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Preparation of reduced molybdenum oxides (MoO2MoO3) by a sol-gel method
Journal of Non-Crystalline Solids 101 (1988) 255-262 North-Holland, Amsterdam
255
PREPARATION OF REDUCED MOLYBDENUM OXIDES (MoOz-MoO3) BY A SOL-GEL METHOD Masamitsu NAGANO * and Martha GREENBLATT Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA Received 19 May 1987 Revised manuscript received 11 December 1987
Molybdenum oxides were prepared by a sol-gel method using molybdenum chloro-ethoxide as a precursor. MoC15, ethanol and potassium metal were used as starting materials. KCl by-product was filtered out. All reactions proceeded in methane dichloride solvent at room temperature. Blue amorphous precipitates which look like "molybdenum blue" were obtained after evaporation of solvent. This product crystallized to MoO 3 on heating to 300 ° C in air, to MoO 2 on heating in vacuum and to Mo4Oal together with MoO 3 and MoO 2 when heated in He gas. The valence of molybdenum in the blue, as-prepared sample, ranges from 4 to 6 and could not be determined unambiguoulsy by IR and XPS spectra. The molybdenum oxide products incorporated 25-35 wt% of physically adsorbed solvent and/or reaction product (H20, CEHsOH ) and retained ethoxy and hydroxyl groups; the adsorbed species and retained ethoxy groups are lost on heating at - 2 0 0 ° C and at - 4 0 0 o C, respectively. About 1-6 wt% of chlorine was also retained in the product, most of which was removed on heating to 160 ° C in air.
1. Introduction
The sol-gel synthesis of glasses and ceramics has received a great deal of interest recently. The advantages of the sol-gel method have been reviewed in many publications [1-5]. Gel-derived transition metal oxides obtained with characteristic electrical and optical properties include: semiconducting V205 [6], electrochromic WO 3 display devices [6,7], photoelectrode of TiO 2 thin film [8], monolithic gels of TiO 2 [9], ZrO 2 [10] and NbzO 5 [111. MoO 3 amorphous films obtained by vacuum evaporation show photo- and electrochromism as do amorphous films of WO 3 and V205 [12]. In this experiment, the preparation of molybdenum oxide with the composition ranging from MoO 3to MOO2, was attempted by a sol-gel method using molybdenum chloro-ethoxide as a precursor. There * Present address: Department of Industrial Chemistry, Faculty of Science and Engineering, Saga University, Saga, Japan. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
is little information about molybdenum oxide gels, even though many other metalfic oxides have been reported by sol-gel procedures [2]. Molybdenum(V) ethoxide is not available commercially. Metal alkoxides may be synthesized by the reaction of metal halides with alcohol. However, most of the transition metal halides undergo only partial solvolysis with alcohol [13]. MoC15 reacts with alcohols to form dimeric products [MoC13(OR)212 ( R = alkyl group) [14]. A base such as sodium ethoxide has been used successfully for the preparation of metal alkoxide free from halides [13], but little information can be found in the literature on the preparation of molybdenum alkoxides [15]. Potassium ethoxide, prepared form potassium metal and ethanol, is more basic than the sodium analog and therefore was chosen for the preparation of molybdenum chloro-ethoxide from MoC15. The reaction of MoC15 and potassium ethoxide in ethanol results in the formation of molybdenum tetraethoxide. The reduction of Mo(V) to Mo(IV) is probably related to the oxidation of ethanol to acetaldehyde
256
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
[15]. Therefore, in this experiment, methane dichloride was used as solvent in an attempt to prevent the reduction of Mo(V). As-prepared sample was a blue amorphous material, which looked like "molybdenum blue", mixed valent oxomolybdenum species [16,17]. The preparation and characterization are reported here.
2. Experimental The experiments began with the preparation of potassium ethoxide. It was synthesized from potassium metal (Aesar) and anhydrous ethanol (Aldrich, H20 < 0.005%) in anhydrous methane dichloride (Aldrich, H20 < 0.005%), according to the reaction in eq. (1): K + EtOH CH2C12)EtOK + ½H2,
(1)
where Et is an ethyl group (-C2Hs). EtOK does not dissolve in methane dichloride but is dispersed in it. The cloudy solution of EtOK was added to MoC15 (Aesar, 99.6%) dissolved in methane dichloride (0.2 g MoC15/100 ml CH2C12). An equimolar quantity of EtOK, required for complete replacement of chloride in MoC15, was used according to eq. (2) with x = 5.
In some cases stoichiometric amount of H20, required to hydrolyze molybdenum chloro-ethoxide, was intentionally added to the filtrate. Blue precipitates were obtained after evaporation of the solvent from the colloidal solution. Some of the precipitates were heated up to 600 °C at a heating rate of 1° C/rain in air, He or vacuum ( - 10 -2 Torr). X-ray powder diffraction (XPD, Paar, HTK-10) using a computer controlled Scintag PAD V system with monochromatized CuKa radiation, IR spectroscopy using a Perkin-Elmer Model 283 spectrometer and X-ray photoemission spectroscopy (XPS) using a PHI 550 ESCA/SAM system were employed for sample characterization. Differential scanning calorimetry (DSC) and thermal gravimetrical analysis (TGA) data were obtained using a DuPont 9900 and 1090B automatic thermal analyzer systems, respectively. A Beckman-Spectrometrics Spectra Span IIIB DCT Basic Multi dc-Argon plasma emission spectrometer (PES) and ion chromatography (Dionex Model 2010i) were used for analysis of potassium and chlorine retained in the products.
3. Results and discussion
MoC15 + xEtOK ~ MoC15_x(OEt)x + xKCL.
