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1 Introduction▲
2 Experimental▲
3 Results and discussion▲
4 Conclusion▲
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Rare Metals2018年第7期

Nanosized GdVO₄ powders synthesized by sol-gel method using different carboxylic acids

Sulawan Kaowphong Nawapong Chumha Piyarat Nimmanpipug Sila Kittiwachana

Department of Chemistry, Faculty of Science, Chiang Mai University

作者简介：*Sila Kittiwachana e-mail:silacmu@gmail.com;

收稿日期：30 May 2015

基金：financially supported by the Chiang Mai University (CMU) Junior Research Fellowship Program;the National Research University (NRU) Project from Thailand's Office of the Higher Education Commission;

Nanosized GdVO₄ powders synthesized by sol-gel method using different carboxylic acids

Sulawan Kaowphong Nawapong Chumha Piyarat Nimmanpipug Sila Kittiwachana

Department of Chemistry, Faculty of Science, Chiang Mai University

Abstract：

Nanosized GdVO₄ powders were synthesized via a sol-gel method using different carboxylic acids as chelating agent, followed by calcination at 600 ℃ for 3 h.The effect of different carboxylic acids such as citric acid,malic acid, and tartaric acid on the characteristics of the nanosized GdVO₄ powders was investigated. The GdVO₄ powder was also synthesized without carboxylic acid for comparison. The thermal decomposition process of the carboxylate precursors was investigated by thermogravimetric differential thermal analysis(TG-DTA). X-ray diffraction(XRD), Fourier transform infrared spectroscopy(FTIR),field emission scanning electron microscope(FESEM),transmission electron microscope(TEM), and surface area measurement data were used to confirm the formation of nanocrystalline GdV04 powders. It is found that the synthesis using the carboxylic acid with higher heat of combustion results in the powder with larger crystallite size. The difference in the steric effect of the acids used,which was evaluated by a computational method, also affects the degree of agglomeration of the synthesized powders.

Keyword：

GdVO₄; Electron microscopy; Sol-gel processes; X-ray method;

Received： 30 May 2015

1 Introduction

Gadolinium vanadate (GdVO₄) has been extensively used as laser material [ 1] and host material for rare earth ions which find application in luminescent displays [ 2] .In addition,much attention was paid on its catalytic properties [ 3, 4, 5, 6] .As for its catalytic applications,the nanocrystalline material is among the favorites due to its larger surface areas which increase the catalytic activity [ 6, 7] .It is well known that both of the physical and the chemical properties of nanomaterials are strongly dependent on their structure,crystallinity,particle sizes,and micros truc tures which are usually sensitive to the synthesis methods as well as the synthesis conditions chosen during the synthesis process [ 8] .Therefore,the fabrication techniques for controllable size and morphology of the nanomaterials were developed.For example,Huang et al. [ 9] reported that the porous FeV_xO_y,one-dimensional (1D) nanostructures with different lengths were controllably synthesized through a hydrothermal method.The length of the 1D nanostructures could be tuned by adjusting pH value.Demonstrated by Zhang et al. [ 10] ,the CeVO₄ nanoparticles were hydrothermally synthesized.Zhang et al. [ 8] also reported the shape-controlled synthesis of ceria nanomaterials for catalytic application.In addition,Huang et al. [ 11] reported a method to synthesize hierarchical transition-metal vanadate (Cu,Fe,and Ni) nanosheets on metal mesh supports.The sizes and the thicknesses of the nanosheets could be controlled by duration of the reaction,acidity of the solution,and concentration of vanadate precursor.

A sol-gel method is a promising method for synthesizing nanosized powders.For the sol-gel synthesis,a chelating agent is one of the factors crucial to the success of the process.Generally,citric acid is used as the chelating agent since theα-hydroxycarboxylic functional groups of this acid could easily form stable complexes with the metal ions,thereby facilitating homogeneous mixing of the cations in the solution which assists good stoichiometry control [ 12, 13] .Moreover,the combustion of the C and the H atoms from the acid provides an amount of liberated heat;^therefore,highly pure powders could be obtained at considerably lower calcination temperature compared to the conventional solid-state method [ 14] .

