Morphological Characterization of Tungsten Trioxide Nanopowders Synthesized by Sol-Gel Modified Pechini's Method

Abstract

Sol-gel modified Pechini's method was used to prepare WO₃ nanopowders using dicarboxylic acid and polyethylene glycol as the chelating agent and polymeric source, respectively. WO₃ powders were first prepared by calcination of resin precursor at 550ºC under various initial concentrations of metal ion (12.5-50 mmol), acid (125-500 mmol), a complexing agent (32-262 mmol), and polyethylene glycol (1-16.5 mmol) in the air atmosphere. The products were characterized using X-ray powder diffraction, field emission scanning electron microscopy, and energy dispersive spectroscopy. The results revealed that the WO₃ nanopowders prepared with different amounts of chelating agent and polyethylene glycol, crystallized in monoclinic phase. The nanopowders were impurity-free due to the presence of the complexing agent and polyethylene glycol as carbon sources. Morphological evolution indicated that the nanopowders evolved from rod-like to regular and spherical shapes, depending on complexing agent and polyethylene glycol amounts. Nanopowders with an average particle size of approximately 58 nm and a narrow size distribution were obtained.

Keywords:
Tungsten trioxide; Nanomaterials; Sol-gel; Morphology; Pechini

1. Introduction

Tungsten trioxide (WO₃) as one of the most interesting material in the fields of material science and metallurgy is used in various industrial applications. WO₃ films are utilized in electrochromic devices, photocatalysis, and gas sensors^1-3. In addition, the production of tungsten powder from high purity WO₃ through hydrogen reduction is still the most important application in the field of powder metallurgy⁴. More important, it has been confirmed that nanostructured WO₃ exhibits improved properties in comparison to the conventional coarse-grained structures. This is believed to result from the greatly increased surface area, which provides a large interface between the solid and a gaseous or liquid medium to react^5,6.

Nowadays, nanopowders are produced by using different physical, mechanical, and chemical methods. Among the chemical processes, the sol-gel technique is known as a very simple and inexpensive approach in which shape and size of powder can successfully be controlled⁷. This technique uses three main routes for preparing metal oxides. The routes include (1) hydrolysis and condensation of metal alkoxides; (2) gel making of aqueous solutions contain metal-chelates; and (3) esterification reaction between carboxylic acids (acetic, formic, oxalic) and polymer molecules (ethylene glycol) also known as the Pechini's method^8,9. This method is known as a simple approach for preparing metal oxide powders where polymeric precursors are made from metal salts, ethylene glycol, and citric acid during a low-temperature heat treatment. The main advantage of this approach is that metal ions are often mixed in atomic level, resulting in an increase in reaction rate. In this method of powder preparation, two reactions occur: at first complex formation between citric acid and metals, and subsequently esterification between citric acid and ethylene glycol (EG). As a result, the polymeric organic net formed by the esterification reaction decreases the ion segregation¹⁰. Many researchers have studied powder preparation of WO₃ by sol-gel method^11-15. Han et al.¹⁵ reported that nanometric WO₃ powders with uniform size and spherical shape can be synthesized in the presence of oxalic acid and cetyltrimethyl ammonium bromide as the complexing and surface active agents, respectively. Results of the investigations revealed that the morphological characteristics of the powder like particle size and shape depend strongly on the preparation conditions such as nature of precursor, solvent, the surface modifier and quantities of them. To the best of author's knowledge, the application of sol-gel modified Pechini's method in the usage of sodium tungstate solution (Na₂WO₄) and oxalic acid as the precursor and chelating agent, respectively, to prepare of WO₃ nanopowder has not been previously reported. Oxalic acid has been reported to have a better complexing effect than monofunctional citric acid¹⁵, therefore it was selected to be used in the present investigation. The sol-gel modified Pechini's method was applied to produce nanosized WO₃ using sodium tungstate dehydrated (Na₂WO₄.2H₂O) as the starting material. Although there are other starting materials such as alkoxides, which are very common in use in the sol-gel method, there are some disadvantages in the case of tungsten alkoxide precursors. These materials not only are not easily available; but also storing and handling of these materials are difficult due to their high chemical reactivity and sensitivity to moisture. In contrast, the alkaline precursor is inexpensive and easily accessible¹⁴. This study mainly aimed at evaluating the morphology and size of the WO₃ nanopowders synthesized by the sol-gel modified Pechini's method.