(2) In order to avoid moisture, the above mentioned procedures were performed in a dry box. The precipitate KC1 was removed by filtration. The filtrate was allowed to stand for nearly one week in ambient air, while stirring continuously, to hydrolyze the molybdenum chloro-ethoxide by the spontaneous uptake of moisture, to polymerize the resulting molybdenum hydroxide ethoxide, and finally remove the solvent by evaporation according to the following reactions (eqs. 3, 4): MoC15_x(OEt), + (y + z)H20 M°C15 -~-y (OEt)x-z (OH)v +z + yHC1 A +zEtOH, A -EtOH and/or -HzO)colloidal product. polycondensation
(3) (4)
In reaction (1) the resulting EtOK does not dissolve in methane dichloride. When an excess of EtOH over that required for equimolar addition to K is used, a clear solution resulted. However, as-prepared sample was contaminated with potassium. Part of the as-prepared sample was converted to K2Mo20 7 on heating to 600°C in air. This is probably because the excess ethanol dissolved the KC1 formed in reaction (2) to some extent, or because of the formation of a soluble potassium salt such as KMo(OEt)6, a molybdenum analogue of KW(OEt)6 (the latter was probably formed by the reaction of WC15 with potassium ethoxide in ethanol similar to reaction (2)) [18]. Thus potassium remained in the filtrate. The filtrate was greenish-yellow soon after filtration and then gradually turned blue as it took up water from the ambient atmosphere. Blue colloidal dispersion appeared after - 1 0 h. It took
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
nearly one week to remove the solvent by evaporation at room temperature. The precipitate products after evaporation were in powder form. When the filtrate was intentionally protected from ambient air, neither precipitate nor color change occurred even in the presence of light. Sol-to-gel transition was not clear. However, when a drop of HNO 3 was added to the blue colloidal dispersion, a yellowish sol precipitated. Acid catalysts may promote the gelation. The effect of acid catalysts is under further investigation. The intentional addition of water to the filtrate produced a small amount of oily phase and a yellowish-brown precipitate, which transformed into a blue precipitate after standing for a day, similar to that obtained by spontaneous uptake of water. As-prepared sample may consist of different molybdenum oxides and hydrates resulting from a variety of the extent of hydrolysis and polymerization. Blue color suggests the formation of so-called "molybdenum blue" (for example, MO02.75_2.93 • x H 2 0 [17]) which contains mixed valent molybdenum. As-prepared sample is expected to incorporate the solvent and reaction products (EtOH, H20), and is probably contaminated with retained chloride, ethoxy and hydroxyl groups. The structure and composition of the products were investigated by XPD, TGA, DSC, IR and XPS measurements.
3.1. X P D
All the precipitates, "as-prepared", are amorphous as shown in fig. 1A. The amorphous products are converted to MoO 3 on heating to 300°C in air (fig. I(B,C)) and to a mixture of crystalline MOO3, (I,- and ,/-) Mo40 n and/or M o O 2 o n heating in He (fig. 1D). The relative amount of each phase present varied from sample to sample. On heating the initial amorphous phase in vacuum, M o O 2 formed (fig. 1E). The valence of Mo in the as-prepared sample should be greater than 4. The exact value could not be determined from the above results, because the retained hydroxyl and ethoxy group affect the redox reactions of Mo compounds when they decompose.
257
as-prepared
A
4 0 0 ° C in air llJ
I"
•
%°11°~ Ill ~.
D
I~ I I
o 1
~11 ~
• 600°C in He
o~ ~ I o ?. o°,1o ~ . P o ~
@l'l~li',
•
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20
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r
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'
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60
Fig. 1. X-ray powder diffraction patterns of samples heated at various conditions, O; MOO3, ©; 7l-Mo4On, A; ~-Mo4On, A; MOO2.
3.2. TGA
A sample prepared by the spontaneous uptake of H20 lost about 13% of its original weight gradually as it was heated to 180°C and then it lost about 15% of its original weight on heating to 370°C (fig. 2A). The first weight loss may be ascribed to the desorption of physically adsorbed solvent and reaction product (water, ethanol), the second to the loss of ethoxy group retained in the sample. Similar weight loss due to loss of solvents, followed by the loss of ethoxy groups are reported in the TGA of SiO2 gels [19]. The gradual weight losses suggest that the as-prepared sample is composed of different molybdenum oxide hydrates with various extent of hydration and polymerization, and with a variety of the retained hydroxyl and ethoxy group content, so that these groups exist in a variety of environment in the molybdenum oxide hydride matrix. The total weight loss on heating to 600 °C ranged between 25-35% depending on the content of the incorporated ethoxides or hydroxides.
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
258
Temp (°C ) 300
0
600
sample as in specimens formed by the spontaneous uptake of H20.
exothemlic 3.3. D S C endothermic
~ 9o .(
8o 7o 3oo
A '
'
zbo
'
Temp (K) Fig. 2. TGA curves (A and B) and DCS curves (C, D and E) in air (except for sample D). Samples A, C and D obtained by spontaneous uptake of water while samples B and E by the intentional addition of H20.
The sample prepared by the intentional addition of water lost 14% of its original weight on heating to 150°C and 7% more upon heating to about 220 ° C (fig. 2B). The intentional addition of H 2 0 promotes the hydrolysis of ethoxy groups (eq. 3). Dehydration of molybdenum oxide hydrates, as well as the desorption of the adsorbed solvent and reaction product, and the decomposition of ethoxy group as mentioned above may be a cause for the weight loss. For example, dehydration of MoO 3 • 2 H 2 0 proceeds in two steps; the 1st step occurs at 6 0 - 8 0 ° C , the second step to MoO 3 at 110 to 125°C [17]. Mo4010(OH)2 (MOO2.75 • 0.25H20 ) loses the water at 230-240 ° C ( P r h o = 10 Torr) [20]. The H 2 0 retained in this sample might also promote the hydrolysis of ethoxy groups on heating. The rapid weight losses in this sample, compared with the case when the water uptake is spontaneous (fig. 2A), suggests that the solvents or the hydroxyl and ethoxy groups retained do not have as complicated interactions with the molybdenum oxide hydride matrix in this
The DSC curve of a sample, obtained by spontaneous uptake of water, shows two broad exothermic peaks at - 1 5 0 ° C and - 3 3 0 ° C on heating in air (fig. 2C). The desorption of physically adsorbed solvent and reaction products which should take place endothermically below 150°C [19] was not clearly evident, although it was assumed in the T G A curves as discussed above. The peak at - 150 ° C may be due to the degradation of the solvent, CH2C12, which thermally decompose around 120 ° C in air [21]. Thus, the expected endothermic peak in the vicinity of - 1 5 0 ° C corresponding to the desorption of various adsorbed species may be masked by the exothermic decomposition of CH2C12. The other broad exothermic peak at - 3 3 0 ° C corresponds to the oxidation of residual ethoxy groups as presumed from the weight loss in the T G A around this temperature. There are two exothermic peaks on heating in He (fig. 2D). These are weaker and appear at 2 0 - 3 0 ° C higher temperatures than the corresponding peaks in air. In He, the degradation of methane dichloride and ethoxy groups may be somewhat depressed. The samples prepared by the intentional addition of water, indicated an endothermic peak at 1 0 0 ° C (fig. 2E), which is attributed to dehydration or the evaporation of the absorbed species. The weak broad exothermic peak at - 2 0 0 ° C corresponds to the 2nd step of the weight loss in the T G A curve (fig. 2E). This weak peak suggests that of the two possible causes, attributed by the T G A results above - endothermic dehydration and exothermic removal of organic species (CH2C12, ethoxy group) by decomposition in ambient oxygen, - the latter is the more important in the process. However, the disappearance of the exotherrnic peak at - 330 ° C implies that hydrolysis is promoted and the amount of ethoxy groups retained in this sample are less than in the sample obtained by the spontaneous uptake of water. The final products, after heating to 600 o C, were the
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
g
I,--
4000
3000
2000 1600 1200 Wave number (cm -I )
800
400
Fig. 3. Changes in IR spectra of "as-prepared" sample on heating in air.