However,the effect of changing the chelating agent on the synthesis of the nanosized GdVO₄ particles is interesting.This is because different chelating agents differ in both the number of carboxylic groups and the combustion heat provided,which can have an effect on the characteristics of the synthesized products.In this research,the nanosized GdVO₄ particles were synthesized by a sol-gel method using different carboxylic acids such as citric acid,malic acid,and tartaric acid as chelating agents.The characteristics such as purity,crystallite size,morphology,specific surface area,and nature of agglomeration of the synthesized GdVO₄ powders were examined in relation to the chelating agent used.

2 Experimental

Gd(OAc)3 and NH₄VO₃ with 1:1 in mole ratio were separately dissolved in deionized water and mixed into a homogeneous solution.The carboxylic acid (citric acid,malic acid,and tartaric acid) was then added into the solution,where the mole ratio of the total metal ions to the acid was 1:2.Then,the solution was heated under continuous stirring at 80℃.After a viscous gel was formed,the gel was subsequently dried at 80℃overnight to obtain a carboxylate precursor.The precursor was calcined at 600℃for 3 h,which resulted in a yellow powder.The GdVO₄ powder was also synthesized without the carboxylic acid serving as chelating agent for comparison.

Thermogravimetric (TG) analysis and differential thermal analysis (DTA) of the carboxylate precursors were measured using PerkinElmer TGA7 and DTA7 devices,respectively,with a heating rate of 10℃·min^-1 from 50 to800℃under a flowing nitrogen atmosphere.Structure,purity,and crystallinity of the products were analyzed by X-ray diffraction (XRD,Rigaku MiniFlex II) using Cu Ka radiation (λ=0.154 nm).Fourier transform infrared spectroscopy (FTIR,Bruker TENSOR27) was used to investigate the vibration modes and the residual organic molecules.Particle size and morphology of the products were investigated by field emission scanning electron microscope (FESEM,JEOL JSM-6335F) operating at accelerating voltage of 15 kV and transmission electron microscope (TEM,JEOL JEM-2010) operating at 200 kV.Specific surface areas of the nanopowders were obtained by the Brunauer-Emmett-Teller (BET) method using QuantaChrome AutoSorb-1-MP analyzer.

3 Results and discussion

The thermal decomposition behaviors of the carboxylate precursors between 50 and 800℃were investigated by TG-DTA,as shown in Fig.1.All precursors show similar behavior,with three main steps of thermal decomposition described as follows.The first step is weight loss in the temperature range of 50-200℃,with weight loss of about10%-11%,and accompanied by the endothermic peak at about 100℃in the DTA curve of the precursors.This is attributed to the vaporization of the physically absorbed water.The second step is in the temperature range of200-470℃,with weight loss of about 43%-49%,and accompanied by an exothermic peak in the DTA curve.This implies the decomposition and the combustion reactions of acetate ions,carboxylate ions,and residual organic constituents [ 13, 15] .A large exothermic peak width suggests that the reactions occur at a slightly low rate.In addition,large volume of gases such as carbon dioxide and water vapor could gradually release during the reaction [ 15, 16] ,resulting in a noticeable weight loss in the TG curve.In the last step,in the temperature range of 470-800℃,a mild exothermic peak is observed at about 530℃,indicating that a gradual crystallization of the GdVO₄ has taken place.For all the three TG curves,only slight weight loss curves could be observed after 550℃,suggesting that purified GdV0₄ compounds have already formed.

