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

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.


Introduction
Niobium Pentoxide (Nb 2 O 5 ) 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 Nb 2 O 5 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][2][3][4][5][6] .Studies were conducted in the past on the use of Nb 2 O 5 nanoparticles for environmental remediation in water through of photocatalytic processes.In these technologies Nb 2 O 5 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][8][9] .
The main phases reported in literature for Nb 2 O 5 are TT-Nb 2 O 5 (pseudohexagonal) at low temperatures, T-Nb 2 O 5 (orthorhombic structure) heating the amorphous oxide to 600 and 800°C, and H-Nb 2 O 5 (monoclinic structure) heating to about 1100°C 10 .In general the crystallization conditions of each Nb 2 O 5 structure depends on the starting materials, synthesis methods and heat treatment conditions.In particular the T-Nb 2 O 5 net parameters are: a = 6.17Å; b = 29.32Å;c = 3.94Å, and its crystalline structures consist of 4 × 4 blocks of corner-shared NbO 6 octahedra, with connected blocks sharing the edges of the octahedron 11 .
In order to get thin films of Nb 2 O 5 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 phase 12 .Additionally, nanostructures of Nb 2 O 5 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 Nb 2 O 5 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 15 ) 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-Nb 2 O 5 and T-Nb 2 O 5 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.

Preparation of the nanosize powders
Tables of the synthesis procedure are shown in (1) and (2).The samples 1−Nb 2 O 5 and 2−Nb 2 O 5 were synthesized by the Sol-gel method according to literature procedures 16 .Niobium (V) ethoxide, (Nb(OC 2 H 5 ) 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−Nb 2 O 5 were prepared by adding 5g of Nb(OC 2 H 5 ) 5 to 90ml of absolute ethanol which was stirred vigorously, then 5ml of Ammonia hydroxide was added to the precipitation system.The samples 2−Nb 2 O 5 were prepared by adding 5g of Nb(OC 2 H 5 ) 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 were prepared using the Pechini method adapted from previous procedures reported 18 .
Niobium chloride (NbCl 5 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−Nb 2 O 5 and aqueous solutions of 65% HNO 3 , 37% HCl and 30% NH 4 OH for the samples B−Nb 2 O 5 , C−Nb 2 O 5 and D−Nb 2 O 5 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).
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 temperature 21 .The XRD patterns were obtained using two different methods.In the first term an Xpert-PRO PANalytical diffractometer (Co Kα 1 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 Kα 2 ) 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.To analyze the crystallite growth, the crystallite size (L) of the Nb 2 O 5 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 scale 22 .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   routes the crystallinity achieved at 650°C is very similar to that achieved at 750°C.The achieved crystallinity with 1-Nb 2 O 5 route is more or less uniform at 650°C and 750°C while the crystallinity increases with the 2-Nb 2 O 5 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-Nb 2 O 5 -750°C and 1-Nb 2 O 5 -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 BET determined 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 BET were less than 10m 2 /g.The S BET dates 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 Nb 2 O 5 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 BET downward trend with increasing temperature, unlike C and D routes which did. S BET of the samples obtained by Sol-gel method did not display a decrease with the temperature.The similarity between specific surface area of the C-Nb 2 O 5 -750°C, D-Nb 2 O 5 -750°C and 2-Nb 2 O 5 -750°C samples agree with the crystallinity achieved at 750°C and information compared with the SEM images show as small S BET measurements are associated with larger particle sizes.This fact is well consistent with the observed results of a higher crystallinity and a larger crystallite size of the Nb 2 O 5 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 BET because at this temperature the particle size is small.

Results and discussion
Figure 3 shows SEM photographs of the selected samples sol-gel and Pechini.Figure 3  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 − Nb 2 O 5 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−Nb 2 O 5 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.   .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 size 25 .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 6 shows the Raman spectra for samples C-Nb 2 0 5 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][27][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.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).

Conclusions
The microstructure of the resulting T − Nb 2 O 5 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 Nb 2 O 5 nanosize powders.For heat treatment temperatures about 500°C the powder has a TT − Nb 2 O 5 phase.The heat treatment process allows the formation of T − Nb 2 O 5 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.

Figure 1
Figure 1 XRD patterns peak analysis verifies that those Nb 2 O 5 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-Nb 2 O 5 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-Nb 2 O 5 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-Nb 2 O 5 phase (a = b = 3,607Å, c = 3.925Å).
. The crystallite size of nanoparticles on the T-Nb 2 O 5 phase increases from ~25 to ~65nm with an increase in calcination temperature, as shown table 3. The crystallite size increases more with the A-Nb 2 O 5 route than the C-Nb 2 O 5 route; with the B-Nb 2 O 5 and D-Nb 2 O 5 also shows the SEM micrographs of samples Nb 2 O 5 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-Nb 2 O 5 ) 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.

Figure 5 (
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),Nb 2 O 5 is the most thermodynamically stable phase among various niobium oxides24 .As shown in figure5the 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.Figure6shows the Raman spectra for samples C-Nb 2 0 5 by Pechini method at different calcination temperatures.As shown in figure6(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 figure6(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][27][28] .In the spectrum of Figure6(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, figure6(a-b) at 521, 642.6, 689.3 and 843cm -1 .With the decrease of the calcination temperature, the peaks decrease rapidly.

Figure 7 (
Figure 7 (a) and (b) shows the Raman spectra of sample 2-Nb 2 O 5 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 Nb 2 O 5 .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
prepared Nb 2 O 5 powders and films using the niobium salt NH 4 H 2 [NbO(C 2 O 4 ) 3 ].3H 2 O as starting material.Nb 2 O 5 nanoparticles obtained by the oxidant-peroxo method 14 were synthesized employing ammonium niobium oxalate (NH 4 [NbO(C 2 O 4 ) 2 (H 2 O) 2 ].nH 2 O as a starting precursor.These nanoparticles presented mixed phases of Nb 2 O 5 orthorhombic and Nb 2 O 5 nH 2 O. 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 work 15 Nb 2 O5 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(OC 2 H 5 ) 5 was used as a starting precursor in the preparation of the Nb 2 O 5 powders using the Sol-gel procedure.The initial Nb 2 O 5 a Departamento de Física, Universidad Francisco de Paula Santander, Cucuta, North Santander, Colômbia b Departamento de Física, Facultad de Ciencias, Universidad Nacional de Colombia, Cra 45, Bogotá, Colômbia powders were amorphous and by heating them at 500° C contained Nb 2 O 5 (TT-phase), whereas at 650°C the Nb 2 O 5 (T-phase) was obtained.Uekawa et al. 17 reported that the synthesis of highly crystallized Nb 2 O 5 nanoparticles with a diameter of 4.5nm was based also on a Sol-gel route.

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

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

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

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.