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ARTICLES Redox Behavior of Molybdenum and Tungsten in Phosphate Glasses Gae1 l Poirier* and Fabiola S. Ottoboni Departamento de Cieˆ ncias Exatas, UNIFAL-MG, CEP 37130-000, Alfenas-MG, Brazil
Fa´ bia Castro Cassanjes, A Ä damo Remonte, Younes Messaddeq, and Sidney J. L Ribeiro Laboratory of Photonic Materials, Instituto de Quı´mica, UNESP, PoBox 355, CEP 14800-900, Araraquara - SP, Brazil ReceiVed: December 13, 2007; In Final Form: January 30, 2008
In this work, vitreous samples were prepared in the binary system (100 - x)NaPO3-xMO3 with M ) Mo and W and x varying from 10 to 60. The transmittance properties in the UV, visible, and near-infrared were monitored as a function of MO3 concentration. In both cases, an increase in the amount of transition metal results in an intense and broad absorption band in the visible and near-infrared attributed to metal reduction under synthesis conditions. It was shown that this large absorption can be partially or totally removed using specific oxidizing agents or by improving synthesis parameters such as melting temperature or cooling rate of the melt. In addition, structural investigations by Raman and X-ray absorption spectroscopy suggest that reduction only occurs when the metal cation is in octahedral geometry and that the transmittance improvement is not related with any structural changes. These results were explained in terms of thermodynamic equilibrium of redox species in the melt and allowed to obtain for the first time transparent and chemically stable glasses containing high concentrations of MO3 with transition metals in octahedral geometry inside the glass network.
1. Introduction Phosphate compounds based on sodium polyphosphate NaPO3 are well-known glass formers and are being intensively used in glass science as host matrixes because of their high vitrifying ability and unusual capacity to dissolve high amounts of other glass formers, modifiers, or intermediate compounds without reduction of glass forming ability. For that reason, phosphate glasses found many applications for UV transmission, as laser hosts or for ionic conductivity and sealing.1-3 Among this large number of vitreous phosphate compositions, phosphate glasses containing transition metal oxides such as MoO3 or WO3 display a special scientific interest because of potential applications in several fields of optics. The incorporation of these transition metals is generally known to improve mechanical properties, chemical resistance against atmospheric moisture, as well as thermal stability against devitrification.4,5 Particularly, some tungstate phosphate compositions were thermally investigated and do not exhibit the expected crystallization event.6 This unusual behavior makes these compositions suitable for optical fiber drawing. Besides these thermal properties, it was already shown that tungsten phosphate glasses exhibit interesting optical properties such as upconversion phenomena when doped with Tm3+7 or two-photon nonlinear optical absorption.8 In addition, a new volumetric photochromic effect was observed in samples with high levels of WO3 incorporation under exposure to visible lasers.9 These characteristics are strongly related with the * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +55-35 32 99 12 61. Fax: +55-35 32 99 12 62.
structural particularities of the vitreous network. In fact, structural studies of these tungsten phosphate glasses indicated the formation of WO6 clusters10,11 in the most WO3-concentrated samples, and it has been suggested that these highly polarizable clusters are responsible for the nonlinear and photochromic properties. Recent structural studies by Raman and NMR suggest a similar behavior in molybdate phosphate glasses.12 However, glasses containing transition metals often exhibit an intense and broad absorption band in the visible and near-infrared attributed to metal reduction during melting.6,13 As redox effects are difficult to control, this undesirable absorption band often limits or even prohibits optical applications in this spectral range. In this work, vitreous samples were prepared in the binary systems (100-x)NaPO3-xMO3 with M ) Mo and W and x varying from 10 to 60. Transmittance spectra were recorded as a function of MO3 concentration. Oxidizing agents were used in the most concentrated samples to check the possibility of obtaining transparent glasses. Synthesis parameters such as melting temperature and cooling rate were varied, and it was shown that appropriate synthesis parameters allow the preparation of transparent glasses in the visible. Structural investigations suggest that the improvement of transparency is not accompanied by a measurable structural change of the vitreous network and that redox phenomena are related with the octahedral configuration of transition metal. This work presents for the first time the preparation of high MO3 concentrated phosphate glasses with good transparency in the visible.