same as those obtained from a sample prepared by the spontaneous uptake of water. 3.4. I R spectra
The IR spectra of the samples obtained by spontaneous uptake of water and corresponding spectra of a sample heat treated in air and in vacuum are shown in fig. 3 and fig. 4, respectively. Similar spectra were observed for the "asprepared" sample by the intentional addition of water. The difference in the extent of hydration by the two procedures suggested by the TGA and DSC data is not confirmed by IR measurement.
4000
3000
2000 1600 1200 Wave number (cm -~ )
800
400
Fig. 4. Changes in IR spectra of "as-prepared" sample on heating in vacuum ( - 10- z Tort).
259
"As-prepared" samples show O - H stretch absorption ( - 3450 cm -1) and C - H stretch absorption (2860-2940 cm-1), which are due to the solvent (CH2C12) and reaction products (H20, EtOH) (figs. 3, 4). These peaks disappear on heating up to 400 °c in air (fig. 3). The broad peak at 1600 cm 1, which also becomes weaker on heating, may be assigned to a bending vibration of H 2 0 [22]. These absorption bands, attributed to water or hydroxyl group are still observed on heating up to 300-to-400 ° C. This may be due to some water a n d / o r hydroxyl groups which are constitutionally incorporated (in contrast to adsorbed) in the structure of the molybdenum oxide hydrates. The evolution of the IR spectra with temperature seems to correspond to the polymerization of polymolybdate hydrates to MoO3-MoO 2, accompanied by the condensation reaction of the retained hydroxyl and ethoxy groups. The characteristic features of the polymolybdate structures are MoO 6 octahedra sharing edges (and occasionally comers). Polymolybdates usually have three nonequivalent type of oxygens; terminal oxygen (Or), bridging oxygen (Ob) and central oxygen (Oc) [23]. The basic elements of MoO 3 structure are similar MoO 6 octahedra, which form infinite sheets by edge and comer sharing. To clarify the observed IR bands in the products, a section of the unit cell of MoO 3 is shown in fig. 5. There also are terminal oxygen (O 1) and bridging oxygens (O 2, 03, 03). The as-prepared sample could not be identified as a specific polymolybdate, probably because it consists of a variety of polymolybdates. The IR spectra were interpreted based on the group frequencies of polymolybdates given in ref. [24]. The shoulder peak seen at 980 cm -1 in the "as-prepared" sample becomes sharp and strong after crystallization to MoO 3 (fig. 3); it is assigned to the t ' ( M o - O t ) stretching mode of as-prepared polymolybdate and converts to the p(Mo-Ol) stretching mode of MoO 3 upon heating [25]. There is a broad band centered around 760 cm -1 in the "as-prepared" sample. Most polymolybdates (Mo6029--[M036O112 - 16H20] 8- have v ( M o - O - M o ) stretching modes in this region (870-400 cm -~) and the absorption peaks are more or less sharper than the band at - 760 cm-a
260
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
,
/
I
........................... ® ....................... o
/1.73
Mo,,Z~__
x
...................
2.33 A
/ . . . . . . . . . . . . . . . . . .
/
........ 1)2
. . . . . . .
0 . . . . . . . . . . . . . . . . . . . .
Fig. 5. A section of the unit cell of MoO3 drawn schematically (at y = 1/2), to help identify vibrational modes assigned in the IR spectra. Dotted lines are unit-cell edges. Solid lines are Mo-O bonds at z = 1/4 and broken lines Mo-O bonds at z = 3/4 (after ref. [27]).
in fig. 3 [23,24]. The broad band may be due to the formation of a variety of polymolybdate structures with different degrees of polymerization. This band becomes gradually stronger on heating the sample, which suggests further polymerization. Finally, at around 4 0 0 ° C the 760 cm -1 band is displaced by two peaks; a sharp peak at 890 cm - t with a shoulder at 820 cm - ] and another broad peak centered at 650 cm -1 (fig. 3). The former is assigned to the v(Mo-O3) stretching and the shoulder to the ~,(Mo-O2) stretching modes of MOO3124 ]. The broad band at 650 cm - ] is due to the vibrational mode [26] associated with an oxygen coordinated to three different Mo ions (i.e. O3-3Mo, fig. 5). The bands at 480 c m - x and at 2350 c m - a seen in the sample which is heated to 400 ° C, are also observed in MoO 3 but these have not been assigned to any specific vibrational modes so far [22,27]. The two peaks at 370 cm -x and at 310 cm -1, which appeared on heating the sample to 400 ° C (fig. 3) may be assigned to 8 ( O 1 - M o - O 2 ) (362 and 367 cm -~) and 8(O1-Mo-O3) (285 cm -I) bending modes in MOO3, respectively [27]. These bands were weak and broad before crystallization (below 300 ° C) of the sample.
When the sample was heated in vacuum, both the stretching ~,(Mo-O) and deformation absorptions 8 ( O - M o - O ) associated with MoO 3 disappear (fig. 4). MoO 2 has no strong absorption in this I R range while it has weak absorptions at 892, 950 and 980 cm -1 [25]. It has been indicated above that "as-prepared" samples heated in vacuum yield MoO 2. The resemblance of the I R spectra of the "as-prepared" sample to that of MoO 3 (fig. 3) suggests that the gel-prepared m o l y b d e n u m oxides are structurally more closely related to MoO 3 than M o O 2. 3.5. X P S
In order to obtain some information of the valence state of m o l y b d e n u m in these products, XPS spectra were measured. Besides molybdenum and oxygen, carbon was detected in all samples. Mo(3d) lines are shown in fig. 6. The binding energy was corrected for the static charge of the sample based on the C(ls) line. It is known that the 3d peak of MoO 3 appears at higher binding energy by a few eV than that of MoO 2 [28]. However, no significant differences in 3d3/2
,
240
,
,
,
1
,
I
,
,
r
.