2. Material and Methods

2.1 Preparation of resin precursor

Sodium tungstate dehydrates (Na₂WO₄.2H₂O, 99.99%, Merck) powder as the starting material, and hydrochloric acid (HCl, 37%, Merck) as well as nitric acid (HNO₃, 68%, Merck) as the chemical materials for ion exchange reaction, were used during the research. Oxalic acid dihydrate powder (H₂C₂O₄.2H₂O, 99.99%, Merck) and polyethylene glycol with a chain length of 200 (PEG 200, 99.99%, Merck) were used as the complexing agent and polymeric source, respectively. Deionized water was used for solution preparation and washing steps. All the chemicals and solvents were used without further purifications. The Na₂WO₄.2H₂O powder was dissolved in water, according to Table 1, then heated at 80ºC. This table summarizes the preparation conditions for the experiments. Next, a mixture of HCl and HNO₃ (molar ratio of 1:1) was added to the solution at the same temperature, while it was being stirred, to adjust the pH to 1-2 and enhance ion exchange process. After a while, the yellow precipitate was formed and converted into a soft gel through aging. Then the gel was washed by the addition of hot distilled water or 0.1 N HCl solution to the gel followed by stirring and aging the soft gel at the same temperature. Several washing steps were applied to remove residual acid and by-products of the reaction. Oxalic acid and PEG were then added as chelating and resin agents, respectively. Finally, the WO₃ nanoparticles were synthesized by heat treatment of resin precursor prepared from hydrated tungstic precipitates. The samples were first heated at 150 ºC to remove residual water, then heated to 550 ºC, and kept at this temperature for 90 min followed by cooling to room temperature.

Table 1
Summary of different preparation conditions

Sample	Precipitation condition			Resin preparation
	Na2WO4.2H2O (mmol)	H+ (mmol)	Number of washing steps	Oxalic acid (mmol)	PEG (mmol)
A1	12.5	125	6 (3 by 0.1 N HCl)	32.7	1
A2	12.5	125	5	49	2
A3	12.5	125	5 (by filter)	65	4
B1	25	250	7 (3 by 0.1 N HCl)	65	4
B2	25	250	5	98	8.25
B3	25	250	5 (1 by 0.1 N HCl)	131	2
C1	50	500	6	131	16.5
C2	50	500	5	196	4
C3	50	500	5 (by filter)	262	8.25

2.2 Characterization

The particles in the samples were dispersed using ultrasonic separation. The structural characterization of the powder products was conducted using X-ray diffraction (Philips by a K_Cu source with a wavelength of 1.54056 Aº). The morphology and particle size of the produced tungsten trioxide nanopowders were studied using field emission electron microscopy (FESEM). The compositional properties were investigated by using energy dispersive spectroscopy (EDS) coupled with the FESEM. The crystallite size (L, in Aº) was calculated from peak broadening using the Scherrer's approximation given in Eq. 1.

(1) $\mathit{L}=\frac{0.89\mathit{\lambda }}{\mathit{B}\cos \mathit{\theta }}$

In this equation λ is the wavelength of the X-ray (1.5418 Aº), B is the full width at half maximum (FWHM, radian) and θ is the Bragg angle (degree).

3. Results and Discussion

3.1 SEM analysis

Morphological development for WO₃ nanopowders (produced using different amounts of Na₂WO₄.2H₂O, oxalic acid, and PEG 200), after calcination at 550ºC for 90 min, is illustrated by a series of FESEM images in Figs. 1-3. The images display a mixture of the rod-shaped or elongated sphere, regular, and sphere powders synthesized by this method. Figure 1(a, b) shows a mixture of rod-shaped and fine sphere particles produced using a minimum amount of chelating agent (32.7 mmol) and PEG (1 mmol). The average particle size is 98 nm, and the aspect ratio of the particles is calculated in the range of 1.00-4.78, which confirms the presence of rod-like morphology besides fine sphere particles. As Figure 1(e) shows the powders have a wide range of size distribution, from 40 to 200 nm. From Figure 1(a-d), it can be observed that the particles have been transformed into granular and spherical morphologies by increasing the quantity of oxalic acid and PEG. The average particle sizes reduced from 98 nm (sample A1, Figure 1(a)) to 84 nm (sample A2, Figure 1(c)) when the amount of chelating agent and PEG increased from 32.7 and 1 mmol to 49.2 and 2 mmol, respectively. Furthermore, particles with the sizes of 40-130 nm and an average aspect ratio of about 1.18 are inferred from Figure (f) for the sample A2, which concedes morphology evaluation in the sample A2 compared to the sample A1.

