EffectofDryingTimeontheAntimonyDopedTinDioxide.doc

精品论文effect of drying time on the antimony doped tin dioxidecoated titanium electrode5wang yun-hai1, zhou zhe1, li guo1, chen qing-yun2(1. department of environmental engineering, school of energy and power engineering, xianjiaotong university, xian 710049;2. state key lab of multiphase flow and power engineering, xian jiaotong university, xian710049)10abstract: the antimony doped tin dioxide coated titanium electrodes were prepared by dip-coating pyrolysis method with different drying time. the effect of drying time on the electrode activity,morphology, crystallinity and composition were discussed. the results indicated that electrode prepared by drying 10 minutes had higher activity towards phenol degradation than those prepared bydrying 0 or 30 minutes. the relation between the activity and oxygen vacancies concentration was15discussed. the tg-dta analysis was used to monitor the reactions occurred during the electrode preparation. the results showed that there were three endothermic or exothermal peaks and the possiblereactions were discussed.key words: environmental engineering; antimony doped tin dioxide; drying; electrochemical; coating200introductionantimony doped tin dioxide coated titanium electrode has attracted considerable research interest in recent years in electrochemical degradation of toxic organic contaminants in wastewater 1-5 and electrolytic generation of ozone due to its high efficiency and non lead-containing 6,7. it has higher oxygen evolution potential which25can hinder the thermodynamically favored oxygen evolution reaction, hence improve the efficiency for organic destruction 8-10. the electrodes have been usually prepared by dip-coating pyrolysis method, sputtering method and chemical vapor deposition method etc 1,11-13. the dip-coating pyrolysis method has been often used to prepare antimony doped tin dioxide electrodes due to its easy operation 14, 15. this method30always includes coating the metal chloride alcoholic solution onto titanium substrate, then drying at a temperature ranging from room temperature to 150 in an oven, and finally baking in a furnace at a higher temperature of 400-600 . this coating, dryingand baking process is repeated for 7-30 times to get the doped tin dioxide film on titanium substrate. in this pyrolysis method, different drying time had been used to35prepare the electrodes in different research work 6, 15, 16. the drying time effect on theelectrode activity and the reactions occurred during the drying and baking process are still not clear. this may cause the lower reproducibility of the electrodes, which was already observed in our previous experimental work. in this work, the drying time effect on the electrode activity will be discussed and the electrodes prepared with40different drying time will be characterized. the tg-dta analysis will be used tomonitor the possible reactions occurred during the electrode preparation.foundations: research fund for the doctoral program of higher education of china (no.20090201120010); natural science foundation of china (no.21206134)brief author introduction:dr. wang yun-hai was born in 1977. now he is an associate professor interested inelectrochemical technology for waste water treatment in department of environmental engineering, xian jiaotonguniversity. e-mail: - 8 -1experimentstitanium grid (dexmet) spot welded with a titanium wire was etched in 10%boiling oxalic acid for 1 hour, then rinsed with alcohol and used as the electrode45substrate. the coating solution was made of 1 m sncl45h2o (98.5%, wako chemical) and 20 mm sbcl3 (99.9%, wako chemical) in absolute ethanol with a few drops of concentrated hcl. the titanium substrate was dipped into the coating solution and drawn out with a uniform liquid film on it. then the substrate was driedunder 80 for a given time. then it was moved to a furnace to bake at 500 for 1050minutes. this dip-coating, drying and pyrolysis procedure was repeated for 15 times and the last pyrolysis was heated with a longer duration of 1 hour. three drying time of 0, 10 and 30 minutes were used to prepare three electrode sample series marked a, b and c, respectively.the electrode activity was evaluated by phenol degradation. the electrochemical55degradation of phenol was performed with a constant current density of 40 ma cm-2 on iviumstat electrochemical workstation. the initial phenol concentration was 200 mg l-1 and the electrolyte was 0.1 m h2so4. the phenol concentration was measured by uv-vis spectroscopy (unico uv-3802).the electrode morphology was characterized by scanning electron microscopy60(sem) on jsm-6700f (jeol). x-ray diffraction (xrd) profiles were obtained on xrd-7000 diffractometer (shimatsu) with cu-ka1 radiation (0.1541 nm) at room temperature in a step-scanning mode, with a step length of 0.02 degree. the composition of the electrode surface was studied by x-ray photoelectron spectrometry (xps). the spectra were obtained on axis ultra (kratos analytical) using65monochromatic al ka radiation (150 w, 15 kv, 1486.6 ev). the vacuum in the spectrometer was 10-9 torr and the binding energies were calibrated relative to the c1s peak (284.6 ev).the tg-dta analysis was performed on hct-2 thermal analyzer (heaven, china)and used to monitor the reactions occurred during the electrodes preparation. the70sample for tg-dta experiment was 5 ul freshly