°), as observed for non-crystalline compounds [13]. X-ray photoelectron spectroscopy analyses were further conducted with the aim of obtaining a more detailed information of the speciation
Fig. 4. UV–vis spectroscopy absorption spectrum of molybdenum blue solutions (in deionized water).
Fig. 5. X-Ray diffraction pattern of the molybdenum blue (prepared at room temperature for a deposition time of 20 min).
with a spatula. It is thought that MB is carbon-contaminated when stripped from the PGE surface, which would make characterization difficult, but this problem is solved by using the Pt plate electrode. Infrared spectroscopy was used in the characterization of MB, and satisfactory results were obtained [11,19]. The FT-IR spectrum of the
Fig. 6. Experimental and deconvoluted Mo3d X-ray Photoelectron spectroscopy bands corresponding to molybdenum blue.
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[13,14,23,24]. In the present study, samples for SEM characterization were prepared by putting a drop of a highly dilute aqueous solution of MB onto a copper grid and allowing the solution to vaporize from the surface of the copper grid. Fig. 8 shows SEM images obtained at different magnifications of the sample. In addition, as the images show, they are conglomerated to form chains, and a wheel-shaped structural model was observed on the entire surface. When examining the SEM images, so-called big wheel-like structures (containing a large number of molybdenum atoms) were observed. The surface composition of electrochemically synthesized MB was qualitatively and semi-quantitatively determined by SEM/EDX. EDX analysis was conducted in a selected area via SEM images. The MB synthesized in the solid state was brought to the appropriate sizes for measurement and distributed on the sample holder. The appropriate vacuum was waited before taking the images of the sample, which were obtained where the sample surface was perfectly flat to ensure satisfactory analysis, before which EDX analysis was performed for the selected area. This analysis showed that high-intensity Mo, O and Na peaks were achieved (Fig. 9). Fig. 9 also shows that the MB weight percentages of Mo, O, Na and C were 57.9%, 34.6%, 6.0% and 1.5% respectively. In addition, it was observed that the MB atomic percentages of Mo, O, Na and C were 19.1%, 68.6%, 8.3% and 4.0% respectively. Therefore, it can be assumed that the material synthesized electrochemically was MB, as both qualitatively and semi-quantitatively determined.
Fig. 7. X-ray photoelectron spectroscopy survey spectrum of molybdenum blue.
of Mo compounds in the MB. Therefore, the XPS measurements were performed to characterize the MB, primarily from the perspective of the oxidation state of the principal elements. Fig. 6 show the experimental and deconvoluted Mo3d core electronic transitions for the MB. The presence of Mo6+ and Mo5+ was confirmed. The XPS Mo3d region is shown in Fig. 6, and reveals that the Mo was characterized by a double peak of 232.98 eV for Mo3d5/2 and 236.08 eV for Mo3d3/2 at binding energy. The ratio between the areas of the peaks was 1.5, which is the typical theoretical value for a three-dimensional doublet [18,21,22]. The Mo3d, 01s and N1s regions, together with the C1s region, were recorded (Fig. 7). We used SEM imaging to examine the nanostructure of the MB that we synthesized electrochemically. The first supporting structural models of MB, published by Müller et al. In 1995, were mainly based on powder X-ray diffraction, and the results showed that the material was formed complex constructions of wheel-shaped mixed-valence polyv 14− oxomolybdate [Moıv anion clusters 126Mo28O462H14H2O70]
3.3. Investigation of the cyclic voltammetric behavior of MB The electrochemical response of a solution with homogeneous MB was determined by CV 100 mV s−1 under optimized parameters using PGE. Phosphate buffer solutions and Britton-Robinson buffer solutions at different pH levels were tested to examine the MB of the redox behavior (for the acidic and basic medium). However, this resulted in a complexation reaction between phosphate ion-containing solutions and MB. Therefore, the electrochemical behavior of MB was not efficiently Fig. 8. Scanning electron microscopy micrographs of the molybdenum blue.
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Fig. 9. Scanning electron microscopy photograph and energy-dispersive X-ray spectrum of the surface of the molybdenum blue.
