Synthesis and Structural Properties of Niobium Pentoxide Powders: A Comparative Study of the Growth Process

Abstract

Powders Nb₂ O₅ were prepared by two different synthesis method, Sol-Gel and polymeric precursors (Pechini). In the Pechini method before adding the citric acid in the process, four different solutions were used to get the samples. For Sol-gel method, two different processes were also used in obtaining powders. The precursor was completely solubilized in ethanol and then hydrolyzed with ammonia and water. The calcination of the samples was between 500 and 750°C. The resulting powders were characterized by Scanning Electron Microscopy (SEM), Brunauer, Emmett and Teller (BET) surface area measurements, UV-visible and Raman spectroscopy. The formation of T−Nb₂ O₅ orthorhombic took place upon calcination at 7500C. Crystallite sizes were determined using the Scherrer method which resulted in an uniformed size of about 25 − 65nm. Ultraviolet-Visible diffuse reflectance spectroscopy indicated a variation in the optical band gap values (3.32-3.40 eV) in crystal growth process. The Raman vibrational modes indicate the presence of the orthorhombic phase of the material.

Keywords:
comparative study; sol-gel; Uv-Vis; Inelastic light scattering and Nb₂ O₅

1. Introduction

Niobium Pentoxide (Nb₂O₅) is an n-type semiconductor with a band gap of about 3.4 eV, low in comparison to other oxides. The interest in studying the Nb₂O₅ is due to its remarkable physicochemical properties and structural isotropy suitable for a wide range of applications in the construction of gas sensing, electrochromics display and photoelectrodes, as well as in field-emission displays and microelectronics^1-6. Studies were conducted in the past on the use of Nb₂O₅ nanoparticles for environmental remediation in water through of photocatalytic processes. In these technologies Nb₂O₅ shows great potential because of its stability in an aqueous medium, its surface acidity, redox and photocatalytic properties, which are instrinsically linked to its structure^7-9.

The main phases reported in literature for Nb₂O₅ are TT-Nb₂O₅ (pseudohexagonal) at low temperatures, T-Nb₂O₅ (orthorhombic structure) heating the amorphous oxide to 600 and 800°C, and H-Nb₂O₅ (monoclinic structure) heating to about 1100°C ¹⁰. In general the crystallization conditions of each Nb₂O₅ structure depends on the starting materials, synthesis methods and heat treatment conditions. In particular the T-Nb₂O₅ net parameters are: a = 6.17Å; b = 29.32Å; c = 3.94Å, and its crystalline structures consist of 4 × 4 blocks of corner-shared NbO₆ octahedra, with connected blocks sharing the edges of the octahedron¹¹.

In order to get thin films of Nb₂O₅ there are two phase deposition methods. Liquid phase deposition and vapour phase deposition. For the liquid phase methods such as hydrotermal, solvothermal, anodization, sol-gel and electrodeposition exist and the physical and chemical methods for the vapour phase¹². Additionally, nanostructures of Nb₂O₅ are obtained through sol-gel and precipitation methods. Besides, the preparation of niobium base materials also the Pechini route is used.

Soares et al.¹¹ reported the synthesis of Nb₂O₅ samples by means of the Laser Floating Zone (LFZ) technique and the solid-state reaction in order to study some of their physical properties as a function of synthesis conditions. The authors found a structural orthorhombic to monoclinic phase transition observed in samples sintered at temperatures higher than 800°C. Rosario et al.¹³ prepared Nb₂O₅ powders and films using the niobium salt NH₄H₂[NbO(C₂O₄)₃].3H₂O as starting material. Nb₂O₅ nanoparticles obtained by the oxidant-peroxo method¹⁴ were synthesized employing ammonium niobium oxalate (NH₄[NbO(C₂O₄)₂(H₂O)₂].nH₂O as a starting precursor. These nanoparticles presented mixed phases of Nb₂O₅ orthorhombic and Nb₂O₅ nH₂O. Sol- gel methods have been widely reported in the production of porous and high surface area niobium oxides. Molecular precursors, mainly metal alkoxides, are generally used as starting materials. In the work¹⁵Nb₂O5 amorphous powders prepared by the Sol-gel technique and their crystalline structures were analyzed through a controlled heat-treatment process. Ristic et al.¹⁶ reported that the Nb(OC₂H₅)₅ was used as a starting precursor in the preparation of the Nb₂O₅ powders using the Sol-gel procedure. The initial Nb₂O₅ powders were amorphous and by heating them at 500° C contained Nb₂O₅ (TT-phase), whereas at 650°C the Nb₂O₅ (T-phase) was obtained. Uekawa et al.¹⁷ reported that the synthesis of highly crystallized Nb₂O₅ nanoparticles with a diameter of 4.5nm was based also on a Sol-gel route.