.
.
.
230
i
i
i
i
i
I
220
Binding energy (eV) Fig. 6. Change in XPS lines of Mo(3d) on heating in air (A, B
and C) and in vacuum ( - 1 0 -2 Torr) (D and E). A: "as-prepared", B: 200 o C, C: 400 o C, D: 300 o C, E: 500 o C. Sample C is MoO3 and E is MoO2 (identified by XPD).
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
binding energy were found between MoO 3 (fig. 6C) and MoO 2 (fig. 6E) which were obtained by heating "as-prepared" samples in air and in vacuum, respectively. The 3d 5/2 binding energy of - 2 3 0 eV observed is close to that reported for MoO 2 (229.4 eV) rather than that of MoO 3 (232.7 eV) [28]. The 3d 3/2 peak of MoO 3 (fig. 6C) shows a shoulder at higher energy and the 3d5/2 peak is weaker than the 3d 3/2 peak. Both peaks are poorly resolved. The 3d3/2 peak of the sample heated to 200 o C in air also shows a weak shoulder at higher energy (fig. 6B). On the other hand, the 3d5/2 peak of MoO 2 is stronger than the 3d3/2 peak. The XPS peaks corresponding to MoO 2 are better resolved than those of MoO 3. The XPS peaks of "as-prepared" sample (fig. 6A) and the sample heated to 3 0 0 ° C in vacuum (fig. 6D) resembles that of MoO 2 (fig. 6E) as far as the shape and intensity of the 3d peaks are concerned. There remains some uncertainty in the above analysis; the binding energy of C(ls), which was used as the reference for correction of the static charge of samples, might not be the same for each XPS experiment as the chemical species containing the C atom may have reacted to form different species. In that case, the position of the peaks in (fig. 6 A - D ) is not certain. Furthermore, XPS gives information about the surface only tens of angstroms below the sample surface. A partly reduced MoO 3 crystal with mixed oxidation state of molybdenum ( + 4, + 5, + 6) is reported to show a poorly resolved 3d spectrum with shoulders [29]. The shoulders and the poor resolution in the 3d spectra of MoO 3 obtained in this experiment may be due to partial reduction of Mo 6+ to Mo 4÷ on the sample surface by evacuation to 10 -6 Torr during XPS measurement; the 3d5/2 peak (232.7 eV) and 3d3/2 peak (235.9 eV) of Mo 6+ [28] may be superimposed by the doublet of Mo 4÷ (229.4 eV and 232.6 eV). A MoO 3 crystal reduced by UV irradiation in vacuum is reported to show a XPS doublet in which the 3ds/2 binding energy is 231.4 eV; this peak was assigned to Mo 5+ [29]. There is little evidence for the existence of Mo 5÷ in fig. 6, however, it cannot be ruled out unambiguously without a more accurate analysis and a thorough fitting of the data. The blue molybdenum bronze, K03MoO 3 shows an asym-
261
metric and poorly resolved XPS 3d doublet which has been attributed to Mo 6 + and Mo 5+ distributed on three inequivalent Mo sites in the structure
[301. Inequivalent Mo sites are certain to exist in the molybdenum oxides obtained here by the sol-gel method. Possibly, because of the poor crystallinity and partial reduction of the sample the number of crystallographically inequivalent molybdenums is very large, which may be responsible for the appearance of the shoulders and the poor resolution seen in fig. 6. However, the "as-prepared" sample does not show significantly asymmetrical peaks (fig. 6A) and resembles the XPS corresponding to MoO 2 remarkably well (fig. 6E). 3.6. Chemical analysis
The precipitate in reaction (2) was identified as KC1 by XPD but it had a faint brown color. A few at% of molybdenum (compared with potassium) was detected in KC1 by plasma emission spectroscopy (PES). Potassium was not detectable in "as-prepared" samples by PES. However, 1-6 wt% of chlorine was found in an "as-prepared" sample by PES. The chlorine content decreased to 0.8 wt% when the sample containing 5wt% chlorine was heated to 160 ° C, and to 0.3 wt% when heated to 600 o C. It is not clear yet whether the chlorine originates from CHzC12 or MoC15_x(OR)x.
4. Conclusion Molybdenum oxides were prepared by a sol-gel method using molybdenum chloro-ethoxide as a precursor. Molybdenum chloro-ethoxide was prepared by the reaction of MoC15 and potassium ethoxide. Potassium ethoxide was synthesized from ethanol and potassium. All reactions proceeded in methane dichloride solvent at room temperature. By-product (KC1) was filtered out. Hydrolysis of molybdenum chloro-ethoxide took place by the uptake of water from ambient air or by the intentional addition of water. Solvent was removed by evaporation at room temperature. Blue amorphous products were obtained, which look like " m o l y b d e n u m blue," mixed valent
262
M. Nagano, M. Greenblatt /Preparation of reduced molybdenum oxides
oxomolybdenum species. These were converted to MoO 3 on heating to 300 ° C in air, to MoO 2 on heating to 4 0 0 ° C in vacuum, and to Mo4Oll together with MoO 3 and MoO 2 on heating in He. IR and XPS measurement suggested the valence of Mo ranging from 4 to 6 in the as-prepared sample. "As-prepared" sample incorporated 25-35 wt% of residual solvent, reaction products and retained hydroxyl and ethoxy groups. Residual solvent and reaction products ( H 2 0 , C 2 H s O H ) were lost on heating to - 2 0 0 ° C and ethoxy groups were lost on heating to - 400 ° C. Samples also contained about 1-6 wt% of chlorine, most of which was removed on heating to 160 o C. We thank Dr. Robert Caracciolo for XPS measurement, Dr. Pin Ping Tsai Lu for ion chrom a t o g r a p h y measurements, and Prof. D o n Schleich for helpful suggestions on the preparation method. This research was supported in part by the Office of Naval Research. The automatic Scintag Powder Diffractometer was purchased partly from an NSF DMR-84-08266 Material Science Division Instrumental grant, and from funds provided by Rutgers University FAS and the Research Council.