FTIR spectra of the carboxylate precursors (Fig.2a)show that O-H stretching band of residual water and the acid is detected at 2700-3700 cm^-1 [ 14] .The absorption bands in the range of 1590-1600 and 1395-1410 cm^-1,respectively,reflect the characteristic asymmetrical and symmetrical stretching vibration modes for carboxylates(-COO^-) [ 14] ,confirming the coordination between the metal ions and the carboxyl groups through the chelating reaction.Notably,the stretching band at 1725 and1230 cm^-1 can be attributed to the C=O and C-O,respectively,in the carboxylic group of the remaining unreacted acids [ 17] .After calcination,no absorption peaks of the carboxylate group and the acids are detected in the FTIR spectra of the powders synthesized using carboxylic acids,as shown in Fig.2b.A strong absorption band in the range of 770-850 cm^-1 and a weak absorption peak at450 cm^-1 are assigned,respectively,to the vibration modes of V-O bonds from groups [ 1, 18] and Gd-O bonds.The small peak at 1384 cm^-1 corresponds to the stretching vibrations of V=O bonds.These results confirm that the GdV0₄ powders are formed and that the carboxylic molecules are completely decomposed at the given thermal treatment,in accordance with the TG-DTA results.The broad peak from 3670 to 2920 cm^-1 and a weak band at1640 cm^-1 correspond to the O-H stretching and bending vibrations,respectively,of the water physically absorbed on the sample surface.For comparison,the FTIR of the asprepared powder synthesized without chelating agents before (Fig.2a) and after (Fig.2b) calcination is shown.In Fig.2a,the intense band at 770-850 cm^-1 and the very small peak at 450 cm^-1 of the stretching vibrations of V-O and Gd-O bonds suggest that the GdVO₄ compound already forms before the calcination.The small peaks at1402-1555 cm^-1 are detected,which could be corresponded to the vibrations of the carboxylate groups of the starting reagent adsorbing on the powder surface.After the calcination,the peaks of stretching vibrations of V-O and Gd-O bonds could be profoundly observed.

Fig.1 TG-DTA curves of a citrate,b malate,and c tartrate precursors

Fig.2 FTIR spectra of powders a before and b after calcination

The XRD spectra of the powders before calcination are shown in Fig.3a.Corresponding to the FTIR results in Fig.2a,although the GdVO₄ signals could be observed only in the XRD pattern of the powder synthesized without chelating agent,they are not sharp and intense.The XRDspectra of all powders after the calcination at 600℃are shown in Fig.3b.It can be seen that all diffraction peaks are found to match well with JCPDS No.17-0260,whichcorresponds to the GdVO₄ tetragonal structure.No diffraction peaks from any other impurities are detected,confirming high purity of the products.The intensities of the diffraction peaks of the GdVO₄ powders synthesized using the carboxylic acids as chelating agents are relatively higher than that synthesized without the acid,suggesting the improvement of the crystallization of the GdVO₄ particles.It could be that the energy evolved from the combustion reaction which can rapidly heat the system to a high temperature and raise the local temperature during calcination [ 19] ,facilitating/as sis ting the formation of the well-crystallized GdVO₄ at low calcination temperature.The crystallite sizes of the particles can be estimated by applying the Debye-Scherrer equation [ 20] ;

Fig.3 XRD patterns of powders a before and b after calcination

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Table 1 Crystallite size (D_XRD) based on Debye-Scherrer equation,specific surface area (SSA_BET),a^verage particle size (D_BET),and degree of agglomeration (D_BET/D_XRD) of synthesized GdVO₄powders

where D_XRD is the crystallite size in nanometers,λis the wavelength of Cu Kα,βis the full width at half-maximum(FWHM),andθis the half-diffraction angle.The most intense peaks corresponding to (200) plane were used to calculate the crystallite sizes.The calculated crystallite sizes are shown in Table 1.The GdVO₄ powder synthesized using citric acid presents a larger crystallite size,whereas those synthesized using malic acid and tartaric acid is smaller.Since the acids during the calcinations act as energy suppliers or fuels [ 14, 21] ,the difference in the crystallite sizes of the synthesized powers could be due to the difference in the relative exothermicity of combustion reactions from the acids.The heat values of combustion of the citric acid,malic acid,and tartaric acid are-1951.75,-1325.83,and-1140.56 kJ·mol^-1,respectively [ 22] .The higher heat of combustion of the acid giving greater heat generated from the exothermic reaction could raise the temperature of the particles,thus promoting the crystallite growth [ 23] .

Fig.4 FESEM images of GdVO₄ particles synthesized with a citric acid,b malic acid,c tartaric acid,and d without acid

The FESEM images of the GdVO₄ powders synthesized using different carboxylic acids,as presented in Fig.4a-c,demonstrate a homogeneous distribution of spherical particles with average particle sizes of about 50-100 nm.To present statistically reliable average size data,the histograms of the particle size distribution of the GdVO₄nanoparticles obtained by measuring 800 particles are presented in Fig.5.The particle size distributions are very close to normal curves.The average particle sizes are(62.43±11.62),(73.85±16.79),(55.25±10.78),and(86.13±18.47) nm for the powders prepared using citric acid,malic acid,tartaric acid,and without acid,respectively.The specific surface areas (SSA_BET) were obtained using BET adsorption of nitrogen gas at the temperature of liquid nitrogen,and the average particle sizes (D_BET) were calculated using the following equation;