prepared coating solution. the temperature program was set to be heating up to 600 with heating rate of 20 min-1, with 0, 10, 30 minutes durations at 80 to simulate the electrodes preparationprocedure, respectively.2results and discussions75the electrodes activities toward phenols degradation were shown in fig. 1. the results obviously showed that drying time had effect on the electrodes activities toward phenol destruction. sample b, prepared by drying 10 minutes, had the highest activity while sample a, prepared by drying 0 minutes had the lowest activity. the antimony doped tin dioxide electrode activity towards organics destruction was80usually affected by the nanostructure, oxygen vacancies, antimony concentration andvalence etc. 17.200180phenol concentration /ppm16014012010080604020a cb0 1 2 3 4 5 6 7 8 9 10electrolysis time /h859095100fig. 1 the activity toward phenol destruction on electrodes prepared by different drying timethe typical high resolution sem image in fig. 2 for the electrodes prepared under different drying time showed that the electrode surface was covered with connected uniform nanoparticles 5-10 nm in diameter.fig. 2 typical sem for electrode with connected nanoparticles on the surface.by using a lower magnification, the electrode showed much different morphology for samples prepared by different drying time. for sample a, by drying 0 minutes, the electrode showed usually observed cracks and island structure. the cracks were more uniform and the islands were smaller in sample a than that in sample b and c. the cracks were caused by the coating film inside stress 9. the drying time may affect the hydrolysis of metal chloride and the doped tin oxide gel structure. so the coating filminside stress would be also affected.abcfig. 3 the cracks and islands structure observed on the electrode samples prepared by drying 0 min (a), 10 min (b), 30 min (c).the drying time effect on antimony doped tin dioxide crystallinity was studied by xrd. as shown in fig. 4, the fringes 110 and 101 of tin dioxide with cassiterite structure were highlighted. in sample a, there was also a small signal for sb2o5 (111) indicated that some sb2o5 crystallites formed and were not doped into the sno2105110lattice. the electrode sample a prepared by drying 0 minutes had the lowest sno2 crystallinity and the sample b prepared by drying 10 minutes had the highest crystallinity. the crystallite size calculated according to scherrer formula was 3.6 nm,4.7 nm and 4.4 nm for sample a, b and c, respectively. the diffraction angles for the fringes did not change, which indicated the lattice constants of doped tin dioxide didnot change.220200180intensity / cps160140sb o2 5sno2sno2120100806040200111101110tia bc1151202025 30 35 40 45 50552 theta/(o)fig. 4 xrd profiles for electrodes prepared by different drying timesin order to analyze the drying effect on the electrode surface composition and elemental valences, the electrode samples were characterized by xps and a typical xps spectrum was shown in fig. 5. the sb3d3/2 could fit well with 1 peak for sb(v)at 540.1 ev, which indicated that all the doped antimony existed as sb5+. the sn3d5/2and sn3d3/2 could be separated into two peaks for sn(iv) (486.7 ev for sn3d5/2 and495.1 ev for sn3d3/2) and sn(ii) (486.0 ev for sn3d5/2 and 494.4 ev for sn3d3/2), respectively, which suggested the existence of sn2+ and sn4+. the quantitative analysis results were listed in table 1.fig. 5. typical xps spectra for antimony doped tin dioxide electrodetable 1. electrode surface composition from xps dataitemso%sn%sb%a65.0433.151.81b66.9532.430.63c62.4737.050.48125from table 1, it was easy to see that drying time increased from 0 to 30 minutes,the antimony content in the electrode surface decreased from 1.81% to 0.48%. in sample a, the antimony to tin enrichment was obviously observed and it should be130135140145due to the violent vaporization of solvent and tin chloride at over heated temperature, which would be discussed in the following tg-dta experiments. according to the electrode surface composition, antimony doped tin dioxide in sample a, b and c could be approximately written in chemical formula as (sno1.8)33(sb2o5)0.91, (sno2.0)32(sb2o5)0.32 and (sno1.7)37(sb2o5)0.24. the ratio of tin to oxygen in sample b was the closest to the stoichiometrical ratio of 0.5. the variations from the stoichiometrical ratio of 0.5 were due to the oxygen vacancies in tin dioxide 17.from the above results, in sample b, the oxygen vacancies concentration was the lowest, while its activity was the highest. the vacancies seemed to have negative effect on the electrode activity. generally the vacancies were negative charged and would reduce adsorbed active hydroxyl free radicals to hydroxyl ion, which would have no activity for organics destruction. doped sb5+, of course would function as active sites for organics destruction as other reports 3. sample a had the highest concentration of sb5+, but some sb5+ was not doped into the sno2 lattice, also more oxygen vacancies existed, so it would not have the highest activity.in order to understand the reactions occurred during the electrode preparation, the tg-dta analysis was performed directly to 5 ul freshly prepared coating solution. three temperature programs were set to simulate the electrode preparation procedures.that was heating up to 600 with heating rate of 20 min-1 with additionaldifferent