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1、附录：英文文献How photocatalytic activity of the MAO-grown TiO2 nano/micro-porous films is influenced by growth parameters?M.R. Bayati a,b, F. Golestani-Fard a,b, A.Z. Moshfegh a,d*a School of Metallurgy and Materials Engineering, Iran University of Science and Technology, P.O. Box 16845-161, Tehran, Iranb
2、 Center of Excellence for Advanced Materials, Iran University of Science and Technology, P.O. Box 16845-195, Tehran, Iranc Department of Physics, Sharif University of Technology, P.O. Box 11155-9161, Tehran, Irand Institute for Nanoscience and Nanotechnology, Sharif University of Technology, P.O. Bo
3、x 14588-89694, Tehran, IranA R T I C I E I V F OArticle history:Received 19 October 2009Received in revised form 27 January 2010Accepted 2 February 2010Available online 10 February 2010Keywords:Oxide materialsTitanium dioxidePhotocatalysisMicro-arc oxidationA R S T R A C T Pure titania porous layers
4、 consisted of anatase and rutile phases, chemically and structurally suitable fo rcatalytic applications, were grown via micro-arc oxidation (MAO). The effect of applied voltage, process time, and electrolyte concentration on surface structure, chemical composition, and especially photocatalytic act
5、ivity of the layers was investigated. SEM and AFM studies revealed that pore size and surface roughness of the layers increased with the applied voltage, and the electrolyte concentration.Moreover, the photocatalytic performance of the layers synthesized at medium applied voltages was significantly
6、higher than that of the layers produced at other voltages. About 90% of methylene blue solution was decomposed after 180 min UV-irradiation on the layers produced in an electrolyte with a concentration of 10 g/ l at the applied voltage of 450 V.2010 Elsevier B.V. All rights reserved.1. Introduction
7、Titanium dioxide has been widely used in a wide range of applications such as gas sensors, photovoltaic solar cells and photocatalysis due to its biological and chemical inertness, strong oxidizing power, cost-effectiveness, and long-term stability against photo-corrosion and chemical corrosion 1-3.
8、 However, the practical applications of this material are limited by its low surface area; hence, many efforts have been focused on increasing its surface area by developing porous structures 4.Titanium dioxide can crystallize in three different phases, i.e.rutile (tetragonal), anatase (tetragonal),
9、 and brookite (orthorhom-bic). Among these crystalline structures, rutile is the most stable one, can be obtained via miscellaneous growth methods, and is thermodynamically stable at all temperatures, while anatase is less dense and less stable 5. Among these phases, anatase has the most photoactivi
10、ty due to its low recombination rate of photo-generated electronhole pairs. In contrast, the most stable rutile phase is least active or not active at all 6,7. TiO2 layers with various morphologies and chemical compositions have been synthesized via different methods including sol-gel 8-10,chemical
11、vapor deposition 11,12, physical vapor deposition 1316, hydrothermal process 17, electrochemical methods 18, liquid phase deposition 19, spray pyrolysis 20,21,and also micro-arc oxidation (MAO) process 2224 which is an in situ growth process for producing oxide films on the surface of nonferrous met
12、als such as aluminum, titanium, magnesium and zirconium. The grown layers have a good combination with the substrates, and are porous and uniformly coated on metal surface25,26. MAO is an electrochemical technique for formation of anodic films by spark/arc micro-discharges which move rapidly on the
13、vicinity of the anode surface 2730. It is characterized by high productivity, economic efficiency, ecological friendliness, high hardness, good wear resistance, and excellent bonding strength with the substrate 3133. This process is carried out at voltages higher than the breakdown voltage of the ga
14、s layer enshrouding the anode. Since the substrate is connected to positive pole of the rectifier as anode, the gas layer consists of oxygen. When the dielectric gas layer completely covers the anode surface, electrical resistance of the electrochemical circuit surges and the process continues provi
15、ding that the applied voltage defeats the breakdown voltage of the gas layer. Applying such voltages leads to formation of electrical discharges via which electrical current could pass the gas layer. MAO process is characterized by these electrical sparks 28,34.It is established that the electrical
16、parameters play an important role in the formation of phases of ceramic films made by MAO technique 35. Although TiO2 layers of varying morphologies and textures have been developed and investigated by many researchers, few studies have been fulfilled on producing TiO2 layers via MAO process among w
