Micro-arc oxidization fabrication and ethanol sensing performance of Fe-doped TiO2 thin films

International Journal of Minerals, Metallurgy and MaterialsVolume 19, Number 5, May 2012, Page 461Do|:10.1007/s12613-0120580-8Micro-arc oxidization fabrication and ethanol sensing performance ofFe-doped TiO2 thin filmsFu-jian Ren, Xiao-bai Yu, Yun-han Ling and Jia-you FengLaboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, ChinaAbstract: In-situ pure TiO and Fe-doped TiO thin films were synthesized on Ti plates via the micro-arc oxidation (MAO)technique. Theas-fabricated anatase TiO thin film-based conductometric sensors were employed to measure the gas sensitivity to ethanol. The resultsshowed that Fe ions could be easily introduced into the MAO-Tio2 thin films by adding precursor K,(FeCN) 3H20 into the NayPO4 elec-trolyte. The amount of doped Fe ions increased almost linearly with the concentration of K (FeCN) 3H2O increasing, eventually affectingthe ethanol sensing performances of TiO, thin films. It was found that the enhanced sensor signals obtained had an optimal concentration ofFe dopant(1.28at%), by which the maximal gas sensor signal to 1000 ppm ethanol was estimated to be 7.91 at 275C. The response time wasnerally reduced by doped Fe ions, which could be ascribed to the increase of oxygen vacancies caused by Fe substituting for TiKeywords: thin films; titanium dioxide; doping: iron; micro-arc oxidation; ethanol sensorsThis work was financially supported by the National Basic Research Priorities Program of China(No 2007CB936601) and the NationalNatural Science Foundation of China(as 10876017 and 91023037).fact that extensiveof these sensors have been em1 Introductionployed [17], the sensitivity is far from satisfactory and fur-Recently, metal oxides have been widely investigatedther investigations need to be carried out to analyzegas sensors because of their appropriate energy band gaps doping mechanism for promoting the gas sensing perfor(Eg[1-2]. TiO2 is a widely studied functional material be- ance ofTiOz sensorscause of its many practical and potential applications [3-41The Fe-doped metal oxide semiconductor sensor hAnatase TiO2 thin films have been used especially as gas been found to show enhanced gas sensitivity [18].Insensors for detecting reductive gases such as CO [5], H2[6], study, Fe-doped TiO2 thin films were prepared via the mi-and H2S [7]. The space-charge layer model is commonly cro-arc oxidation(MAO) technique, which is convenientutilized to explain the gas sensing mechanisms of these and effective for the synthesis of metal ion-doped TiO, thinconductance-based sensors, in which the electrical proper- films by adjusting electrolyte compositions [19].Ethanolties vary in different gas atmospheres with certain concen- was utilized as the sample gas to investigate the influence oftrations[8]. As the gas sensor signals of pure TiO, are rela- Fe doping on the gas sensing properties of TiO2 thin filmstively low [9], various approaches have been adopted to en-hance its gas sensitivity [101, and metal-ion doping has been 2. Experimentalwidely investigated [11]. The enhanced gas sensitivity ofTiOr-based sensors by doping with Cu2*[ 2], w*[13], Cr2. 1. Synthesis and characterization[14], Nb[15], and Sn[16] has been reported. Despite the中国煤化工) was used for the fab.orresponding author: Yun-han Ling E-mail: yuling amail tsinghua. edu. cnCNMHGringere University of Science and Technology Beijing and Springer-Verlag Berlin Heidelberg 2019 Spi462nt. J. Miner. Metall. Mater VoL 19, No, 5, May 2012rication of pure TiOz and Fe-doped TiO2 thin films via the tainer, in which the fabricated sensors were fixed. A desig-MAO process. The experimental equipment for MAO was nated volume of ethanol was injected using a microsyringeillustrated in detail in a previous paper [20]. Ti plates were upon a heater and gasified to gain the required concentrationcut into small samples(20 mmx30 mm). a dilute Kroll's of sample gas with the ppm level balanced with air. Differacid(a mixture of vol% HF and 30vol% HCD)was used to ent measured temperatures were realized by using a back-remove the oxide