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Rare Metals2018年第7期

Preparation and electrochromism of pyrochlore-type tungsten oxide film

Qiu-Sheng Zhou Yong-Kun Chen Xiao-Bin Li Tian-Gui Qi Zhi-Hong Peng Gui-Hua Liu

School of Metallurgy and Environment, Central South University

作者简介：*Qiu-Sheng Zhou e-mail:qszhou@csu.edu.cn;

收稿日期：13 May 2016

基金：financially supported by the National Natural Science Foundation of China (No. 51274243);the Project of Innovation-Driven Plan in Central South University, China (No. 2015CX001);

Preparation and electrochromism of pyrochlore-type tungsten oxide film

Qiu-Sheng Zhou Yong-Kun Chen Xiao-Bin Li Tian-Gui Qi Zhi-Hong Peng Gui-Hua Liu

School of Metallurgy and Environment, Central South University

Abstract：

Pyrochlore-type WO₃ powder was synthesized via hydrothermal method using aqueous sodium tungstate solution and oxalic acid as raw materials. The as-prepared powder was made into a soliquoid,from which films were made by dip coating process with indium-tin oxide(ITO).The obtained films were characterized by thermogravimetric and differential thermal analysis(TG-DTA), X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM), cyclic voltammetry(CV), chronoamperometry(CA) and ultravioletvisible(UV-Vis) absorption. Results show that the crystal of the pyrochlore-type WO₃ powder is perfect. When the calcination temperature rises from room temperature to500 ℃,the pyrochlore-type structure first becomes deformed, then it is destroyed and turns into amorphous phase,finally it will completely convert to WO₃ with a monoclinic structure. Electrochemical and optical tests demonstrate that the film calcined at 300 ℃ exhibits the best electrochromic performance and has a coloration efficiency of up to 68.5 cm² C^-1 at 884 nm.

Keyword：

Pyrochlore-type WO₃; Sodium tungstate solution; Films; Electrochromic properties;

Received： 13 May 2016

1 Introduction

Electrochromism refers to the phenomenon in which the optical properties can be switched reversibly and persistently in a material induced by an external voltage [ 1, 2, 3, 4, 5] .Electrochromic materials draw plenty of attention in recent years as they could be used in information display devices,smart windows,self-dimming rear mirrors and military camouflage,etc. [ 6, 7, 8, 9, 10, 11, 12, 13, 14, 15] .Many transition metal oxides can be used as electrochromic materials,such as tungsten oxide,nickel oxide,cob altous oxide,iridium oxide,vanadic anhydride,titanium dioxide,and molybdenum trioxide [ 16] .However,as electrochromic materials,part of the oxides has intrinsic disadvantages,such as high cost (iridium oxide),poor cycle performance(molybdenum trioxide),poor discoloration performance(cobaltous oxide,titanium dioxide) [ 16] .Compared with these oxides,tungsten oxide shows a better performance and has a promising application in the field of electrochromic.

As one of the most important electrochromic materials,WO₃ shows potential applications in photochromic devices,photocatalysis and batteries [ 17, 18, 19] .Since Deb [ 20] first reported the electrochromic phenomenon of WO₃ in1969,many researches on the electrochromic property of WO₃ have been conducted [ 21, 22, 23, 24, 25] .However,most studies were performed on monoclinic or amorphous WO₃,and little work was reported on the cubic phase [ 26] ,especially on pyrochlore-type WO_3.