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255
PREPARATION OF REDUCED MOLYBDENUM OXIDES (MoOz-MoO3) BY A SOL-GEL METHOD Masamitsu NAGANO * and Martha GREENBLATT Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, NJ 08903, USA Received 19 May 1987 Revised manuscript received 11 December 1987
Molybdenum oxides were prepared by a sol-gel method using molybdenum chloro-ethoxide as a precursor. MoC15, ethanol and potassium metal were used as starting materials. KCl by-product was filtered out. All reactions proceeded in methane dichloride solvent at room temperature. Blue amorphous precipitates which look like "molybdenum blue" were obtained after evaporation of solvent. This product crystallized to MoO 3 on heating to 300 ° C in air, to MoO 2 on heating in vacuum and to Mo4Oal together with MoO 3 and MoO 2 when heated in He gas. The valence of molybdenum in the blue, as-prepared sample, ranges from 4 to 6 and could not be determined unambiguoulsy by IR and XPS spectra. The molybdenum oxide products incorporated 25-35 wt% of physically adsorbed solvent and/or reaction product (H20, CEHsOH ) and retained ethoxy and hydroxyl groups; the adsorbed species and retained ethoxy groups are lost on heating at - 2 0 0 ° C and at - 4 0 0 o C, respectively. About 1-6 wt% of chlorine was also retained in the product, most of which was removed on heating to 160 ° C in air.
1. Introduction
The sol-gel synthesis of glasses and ceramics has received a great deal of interest recently. The advantages of the sol-gel method have been reviewed in many publications [1-5]. Gel-derived transition metal oxides obtained with characteristic electrical and optical properties include: semiconducting V205 [6], electrochromic WO 3 display devices [6,7], photoelectrode of TiO 2 thin film [8], monolithic gels of TiO 2 [9], ZrO 2 [10] and NbzO 5 [111. MoO 3 amorphous films obtained by vacuum evaporation show photo- and electrochromism as do amorphous films of WO 3 and V205 [12]. In this experiment, the preparation of molybdenum oxide with the composition ranging from MoO 3to MOO2, was attempted by a sol-gel method using molybdenum chloro-ethoxide as a precursor. There * Present address: Department of Industrial Chemistry, Faculty of Science and Engineering, Saga University, Saga, Japan. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
is little information about molybdenum oxide gels, even though many other metalfic oxides have been reported by sol-gel procedures [2]. Molybdenum(V) ethoxide is not available commercially. Metal alkoxides may be synthesized by the reaction of metal halides with alcohol. However, most of the transition metal halides undergo only partial solvolysis with alcohol [13]. MoC15 reacts with alcohols to form dimeric products [MoC13(OR)212 ( R = alkyl group) [14]. A base such as sodium ethoxide has been used successfully for the preparation of metal alkoxide free from halides [13], but little information can be found in the literature on the preparation of molybdenum alkoxides [15]. Potassium ethoxide, prepared form potassium metal and ethanol, is more basic than the sodium analog and therefore was chosen for the preparation of molybdenum chloro-ethoxide from MoC15. The reaction of MoC15 and potassium ethoxide in ethanol results in the formation of molybdenum tetraethoxide. The reduction of Mo(V) to Mo(IV) is probably related to the oxidation of ethanol to acetaldehyde
256
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
[15]. Therefore, in this experiment, methane dichloride was used as solvent in an attempt to prevent the reduction of Mo(V). As-prepared sample was a blue amorphous material, which looked like "molybdenum blue", mixed valent oxomolybdenum species [16,17]. The preparation and characterization are reported here.
2. Experimental The experiments began with the preparation of potassium ethoxide. It was synthesized from potassium metal (Aesar) and anhydrous ethanol (Aldrich, H20 < 0.005%) in anhydrous methane dichloride (Aldrich, H20 < 0.005%), according to the reaction in eq. (1): K + EtOH CH2C12)EtOK + ½H2,
(1)
where Et is an ethyl group (-C2Hs). EtOK does not dissolve in methane dichloride but is dispersed in it. The cloudy solution of EtOK was added to MoC15 (Aesar, 99.6%) dissolved in methane dichloride (0.2 g MoC15/100 ml CH2C12). An equimolar quantity of EtOK, required for complete replacement of chloride in MoC15, was used according to eq. (2) with x = 5.
In some cases stoichiometric amount of H20, required to hydrolyze molybdenum chloro-ethoxide, was intentionally added to the filtrate. Blue precipitates were obtained after evaporation of the solvent from the colloidal solution. Some of the precipitates were heated up to 600 °C at a heating rate of 1° C/rain in air, He or vacuum ( - 10 -2 Torr). X-ray powder diffraction (XPD, Paar, HTK-10) using a computer controlled Scintag PAD V system with monochromatized CuKa radiation, IR spectroscopy using a Perkin-Elmer Model 283 spectrometer and X-ray photoemission spectroscopy (XPS) using a PHI 550 ESCA/SAM system were employed for sample characterization. Differential scanning calorimetry (DSC) and thermal gravimetrical analysis (TGA) data were obtained using a DuPont 9900 and 1090B automatic thermal analyzer systems, respectively. A Beckman-Spectrometrics Spectra Span IIIB DCT Basic Multi dc-Argon plasma emission spectrometer (PES) and ion chromatography (Dionex Model 2010i) were used for analysis of potassium and chlorine retained in the products.
3. Results and discussion
MoC15 + xEtOK ~ MoC15_x(OEt)x + xKCL.
