Porous Ti Electrode and Its Degradation of Methylene Orange

Rare Metal Materials and Engineering Volume 44, Issue 6, June 2015 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2015, 44(6): 1326-1330.

ARTICLE

Preparation of Sb Doped Nano SnO2/Porous Ti Electrode and Its Degradation of Methylene Orange Li Guangzhong1,

Li Gang1,

Wang Hui1,

Xiang Changshu1,

Zhuang Jiandong2,

Liu Qian2,

Tang Huiping1 1

2

State Key Laboratory of Porous Metals Materials, Northwest Institute for Non-ferrous Metal Research, Xi’an 710016, China; State Key

Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Abstract: Sb doped nano SnO2 coating electrode was prepared on the porous Ti by a simple thermal decomposition method and then its electrochemical property for degradation of methylene orange was also investigated. SEM and XRD measurements results show that an integrative and crackless coating layer on porous Ti substrate can be obtained. The crackless coating surface is composed of Sb doped SnO2 nanoparticles with a size range from 80 nm indicates to 230 nm. HRTEM test suggests that these SnO2 nanoparticles consist of coating grains with 5~6 nm. Intensified life time test indicates that SnO2/porous Ti electrode has a longer service life time than the coatings on dense titanium plate. Moreover, the as-prepared Sb doped SnO2/porous Ti electrode can degrade the methylene orange with 100 mg/L concentration into that with 8 mg/L, which reveals that the electrode has a strong electrochemical ability to degrade organic pollutants. A simple surface treatment of porous Ti for organic pollutants degradation was presented in the paper, showing its great application potential in the area of pollutants degradation. Key words: Sb doped; nano SnO2; porous Ti electrode; methylene orange

Tin oxide (SnO2), a wide band gap (3.6 eV) n-type semiconductor, has been widely used in optoelectronic applications, gas sensors and organic pollutants degradation [1-5]. In particular, due to its high overpotential for oxygen evolution, SnO2 can produce high concentration OH radicals, making it an attractive electrode material for the anodic oxidation of organic pollutants [6,7]. SnO2-based electrode can be prepared by thermal deposition on Ti substrate. However, the Ti/SnO2 electrode has been restricted by the short service life [8]. In order to make the electrodes steady in solution and have a long service time, new methods for preparing electrodes have been developed to enhance the service life of electrode, such as electrodeposition, spray pyrolysis, and sol-gel process [4, 9-12]. Recently, Hossain et al. [13] have reported that the size and the morphology of SnO2 strongly affected the

properties and the application of SnO2 devices. They found that large surface area is essential for the photoanode of sensitized solar cells, and 1D nanostructured SnO2 has demonstrated much facilitated charge transport in optoelectronics. Furthermore, through thermal decomposition of tin oxide on Ti substrate with TiO2 nanotubes could be formed using titanium foil [4], and the process can be readily extended to make a thin layer of porous tin oxide on a suitable substrate for a specific application, by anodizing the pre-deposited tin film. However, it is not easy to prepare a well-defined porous tin oxide layer on a conducting substrate. In this paper, the origin of the cracks and blocked surface was systematically explored to clarify the key factors involved in the typical imperfections of thermal decomposition preparation of tin oxides coating electrode.

Received date: May 25, 2014 Foundation item: National Natural Science Foundation of China (50674076, 50902115); Natural Science Foundation of Shaanxi Province (2013JZ015) Corresponding author: Tang Huiping, Professor, Northwest Institute for Non-ferrous Metal Research, Xi’an 710016, P. R. China, Tel: 0086-29-86231095, E-mail: [email protected] Copyright © 2015, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Moreover, a viable way was suggested to obtain a crack-free SnO2-Sb electrode with the nanostructure on the porous titanium substrate [1,4]. The service lifetime and the electrochemical performances of the as-prepared porous Ti/SnO2-Sb electrode were also tested. The obtained Sb doped SnO2 nanostructure electrodes were employed as an electrode material for the anodic oxidation of organic pollutants.

1

2.1

Intensity/a.u.