As a kind of metastable cubic WO₃,pyrochlore-type WO₃ (c-WO_3.0.5H₂O) can be used not only as an intermediate to produce WO₃ but also as an important promising functional material used in sensors,catalysis and batteries.These properties or applications relate to its special structure:W-O skeleton is built up of layers made up of slightly distorted (WO₆) octahedra sharing their corners and arranged in six-membered rings.These layers are linked along the[111]direction,and 3D interconnected tunnels are formed.The structure withW-O skeleton and empty tunnel in three dimensions is conductive to the motion of ions in the solid state.Li and Tsaistudied the humidity-sensing properties of M-pyroWO₃ (M=H⁺,Na⁺,Li⁺,Ag+,NH₄⁺),and it took 48 h to synthesize the Na-pyroWO₃ via the hydrothermal method using sodium tungstate and glacial acetic acid as raw materials.Zheng et al.spent 72 h synthesizing c-WO₃·0.5H₂O using sodium tungstate,urea and hydrochloric acid as raw materials.He studied the photocatalytic activity of c-WO₃·0.5H₂O,and the results showed that c-WO₃.0.5H₂O displayed an excellent photocatalytic activity and crystal water played a key role in the photocatalytic activity.Yang et al.carried out a study on Pt-WO_3-C composite catalysts with pyrochloretype WO₃ synthesized by hydrothermal method using sodium tungstate and hydrochloric acid as raw materials,and the catalyst exhibited a good performance.Guo et al.first reported the electrochromic property of pyrochlore-type WO₃,but no other researches were performed to study this property.Although pyrochlore-type WO₃,as a functional material,can be used in many fields,few researches were carried out on its properties.This loss in interest is probably attributed to the long preparation time (～72 h),low tungsten crystallization ratio and high cost.

In recent work,it was proposed a novel hydrothermal technique for preparing pyrochlore-type WO₃ powder [ 32, 33] .According to the research of Guo et al. [ 34] ,pH was an important factor that affected the preparation and the pyrochlore-type WO₃ could only be synthesized in a low pH of 2.5-4.5.However,in previous work,well-crystallized pyrochlore-type WO₃ with a high purity could also be prepared in an alkaline environment(7.0<pH<8.9).The crystallization ratio of tungsten in the tungstate solution could reach 91%with a low cost and short time (～12 h).This technique may not only encourage researches on the properties of the pyrochlore-type WO₃ but also promote its potential industrial applications.

In this work,it was synthesized the pyrochlore-type WO₃ powder by the novel hydrothermal technique,and then the powder was made into a soliquoid.The thin films were prepared from the soliquoid by sol-gel dip coating with indium-tin oxide (ITO).Moreover,it was discussed the influence of the calcination temperature on the surface,internal structure and the electrochromic property of the pyrochlore-type WO₃ films for the first time.It was found that the electrochromic property of the film calcined at an appropriate temperature was outstanding,including fast response time,great optical modulation and coloration efficiency.

2 Experimental

2.1 Preparation of pyrochlore-type WO3 film

2.1.1 Preparation of pyrochlore-type WO3 powder

Oxalic acid[H₂C₂O₄·2H₂O,analytical reagent (AR)]was gradually added into a 0.65 mol·L^-1 sodium tungstate(Na₂WO₄·2H₂O) solution until the pH of the mixture reached 5.5.The obtained mixture was introduced to a noncorrosive pressure bomb with several small steel balls for agitation.The bomb was immersed in glycerol and then rotated in a low-pressure vessel.The hydrothermal reaction was maintained at 120℃for 10 h.The hydrothermal product was washed with deionized water followed by ethanol several times and subsequently dried at 80℃for1 h to obtain the pyrochlore-type WO₃ powder.

2.1.2 Hydrochloric acid treatment of pyrochlore-type WO3powder

30 ml 6 mol-L^-1 hydrochloric acid and 5 g as-prepared pyrochlore-type WO₃ powder were mixed in a 100-ml beaker and agitated by a magnetic stirrer for 1 h,and the beaker was left to settle for 10 min,then the above solution was removed.The powder was treated by hydrochloric acid 4-5 times according to the operations mentioned above.Subsequently,the deposit in the beaker was washed with water 4-5 times according to the following procedures:The deposit obtained in the beaker was washed with deionized water and agitated by the magnetic stirrer for 30 min,then the solid and liquid were separated.Finally,50-ml deionized water was added into the beaker and the pyrochlore-type WO₃ soliquoid was prepared.