skin, and acetone, ethanol and distilled side heater sheet and an infrared thermometer was used towater were used to clean the Ti plates for 10 min in an ul- verify the temperature. The applied dc bias for the ethanoltrasonic machine, respectively. During the oxidation process, sensing tests was performed at a constant of 1 V, thus thethe cleaned Ti plates were utilized as the anode and a Mo sensing current was obtained and recorded using an electro-plate was used as the cathode. The detailed parameters for chemical workstation(Zahner IM6e ). Gas sensitivity wasthe fabrication of TiOz and Fc-doped TiO2 thin films are defined as the ratio of sensing signal Rai/Rgas, where rair andlisted in Table 1. The concentration of Nag POA was kept at a Reas represent the resistances in air and in a sample gas, re-constant 0.5 M and a pure TiO2 thin film was obtained, spectively. The response time, as well as the recovery time,which was identified as SI in this paper. Four Fe-doped was determined as the time from the switch between theTiO2 thin films were fabricated by the introduction of dif- sample gas and air to 90% of the balanced valueferent concentrations of K(FeCN) 3H2O into the Na, POelectrolytic solution, and were identified as S2-S53. Results and discussionTable 1. Parameters of MAO process for fabricating TiOThe MAO technique is commonly employed to processand Fe-doped TiO, thin filmsvalve metals and alloys in aqueous electrolyte solutions.Concentration of theMAO is a general method for the anodic formation of crysSampleelectrolytic solution Voltage/ Oxidation talline TiO, thin films without the use of subsequent thermalnumber Na PO4/K4()% 3H,O/ V time/minannealing and the crystal phase depends on the applied voltage and the composition of the electrolyte. Fig. 1 shows the0:0250XRD patterns of the prepared samples in this study. It can250be seen that all the viewed peaks can be indexed as indicatedwhich correspond to the anatase phase Tio, and the sub-0.51.0strate reflections. Compared with pure Tio2 thin films, im-purity phases are not observed in the patterns of Fe-dopedX-ray diffraction(XRD, D/max-RB) was used to detTiO2 thin films, indicating that the doped Fe ions might havemine the crystal phase of the as-fabricated thin films using substituted Ti ions or were incorporated in the as-preparedCu-Ka radiation at a scanning speed of 6%/min. The mor- T1O2 thin films but could not be detected because of its lowphologies of the thin films were characterized by scanning doped valueelectron microscopy(SEM, Hitachi $-450) In addition, thechemical compositions and doped ion concentrations of theAnatasesample surfaces were detected by energy dispersive X-rayo Titaniumspectroscopy (EDS). X-ray photoelectron spectroscopy(XPS, PHI 5300) was used to analyze the chemical states ofelements on the oxide surfaces2. 2. Fabrication and test of TiO, and Fe-TiO thin filmsensorsFor testing the gas sensing properties, a pair of gold elec-人trodes was sputtered on the top of the prepared oxide thinfilms and attached with Cu wires by Ag paste. The spacebetween the fingers was 0.4 mm and the thickness wasabout 100 nm. Then the sensors were heated at 300 C in a中国煤化工drying oven for I h to obtain a well electrical contact.CNMHGTIO thin films fabri-aing different concentra-a plexiglass chamber(18 L) was utilized as the test con-tions of K(FeCN) 3H,0F.J. Ren et aL, Micro-arc oxidization fabrication and ethanol sensing performance of Fe-doped TiOz thin filmsThe surface morphologies of samples Sl-S5 are shown in be noted that distinct cracks are obviously visible on theig. 2. It can be observed from Figs. 2(a)and 2(e)that the surface of Fe-doped TiO2 thin films. The reason should behickness of as-prepared TiO2 thin films is approximately 3ascribed to the thermal stress and bubbles such as o2, Coum under the given conditions. From the top views in Figsand N,, which were released during the MAO process.2(a)-2(e), pores with nano- and micro-dimensional sizes Fig 3 shows the XPS spectra of Fe-doped TiO2 thin filmscould be observed on the surfaces of the samples. It should (sample S5)for Ti2p and Fe2p. The XPS results of theFig. 