17、hich no study has been carried out on the effect of growth conditions on the photocatalytic activity of the layers. In this research, we will present the effect of applied voltage, process time, and electrolyte concentration on the surface morphology,topography, phase structure, chemical composition
18、, and especially photoactivity of the MAO-synthesized TiO2 layers for the first time.2. Experimental procedure2.1. Sample preparation Commercially pure (CP-grade 2) titanium substrates with dimensions of 30 mm30 mm 0.5 mm were used as anode.Prior to MAO treatment, substrates underwent a cleaning pro
19、cess including mechanical polishing followed by washing in distilled water. Afterward, they were chemically etched in diluted HF solution (HF:H2O = 1:20 vol.%) at room temperature for 30 s, and then washed in distilled water again. In the last stage of cleaning procedure, the substrates were ultraso
20、nically cleaned in acetone for 15 min and finally washed by distilled water. An ASTM 316 stainless steel cylindrical container, surrounding the substrates,was also used as cathode. Three sodium phosphate (TSP,Na3PO412H2O, Merck) solutions with different concentrations were used as electrolyte temper
21、ature of which was fixed at 703 employing a water circulating system.A home-made rectifier with a maximum output of 600 V/30 A,able to supply AC, DC and pulse-DC, was used as current source. DC mode of the rectifier was selected, and different voltages were applied to the electrochemical cell, expla
22、ined above, in a range of 250550 V with intervals of +50 V.2.2. Structural and compositional characterization Surface morphology and topography of the layers were evaluated by scanning electron microscopy (TESCAN, Vega II)and atomic force microscopy (Veeco auto probe), respectively.Furthermore, X-ra
23、y diffraction (Philips, PW3710), and X-ray photoelectron spectroscopy (VG Microtech, Twin anode, XR3E2 X-ray source, using Al Ka = 1486.6 eV) techniques were used in order to study phase structure and chemical composition of the synthesized layers.2.3. Photocatalytic studies The photocatalytic activ
24、ity of the layers was evaluated by measuring the degradation rate of aqueous methylene blue (MB,Wako Pure Chemical) solution at room temperature. A UV-vis spectrophotometer (Jascow Y-530) was used to measure the change in concentration, based on the Beer-Lamber equation stating A = ebC where A, e, b
25、, and C are absorption of the solution, molar absorptivity, path length, and solution concentration, respectively. Since b and e are constant, the parameter C is linearly proportioned to the absorption; thus, it can be found by measuring the parameter A. To do that, 50 ml of the MB solution (50 ppm)
26、 and a 1 cm1 cm sample were placed in a quartz cell. A 25W UV lamp was used as an irradiation source during photocatalytic experiments. In each experiment, prior to UV-irradiation, the solution and the catalyst were left in the dark for 60 min (as a reference point) until adsorption/desorption equil
27、ibrium was reached. The solution was then irradiated under UV light.A fixed quantity of the solution was removed every 20 min to measure the absorption and then the concentration. The absorptivity measurements were carried out at a fixed wavelength of 664 nm, because the maximum light absorption by
28、the MB solutions occurs at this wavelength 36. 3. Results and discussion SEM morphologies of the layers synthesized with applying different voltages for 180 s in TSP solutions with a concentration of 10 g/l are shown in Fig. 1. No any pore is observed in the structure of the layers grown under appli
29、ed voltages less than 300 V, yet the samples which were made at higher applied voltages.It is necessary to mention that no electrical sparking occurred at the applied voltages less than 300 V, and increasing the applied voltage resulted in generation of stronger and long-living sparks.The voltage be
30、fore which no pore was formed, and no sparking was observed was named critical voltage (Vc). In addition to the mentioned electrolyte concentration, 5, 20,and 50 g/l concentrations were also examined and a similar behavior was observed; in other words, it was confirmed that there is a Vc at which el
31、ectric discharges began to appear on the vicinity of the anode. SEM imaging of the samples which were produced using electrolytes with concentrations of 5, 20, and 50 g/l also showed that the pores solely existed in the structure of the samples which were made by applying voltages higher than Vc.Sin
32、ce no pore was observed in the absence of electric sparks, it can be concluded that electrical sparks has the most critical role in formation of the structural pores, and other phenomena such as corrosion has trivial or no effect on the structural pores formation.That is, surface oxide layers are de
33、veloped by corrosion, namely electrochemical reactions; however, structural pores form by electric sparks. Meanwhile, it was observed that the pore size increased with the applied voltage. It is necessary to emphasize that applying higher voltages causes electrical sparks with higher energy due to h