SnO2 200 SnO2 101

SnO2 211

SnO2 110

Ti

Results and Discussion XRD analysis

XRD patterns of SnO2-Sb electrodes prepared on porous titanium substrate and dense titanium substrate are shown in Fig.1. Although there are little differences in the intensity and offset of the diffraction peak, the two XRD patterns of both electrodes are almost the same. As X-ray penetrates the coating, the sharp peaks related to the titanium substrate can be clearly seen. Except these sharp titanium peaks, the related XRD patterns exhibit the diffraction peaks only ascribed to SnO2 crystals with the tetragonal rutile structure (JCPDS No. 41-1445). The grain size estimated from the Scherrer eq uation is about 6 n m, suggestin g the nanocrystalline nature of SnO2. It is noted that no peaks

Ti

a b

Experiment

Porous titanium sheets (1 mm thick) were cut into samples with 10 mm×50 mm and then ultrasonically cleaned in deionized water and acetone. SnCl4·4H2O and SbCl3·3H2O were dissolved in isopropyl alcoholhydrochloric acid solution, where the mass ratio of Sn:Sb was 20:1. The electrodes were prepared by brush coating process on the porous Ti sheets using the above mentioned solution. The electrodes were dried in a drying oven at 110 °C for about 10 min and then placed into a muffle furnace at 450 °C for 10 min followed by air cooling. This procedure was repeated for several times in order to obtain the coating with the required thickness. Finally, the electrodes were treated at 450 °C for 1 h in the muffle furnace in order to make the coating fully oxidized. As comparison, the electrodes of dense titanium substrates were prepared under the same condition. The morphologies of the substrates and the coatings were examined by SEM (JEOL, 6700, Japan). The X-ray diffractometer (ADVANCE D8, bruker ， Germany) and transmission electron microscope (TEM, JEOL JEM 2100 F) were employed to analyze the crystalline structure of the electrode coatings. The cyclic voltammetric (CV) behavior was investigated in 0.5 mol//L H2SO4 solution at a scan rate of 40 mV/s on IM6e electrochemical workstation. Accelerated service life test was conducted in 0.1 mol//L Na2SO4 solution. Methylene orange was adopted as a model pollutant in electrochemical degradation testing using 1 mol/L Na2SO4 as the supporting electrolyte.

2

Ti

20

Fig.1

30

40

50 2θ/(º)

60

70

80

X-ray diffraction patterns of the Sb doped SnO2 coatings (a-porous titanium substrate and b-titanium substrate)

corresponding to antimony oxide are detected, which suggests that the antimony has been dissolved in solid solution with SnO2. 2.2 Morphology change of electrode The morphology of the SnO2-Sb electrodes was characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig.2 shows the SEM images of SnO2-Sb electrodes after thermal decomposition seven times. It can be seen that the crackless coating on porous titanium substrate is obtained although the porous Ti substrate has different surface morphologies, which is quite different from the “cracked-mud” coating on the typical Ti substrate, as shown in Fig.2c. The crackless coating surface is composed of nanoparticles with size range from 80 to 230 nm (Fig.2b). This indicates that porous titanium substrate can improve the coating structure effectively. Fig.3a shows the SEM image of porous-Ti/SnO 2 -Sb electrode prepared by 1 time thermal decomposition process. It is clearly seen that the surface morphology is compact and continuous, and there are no cracks on the surface. Then the property nature of porous Ti substrate can provide a larger surface area and an ideal support for the coating of the Sb-doped SnO2 nanoparticles. Fig.3a also reveals that the coating with somewhat spheres has a rough and granular surface, implying that it is built up by the agglomeration of small colloidal particles of SnO2. The sizes of the spheres are found to be in the range from 20 nm to 260 nm. The surface of porous-Ti/SnO2-Sb electrode prepared by 3 times thermal decomposition process is also compact, and the Sb-doped SnO 2 coating are tightly combined with the substrate, as shown in Fig.3b. Compared with the formers sample, the spheres prepared by 3 times decomposition remarkably increase on the surface. Although the number of thermal decomposition process increases, no cracks of Sb-doped SnO 2 coatings are

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a

b

c

and continuous. Selected area electron diffraction (SAED, Fig.4a) measurement indicates that all the annealed spherical particles are polycrystalline in nature, which is consistent with the result of the XRD measurement. These SnO2 spherical particles apparently consist of tiny nanocrystallites, indicating a polycrystalline nature of the spherical particles, which is confirmed by the corresponding SAED pattern, as presented in the inset of Fig. 4a. HRTEM measurements (Fig.4b, 4c) suggest that these nanoparticles consist of coating grains of 5~6 nm, presumably being tin(II) oxide sintered as indicated by XRD measurements, presenting a characteristic of cascade structure. It will be shown later that these agglomerated crystal grains serve as building blocks which can form various SnO2 superstructures.