2. SEM images of as-prepared TiO thin filmsfabricated in the Na PO4 electrolyte containing dif-ferent concentrations of K(FeCN)6 3H20:(a) sampleS1;(b)sample S2; (c)S3;(d)Ti2p,Fe2p,z[12p456中国煤化工Binding energy/evCNMHGFig. 3. XPS spectra of the surface of Fe-doped TiO2 thin films(sample S5): (a) Ti2p;(b)Fe2p.464Int J. Miner. Metall. Mater., voL 19, No 5, May 2012Fe-doped TiO2 thin films further show that the binding en- Fe-doped TiO2 thin films were calculated and are illustratederies of Ti2p3 and Ti2pI of the as-synthesized thin films in Fig. 6. It can be seen that the sensing current towardsare approximately 458.3 and 464. 1 eV, respectively, with a 1000 ppm ethanol of all the samples reaches the maximumpeak separation of 5.8 eV, assigned to Ti. The values of at 275.. Samples S2 and S3( the values of doped Fe arethe binding energies of Fe2p32 and Fe2p1n of Fe-doped TiO2 0.98at% and 1.28at%, respectively) present enhanced etha-thin films are approximately 710.9 and 724.5 eV, respec- nol sensing performance. The gas sensor signals of S2 andtively. The peak separation is 13.6 ev, which indicates that $3 thin film sensors toward 1000 ppm ethanol reach theirthe doped Fe ions in the TiOz thin films exhibit a chemical maximum at 275.C, estimated to be 4.45 and 7.91, respec-state of Fe". As the Fe ion shows a Fe2"in K4(FeCN)6 3H20, tively. However, the sensor signals of samples S4 and S5it could be implied that the oxidation reaction of Fe" to Fe decrease compared with the other sensors. The inset patternoccurred during the MAO process. The compositions of shows the gas response time to 1000 ppm ethanol at 275.CFe-doped TiO2 thin films determined by EDS are shown in as a function of the addition of K, ( FeCN)6 3H,O concentra-Fig 4. From the inset EDS spectrum of sample S5, it can be tion in the Na PO4 electrolyte. The response time of theobserved that the Fe-doped TiO2 thin films appear to contain Fe-doped TiO2 thin films to 1000 ppm ethanol at 275.C is inTi,O, Fe, and P. Phosphorus(P)was incorporated into the the range of 180-130 s and is generally reduced by increas-TiO2 thin films from Na,PO4 in the electrolyte during theMAO treatment process [21]. The amount of Fe dopant in1000TiO2 thin films increases almost linearly with the concentration of (Fe CN)6 3H20 increasing45三250°CEnergy /keV0200400600800100012001400Inear fittingFig. 5. Response transient curves of Fe-doped TiO thin films(sample $3)to 1000 ppm ethanol as a function ofConcentration of k Fe(CN).3H. O/(mM.L-). 275C ethanol- SSFig. 4. Amount of doped Fe ions as a function ofK,(FeCN) 3H,0 concentration in the NayPO4 electrolyte. Theinset is the EDS spectrum of Fe-doped TiO2 thin films(sampleS5)K, FeCNI 3H O/(mM.IFig. 5 illustrates the ethanol sensing properties ofsample S3-based sensor in this study. From the transientcurve, it is obvious that the sensing current is stable and in-creases rapidly after injecting ethanol, and the sensing cur-rent could be nearly recovered to the initial value afterre-injecting air. It indicates that the sensing signal of theMeasuring temperature /Cas-prepared sensors could be reversible after switching thegas between air and ethanol. The gas sensitivity is enhancedils nf nure Tin. and Fe-doped TiO, thi中国煤化工 tion of measuring temand the gas response time is generally reduced by increasing peatthe measuring temperature. The sensor sensitivity and gas TiOCNMH Gse time of as-preparedt275° C as a function ofsponse time toward 1000 ppm ethanol of pure TiO2 and K, ( FeCN 3H, 0 concentration in the electrolyte solutionF.J. Ren et al, Micro-are oxidization fabrieation and ethanol sensing performance of Fe-doped TiOz thin filmsthe concentration of K4(FeCN)6 3H20. This phenome- Tio2 thin film sensors is generally reduced and the reasonnon results from the creation of oxygen vacancies by Fe might be ascribed to oxygen vacancies resulting from a sub-stitution by Fe*for TiThe conductance of oxide thin films that could be af- referefected by doped metal ions have a close relationship withthe gas sensor signal [22]. The doped ions in TiO2 generally [l G. 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