2.1.3 Preparation of pyrochlore-type WO3film

ITO glasses (15Ω·(sq)^-1) were successively cleaned with acetone,ethanol and deionized water several times and then dried in a nitrogen atmosphere.The cleaned ITO glass was dipped into the soliquoid for a duration of 5 min,then pulled up at a speed of 4 cm·min^-1 and kept in the air for 5 min.The coated ITO was dipped into the soliquoid and pulled up at the same speed again.The obtained ITO was kept in the air for5 min and then dried at 80℃,and finally calcined at a preset temperature for 1 h.The prepared pyrochlore-type WO₃films were used for electrochromic characterizations.

2.2 Tests and characterization

The thermal stabilization of the pyrochlore-type WO₃powder was tested in a thermal analyzer (SDTQ600,TA,USA) with the heating rate of 10℃·min^-1 in a nitrogen atmosphere.The crystalline phases of the films were determined by X-ray diffractometer (XRD,D8-Advance,Bruker) with Cu Kαradiation.Microstructural analyses of the powder and films were conducted on a scanning electron microscope (SEM,JSM-6360LV,JEOL,Japan)operated at 20 kV and transmission electron microscope(TEM,Tecnai G2 F20,FEI,Japan).

Electrochemical measurements were carried out on an electrochemical analyzer (PARSTAT4000,AMETEK,USA) by three electrode cyclic voltammetry (CV).The working electrode was made of prepared pyrochlore-type WO₃ film.The saturated calomel electrode and graphite served as the reference electrode and counter electrode,respectively.All electrodes were performed in 1.0 mol·L^-1H₂SO₄.The absorption spectra of the films were recorded in ultraviolet-visible (UV-Vis) spectrophotometer (U-4100,Hitachi,Japan).

The coloration efficiency was calculated according to the Beer-Lambert law:

whereηdenotes the coloration efficiency (cm².C^-1),Q represents the charge density (C·cm^-2),and T_b and T_cdesignate the light transmittance of the bleached and colored states,respectively.

3 Results and discussion

3.1 Thermal analyses of pyrochlore-type WO3 powder

Figure 1 presents thermogravimetric and differential thermal analysis (TG-DTA) curves of the pyrochlore-type WO₃ powder.As shown in TG curve,the powder loses weight from room temperature to approximate 400℃.This weight loss can be attributed to the removal of the adsorbed water on the powder surface and part of the crystallization water.It is noted that several studies [ 26, 35] observed a distinct endothermic peak at about 250℃in DTA curve,and the net structure made of hexatomic ring and tunnels were destroyed.However,no obvious peaks in DTA curve were detected from room temperature to400℃after repeated tests.The possible reason is that the net structure and 3D tunnels are deformed instead of destroyed as the calcination temperature rises to 400℃.No remarkable changes in weight are observed from 400 to600℃in TG curve;however,two evident endothermic peaks at about 440 and 550℃are observed in DTA curve,indicating that the crystalline phase is changed and the pyrochlore structure may be destroyed.The weight decreases again when the temperature exceeds 900℃,and an endothermic peak is observed in DTA curve.This behavior may be caused by the decomposition of the impurities.According to the analyses of TG-DTA curves,the films and the powder were calcined at 260,300,400and 500℃for 1 h to determine the influence of calcination temperature on the electrochromic property of the pyrochlore-type WO₃.