(2) In order to avoid moisture, the above mentioned procedures were performed in a dry box. The precipitate KC1 was removed by filtration. The filtrate was allowed to stand for nearly one week in ambient air, while stirring continuously, to hydrolyze the molybdenum chloro-ethoxide by the spontaneous uptake of moisture, to polymerize the resulting molybdenum hydroxide ethoxide, and finally remove the solvent by evaporation according to the following reactions (eqs. 3, 4): MoC15_x(OEt), + (y + z)H20 M°C15 -~-y (OEt)x-z (OH)v +z + yHC1 A +zEtOH, A -EtOH and/or -HzO)colloidal product. polycondensation
(3) (4)
In reaction (1) the resulting EtOK does not dissolve in methane dichloride. When an excess of EtOH over that required for equimolar addition to K is used, a clear solution resulted. However, as-prepared sample was contaminated with potassium. Part of the as-prepared sample was converted to K2Mo20 7 on heating to 600°C in air. This is probably because the excess ethanol dissolved the KC1 formed in reaction (2) to some extent, or because of the formation of a soluble potassium salt such as KMo(OEt)6, a molybdenum analogue of KW(OEt)6 (the latter was probably formed by the reaction of WC15 with potassium ethoxide in ethanol similar to reaction (2)) [18]. Thus potassium remained in the filtrate. The filtrate was greenish-yellow soon after filtration and then gradually turned blue as it took up water from the ambient atmosphere. Blue colloidal dispersion appeared after - 1 0 h. It took
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
nearly one week to remove the solvent by evaporation at room temperature. The precipitate products after evaporation were in powder form. When the filtrate was intentionally protected from ambient air, neither precipitate nor color change occurred even in the presence of light. Sol-to-gel transition was not clear. However, when a drop of HNO 3 was added to the blue colloidal dispersion, a yellowish sol precipitated. Acid catalysts may promote the gelation. The effect of acid catalysts is under further investigation. The intentional addition of water to the filtrate produced a small amount of oily phase and a yellowish-brown precipitate, which transformed into a blue precipitate after standing for a day, similar to that obtained by spontaneous uptake of water. As-prepared sample may consist of different molybdenum oxides and hydrates resulting from a variety of the extent of hydrolysis and polymerization. Blue color suggests the formation of so-called "molybdenum blue" (for example, MO02.75_2.93 • x H 2 0 [17]) which contains mixed valent molybdenum. As-prepared sample is expected to incorporate the solvent and reaction products (EtOH, H20), and is probably contaminated with retained chloride, ethoxy and hydroxyl groups. The structure and composition of the products were investigated by XPD, TGA, DSC, IR and XPS measurements.
3.1. X P D
All the precipitates, "as-prepared", are amorphous as shown in fig. 1A. The amorphous products are converted to MoO 3 on heating to 300°C in air (fig. I(B,C)) and to a mixture of crystalline MOO3, (I,- and ,/-) Mo40 n and/or M o O 2 o n heating in He (fig. 1D). The relative amount of each phase present varied from sample to sample. On heating the initial amorphous phase in vacuum, M o O 2 formed (fig. 1E). The valence of Mo in the as-prepared sample should be greater than 4. The exact value could not be determined from the above results, because the retained hydroxyl and ethoxy group affect the redox reactions of Mo compounds when they decompose.
257
as-prepared
A
4 0 0 ° C in air llJ
I"
•
%°11°~ Ill ~.
D
I~ I I
o 1
~11 ~
• 600°C in He
o~ ~ I o ?. o°,1o ~ . P o ~
@l'l~li',
•
'
"
'
'
I
20
'
'
'
-
'
~
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o o°o~oo °, OOo
4.00°C in vacuum
r
40 20 (Cu K~)
'
~
~
60
Fig. 1. X-ray powder diffraction patterns of samples heated at various conditions, O; MOO3, ©; 7l-Mo4On, A; ~-Mo4On, A; MOO2.
3.2. TGA
A sample prepared by the spontaneous uptake of H20 lost about 13% of its original weight gradually as it was heated to 180°C and then it lost about 15% of its original weight on heating to 370°C (fig. 2A). The first weight loss may be ascribed to the desorption of physically adsorbed solvent and reaction product (water, ethanol), the second to the loss of ethoxy group retained in the sample. Similar weight loss due to loss of solvents, followed by the loss of ethoxy groups are reported in the TGA of SiO2 gels [19]. The gradual weight losses suggest that the as-prepared sample is composed of different molybdenum oxide hydrates with various extent of hydration and polymerization, and with a variety of the retained hydroxyl and ethoxy group content, so that these groups exist in a variety of environment in the molybdenum oxide hydride matrix. The total weight loss on heating to 600 °C ranged between 25-35% depending on the content of the incorporated ethoxides or hydroxides.
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
258
Temp (°C ) 300
0
600
sample as in specimens formed by the spontaneous uptake of H20.
exothemlic 3.3. D S C endothermic
~ 9o .(
8o 7o 3oo
A '
'
zbo
'
Temp (K) Fig. 2. TGA curves (A and B) and DCS curves (C, D and E) in air (except for sample D). Samples A, C and D obtained by spontaneous uptake of water while samples B and E by the intentional addition of H20.
The sample prepared by the intentional addition of water lost 14% of its original weight on heating to 150°C and 7% more upon heating to about 220 ° C (fig. 2B). The intentional addition of H 2 0 promotes the hydrolysis of ethoxy groups (eq. 3). Dehydration of molybdenum oxide hydrates, as well as the desorption of the adsorbed solvent and reaction product, and the decomposition of ethoxy group as mentioned above may be a cause for the weight loss. For example, dehydration of MoO 3 • 2 H 2 0 proceeds in two steps; the 1st step occurs at 6 0 - 8 0 ° C , the second step to MoO 3 at 110 to 125°C [17]. Mo4010(OH)2 (MOO2.75 • 0.25H20 ) loses the water at 230-240 ° C ( P r h o = 10 Torr) [20]. The H 2 0 retained in this sample might also promote the hydrolysis of ethoxy groups on heating. The rapid weight losses in this sample, compared with the case when the water uptake is spontaneous (fig. 2A), suggests that the solvents or the hydroxyl and ethoxy groups retained do not have as complicated interactions with the molybdenum oxide hydride matrix in this
The DSC curve of a sample, obtained by spontaneous uptake of water, shows two broad exothermic peaks at - 1 5 0 ° C and - 3 3 0 ° C on heating in air (fig. 2C). The desorption of physically adsorbed solvent and reaction products which should take place endothermically below 150°C [19] was not clearly evident, although it was assumed in the T G A curves as discussed above. The peak at - 150 ° C may be due to the degradation of the solvent, CH2C12, which thermally decompose around 120 ° C in air [21]. Thus, the expected endothermic peak in the vicinity of - 1 5 0 ° C corresponding to the desorption of various adsorbed species may be masked by the exothermic decomposition of CH2C12. The other broad exothermic peak at - 3 3 0 ° C corresponds to the oxidation of residual ethoxy groups as presumed from the weight loss in the T G A around this temperature. There are two exothermic peaks on heating in He (fig. 2D). These are weaker and appear at 2 0 - 3 0 ° C higher temperatures than the corresponding peaks in air. In He, the degradation of methane dichloride and ethoxy groups may be somewhat depressed. The samples prepared by the intentional addition of water, indicated an endothermic peak at 1 0 0 ° C (fig. 2E), which is attributed to dehydration or the evaporation of the absorbed species. The weak broad exothermic peak at - 2 0 0 ° C corresponds to the 2nd step of the weight loss in the T G A curve (fig. 2E). This weak peak suggests that of the two possible causes, attributed by the T G A results above - endothermic dehydration and exothermic removal of organic species (CH2C12, ethoxy group) by decomposition in ambient oxygen, - the latter is the more important in the process. However, the disappearance of the exotherrnic peak at - 330 ° C implies that hydrolysis is promoted and the amount of ethoxy groups retained in this sample are less than in the sample obtained by the spontaneous uptake of water. The final products, after heating to 600 o C, were the
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
g
I,--
4000
3000
2000 1600 1200 Wave number (cm -I )
800
400
Fig. 3. Changes in IR spectra of "as-prepared" sample on heating in air.