Figure 1
FESEM images of A1, A2 and A3 samples with different amount of oxalic acid and PEG: (a) and (b) 32.7 and 1; sample A1, (c) 49 and 2; sample A2, and (d) 65 and 4 mmol; sample A3, respectively; (e) and (f) particle size distribution for Figure 1(b) and Figure 1(c), respectively

Further increase in the additives did not lead to apparent changes in shape and size of the particles, but it seems some particles possess regular form, as illustrated in Figure 1(d). It is worth noting that oxalic acid chelates metal cations, allowing a homogenous distribution of the cations through the polymeric chains, or reacting with the alcohol functions of the polyhydroxyl alcohol to form organic esters. Thus, when the oxalic acid increases, the quantity of hydroxyl carboxylic acid which can react with polyhydroxyl alcohol increases¹¹. Oxalate anions [(C₂O₄)^2-] as a member of dicarboxylic acid groups have been shown more effective complexing agents than the acetate anions as the monofunctional acids¹⁶. Oxalic acid has a better complexing effect than citric acid as well¹⁵. In terms of chemical structure, the chelating agents bind with H₂WO₄.2H₂O [or W(OH)₆] and exchange with the OH^- ligands attached to the central atom, and in the case of oxalic acid, the oxalate anion strongly binds with the central W atom through bidentate ligands, as illustrated in Reaction 1 ¹⁷.

Reaction 1

This strong hydrogen bonding with the carboxyl of oxalic acid, makes WO₃ colloid particles segregate from each other and inhibits the condensation and growth of individual particles¹⁸. The complex formation between tungsten species and carboxylic acids are known^15,19-21. In the presence of the complexing agent, it is expected that growth of particles stops in all directions and powders with a spherical morphology are obtained. But it can be seen that powders with rod-shaped morphology have earned in sample A1 (Figure 1 (a and b)), suggesting a different result. Patil²⁰ reported achieving nanorods of WO₃ by the addition of oxalic acid during hydrothermal reaction. Selectively binding of the carboxylic acid with the particular facets of the newly formed tungsten oxide hydrate seed particles is considered as the reason for this phenomenon. It leads to the stabilization of specific planes and sometimes limits the growth. Meanwhile, some other crystallographic facets continue to grow in different particular directions to achieve rod-shaped particles eventually. Figure 2(a-c) presents FESEM images showing the particles corresponding to the samples B1, B2 and B3 prepared based on the preparation conditions given in Table 1. Regular-shaped and fine particles (Figure 2(a)) were transformed to approximately spherical-shaped particles with a uniform size as seen in Figure 2(b). In addition, as Figure 2(b) shows, particles prepared under the conditions corresponding to that of sample B2 (Table 1), agglomerated to a large size particle. Changing oxalic acid and PEG contents from 98 and 8.25 to 131 and 2 mmol, respectively resulted in the formation of almost fine regular and spherical particles as shown in Figure 2(c). It can certainly be concluded that the morphology of final powders has been affected by the quantity of both the oxalic acid and PEG.

Figure 2
FESEM images of B1, B2 and B3 samples with different amount of oxalic acid and PEG: (a) 65 and 4, (b) 98 and 8.25, and (c) 131 and 2 mmol, respectively.

Figure 3(a-c) shows the particles with different morphologies produced using the conditions corresponding to those of samples C1, C2, and C3. Figure 3(a) exhibits the particles with regular and spherical morphologies obtained using 131 and 16.5 mmol oxalic acid and PEG, respectively. As Figure 3(d) shows the size of particles is distributed between 40-160 nm and it can be seen that the presence of larger particles caused lack of uniformity in the particle size distribution. Figure 3(b) demonstrates elongated curved spheres along with a few large particles synthesized using the preparation condition corresponding to that of sample C2. The aspect ratio of the particles is in the range of 1.04-4.10, and the average of 2.69 supports the morphology observations. But, in a completely different FESEM image shown in Figure 3(c), spherical shape particles with a uniform size distribution were obtained.