In this research, Nb₂O₅ nanosize powders were prepared by two different routes: a) the Sol-gel technique, for its similarities with the Pechini route (in other research also known as the citrate gel route¹⁵⁾ and b) the Pechini method, using different sintering temperatures (500-750°C). The powders obtained were characterized by X-ray diffraction (XRD) to evaluate the influence of heat treatment on the formation of the TT- Nb₂O₅ and T-Nb₂O₅ phases. Also by the BET method in order to obtain measurements of surface area, the UV-vis diffuse reflectance spectroscopy (DRS) to identify the band gap energy and the Raman spectroscopy to perform a vibrational characterization.

2. Experimental details

2.1. Preparation of the nanosize powders

Tables of the synthesis procedure are shown in (1) and (2). The samples 1−Nb₂O₅ and 2−Nb₂O₅ were synthesized by the Sol-gel method according to literature procedures¹⁶. Niobium (V) ethoxide, (Nb(OC₂H₅)₅ 99.999% Alfa Aesar) was employed as the starting precursor and absolute ethanol for analysis (Merck) and Ammonia hidroxide 30% (Panreac) were also used. The samples 1−Nb₂O₅ were prepared by adding 5g of Nb(OC₂H₅)₅ to 90ml of absolute ethanol which was stirred vigorously, then 5ml of Ammonia hydroxide was added to the precipitation system. The samples 2−Nb₂O₅ were prepared by adding 5g of Nb(OC₂H₅)₅ to 94ml of absolute ethanol under vigorous stirring, then 1ml of distilled water was added to the precipitation system. After firmly shaking, the precipitation systems were transferred to petri dishes and left to evaporate at 60°C on a stove; the solid residue was subsequently dried at 160°C for 4h in an oven. The powders obtained were calcined at 500, 650 and 750°C respectively for 2h. The samples A−Nb₂O₅, B−Nb₂O₅, C−Nb₂O₅ and D−Nb₂O₅ were prepared using the Pechini method adapted from previous procedures reported¹⁸.

Niobium chloride (NbCl₅ 99% Sigma-Aldrich) was employed as the starting precursor, hydrated citric acid (99.5% Panreac) as a chelating agent and ethylene glycol (99.8% Panreac) as polymerizing agent. The employed solvents were: distilled water for the samples A−Nb₂O₅ and aqueous solutions of 65% HNO₃, 37% HCl and 30% NH₄OH for the samples B−Nb₂O₅, C−Nb₂O₅ and D−Nb₂O₅ respectively. The precalcination was performed at 300°C for 4 hours. The powders that were obtained were calcined at 500, 650 and 750°C for 2h respectively (table 2).

Table 1
Different conditions of the preparation samples in the sol-gel method.

sample	Hydrolysis	Temperature °C
1-Nb2O5 - 500°C	NH4OH	500
2-Nb2O5 - 500°C	H2O
1-Nb2O5 - 650°C	NH4OH	650
2-Nb2O5 - 650°C	H2O
1-Nb2O5 - 750°C	NH4OH	750
2-Nb2O5 - 750°C	H2O

Table 2
Different conditions of the preparation samples in the Pechini method.