10.1021/jp711709r CCC: $40.75 © 2008 American Chemical Society Published on Web 03/22/2008
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TABLE 1: Sample Labels, Molar Compositions, and Color sample
composition (mol %)
color
N100 NW10 NW20 NW30 NW40 NW50 NW60 NMo10 NMo20 NMo30 NMo40 NMo50 NMo60 NW50Ce NW60Ce NW50SN NW60SN NKLW60 (ref 21) NKLMo60 (ref 21)
NaPO3 90 NaPO3-10 WO3 80 NaPO3-20 WO3 70 NaPO3-30 WO3 60 NaPO3-40 WO3 50 NaPO3-50 WO3 40 NaPO3-60 WO3 90 NaPO3-10 MoO3 80 NaPO3-20 MoO3 70 NaPO3-30 MoO3 60 NaPO3-40 MoO3 50 NaPO3-50 MoO3 40 NaPO3-60 MoO3 49.8 NaPO3-50 WO3-0.2 CeO2 39.8 NaPO3-60 WO3-0.2 CeO2 48.5 NaPO3-50 WO3-0.5 Sb2O3-1 Na2O 38.5 NaPO3-60 WO3-0.5 Sb2O3-1 Na2O 60 WO3-10 Li2O-10 Na2O-20 K2O
colorless colorless colorless colorless colorless blue deep blue green deep green dark dark dark dark deep yellow orange yellow deep green colorless
60 MoO3-10 Li2O-10 Na2O-20 K2O
colorless
2. Experimental 2.1. Sample Preparation. The glasses were first synthesized by the conventional melt-quenching method. Powdered starting materials such as tungsten oxide WO3 from Alpha (99,8% pure), molybdenum oxide MoO3 from Aldrich (99,9% pure), and sodium polyphosphate NaPO3 from Acros (99+% pure), were mixed and heated at 400 °C for 1 h to remove water and adsorbed gas. Then, the batch was melted at 1000 °C for all compositions. Liquid was kept at this temperature for 30 min to ensure homogenization and fining. Finally the melt was cooled in a brass mold at room temperature to check sample color under the cooling conditions. The noncrystalline state of each sample was checked by X-ray diffraction. Glass samples were synthesized in the binary systems NaPO3-WO3 and NaPO3-MoO3. The molar concentrations, colorations, and visual aspect of vitreous samples are presented in Table 1, Figure 1, and Figure 2, respectively. Samples NW50Ce and NW60Ce were prepared in the same experimental conditions as the original compositions NW50 and NW60, but the starting compositions were doped with 0.2% of CeO2, which is a wellknown oxidizing agent. In another test to oxidize these high MO3 concentrated samples, the oxidizing couple Sb2O3/NaNO3 was added to the starting materials in compositions NW50 and NW60 as shown in Table 1. The starting powders were first kept at an intermediary temperature (750 °C) for 2 h in order to promote the chemical reaction
4NaNO3 + Sb2O3 f 2Na2O + Sb2O5 + 4NO2
(1)
After this first step, the mixtures were melted at 1000 °C for 30 min. During melting, O2 is produced in the liquid by dissociation of Sb2O514
Sb2O5 f Sb2O3 + O2
(2)
The formation of O2 bubbles in the melt promotes homogeneization of the melt as well as oxidation of reduced species (M5+) by the reaction
4M5+ + O2 f 4M6+ + 2O2Finally, glass synthesis parameters such as melting temperature and cooling rate were varied to investigate their influence on the final transmittance of the glass. For the system NaPO3-
WO3, composition NW50 was chosen because of its high absorption in the visible and prepared using the same conventional melting-quenching method but using 3 different melting temperatures: 850, 1000, and 1150 °C. The resulting glasses were labeled as NW50-850, NW50-1000, and NW50-1150, respectively. In the system NaPO3-MoO3, composition NMo30 was chosen for the same reasons, melted at 2 different temperatures (850 and 1150 °C), and quenched as described below. The resulting samples were labeled NMo30-850 and NMo30-1150, respectively. A third sample of composition NMo30 was prepared by an unusual technique. The starting materials were mixed and melted at 1150 °C for 30 min. The melt was then slowly quenched to room temperature using a cooling rate of 1 °C/min. The vitreous nature of the resulting sample labeled NMo30-SQ (Slow Quenching) was checked by thermal analysis and X-ray diffraction. 