same as those obtained from a sample prepared by the spontaneous uptake of water. 3.4. I R spectra
The IR spectra of the samples obtained by spontaneous uptake of water and corresponding spectra of a sample heat treated in air and in vacuum are shown in fig. 3 and fig. 4, respectively. Similar spectra were observed for the "asprepared" sample by the intentional addition of water. The difference in the extent of hydration by the two procedures suggested by the TGA and DSC data is not confirmed by IR measurement.
4000
3000
2000 1600 1200 Wave number (cm -~ )
800
400
Fig. 4. Changes in IR spectra of "as-prepared" sample on heating in vacuum ( - 10- z Tort).
259
"As-prepared" samples show O - H stretch absorption ( - 3450 cm -1) and C - H stretch absorption (2860-2940 cm-1), which are due to the solvent (CH2C12) and reaction products (H20, EtOH) (figs. 3, 4). These peaks disappear on heating up to 400 °c in air (fig. 3). The broad peak at 1600 cm 1, which also becomes weaker on heating, may be assigned to a bending vibration of H 2 0 [22]. These absorption bands, attributed to water or hydroxyl group are still observed on heating up to 300-to-400 ° C. This may be due to some water a n d / o r hydroxyl groups which are constitutionally incorporated (in contrast to adsorbed) in the structure of the molybdenum oxide hydrates. The evolution of the IR spectra with temperature seems to correspond to the polymerization of polymolybdate hydrates to MoO3-MoO 2, accompanied by the condensation reaction of the retained hydroxyl and ethoxy groups. The characteristic features of the polymolybdate structures are MoO 6 octahedra sharing edges (and occasionally comers). Polymolybdates usually have three nonequivalent type of oxygens; terminal oxygen (Or), bridging oxygen (Ob) and central oxygen (Oc) [23]. The basic elements of MoO 3 structure are similar MoO 6 octahedra, which form infinite sheets by edge and comer sharing. To clarify the observed IR bands in the products, a section of the unit cell of MoO 3 is shown in fig. 5. There also are terminal oxygen (O 1) and bridging oxygens (O 2, 03, 03). The as-prepared sample could not be identified as a specific polymolybdate, probably because it consists of a variety of polymolybdates. The IR spectra were interpreted based on the group frequencies of polymolybdates given in ref. [24]. The shoulder peak seen at 980 cm -1 in the "as-prepared" sample becomes sharp and strong after crystallization to MoO 3 (fig. 3); it is assigned to the t ' ( M o - O t ) stretching mode of as-prepared polymolybdate and converts to the p(Mo-Ol) stretching mode of MoO 3 upon heating [25]. There is a broad band centered around 760 cm -1 in the "as-prepared" sample. Most polymolybdates (Mo6029--[M036O112 - 16H20] 8- have v ( M o - O - M o ) stretching modes in this region (870-400 cm -~) and the absorption peaks are more or less sharper than the band at - 760 cm-a
260
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
,
/
I
........................... ® ....................... o
/1.73
Mo,,Z~__
x
...................
2.33 A
/ . . . . . . . . . . . . . . . . . .
/
........ 1)2
. . . . . . .
0 . . . . . . . . . . . . . . . . . . . .
Fig. 5. A section of the unit cell of MoO3 drawn schematically (at y = 1/2), to help identify vibrational modes assigned in the IR spectra. Dotted lines are unit-cell edges. Solid lines are Mo-O bonds at z = 1/4 and broken lines Mo-O bonds at z = 3/4 (after ref. [27]).
in fig. 3 [23,24]. The broad band may be due to the formation of a variety of polymolybdate structures with different degrees of polymerization. This band becomes gradually stronger on heating the sample, which suggests further polymerization. Finally, at around 4 0 0 ° C the 760 cm -1 band is displaced by two peaks; a sharp peak at 890 cm - t with a shoulder at 820 cm - ] and another broad peak centered at 650 cm -1 (fig. 3). The former is assigned to the v(Mo-O3) stretching and the shoulder to the ~,(Mo-O2) stretching modes of MOO3124 ]. The broad band at 650 cm - ] is due to the vibrational mode [26] associated with an oxygen coordinated to three different Mo ions (i.e. O3-3Mo, fig. 5). The bands at 480 c m - x and at 2350 c m - a seen in the sample which is heated to 400 ° C, are also observed in MoO 3 but these have not been assigned to any specific vibrational modes so far [22,27]. The two peaks at 370 cm -x and at 310 cm -1, which appeared on heating the sample to 400 ° C (fig. 3) may be assigned to 8 ( O 1 - M o - O 2 ) (362 and 367 cm -~) and 8(O1-Mo-O3) (285 cm -I) bending modes in MOO3, respectively [27]. These bands were weak and broad before crystallization (below 300 ° C) of the sample.
When the sample was heated in vacuum, both the stretching ~,(Mo-O) and deformation absorptions 8 ( O - M o - O ) associated with MoO 3 disappear (fig. 4). MoO 2 has no strong absorption in this I R range while it has weak absorptions at 892, 950 and 980 cm -1 [25]. It has been indicated above that "as-prepared" samples heated in vacuum yield MoO 2. The resemblance of the I R spectra of the "as-prepared" sample to that of MoO 3 (fig. 3) suggests that the gel-prepared m o l y b d e n u m oxides are structurally more closely related to MoO 3 than M o O 2. 3.5. X P S
In order to obtain some information of the valence state of m o l y b d e n u m in these products, XPS spectra were measured. Besides molybdenum and oxygen, carbon was detected in all samples. Mo(3d) lines are shown in fig. 6. The binding energy was corrected for the static charge of the sample based on the C(ls) line. It is known that the 3d peak of MoO 3 appears at higher binding energy by a few eV than that of MoO 2 [28]. However, no significant differences in 3d3/2
,
240
,
,
,
1
,
I
,
,
r
.