Fig.5 Particle size distribution curves of GdVO₄ particles synthesized with a citric acid,b malic acid,c tartaric acid,and d without acid (d_aveaverage particle size,SD standard deviation,d_max maximum particle size,d_min minimum particle size,N total particle numbers)

whereρis the theoretical density of GdVO₄ (5.47 g·cm^-3) [ 24] .The SSA_BET and D_BET values of the synthesized GdVO₄ powders are summarized in Table 1.As can be seen from Table 1,D_BET values determined using SSA_BETare higher than crystallite sizes (D_XRD) calculated using the Debye-Scherrer equation.This implies that the particles are composed of agglomerated crystallites.To confirm the agglomeration level of the synthesized powders,the degrees of agglomeration (D_BEE/D_XRD) of the GdVO₄powders were calculated.The powders synthesized using carboxylic acids provide low values of D_BET/D_XRD compared to that synthesized without acid.This implies that they exhibit weaker particle agglomeration.Among the powders synthesized using acids,the highest value of D_BET/D_XRD can be observed from the powder synthesized using malic acid as the chelating agent,suggesting the strongest particle agglomeration.This is in accordance with the FESEM results shown in Fig.4b.These observations could be associated with the lowest steric structure of malic acid which leads to close contact of the inpidual particles.The steric effect of the acids used in this research was evaluated via relaxed conformational energy scans at the AM1 level of the theory,carried out by GAUSSIAN03software.As the profiles were also dependent on the starting points,a full optimization was carried out.The torsion by the end-carboxylic group,which is common for all these acid series,was scanned to shift everyπ/6.Citric,malic,and tartaric acids need to overcome the potential barriers of 10.65,9.67,and 11.71 kJ·mol^-1,respectively.The lowest potential barrier of malic acid indicates the lowest steric structure.The degree of agglomeration of the powder synthesized without the acid is relatively high since there is no steric effect from the acid preventing the agglomeration of the particles in the reaction process,as shown in Fig.4d.

The TEM images of the GdVO₄ powders prepared with and without the acids are shown in Fig.6.The powders are composed of nanoparticles with diameter in the range of50-70,70-110,40-65,and 85-120 nm for the powders prepared using citric acid,malic acid,tartaric acid,and without acid,respectively.The selected area electron diffraction (SAED) patterns illustrated as the insets in Fig.6 show bright concentric rings of the poly crystalline particles.Lattice spacing (d) of the nanoparticles is determined using the following equation [ 25] ;

Fig.6 TEM images of GdVO₄ particles synthesized with a citric acid,b malic acid,c tartaric acid,and d without acid.Insets in TEM images being SAED patterns (left) and HRTEM images (right)

Lλ=Rd (3)where Lλis the camera constant (2.49630 mm-nm)and R is the radius distance between the rings.The calculated lattice spacing corresponds to (101),(200),(112),(301),and (312) diffraction planes,in agreement with the XRD results,confirming the tetragonal structure of the GdVO₄ nanocrystals.High-resolution TEM (HRTEM)images indicate that the GdVO₄ nanoparticles have good crystallinity and the crystal lattice fringes of the 0.36,0.25,and 0.28 nm planar spaces correspond well with the (200),(220),and (211) planes of tetragonal GdVO₄,respectively.

4 Conclusion

The carboxylic acid-as sis ted sol-gel method is a promising way to synthesize the nanosized GdVO₄ powders.The energy released from the combustion process and the steric structures of the carboxylic acids are found to have an effect on the crystallite sizes and the degree of agglomeration of the synthesized nanoparticles.The GdVO₄ powder synthesized using citric acid exhibits larger crystallite size than those derived from malic acid and tartaric acid due to the intense heat released during the combustion process which causes the crystallites to grow in size.Considering the synthesis route using the acids,the powder synthesized using malic acid demonstrates the highest degree of agglomeration due to the least steric structure of the chelating agent used.

Acknowledgments This work was financially supported by the Chiang Mai University (CMU) Junior Research Fellowship Program and the National Research University (NRU) Project from Thailand's Office of the Higher Education Commission.