34、igher electrical current passing the electrochemical cell. Stronger electric avalanches result in formation of the wider pores. The change of the Vc versus electrolyte concentration is plotted in Fig. 2. Concerning data analysis, the Vc was reduced by using higher concentrated electrolytes. As the e
35、lectrolyte electrical resistance, and, consequently, total resistance of the electrochemical cell decrease with increasing the electrolyte concentration, the voltage which is applied on the anode surface increases.Furthermore, any decrease in circuit resistance results in increasing the current whic
36、h passes the cell, and, therefore, the power of the electrical avalanches. As a consequence, larger pores are formed when thicker electrolytes are utilized. The SEM images of the layers synthesized with applying 400V in the electrolytes with concentrations of 10 and 20 g/l for different treatment ti
37、mes are exhibited in Fig. 3. It is observed that the pore size increases with the treatment time. When a structural pore forms by an electrical spark it is more susceptible for next electron avalanches, for it has a lower breakdown voltage in comparison with other areas which are not porous. Sequenc
38、e of the electrical sparks in one point makes the pores larger and increases the pore size. Fig. 4 shows the surface topography of the layer synthesized with different applying voltages in electrolytes containing TSP with a concentration of 10 g/l in a scale of 10 mm10 mm. The results depict a rough
39、 surface which is usual for MAO-grown layers. Using statistical analysis, it was found that the average surface roughness of the layers (ASR) increases with the applied voltage. The reason for such a behavior is that electrical current passing the electrochemical cell increases with the applied volt
40、age,and, therefore, more heat is generated in the oxide layer. This extra heat results in sequential melting and solidifying of the growing layers in the surrounding electrolyte which makes the layers roughened. The XRD results, depicted in Fig. 5, demonstrate that thesynthesized layers contain both
41、 the anatase and the rutile phases.Since the anatase form of the TiO2is known as the only photoactivephase among its other forms, the mass fraction of anatase in thesynthesized layers determines the sample photocatalytic perfor-mance; hence, anatase mass fraction of the layers was calculatedusing th
42、e formula WA= (1 + 1.265IR/IA)-137 where the IR and IA are the normalized XRD peak intensities of rutile and anatasephases, respectively, and the results are shown in Fig. 6. The resultsrevealed that the anatase mass fraction reaches a maximum valueat a certain applied voltage (Vm) and then decrease
43、s at very highvoltages. Applying higher voltages warms the anode up more dueto higher electrical current passing the electrochemical circuit andmore electrical sparks taking place on the vicinity of the anode28. Because of this extra heat, the anatase which is a metastablephase transforms to rutile
44、stable phase at higher temperatures.In order to investigate the effect of treatment time on the phasestructure of the layers, some samples were prepared with applyingdifferent applied voltages in electrolytes with various concentra-tions for 60, 300, and 600 s in addition to 180 s which was checked
45、before. Of course, we only present the XRD patterns of the layerssynthesized in the electrolyte with a concentration of 10 g/l(Fig. 7). Meanwhile, the fraction of the anatase phase werecalculated and the results are depicted in Fig. 8. The results assertthat the anatase fraction increases and, then,
46、 decreases withtreatment time, because the anatase metastable phase transformsto rutile stable phase after long times. It is also seen that theanatase fraction of the layers synthesized under the appliedvoltage of 550 V decreases continually with treatment time; that is,no maximum point is observed.
47、 The reason for such a behavior isthat the phase transformation of anatase to rutile occurs even atshort treatment times due to high amount of generated heat on thevicinity of the anode at the voltage of 550 V. To confirm the stoichiometry of the synthesized TiO2layers andinvestigate the chemical st
48、ates of the elements, XPS technique,whose result is presented in Fig. 9, was used. It should be noted that all of the binding energies were referenced to the C(1s) peakbinding energy at 285 eV. The survey spectrum is depicted inFig. 9a confirming the existence of Ti and O in the grown layers. Itshou
49、ld be mentioned that a small of P impurity was also detectedby our XPS instrument whose peak (located at the binding energiesless than 100 eV) is not shown here. Fig. 9b shows the Ti(2p) corelevel which is located at the binding energy of 458.7 eV asserting Table 1. According to the obtained results, the photocatalyticactivity increases and then decreases with the applied voltage. Inother words, the ph