durations (0, 10, 30 minutes) at 80 . the results were shown in fig. 6(a), (b) and (c), respectively.10080604020weight /% & dta /uv0weightadtatemperature700600500400300temperature /oc20010001008060weight /% & dta /uv40200bweighttemperaturedta600500temperature /oc40030020010000 4 812 16 20 24 28 32time /min0 4 8 12 16 20 24 28 32 36 40time /min10080weight /% & dta /uv6040200cweightdta800700600temperature /oc500400 temperature 30020010000 102030405060time /min150fig. 6 tg-dta analysis for electrodes prepared by drying 0 min (a), 10 min (b)and 30 min (c).in fig. 6, there were obviously three main endothermic or exothermal peaks during155160165170175180185190the sample heating. the first endothermic peak appeared at about 80 , which wasmainly the vaporization of ethanol solvent. partial hydrolysis of antimony chloride and tin chloride might also occur inevitably with lower reaction rate at thistemperature. the second endothermic peak appeared at about 130 , which might bethe hydrolysis of metal chloride with water molecules from sncl45h2o and hcl solution, then released hydrochloride. the main reaction equation could be written approximately as the following.xsncl4 + ysbcl3 + (4x+3y)h2o snxsby(oh)(4x+3y) + (4x+3y)hcl(1)the releasing of hydrochloride and residual solvent might cause the weight loss during this period. tin chloride has a boiling point of 114 and the vaporization oftin chloride might also occur at this temperature, which would also contribute to the weight loss. the antimony enrichment in the prepared electrode film observed in this experimental work and other work19 might also be caused by tin chloridevaporization. the third exothermal peak appeared at about 450 , which might be thepyrolysis to form metal oxide and the antimony doped tin dioxide crystallites formation. the main reaction equation could be written approximately as the following.snxsby(oh)(4x+3y) + no2 (snom)x (sb2o5)y/2 + (2x+1.5y)h2o(2)it could be seen from fig. 6(b) and (c), when the duration at 80 reached 8 minutes, the gravity of the sample reached a platform which indicated that the ethanol wasalready vaporized mostly. by comparing fig. 6(a), (b) and (c), it could be seen that in sample a, the weight loss was the largest, which was about 70%, while in sample c the weight loss is about 40%. this was because the solvent ethanol had a boiling pointof 78 . if there was no duration at 80 , the ethanol would be over heated andviolent vaporized together with some of tin chloride which would cause a larger weight loss. if there was enough duration time at 80 , the ethanol would bevaporized steadily together with less tin chloride. in another hand, without duration at80 , most of tin chloride molecules were not hydrolyzed and easily vaporized at higher temperature. the loss of tin chloride from the solution would cause a relativeenrichment of antimony as observed from xps data. also the antimony enrichment would cause a lower crystallinity in the electrode as showed in xrd profiles 16, 19. if there was enough duration time at 80 , most of the tin chloride molecules werehydrolyzed. in this case, the tin chloride vaporization would be mostly prevented. so with the duration of 30 minutes at 80 , the dta signal at 130 was not very clearin figure 6(c). in fact, a very wide and low peak could be observed during 15-30 minutes in figure 6(c), indicated the hydrolysis reaction already occurred during thedrying procedure at 80 . the hydrolysis of tin chloride would reduce the tinchloride vaporization, which would cause a smaller weight loss and smaller antimony enrichment in the final product as that in sample c.3conclusions195200205210215220225230235240245the drying time effect on the activity of antimony doped tin dioxide electrode for phenol degradation was observed. the electrode prepared by drying at 80 for 10 minutes had higheractivity than those drying 0 or 30 minutes. the drying time also had effect on the electrode morphology, crystallinity and electrode composition. the antimony concentration in the electrode surface decreased while the islands and cracks size on the electrode surface increased with the drying time increase. the electrode prepared by drying 10 minutes had the highest crystallinity and in this electrode tin to oxygen ratio was closest to theoretical value 0.5. the electrode activity had closed relations with the oxygen vacancies concentration. the lower oxygen vacancies concentration seemed to be helpful to improve the electrode activity. the tg-dta analysis showed that there were three main endothermic or exothermal reactions during the electrodepreparation procedure. they were possible solvent vaporization at 80 , hydrolysis and tinchloride vaporization at 130 , and pyrolysis and crystallite formation at 450 , respectively. the drying time at 80 would affect the hydrolysis process, solvent vaporization and tin chloridevaporization.references1 yao p, chen x, wu h, wang d. active ti/sno2 anodes for pollutants oxidation prepared using chemical vapor depositionj. surf. coat. technol.,2008, 202:3850-3855.2 zanta c, michaud p a, comninellis c, de andrade a r,boodts j. electrochemical oxidation ofp-chlorophenol on sno2-sb2o5 based anodes for wastewater treatmentj. j. appl. electrochem.,2003,33:1211-1215.3 he d, mho s. electrocatalytic reaction of phenolic compounds at ferric ion co-doped sno2:sb5+ electrodesj. j. electroanal. chem., 2004, 568:19-24.