3.4. Phosphate determination The generally applied methods for orthophosphate determination by spectrophotometry are based on the formation of a stable phosphomolybdenum blue complex, which allows the application of the Beer-Lambert Law in its most basic form. This method was originally developed by Osmond [25] in 1887 and Taylor & Miller [26] in 1914, and the number of studies published in this regard has greatly expanded [24,27–30]. The relationship between wavelength and absorbance values was recorded on a UV–vis spectrometer. It was found that the best time for the formation of the phosphomolybdenum blue complex was 40 min after the addition of MB (Fig. 11). Therefore, measurements of all standard solutions were taken at this point. The UV–vis spectra of the phosphomolybdenum blue complexes formed by phosphate-containing solutions at different concentrations are shown in Fig. 12. Although a single band was observed between 500 and 800 nm in the Uv–vis spectrum of the MB, the second band of between 800 and 900 nm was observed with the addition of different amounts of phosphate [29,31]. As shown in Fig. 12 (inset shows calibration graph), the rise in phosphate concentration increased the absorbance of the band of the resulting phosphomolybdenum blue complex.
Fig. 10. A cyclic voltammogram of a homogeneous solution of 0.10 g molybdenum blue in 0.1 M sodium sulfate solution using pencil graphite electrode (potential range 0.2 V and −0.6 V, scan rate 100 mV/s, the number of stop crossings 2, step potential 0.00224 V, vs. Ag/AgCl).
examined at different pH levels. The MB of electrochemical behavior in a 0.1 M sodium sulfate solution, at a potential range of between 0.2 V and −0.6 V, was then examined with the PGE (as shown in Fig. 10). The CV measurement data are presented in Table 1. The anodic peak obtained for oxidation of MB was observed at −0.12 V, and the cathodic peak obtained for reduction of MB was observed at −0.35 V. The difference between the anodic peak potential and cathodic peak potential (ΔE) was calculated as 0.24. This indicates that the happening redox reaction is a quasi-reversible reaction. The reduction and oxidation peaks are shown in Fig. 10, and were the result of the redox behavior of molybdenum metal (reduction of Mo6+ to Mo5+ and oxidation of molybdenum Mo5+ to Mo6+ (Mo6+ + e− ⇋ Mo5+)).
4. Conclusion In summary, we have been the first to show the direct, one-step synthesis of MB in the solid state, via the use of an electrochemical method. The structure of electrochemically synthesized MB was illuminated by spectroscopic techniques. Subsequently, the electrochemical behavior of the synthesized MB was examined using CV. Electrochemically synthesized MB is very well dissolved in water and most organic solvents. In the method described in our study is much easier than ordinary synthesis methods, and the pure product was obtained without the use of a reducing agent, meaning that an additional purification method was not necessary. In addition, the most important feature of the electrochemical method is that it synthesizes MB in a very short period of time, without heating. Furthermore, this study proved that it is possible to determine phosphate by MB that has been electrochemically synthesized, according to the spectrometric results. This also contributed to the characterization of the synthesized product. Finally, the literature entry of the electrochemically synthesized of molybdenum blue modified electrodes is provided thanks to this work.
Table 1 Peak potentials, currents, and areas data of cyclic voltammogram. Oxidation peak potential (V)
Oxidation peak current (A)
Oxidation peak area (C/cm2)
Reduction peak potential (V)
Reduction peak current (A)
Reduction peak area (C/cm2)
−0.12
−0.00012
0.0034
−0.35
0.00017
0.0018
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Fig. 11. The absorption spectra of the phosphomolybdenum blue complexes. The reaction mixture contained a constant concentration of phosphate. Recording began immediately after MB was added, for a total of 75 min with 1-min intervals.
Fig. 12. The absorption spectra for phosphate solutions at 5.0, 10.0, 20.0, 30.0, 40.0, and 50.0 mg/L (inset: calibration graph).
Besides, the proposed synthesis method may be important for further development of such syntheses. When all these results are evaluated, it is considered that the present study will shed light on future works.
[3]
[4]
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