sample	Solvent	Temperature °C
A-Nb2O5 - 500°C	H2O	500
B-Nb2O5 - 500°C	H2O − HNO3
C-Nb2O5 - 500°C	H2O − HCl
D-Nb2O5 - 500°C	H2O − NH4OH
A-Nb2O5 - 650°C	H2O	650
B-Nb2O5 - 650°C	H2O − HNO3
C-Nb2O5 - 650°C	H2O − HCl
D-Nb2O5 - 650°C	H2O − NH4OH
A-Nb2O5 - 750°C	H2O	750
B-Nb2O5 - 750°C	H2O − HNO3
C-Nb2O5 - 750°C	H2O − HCl
D-Nb2O5 - 750°C	H2O − NH4OH

Sol-gel and Pechini methods were chosen for their similarity and since these allow the preparation of homogeneous and highly pure powders and porous and high surface area oxides, with fine particles and adequate chemical composition. The Pechini route consists of the formation of chelates between metal ions and carboxylic acids. These are subsequently polymerized by a reaction with polyalcohols, which have a good distribution of metallic ions in the polymer structure, that when heated at moderate temperatures it favors the condensation reaction causing polyesterification to occur. The excess water is removed and a homogeneous gel is formed^19,20. The sol-gel method enables homogenous samples to be obtained at low temperatures and the starting cationic composition to be maintained by the use of metal salts as raw materials and mixing them in a liquid solution. The principal advantage of the Sol-gel method is that reagents are mostly mixed in atomic levels, which may increase the reaction rate and decrease the heat treatment temperature²¹.

The XRD patterns were obtained using two different methods. In the first term an Xpert-PRO PANalytical diffractometer (Co Kα₁ radiation, λ= 1.78901Å), operating at 40mA. A step of 0.0133° in 1 min, in the 2θ angle range of 10 - 90° was used. That equipment then required technical support. For the remaining analysis an Xpert PANalytical Empyrean Serie II Alpha 1 (Cu Kα₂) radiation, λ=1.54442Å) was used operating at 40mA with a step of 0.05° in 50 seconds, in the 2θ angle range of 5 - 80°. The identification of the crystalline phases was performed using the X'Pert HighScore PANalytical software. Raman spectroscopy was made using a commercial micro-Raman probe setup TG4000 Jobin Yvon spectrometer (Raman excitation line: λ=532 nm) at room temperature. The Brunauer-Emmett-Teller (BET) approach was employed to determine the specific surface area of the nanosize powders, employing a Micromeritics ASAP 2020 − 77 K apparatus with adsorption data at a relative pressure range of 0.03 − 0.15. Before the analysis, each sample was outgassed at 100°C for 2 hours then the temperature was increased to until 300°C. The UV-vis spectra were taken using Cary 5000 spectrometer (varian) in diffuse reflection mode. A field emission scanning electron microscope (SEM Vega3 Tescan) operating at 5.0kV was used to verify the material morphology.

3. Results and discussion

Figure 1XRD patterns peak analysis verifies that those Nb₂O₅ samples obtained by the Pechini method heated at 750°C have the orthorhombic crystalline structure due to the existence of dominant diffraction peaks according to PDF cards No. 30-0873 (contained in the Power Diffraction File) for the T-Nb₂O₅ phase, (a = 6, 175Å, b = 29, 175Å, c = 3, 930Å). The XRD patterns of the samples obtained by routes based on the Pechini method at 650°C also displayed the existence of the T-Nb₂O₅ phase although with less crystallinity. At 500°C spectra obtained from routes based on the Pechini method coincided with PDF cards No. 28-0317 for the low temperature pseudohexagonal TT-Nb₂O₅ phase (a = b = 3,607Å, c = 3.925Å).

Figure 1
XRD diffraction patterns of samples obtained by Pechini method; the samples correspond to T−Nb₂O₅ phase obtained by heating at 750°C. (a) A-Nb₂O₅ - 750°C, (b)B-Nb₂O₅ - 750°C, (c) C-Nb₂O₅ - 750°C and (d) D-Nb₂O₅ - 750°C.