Figure 3
FESEM images of the powders prepared using different amounts of oxalic acid and PEG; (a) 131 and 16.5 (sample C1); (b) 196 and 4 (sample C2), and (c) 262 and 8.25 mmol (sample C3), respectively; (d) and (e) particle size distribution for Figure 3(a) and Figure 3(c), respectively

Moreover, the particle size distribution, shown in Figure 3(e), revealed that powders with an average size less than 100 nm were obtained. The average particle size of 58 nm was measured for the this sample which are lower than what reported by Han et al.¹⁵. It should be noted that all samples were prepared in the presence of 50 mmol sodium tungstate while the quantity of oxalic acid (from sample C1 to C3) was increasing. This result suggests that morphology of the final powders has been affected by both the oxalic acid and PEG quantities. Considering the conditions given in Table 1, and the comparison made between FESEM images of the samples series A to C, it can be concluded that although there was a similar strategy in preparation procedure, particles with different morphologies were obtained due to the variation in PEG content. For instance, in A1, B1, and C1 samples with 32.7, 65, and 131 mmol oxalic acid, respectively, increasing PEG content from 1 to 16.5 mmol resulted in breaking the large particles down. However, there was still large particles caused lack of uniformity in size distribution, in addition to dissimilarity in the morphology of particles. On the other hand, in samples A3, B3, and C3 with 65, 131, and 262 mmol oxalic acid, respectively, the particles produced using 8.25 mmol PEG led to a uniform size distribution as well as spherically shaped particles. PEG is a short-chain polymer with a uniform and ordered chain structure, which can be easily adsorbed on the surface of metal oxide colloid. This strong interaction kinetically decreases the activities of metal oxide colloid, and thus control the growth rates of various crystallographic facets of crystals which can control the morphology^22-25. In this research, regular and spherical particles were obtained at low and high concentrations of PEG (in all A, B, and C sample series), respectively. This corresponds to the fact that the short-chain polymer, PEG, confines the growth rate of the metal oxide colloid in some certain facet at low concentration which finally leads to an anisotropic growth of the crystals. It is found that an enough PEG concentration can confine the growth rate of the colloids in all directions which leave a spherical morphology. In another aspect of view, the formation of oligomers through polyesterification reactions which is obtained by Pechini's method, can decrease ion segregation and finally control the morphology of particles²⁶. From these conclusions, it can be restated that the morphology of powder produced in this experiment depends on the quantity of oxalic acid and PEG as well as the ratios of them.

3.2 Structure analysis

Figure 4 shows a spectrum corresponding to the sample C3 obtained from EDS analysis. It reveals that the powder dominates in W and O elements. Moreover, no impurity originating from carbon additives can be seen in the spectrum.

Figure 4
EDS spectrum of the nano powder prepared by the addition of 262 mmol oxalic acid and 8.25 mmol PEG to the gel and heating at 550 ºC for 90 min.

Figure 5 shows XRD patterns of the samples C1, C2, and C3 prepared by 50 mmol sodium tungstate and 500 mmol tungstic acid with different quantities of oxalic acid and EPG. The XRD patterns show that the products were crystallized in the form of a monoclinic lattice (JCPDS card 83-0950)^27,28. The patterns indicate no considerable apparent difference between the peaks suggesting no significant dissimilarity between the powders synthesized in the samples. No characteristic diffraction peaks from impurities were detected. In addition, the sharp and strong peaks in all the three products signify the synthesis of high crystallized powders. This confirms the relationship between crystallinity and peak intensity or sharpness that has been mentioned in some references^29,30. Similar results from the structure, purity, and crystallinity viewpoints were obtained for the samples A and B. Therefore, in order to avoid lengthening the paper with no significant results, no similar discussions were done for them. When the amount of oxalic acid is the highest (sample C3) -relative to sample C1 and C2- the peak intensity is at the maximum value, while when the amount of PEG is the lowest (sample C2) -relative to sample C1 and C3- the peak intensity is at the minimum value. This proposes that both chelating agent and PEG along with their concentrations only affect the crystallinity of final powder in Pechini's method.