2.2. Physical Measurements. Absorption and transmittance spectra in the UV, visible, and near-infrared range were obtained between 200 and 2500 nm using a spectrophotometer Varian Cary 500 on polished vitreous samples. Raman scattering spectra were recorded at room temperature in the wavenumber range from 1300 to 200 cm-1 using a Renishaw Micro-Raman spectrometer with a single monochromator and a filter. The excitation was provided by a He-Ne laser at 633 nm, and the monochromator was operating with a resolution of about 6 cm-1. All measurements were carried out on bulk vitreous samples. Tungsten LI (12100 eV) and molybdenum K (20000 eV) X-ray absorption near-edge structure (XANES) measurements were performed on the XAS beam line at LNLS (CampinasBrazil) working with an electron energy of 1.37 GeV and a maximum electron current of 250 mA. A double crystal Si (111) monochromator used to obtain the monochromatic X-ray incident beam was first calibrated using metallic tungsten and molybdenum foils at the W L1 and Mo K absorption edges, respectively. The energy steps and counting times were adjusted to improve the resolution. Data were collected at room temperature in transmission mode using ionization chambers filled with Helium. Samples were prepared by grinding and sieving glasses to obtain fine powders with regular grain size of 20 µm. The mass powder necessary for the measurements has been previously computed to avoid saturation effects and to optimize the signal-to-noise ratio. The powders were then dispersed in ethanol and deposited in a microporous membrane to obtain sample deposits. Experimental spectra were treated with the following procedure: the absorption background was subtracted using a linear extrapolation, E0 was determined at the inflection point of the absorption edge, and the spectra then normalized by taking an energy point around 50 eV above the edge. 3. Results and Discussion Vitreous samples can be obtained in the binary system (100 - x)NaPO3-xMoO3 with x varying from 0 to 60%. The structural evolution of the vitreous network in function of the composition was already studied by solid-state NMR, and it was shown that molybdenum can be present both as MoO6 clusters that act as modifier between covalent chains and as MoO4 playing the role of intermediary between phosphate PO4 inside the covalent chains.12 Visual aspect and transmittance spectra are shown in Figures 1 and 3, respectively, in function of the composition. These results show that incorporation of MoO3 in a phosphate glass induces an intense green color and even dark samples for compositions containing more than 20% of MoO3. On the basis of the results shown in Figure 3, it can
Redox Behavior of Molybdenum and Tungsten
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Figure 1. Visual aspect of the vitreous samples in the system (100 - x)NaPO3-xMoO3 with x varying from 0 to 60 mol %.
Figure 2. Visual aspect of the vitreous samples in the system (100 - x)NaPO3-xWO3 with x varying from 0 to 60 mol %.
Figure 3. Transmittance spectra in the UV, visible, and near-infrared ranges for compositions (100 - x)NaPO3-xMoO3 with x varying from 0 to 60 mol %.
be seen that this strong coloration is related with an intense absorption band centered around 800 nm. For samples containing more than 20% in MoO3, this absorption band is clearly broad enough to prevent transmission of visible wavelength witch results in dark samples. This absorption in the visible and near-infrared spectral range is well-known in molybdenumbased materials and is commonly attributed to the presence of reduced molybdenum species such as Mo5+ and Mo4+. In this case, the large absorption band is associated with two distinct absorption mechanisms: the absorption due to electronic d-d transitions and polaron absorption, which can be defined as the transfer of an electron from a reduced species (like Mo5+) to an oxidized species (like Mo6+) after photon absorption. The reason of this tendency to reduce is not well understood in glasses but is probably related to oxygen vacancies in the vitreous network and subsequent reduction to maintain electric neutrality. On the basis of previous structural studies described below,12 all compositions in the system NaPO3-MoO3 contain a proportion of molybdenum atom as MoO6 clusters. These clusters can clearly behave as nonstochiometric oxide entities as in crystalline MoO3 to minimize the free energy of the system. For that reason, the reduction phenomenon can occur in all compositions, and the resulting samples exhibit an intense absorption in the visible. These reduced species are responsible for the well-known dark color of molybdenum-containing glasses and strongly limit applications of these glasses in optical fields. In the same way, vitreous samples can be obtained in the binary system (100-x)NaPO3-xWO3 with x varying from 0 to 60. Previous structural studies by solid-state NMR and Raman
Figure 4. Transmittance spectra in the UV, visible, and near-infrared ranges for compositions (100 - x)NaPO3-xWO3 with x varying from 0 to 60 mol %.