In Figure 2XRD patterns of the Nb₂O₅ samples obtained by the Sol-gel method are presented. The sharpness and intensity of all diffraction peaks improve progressively with the increase in the calcination temperature which proves the crystallite growth of Nb₂O₅ nanoparticles. Figure 2XRD patterns show that at 650 and 750°C the crystalline structures are orthorhombic according to PDF cards No. 30-0873. In the XRD patterns of the 1-Nb₂O₅ - 500°C and 2-Nb₂O₅ - 500°C samples there are no appreciable peaks that specify the crystalline phase that was reached. In general the XRD patterns structurally point out the pureness of the Nb₂O₅ nanosize powders synthesized by Pechini and Sol-gel methods due to the absence of undesired impurities generated from the precursors used in both methods.

Figure 2
Sol-gel method, samples obtained by heating at 500, 650 and 750°C. (a) 1-Nb₂O₅ −500°C, (b) 2-Nb₂O₅ −500°C, (c) 1-Nb₂O₅ - 650°C, (d) 2-Nb₂O₅ −650°C, (e) 1-Nb₂O₅ −750°C and (f) 2-Nb₂O₅ - 750°C.

To analyze the crystallite growth, the crystallite size (L) of the Nb₂O₅ nanoparticles was calculated by the Scherrer's equation L = Kλ/(βcosθ), (for peak broadening due to size effects) where K is the shape correction factor, 0.9 for L taken as the volume-averaged crystallite dimension perpendicular to the hkl diffraction plane; λ is the wavelength used, θ is the Bragg diffraction angle measured hkl peak and β represents the FWHM (Full width at half maximum) measured in radians on the 2θ scale²². The broadening lines chosen for the L estimate correspond to (001), (180), (181), (002), (380), and (212) crystalline planes according to PDF No. 01-071-0336 reported in Inorganic Crystal Structure Database (ICSD)²³. The crystallite size of nanoparticles on the T-Nb₂O₅ phase increases from ~25 to ~65nm with an increase in calcination temperature, as shown table 3.

Table 3
Crystallite size estimated by Scherrer equation from the XRD data

2θ	(hkl)	Crystallite size (nm)
22.5	(001)	A-Nb2O5 - 500°C 11.1	A-Nb2O5 - 650°C 26.6	A-Nb2O5 - 750°C 66.4
36.6	(181)	12.7	30.6	50.9
22.5	(001)	C-Nb2O5 - 500°C 20.9	C-Nb2O5 - 65°C 29.1	C-Nb2O5 - 750°C 35.3
28.5	(180)	11.0	43.9	33.5
22.6	(001)		1-Nb2O5 - 65°C 54.5	1-Nb2O5 - 750°C 54.5
50.7	(380)		52.8	63.8
28.4	(180)		2-Nb2O5 - 65°C 25.4	2-Nb2O5 - 750°C 46.5
50.8	(380)		22.7	35.4

The crystallite size increases more with the A-Nb₂O₅ route than the C-Nb₂O₅ route; with the B-Nb₂O₅ and D-Nb₂O₅ routes the crystallinity achieved at 650°C is very similar to that achieved at 750°C. The achieved crystallinity with 1-Nb₂O₅ route is more or less uniform at 650°C and 750°C while the crystallinity increases with the 2-Nb₂O₅ route when the samples are sintered at 650°C and 750°C. This is due to the fact that the crystallite size is greater when the hydrolisis is catalyzed in basic medium. The samples A-Nb₂O₅ - 750°C and 1- Nb₂O₅ - 750°C have the L larger. In general the route A of the Pechini method allows to obtain a larger crystallite size and 750°C is the more favorable crystallization temperature. Table 4 shows the results of specific surface area, S_BETdetermined by taking data at relative pressures between 0.03 and 0.15. The method had accuracy constraints during the measurement process. This fact was observed because the values of S_BETwere less than 10m²/g. The S_BETdates show that the increase in the calcination temperature from 500°C to 750°C resulted in the progressive decrease of specific surface area of the Nb₂O₅ nanosize powders. This is mainly because a more high calcination temperature can lead to a greater proportion of the pore coalescence due to the further crystallization of walls separating mesopores in their structure which be established through measurement of pore size. The B route Pechini method did not show regularity in S_BETdownward trend with increasing temperature, unlike C and D routes which did. S_BETof the samples obtained by Sol-gel method did not display a decrease with the temperature. The similarity between specific surface area of the C-Nb₂O₅ - 750°C, D-Nb₂O₅ - 750°C and 2-Nb₂O₅ - 750°C samples agree with the crystallinity achieved at 750°C and information compared with the SEM images show as small S_BETmeasurements are associated with larger particle sizes.