Fig.1 TG and DTA curves of pyrochlore-type WO₃ powder

3.2 XRD,SEM and TEM analyses

Figure 2 shows XRD patterns of the products treated in different ways.XRD peaks of the original hydrothermal product (Fig.2(1)) washed without hydrochloric acid are indexed to the phase of c-WO_3·0.5H₂O (JCPDS card No.84-1851).No other impurities are detected.The sharp characteristic peaks indicate that the original hydrothermal product is well crystallized.The diffraction peaks of the ideal pyrochlore-type WO₃ near 2θof 30°are symmetric.However,the peaks of the original product are asymmetric,suggesting that the crystal lattice of the original product is deformed.The diffraction peaks of the product treated by hydrochloric acid (Fig.2(2)) demonstrate that the acidtreated product is also pyrochlore-type WO₃.Compared with the diffraction peaks near 2θof 30°in Fig.2(1),the peaks in Fig.2(2) are symmetrical,showing that the structure of the acid-treated product is improved after hydrochloric acid treatment.Figures 2(3),(4) are XRD patterns of acid-treated product calcined at 260 and300℃,respectively.Both patterns correspond to pyrochlore-type WO₃,and the diffraction peaks of Fig.2(3)near 2θof 30°are nearly symmetrical.This finding verifies the previous TG-DTA analysis that the pyrochlore structure is deformed rather than destroyed when calcined at a temperature below 300℃.Figure 2(5) illustrates that the3D tunnels of the product calcined at 400℃collapse,and the pyrochlore structure is destroyed completely.A portion of the product becomes amorphous,whereas the remaining portion becomes monoclinic WO₃.When the calcination temperature reaches 500℃,the entire product converts to monoclinic WO₃,as shown in Fig.2(6).

Fig.2 XRD patterns of product treated by different methods:(1)original hydrothermal product,(2) product washed with hydrochloric acid and acid-treated product calcined at (3) 260℃,(4) 300℃,(5)400℃and (6) 500℃for 1 h

According to XRD analyses,the pyrochlore-type structure successively undergoes deformation,collapse,conversion to amorphous phase and conversion to monoclinic WO₃ with an increase in calcination temperature.This result differs from those reported by Xu et al. [ 36, 37] and Ali et al. [ 38] ,who concluded that pyrochlore-type WO₃calcined at 500℃was converted to monoclinic Na₂W₄O_13.The reason is that the sodium content in their pyrochlore-type WO₃ is extremely high without sodium removal,and thus the sodium is inserted into 3D tunnels.In this work,as the products were washed or treated with hydrochloric acid before calcination,the sodium content is very low compared with that of their product.Thus,the main phase of the product calcined at 500℃is monoclinic WO₃ instead of Na₂W₄O₁₃.

As shown in SEM image of pyrochlore-type WO₃powder (Fig.3a),the shape of the particle is irregular and the size is not uniform because of agglomeration of the original fine particles.Additionally,it can be seen from Fig.3b that the thickness of the prepared film is about11.6μm.The surface of the film without calcination(Fig.3c) is uniform and compact,whereas the surfaces of the films calcined at 260℃(Fig.3d) and 300℃(Fig.3e)are relatively loose.This change suggests that the calcination of the film is conducive to the diffusion of ions,and thus the film can be more readily colored and bleached.Meanwhile,there are many granules on the crude surface(Fig.3f) of the film calcined at 400℃.Combined with the previous XRD analyses,the pyrochlore structure of the product is destroyed when calcined at 400℃for 1 h and substituted by amorphous phase or monoclinic phase.During this process,the monoclinic WO₃ is recrystallized on the surface of the film,resulting in a crude surface.

The structures of the pyrochlore-type WO₃ calcined at different temperatures were further illustrated by TEM.As shown in the low magnification images (Fig.a₁,b₁,c₁,d₁,e₁),the shape of the particle is irregular.The high magnification TEM images and fast Fourier transform (FFT)patterns of the products (Fig.4a₂,b₂,c₂,d₂,e₂) indicate that the products are well-crystallized,but the product calcined at 400℃(Fig.4d₂) is bad-crystallized and appears amorphous phase,being in agreement with XRD and SEM analyses.