suggested a different behavior of tungsten compared to molybdenum when incorporated in a phosphate glass.15 In this case, structural investigations showed that tungsten oxide entities act only as intermediaries inside the covalent chains and are only octahedral (WO6). At low WO3 concentrations, isolated WO6 are inserted inside the original phosphate chains and form P-O-W bonds. At higher WO3 concentrations (40% WO3), WO6 octahedra not only act as intermediaries inside the covalent chains but start to form clusters of WO6 identified by W-O-W bonds in Raman spectra. These highly polarizable clusters are probably responsible for nonlinear optical properties and photochromic properties.8,9,16,17 Figures 2 and 4 present the visual aspect and transmittance properties in the UV, visible, and nearinfrared of vitreous samples prepared in the system NaPO3WO3. The general behavior is similar to the NaPO3-MoO3 system: an increase in WO3 concentration results in a blue coloration of the samples attributed to a broad absorption band centered around 900 nm. In the same way, this blue coloration of tungsten oxide based materials is well-known and is generally explained in terms of absorption of visible light by reduced species W5+ and W4+. Absorption of photons can occur both by electronic d-d transitions and by polaron absorption between reduced and oxidized species18
hν + W5+ (A) + W6+ (B) f W5+ (B) + W6+ (A) hν + W5+ (A) + W4+ (B) f W5+ (B) + W4+ (A) An important difference in the behavior of both binary systems is that tungsten-containing glasses exhibit the broad absorption band only for high WO3 concentrations (40% WO3). Low WO3
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Figure 5. Transmittance spectra in the UV, visible, and near-infrared ranges for samples NW50 and NW60 undoped and doped with oxidizing agent CeO2.
Figure 6. Transmittance spectra in the UV, visible, and near-infrared ranges for samples NW50 and NW60 untreated and treated with oxidizing couple Sb2O3/NaNO3.
concentrated samples show a high transparency in the visible and near-infrared and no reduction evidence. This singular difference between these two “twin” elements can be explained by their specific behavior in the glass phosphate matrix. Unlike molybdenum, tungsten can be found as WO6 clusters in these glasses only for high WO3 concentrations. Reduction of metal M is probably due to oxygen vacancies formed by oxygen oxidation and results in nonstochiometric MO3-x entities as in crystalline MO3. These oxygen vacancies only decrease the free energy in structures of corner-sharing MO6 octahedra. Consequently, a reduction resulting in clear visible absorption only occurs in vitreous structures containing MO6 clusters. Structural studies described below revealed that vitreous samples in the system NaPO3-WO3 only present WO6 clusters for concentrations of WO3 higher than 30 mol %. Thus, it can be easily understood that only these compositions exhibit the absorption band in the visible-near-infrared spectral range. It has already been said that these glasses should be interesting for applications in optic fields such as nonlinear optics or photochromism. However, this reduction ability of Mo6+ and W6+ and subsequent broad and intense absorption band strongly limits their potential in optics. For that reason, decreasing or even removing the absorption is fundamental for further applications. The first and simpler way to promote oxidation of the batch is the use of oxidizing agents or oxidizing systems that release gaseous O2 during melting. Cerium oxide CeO2 is a well-known oxidizing agent and was used to dope high MO3 concentrated compositions, particularly compositions containing 50 and 60% of MoO3 and WO3. The results showed that CeO2 is efficient to oxidize reduced species in samples NW50 and NW60. The visual aspects and transmittance spectra presented in Figure 5 for undoped and doped NW50 and NW60 samples clearly show that the absorption band is removed in both cases and that the resulting glasses are transparent in the visible. Reduced W5+ cations lose an electron and reduce Ce4+ to Ce3+. In addition, it can be seen from Figure 5 that incorporation of cerium in the samples shifts the band gap position to higher wavelength and results in the yellowish color of glasses. Since the doping concentration of CeO2 is 0.2 mol %, the results suggest that the concentration of reduced W species is lower than 0.2% in the glass. This proportion is consistent with the reduced species proportions in well-known nonstochiometric tungsten oxides WO3-x with 0.2 < x < 0.5. However, further investigations by X-ray photoelectron spectroscopy (XPS) are in progress to quantify the proportion of reduced and oxidized