Figure 5
XRD patterns of WO₃ nano powders after calcination at 550ºC for 90 min with different amounts of oxalic acid and PEG. All peaks match with the standard pattern of monoclinic WO₃ (JCPDS # 83-0950)

The crystallite sizes were found to be 41, 33, and 39 nm for the samples C1, C2, and C3, respectively. Hence, it can be concluded that the additives amount also influences the crystallite size. On the other hand, although there is no apparent difference between the XRD patterns of WO₃ powders synthesized at different preparation condition, a product with various morphologies was obtained as well. Table 2 summarizes these results. Based on previous studies^21,31 and results obtained from this paper, it is clear that organic additive such as oxalic acid and PEG play an important role in controlling the morphology and particle size uniformity of the final products. In the present sol-gel procedure, the simultaneous presence of oxalate ion and PEG chains with different inhibitory mechanisms are able to control the size and morphology of tungsten trioxide particles at the different preparation steps.

Table 2
Characterization of the product under various conditions

Sample	Product of preparation
	XRD Guinier	Color	Crystallite size (nm)	Morphology
A1	-	Yellow	-	Regular, Rod-like
A2	-	Yellow	-	Spheres
A3	-	Yellow	-	Spheroids
B1	-	Yellow	-	Regular
B2	-	Yellow	-	Spheres
B3	-	Yellow	-	Regular, Spheroids
C1	WO3	Yellow	41	Regular, Spheroids
C2	WO3	Yellow	33	Regular, Curved rods
C3	WO3	Yellow	39	Spheres

Oxalic acid species are thermally stable up to 300ºC³². The presence of coordinated oxalate at the elevated temperatures can inhibit the aggregation of particles during the calcination step as well as the aqueous medium. On the other hand, oxalic acid acts as an organic fuel within thermal treatment²¹. Upon calcination in the air, oxalic acid is oxidized to carbon dioxide subsequently, and a significant amount of gases are released during the calcination step, preventing particles from continued growth and helping large agglomerated particles to break down^20,21. In addition, the quantity of it should be controlled because of it is effective on the yield of synthesis. As expected, nanopowders synthesized by this procedure were stoichiometric WO₃ without any impurity. The organic additives (oxalic acid and PEG 200) with different quantities only acts as a crystallinity controller and never involve in changing the crystalline structure. In addition to oxalic acid, PEG with a different mechanism controls the growth of particles too. Hydrogen bonding formation between hydroxyl group in PEG and hydrogen ion in WO₃.H₂O molecule resulting in a highly unstable intermediate product (WO₃.OH-H-[OCH₂CH₂]_n) and H₂O. In acidic condition, this product reduced to WO₃.H₂O and PEG and finally prevails in the growth environment. The process continues until the crystallite reaches its maximum size. The result shows that the oxalic acid and PEG concentration has a strong effect on the particle size and morphology of tungsten trioxide nanopowders. In the presence of polymeric chains of PEG, the crystal nuclei of WO₃ surrounded by this chains prevent their growth and generates the steric impediment effect leading to a reduced particle-particle aggregation³³.

4. Conclusions

In the present work, WO₃ nanopowders were successfully synthesized using the sol-gel modified Pechini's method with the aid of sodium tungstate, oxalic acid, and PEG-200 as the starting material, chelating agent, and polymeric additive, respectively. WO₃ nanopowders with a high purity, and a controllable morphology and particle size attained. The quantity of oxalic acid and PEG in the precursor, have noticeable effects on the particle shape, size, and aggregation. Nanoparticles with high crystallinity obtained via the simple and low-cost sol-gel modified Pechini's method. Particles with an average size of 58 nm and spherical morphology were synthesized by changing the amount of oxalic acid and poly ethylene glycol at the same time.

5. Acknowledgments

The authors are thankful to Iran Nanotechnology Initiative Council (Grant No. 93379) for the financial support and the Faculty of Mechanical Engineering, Shahid Rajaee Teacher Training University (SRTTU) for providing facilities. In addition, the authors would like to appreciate Eng. Ali Jafari for his very helpful assistance in conducting the experiments.

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Publication Dates

• Publication in this collection
  28 Sept 2017
• Date of issue
  Nov-Dec 2017

History

• Received
  04 May 2017
• Reviewed
  07 Aug 2017
• Accepted
  27 Aug 2017