Table 4
S_BET values of the synthesized samples: sol-gel method 1 − 2 and Pechini C − D. The S_BET was determined by taking data at relative pressures between 0.03 and 0.15.

Sample	SBET (m2/g)	Sample	SBET (m2/g)	Sample	SBET (m2/g)
C-Nb2O5 - 500°C	79.50	C-Nb2O5 - 650°C	14.52	C-Nb2O5 - 750°C	4.53
D-Nb2O5 - 500°C	75.85	D-Nb2O5 - 650°C	9.85	D-Nb2O5 - 750°C	4.44
1-Nb2O5 - 500°C	1.70	1-Nb2O5 - 650°C	1.17	1-Nb2O5 - 750°C	1.19
2-Nb2O5 - 500°C	1.30	2-Nb2O5 - 650°C	14.24	2-Nb2O5 - 750°C	4.68

This fact is well consistent with the observed results of a higher crystallinity and a larger crystallite size of the Nb₂O₅ nanoparticles at a higher calcination temperature (figure 2 and figure 3). When comparing the data re- ported in table 4 with the SEM images it is concluded that the oxides prepared at 500°C by the method Pechini had a high S_BETbecause at this temperature the particle size is small.

Figure 3
SEM micrographs of Nb₂O₅ particles: (a) 1-Nb₂O₅ −750°C for a resolution of 2µm, (b) 2-Nb₂O₅ - 750°C for a resolution of 2µm, 750°C for a resolution of 1µm.

Figure 3 shows SEM photographs of the selected samples sol-gel and Pechini. Figure 3 also shows the SEM micrographs of samples Nb₂O₅ by the Pechini (B and D) and sol-gel (1 and 2) method, at a temperature of 750°C respectively. The samples obtained by Sol-gel (1,2-Nb₂O₅) show a structure of the particles consisting of regular spherical particles of well-defined shapes and sizes of approximately 1-2 µm, figure 3 (a), (b). In figure 3(c) and (d) for example, two samples are presented for Pechini method. These samples exhibit the nanoparticles formation, whereas the SEM photograph of sample (a) and (b) shows a unique spherical microstructure. The last two photographs show high-magnification SEM images. Here, it is also shown that particles involved are smaller. This indicates that by virtue of Pechini method, smaller and irregular particles are obtained.

In Figure 4 (a), (c) UV-Vis A − Nb₂O₅ material is observed at different calcination temperatures. By extrapolating the linear part of a curve photon energy to the abscissa, the band gap energy transition can be derived. The band gap energies of the Pechini method, A−Nb₂O₅ were estimated to be 3.40, 3.32 and 3.32eV, respectively to different temperatures. This difference might be due to the different crystal sizes of the samples. The value of the band gap is also an indication that the calcination temperature influences the process for higher calcination temperatures, however the energy band is the same.

Figure 4
UV-Vis Spectra of Nb₂O₅ nanoparticles synthesized by the Pechini method (A-Nb₂O₅ ) at different temperatures: (a) 500, (b) 650 and (c) 750°C.

Figure 5 (a)-(e) UV-Vis for samples to 750°C by sol- gel methods and Pechini. In literature, as a typical n-type wide band gap semiconductor (Eg=3.4eV), Nb₂O₅ is the most thermodynamically stable phase among various niobium oxides²⁴. As shown in figure 5 the band gap energy values for different samples are very close to the value of literature between 3.32 and 3.37eV. Small changes in values can be related to particle size²⁵. There is no clear explanation for the discrepancy of the band gap in this study. As can be seen, the energy band by the two methods of growth varies, being lower for the B method Pechini while it remained practically the same for the other methods. Rigorously, we believe the particle size and band gap depend on: method of preparation, calcination temperature and grain size.