species in such glasses. Another oxidizing system was added to these dark compositions: antimony/nitrate. This chemical couple is well-known and widely employed in glass manufacturing to oxidize and homogenize glass melts. This property is related with the ability of this system to release gaseous O2 in the liquid during melting. The detailed chemical reactions are described in the experimental part. On the basis of spectra and pictures shown in Figure 6, it appears that the chemical system Sb2O3/NaNO3 is efficient to oxidize and improve the transparency of composition NW50. The resulting sample (NW50SN) is highly transparent, yellowish colored, and exhibits a good optical quality with high homogeneity. This sample NW50SN was previously identified as photochromic under laser exposure and is able to be used for 3D optical storage.9,16,17 On the other hand, this chemical system was not able to oxidize composition NW60. As shown in Figure 6, the resulting sample NW60SN presents a higher transparency than NW60 in the visible but still exhibits the intense absorption band centered around 900 nm and is deep green. These oxidizing systems CeO2 and antimony/nitrate were added to molybdenum-containing glasses but were not able to improve the transparency of the samples. This different behavior can be related with different values of redox potentials in couples Mo5+/Mo6+ and W5+/W6+ in function of Ce4+/Ce3+ or to a higher proportion of reduced species in molybdenum phosphate glasses. In the case of Sb2O3/ NaNO3, it is assumed that the oxidizing ability of O2 is weaker than the reduction tendency of Mo in MoO6 clusters. At this point, none external oxidizing agent was useful to successfully oxidize MoO3-containing glasses and internal synthesis parameters such as melting temperature and cooling rate were varied in a tungsten and molybdenum-based glass in order to determine their influence on the redox mechanisms. Figure 7 presents the visual aspect and absorbance spectra of sample NW50 melted at 850, 1000, and 1150 °C, respectively, during 30 min and quenched in a brass mold preheated near Tg. A surprising and unusual result is that the transparency of the glass is strongly improved when the melting temperature decreases. An opposite behavior should be expected considering that a higher melting temperature should favor oxidation of reduced species. However, NW50 melted at 1150 °C exhibits a strong absorption formed by at least 2 bands centered at 600 and 900 nm. The first one is attributed to electronic d-d transitions in W5+, whereas the second band is due to polaron transitions between W5+ and W6+. The intensity of this absorption decreases for sample NW50 melted at 1000 °C and
Redox Behavior of Molybdenum and Tungsten
Figure 7. Absorbance spectra in the UV, visible and near-infrared range for sample NW50 melted at 850, 1000, and 1150 °C.
disappears in a sample melted at 850 °C. This last sample NW50-850 is transparent and colorless as shown in Figure 7. This behavior of tungsten-based glasses has never been reported and can be a very simple way to obtain glasses with a desired transmittance or absorbance depending of the expected application. The redox phenomena involved in the liquid phase must be studied more carefully by electrochemical techniques, but we assume that thermodynamic redox equilibrium takes place in the batch between W5+ and W6+ and that the equilibrium constant value is dependent on the temperature. In this model, it appears that a lower melting temperature promotes the shift of equilibrium to form oxidized species (W6+), whereas a higher melting temperature shifts the equilibrium to reduced species (W5+) and, consequently, the production of colored glasses. For that reason, laws of chemical equilibria can be used to suggest that the oxidation of W5+ to W6+ is an exothermic phenomenon. When the melting temperature is decreased, the system tends to shift the equilibrium to promote an exothermic reaction and to minimize the perturbation. In this case, experimental results show that this exothermic reaction involves the oxidation of tungsten to W6+ and, thus, the production of transparent glasses. On the other hand, an increase of the melting temperature can be minimized by the promotion of an endothermic reaction with seems to be the reduction of tungsten to W5+ and the production of colored glasses. Since every reduction must be associated with an oxidation process, the ions oxidized at higher temperatures in this binary system must be oxygen anions O2- from W-O bonds. When O2- is oxidized, gaseous O2 is formed and induces oxygen deficiency in the liquid phase. After vitrification, this oxygen deficiency can be understood as oxygen vacancies in the glass network. Since the shift in redox equilibrium in function of melting temperature was clear in the system NaPO3WO3, the same procedure has been employed in the system NaPO3-MoO3. Samples of composition NMo30 were melted at 850 and 1150 °C and quenched in a brass mold preheated near Tg. The visual aspect and transmittance spectra of resulting samples NMo30-850 and NMo30-1150 are presented in Figure 8. These results were expected, and it was observed that a lower melting temperature decreases the intensity of the absorption band and favor the formation of a more transparent glass. However, this last sample melted at 850 °C still presented a strong absorption band in the visible and near-infrared regions. An obvious way to minimize this absorption should be the use of a lower melting temperature. Unfortunately, such melting temperature (