.
.
.
230
i
i
i
i
i
I
220
Binding energy (eV) Fig. 6. Change in XPS lines of Mo(3d) on heating in air (A, B
and C) and in vacuum ( - 1 0 -2 Torr) (D and E). A: "as-prepared", B: 200 o C, C: 400 o C, D: 300 o C, E: 500 o C. Sample C is MoO3 and E is MoO2 (identified by XPD).
M. Nagano, M. Greenblatt / Preparation of reduced molybdenum oxides
binding energy were found between MoO 3 (fig. 6C) and MoO 2 (fig. 6E) which were obtained by heating "as-prepared" samples in air and in vacuum, respectively. The 3d 5/2 binding energy of - 2 3 0 eV observed is close to that reported for MoO 2 (229.4 eV) rather than that of MoO 3 (232.7 eV) [28]. The 3d 3/2 peak of MoO 3 (fig. 6C) shows a shoulder at higher energy and the 3d5/2 peak is weaker than the 3d 3/2 peak. Both peaks are poorly resolved. The 3d3/2 peak of the sample heated to 200 o C in air also shows a weak shoulder at higher energy (fig. 6B). On the other hand, the 3d5/2 peak of MoO 2 is stronger than the 3d3/2 peak. The XPS peaks corresponding to MoO 2 are better resolved than those of MoO 3. The XPS peaks of "as-prepared" sample (fig. 6A) and the sample heated to 3 0 0 ° C in vacuum (fig. 6D) resembles that of MoO 2 (fig. 6E) as far as the shape and intensity of the 3d peaks are concerned. There remains some uncertainty in the above analysis; the binding energy of C(ls), which was used as the reference for correction of the static charge of samples, might not be the same for each XPS experiment as the chemical species containing the C atom may have reacted to form different species. In that case, the position of the peaks in (fig. 6 A - D ) is not certain. Furthermore, XPS gives information about the surface only tens of angstroms below the sample surface. A partly reduced MoO 3 crystal with mixed oxidation state of molybdenum ( + 4, + 5, + 6) is reported to show a poorly resolved 3d spectrum with shoulders [29]. The shoulders and the poor resolution in the 3d spectra of MoO 3 obtained in this experiment may be due to partial reduction of Mo 6+ to Mo 4÷ on the sample surface by evacuation to 10 -6 Torr during XPS measurement; the 3d5/2 peak (232.7 eV) and 3d3/2 peak (235.9 eV) of Mo 6+ [28] may be superimposed by the doublet of Mo 4÷ (229.4 eV and 232.6 eV). A MoO 3 crystal reduced by UV irradiation in vacuum is reported to show a XPS doublet in which the 3ds/2 binding energy is 231.4 eV; this peak was assigned to Mo 5+ [29]. There is little evidence for the existence of Mo 5÷ in fig. 6, however, it cannot be ruled out unambiguously without a more accurate analysis and a thorough fitting of the data. The blue molybdenum bronze, K03MoO 3 shows an asym-
261
metric and poorly resolved XPS 3d doublet which has been attributed to Mo 6 + and Mo 5+ distributed on three inequivalent Mo sites in the structure
[301. Inequivalent Mo sites are certain to exist in the molybdenum oxides obtained here by the sol-gel method. Possibly, because of the poor crystallinity and partial reduction of the sample the number of crystallographically inequivalent molybdenums is very large, which may be responsible for the appearance of the shoulders and the poor resolution seen in fig. 6. However, the "as-prepared" sample does not show significantly asymmetrical peaks (fig. 6A) and resembles the XPS corresponding to MoO 2 remarkably well (fig. 6E). 3.6. Chemical analysis
The precipitate in reaction (2) was identified as KC1 by XPD but it had a faint brown color. A few at% of molybdenum (compared with potassium) was detected in KC1 by plasma emission spectroscopy (PES). Potassium was not detectable in "as-prepared" samples by PES. However, 1-6 wt% of chlorine was found in an "as-prepared" sample by PES. The chlorine content decreased to 0.8 wt% when the sample containing 5wt% chlorine was heated to 160 ° C, and to 0.3 wt% when heated to 600 o C. It is not clear yet whether the chlorine originates from CHzC12 or MoC15_x(OR)x.
4. Conclusion Molybdenum oxides were prepared by a sol-gel method using molybdenum chloro-ethoxide as a precursor. Molybdenum chloro-ethoxide was prepared by the reaction of MoC15 and potassium ethoxide. Potassium ethoxide was synthesized from ethanol and potassium. All reactions proceeded in methane dichloride solvent at room temperature. By-product (KC1) was filtered out. Hydrolysis of molybdenum chloro-ethoxide took place by the uptake of water from ambient air or by the intentional addition of water. Solvent was removed by evaporation at room temperature. Blue amorphous products were obtained, which look like " m o l y b d e n u m blue," mixed valent
262
M. Nagano, M. Greenblatt /Preparation of reduced molybdenum oxides
oxomolybdenum species. These were converted to MoO 3 on heating to 300 ° C in air, to MoO 2 on heating to 4 0 0 ° C in vacuum, and to Mo4Oll together with MoO 3 and MoO 2 on heating in He. IR and XPS measurement suggested the valence of Mo ranging from 4 to 6 in the as-prepared sample. "As-prepared" sample incorporated 25-35 wt% of residual solvent, reaction products and retained hydroxyl and ethoxy groups. Residual solvent and reaction products ( H 2 0 , C 2 H s O H ) were lost on heating to - 2 0 0 ° C and ethoxy groups were lost on heating to - 400 ° C. Samples also contained about 1-6 wt% of chlorine, most of which was removed on heating to 160 o C. We thank Dr. Robert Caracciolo for XPS measurement, Dr. Pin Ping Tsai Lu for ion chrom a t o g r a p h y measurements, and Prof. D o n Schleich for helpful suggestions on the preparation method. This research was supported in part by the Office of Naval Research. The automatic Scintag Powder Diffractometer was purchased partly from an NSF DMR-84-08266 Material Science Division Instrumental grant, and from funds provided by Rutgers University FAS and the Research Council.
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