Figure 5
UV-Vis Spectra of Nb₂O₅ nanoparticles at temperature of 750°C. (a) 1-Nb₂O₅ - 750°C, (b) 2-Nb₂O₅ - 750°C, (c) B-Nb₂O₅ - 75°C, (d) C-Nb₂O₅ - 750°C and (e) D-Nb₂O₅ - 750°C.

Figure 6 shows the Raman spectra for samples C-Nb₂0₅ by Pechini method at different calcination temperatures. As shown in figure 6 (a) and (b) the position of the Raman peaks is narrow and well defined which indicates crystallinity in the material of these calcination temperatures. In figure 6 (c) Raman modes for the calcination temperature of 500°C can be observed. This spectrum does not have well-defined peaks which indicates an amorphous or poorly crystalline material. Bands corresponding to vibrations of cations occupying centres of octahedrons and tetrahedrons are found in the 150-400cm^-1 range^26-28. In the spectrum of Figure 6 (a-b), the peaks occur at 78.3, 127.4, 227.3 and 307.4cm^-1. Bands of the oxygen framework vibrations are located in the 500-1100cm^-1 range, figure 6(a-b) at 521, 642.6, 689.3 and 843cm^-1. With the decrease of the calcination temperature, the peaks decrease rapidly.

Figure 6
Raman spectra for C-Nb₂O₅ at (a) 750°C, (b) 650°C and (c) 500°C.

Figure 7 (a) and (b) shows the Raman spectra of sample 2-Nb₂O₅ by Sol-gel method. For Irena et al.²⁹ the bands in the 400 - 800cm^-1 wavenumber region are assigned to the symmetric and antisymmetric stretching mode of the Nb-O-Nb linkage. While the Raman band at 235cm^-1 becomes the bending mode of Nb-O-Nb. The spectra in figure 7, corresponds to a well developed crystalline structure of Nb₂O₅. As shown in figure 6 compared to figure 7 the Raman modes are narrower, also the peak near 667cm^-1 is much more intense. These two figures again show that the calcination temperature of 700°C is ideal for crystalline samples. In general the Raman study shows that an increase of the synthesis temperature causes an increase in the material crystallinity because an increase in the synthesis temperature led to a sharp peak, this fact is verified with the values of the crystallite size that were estimated by the Scherrer equation (Table 3).

Figure 7
Raman spectra for 2-Nb₂O₅ sol-gel method (a) 750º C and (b) 650º C.

4. Conclusions

The microstructure of the resulting T − Nb₂O₅ nanosize powder is linked to the two synthesis methods, because the properties of the niobium oxide powders are strongly dependent on the raw material and on the synthesis route used. The employed Pechini and Sol- gel routes allowed the preparation of Nb₂O₅ nanosize powders. For heat treatment temperatures about 500°C the powder has a TT − Nb₂O₅ phase. The heat treatment process allows the formation of T − Nb₂O₅ nanosize powders with an orthorhombic structure, which was confirmed by XRD. The Raman spectroscopy technique confirmed the phase transformations observed by X-ray diffraction. In the SEM it was observed that the grain size is smaller for the Pechini method, but for the sol gel method it is more uniformed. As is been in the UV-VIS measurements confirmed that with the increase of the calcination temperature decreases the E_gap.

5. Acknowledgements

Authors acknowledge financial support from the agencies; Universidad Francisco de Paula Santander, Cúcuta, Colombia; Universidad Nacional Colombia and Colciencias and CNPq Brazil.

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Publication Dates

• Publication in this collection
  10 Oct 2016
• Date of issue
  Nov-Dec 2016

History

• Received
  02 Dec 2015
• Reviewed
  23 Aug 2016
• Accepted
  10 Sept 2016