Fig.3 SEM surface images of a product washed with acid,b cross section of original film,and films c with no calcination,calcined at d 260℃,e 300℃and f 400℃for 1 h

3.3 CV analysis of pyrochlore-type WO3 films

Figure 5 shows CV profiles of no-calcination pyrochloretype WO₃ film between-0.3 and 1.0 V at different scanning rates.The inset in Fig.5 gives the plot of anodic peak current as a function of square root of scan rate.As Fig.5shows,when the scan rate increases,the anodic and cathodic currents increase,manifesting that the amount of protons and electrons incorporated into the film increases.This phenomenon also implies that the reaction activity of the film increases.The anodic current (Amperes),during anodic scans at different scanning rates,is used to extract the electrode,applying the Randles-Sevcik equation [ 39, 40] :

Fig.4 TEM images of pyrochlore-type WO3 calcined at different temperatures for 1 h (FFT patterns presented in inset):a1,a2 no calcination;b1,b2 260℃;c1,c2 300℃;d1,d2 400℃;e1,e2 500℃

Fig.4 TEM images of pyrochlore-type WO₃ calcined at different temperatures for 1 h (FFT patterns presented in inset):a₁,a₂ no calcination;b₁,b₂ 260℃;c₁,c₂ 300℃;d₁,d₂ 400℃;e₁,e₂ 500℃the diffusion coefficient (D,cm²·s^-1) of H⁺in

where A is the interface between electrolyte and active material,C is the concentration of species in the bulk(mol·cm^-3),n is the number of electrons involved in the redox process (n=1 for W⁵⁺/W⁶⁺redox pair),and v is the potential scan rate (V·s^-1).As shown in inset in Fig.5,I_p is proportional to v^1/2,confirming a diffusion-controlled behavior.

The curve of the current density with time is plotted in Fig.6,and the film tested was calcined at 300℃for 1 h.As shown in Fig.6,the current density changes little after the 10th cycle,revealing that the film has a good electrochemical stability.

Figure 7 presents the 20th CV curves of pyrochlore-type WO₃ films calcined at different temperatures for 1 h.According to the theory of injection and extraction [ 41, 42] ,the color of the film changes from colorless to dark blue as H⁺ is injected into the film with a voltage scan from positive to negative.By contrast,H⁺is extracted from the film,so the color bleaches when changing scan direction.All films tested undergo the color changing process in this study.

Fig.5 CV curves of pyrochlore WO₃ films without calcination at different scan rates (anodic peak current as a function of square root of scan rate presented in inset)

Fig.6 Curve of current density with time for film calcined at 300℃for 1 h

Fig.7 20th CV curves of pyrochlore WO₃ films calcined at different temperatures for 1 h

下载原图

Table 1 Injected and extracted charge at 20th circle of films calcined at different temperatures and their corresponding response time

Q₁-Injected charge,Q₂-Extracted charge,t_c-Coloration time, t_b-Bleaching time

The capacity to hold the ions of the film is proportional to the area of the CV curves [ 43] .In Fig.7a,the enclosed areas of the three CV curves gradually increase as the calcination temperature rises,indicating that the capacity to hold the ions of the films increases gradually with calcination temperature increasing.Being consistent with previous result from SEM analyses,the surface of the film without calcination is uniform and compact with high crystallinity,and it is not conducive to the ion diffusion in the film.By contrast,the surfaces of the films calcined at260 and 300℃become loose and multi-aperture so that the ions are readily inserted into the film.As discussed in XRD analyses,3D tunnels of the pyrochlore-type will be deformed at a high temperature.The deformation may benefit the injection of H⁺,resulting in a larger capacity to hold ions of the film calcined at 300℃than that of the film calcined at 260℃.Compared with Fig.7a,Fig.7b presents smaller area of each enclosed curve.The color change in the films calcined at either 400 or 500℃is also lighter than that of the other three in Fig.7a.This difference may be caused by the collapse of 3D tunnels of pyrochlore WO₃ and the phase change from cubic to either amorphous or monoclinic when the film was calcined at relatively elevated temperatures of 400 and 500℃.In addition,the influence of calcination temperature on the ion capacity of the film is also in agreement with the change in the surface of films discussed in SEM analyses.

Table 1 lists the injected and extracted charge at the20th circle of the films calcined at different temperatures for 1 h.From Table 1 and the previous analyses,it can be drawn a conclusion that the relationship between the capacity to hold ions of the films and the calcination temperature can be represented by a parabola with a downward opening,and the apex is around 300℃.The ratio of the injected charge to the extracted charge at the20th circle of the film calcined at 300℃is 95.7%,showing that the film calcined at 300℃exhibits satisfactory electrochemical reversibility.