2.3 Fig.2

SEM images of SnO2-Sb electrodes: (a, b) surface coatings on porous titanium substrates; (c) surface coatings on titanium substrate

Fig.3

a

b

c

d

SnO2-Sb electrodes prepared on porous titanium substrate by thermal decomposition route: (a) 1 time, (b) 3 times, (c) 5 times, and (d) 7 times

observed on porous Ti substrate. The surface morphology still remains compact when the number of thermal decomposition process increases to 7 times. The surface of Sb-doped SnO2 coating is completely composed of nanoparticles with a size range from 80 nm to 230 nm. So the surfaces of porous Ti /SnO2-Sb electrodes are relatively rough. The time-dependent particle growth on the coating surface suggests that the spherical particles are grown by continuous collection of tin (II) oxide. TEM images were obtained by observing the coatings scraped from the substrate and then well dispersed ultrasonically. As shown in Fig.4a, the coating is compact

Service life test

Fig.5 illustrates the accelerated service life test of SnO2-Sb coating at 100 mA.cm-2 in a 0.1 mol/L Na2SO4 solution on both substrates. It can be seen that the potentials of both electrodes barely increase at the initial stage. The service life of SnO2 electrode obtained on porous Ti substrate reaches 50 h, while it is only 20 h for the electrode obtained on dense Ti substrate. This result demonstrates that the service life of such an electrode coating on porous Ti substrate is greatly improved compared with the traditional electrode on dense Ti substrate. Study also found that accelerated service life would be shortened mainly because of the oxygen evolution reaction when the water penetrates into the inner part of the coating through cracks[4]. The impact action of generated oxygen on the coating would aggravate the crack. On the other hand, the generated nonconductive oxidation film on titanium substrates would also passivate the coating. Therefore, the integrative crackless coating obtained on porous Ti substrate could effectively increase the accelerated service life. Meanwhile, the morphology of the coating plays a decisive role in the electrochemical catalytic behavior of electrodes. For the morphology of the coating, fundamental shift happens in the morphology and the apace structure of the coating obtained on the porous titanium substrates compared with the traditional electrode. Such coating is composed of evenly distributed nanoparticles, which is beneficial to its electrochemical catalytic activity. For the integral structure of the coating, the appearance of SnO2 nanoparticles could ameliorate the crystallization degree of tin oxide as well as improve the intrinsic activity of the coating. Besides that, the appearance of SnO2 nanoparticles also increases the dispersion of active centers on the coating, that is, increase the effective active sites. Therefore, the above mentioned microstructure with Sb dopant SnO2 nanoparticles presents a higher electrochemical catalytic activity and a longer service life.

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Fig.4

a

b

c

20 nm

2.5 nm

5 nm

TEM images of SnO2-Sb electrodes prepared on porous titanium substrate by thermal decomposition route: (a) TEM image and with the selected area diffraction patterns of the SnO2-Sb in the inset, (b, c) HRTEM images

120 100 -2

Titanium plate

j/mA·cm

Potential/V

15

10 Porous titanium plate

Fig.5

60

Porous titanium plate

40 20

5 0

80

0

10

20

30 40 Time/h

50

60

Accelerated service life test of SnO2 coating at 100

Titanium plate

0.0

Fig.6

0.5

1.0 1.5 2.0 E/V (vs Ag/AgCl)

2.5

3.0

Cyclic voltammograms obtained in 0.5 mol/L H 2SO4, V=

mA·cm−2 in a 0.1 mol/L Na2SO4 solution

40 mV/s. SnO2-Sb electrodes prepared on porous titanium and titanium substrate

2.4 Electrochemical properties

2.5

Degradation testing of methylene orange

Methylene orange was used as model pollutant to assess the electrochemical catalytic activity for both electrodes. The initial concentration of methylene orange in the reaction system was 100 mg/L. The acreage of SnO2-Sb electrode was 3 cm2. And the concentration of supporting