Switching speed from one state to another state is of great importance to the electrochromic materials.In addition,the response time of coloration and bleaching is closely related to the charge-transfer rate at the electrochromic material interface,the diffusion coefficient of cations within the electrochromic material,and the diffusion distance to the active sites [ 10] .The response time can be valued by the minimum time for the current of the electrode reducing to zero or 90%of its full value after adding a certain voltage [ 43] .The switching characteristic of pyrochlore-type WO₃ film was investigated by CA,as shown in Fig.8.The values of t_c and t_b for all the films are listed in Table 1.As shown in Table 1,the switching speed of coloration is faster than that of bleaching.Moreover,the film calcined at 300℃has the shortest bleaching time,probably due to its loose porous surface and 3D tunnels.This facilitates ion diffusion and the active surface area can be sufficiently utilized for charge-transfer reactions.

Fig.8 Current responses for pyrochlore-type WO₃ films calcined at different temperatures for 1 h at-0.6 and 1.0 V applications for 30 s per step

3.4 Transmission spectra analysis

Figure 9 shows spectral transmittance at the 20th circle of the pyrochlore-type WO₃ films calcined at different temperatures for 1 h.As can be seen in Fig.9a,the film without calcination has no modulation of light at the wavelength below 620 nm,because the transmittance of color is above the bleached state.When the wavelength is above 620 nm,the modulation of light is also poor with the maximum difference in transmittance between colored and bleached states of 30%.The film calcined at 260℃(Fig.9b) absorbs light from 400 to 900 nm;however,the maximum difference in transmittance is not broad.Compared with that of the film without calcination,the light modulation of the film calcined at 260℃slightly improves.When the film was calcined at300℃(Fig.9c),the highest bleached state transmittance exceeds 90%from 700 to 900 nm,and the maximum difference in transmittance between colored and bleached states is about 65%,indicating that the film calcined at 300℃has a relatively good modulation.Figure 9d illustrates that the highest bleached state transmittance is about 70%,and the maximum difference in transmittance reduces to 50%at a film calcination temperature of 400℃(i.e.,its light modulation decreases compared with that of the film calcined at300℃).With a further increase in film calcination temperature to 500℃,light modulation becomes markedly inferior (Fig.9e).This result agrees with the previous analyses.The pyrochlore-type WO₃ calcined at 500℃converts to monoclinic WO₃,thus the ions it holds are fewer than those of other films,as listed in Table 1.Moreover,the changes in both its surface and internal structure impede coloration bleaching.

From analyses above,the calcination temperature of the film greatly influences the electrochromic property of the pyrochlore-type WO₃ film.The experiments indicate that the pyrochlore-type WO₃ film calcined at 300℃exhibits the best electrochromic property,and the highest coloration efficiency can reach 68.5 cm²·C^-1 at 884 nm,which is close to that of monoclinic and amorphous WO₃ film reported by Shang [ 43] .

Fig.9 Spectral transmittance at 20th circle of pyrochlore-type WO₃ films calcined at different temperatures for 1 h:a without calcination,b 260℃,c 300℃,d 400℃,and e 500℃

4 Conclusion

Pyrochlore-type WO₃ powder synthesized via the hydrothermal method was made into soliquoid,from which pyrochlore-type WO₃ films were prepared by dip coating process with ITO.The results indicate that the pyrochloretype structure varies with the calcination temperature,and it successively involves deformation,collapse,conversion to amorphous phase and transformation to monoclinic phase with calcination temperature increasing from room temperature to 500℃.Moreover,the pyrochlore-type WO₃ film calcined at 300℃for 1 h exhibits the best electrochromic property,with the ratio of injected charge to extracted charge of 95.7%at the 20th circle and the highest coloration efficiency of 68.5 cm²·C^-1 at 884 nm.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No.51274243) and the Project of Innovation-Driven Plan in Central South University,China (No.2015CX001).