100

Titanium plate

-1

Concentration/mg·L

Cyclic voltammetry (CV) measurements were performed to test the oxygen precipitation on SnO2-Sb electrodes. Fig.6 shows the cyclic voltammograms of SnO2-Sb electrodes prepared on both substrates in 0.5 mol/L H2SO4. There are no other peaks appearing in the cyclic voltammograms except the one formed by the oxygen evolution reaction (OER). From Fig.6 we can see that, the current density of SnO2-Sb electrode on porous titanium substrate is larger than that of SnO2-Sb electrode on dense titanium substrate. Concerning the morphologies of SnO2-Sb electrode on the two substrates we can conclude that the “mud-crack” SnO2-Sb coating on dense titanium substrate results in the limited the catalytic active site. In contrast, the crackless SnO2-Sb coating on the porous titanium substrate can effectively increase the actual surface area. So, the catalytic activity of SnO2-Sb electrode is significantly improved [14].

80 60 40 20 0

Fig.7

Porous titanium plate

0

20 40 60 80 100 120 140 160 Time/min

Electrochemical degradation of methylene orange on SnO2-Sb electrodes prepared on porous titanium substrate and titanium substrate

electrolyte Na2SO4 was 1 mol/L. Fig.7 shows the electrochemical degradation of methylene orange on both SnO2-Sb electrodes. Compared with SnO2-Sb electrode obtained on the dense titanium substrate, the SnO2-Sb electrode on the porous titanium substrate shows a higher catalytic activity. And the concentration of methylene orange is reduced to less than 8 mg/L after discomposed for

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120 min. While the SnO2-Sb electrode on dense titanium substrate could only reduce the concentration of methylene orange to 80 mg/L after disposed for the same time. This reveals that such coating (see Fig.7) has a strong electrochemical degradation ability of organic contaminations.

3 Conclusions 1) SnO2/porous Ti electrode can be prepared by a thermal decomposition method. An integrative, crackless coating layer on porous Ti substrate can be obtained, which is composed of nanoparticles with sizes ranging from 80 nm to 230 nm. The nanoparticles are composed of SnO2 grains with sizes of 5~6 nm and cascade structure. 2) SnO2/porous Ti electrode has a longer service life time than that coating by a traditional coating. 3) SnO2/porous Ti electrode can electrochemically degrade 100 mg/L methylene orange into 8 mg/L one, showing that the electrode has a strong electrochemical ability to degrade organic pollutants.

References 1

Cui X, Zhao G H, Lei Y Z et al. Materials Chemistry and Physics [J], 2009, 113: 314

2

Engineering[J], 2011, 40(9): 1638 (in Chinese) 3

Srivastava R A, Jain K. Materials Chemistry and Physics[J], 2007, 105: 385

4

Ding H Y, Feng Y J, Liu J F. Materials Letters[J], 2007, 61: 4920

5

Sun Z Q, Lu H Y, Ren X B et al. Acta Phys-Chem Sin[J], 2009, 25(7): 1385 (in Chinese)

6

Kotz R, Stucki S, Carcer B. J Appl Electrochem[J], 1991, 21: 14

7

Montilla F, Morallón E, Vázquez J L. J Electrochem Soc[J], 2005, 152 (10): B421

8

Chen A C, Nigro S. J Phys Chem B[J], 2003, 107: 13 341

9

Chen X M, Chen G H, Yue P L. J Phys Chem B [J], 2001, 105: 4623

10

Korotcenkov G, Brinzari V, Schwank J et al. Mater Sci Eng C, Biomim Mater, Sens Syst[J], 2002, 19: 73

11

Jin Z, Zhou H J, Jin Z L et al. Sensors and Actuators B[J], 1998, 52: 188

12

Chen Y, Hong L, Xue H M et al. Journal of Electroanalytical Chemistry[J], 2010, 648: 119

13

Zong Y, Cao Y L, Jia D Z et al. Sensors and Actuators B[J], 2010, 145: 84

14

Liang Z H, Ding Y B, Sun Y F. Rare Metal Materials and Engineering[J], 2010, 39(7): 1265 (in Chinese)

Fang Z Q, Yang M, Nan J M et al. Rare Metal Materials and

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