- Email: [email protected]
Nano-structural variation of highly aligned anodic Titania nanotube arrays for gas phase photocatalytic application
Accepted Manuscript Title: Nano-structural variation of highly aligned anodic Titania nanotube arrays for gas phase photocatalytic application Author: Ibrahim Mustafa Mehedi M.F. Hossain H. Okada Md. Shofiqul Islam PII: DOI: Reference:
S1010-6030(16)30752-3 http://dx.doi.org/doi:10.1016/j.jphotochem.2016.11.019 JPC 10443
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
5-9-2016 9-11-2016 18-11-2016
Please cite this article as: Ibrahim Mustafa Mehedi, M.F.Hossain, H.Okada, Md.Shofiqul Islam, Nano-structural variation of highly aligned anodic Titania nanotube arrays for gas phase photocatalytic application, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2016.11.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano-structural variation of highly aligned anodic Titania nanotube arrays for gas phase photocatalytic application
Ibrahim Mustafa Mehedi1, M.F. Hossain2,*, H. Okada3,Md. Shofiqul Islam1 1
Department of Electrical & Computer Engineering Engineering, King Abdulaziz University,
Jeddah 21589, Saudi Arabia 2
Department of Electrical & Electronic Engineering, Rajshahi University of Engineering &
Technology, Rajshahi-6204, Bangladesh. 3
Graduate School of Science and Engineering for Research, University of Toyama, 3190
Gofuku, Toyama 930-8555, Japan
*Corresponding Author: E-mail: [email protected] Or [email protected] Or [email protected] Tel./Fax: +88-01778-400-600
Highlights
Highly aligned TiO2 nanotube (NT) arrays have been grown on titanium sheet by using a self organized anodic oxidation method at different potentials (34 to 74V) in a NH4F and ethylene glycol based electrolyte.
Two sets of TiO2 NTs have been fabricated with and without using cooling systems.
The temperature has been maintained to 25C by using water based cooling system.
The photocatalytic decomposition of methanol (5 μL) has been evaluated by Fourier transform infrared spectrophotometer, measuring its concentration decay in a cylindrical glass cell, which contains the TiO2 NTs.
The TiO2 NT (at 54V) at room temperature shows the highest photocatalytic activity due to its strong rutile phase and large number of TiO2 NTs remain in same area.
Abstract Highly aligned titania (TiO2) nanotube (NT) arrays have been grown on titanium sheet by using a self organized anodic oxidation method at different potentials (34 to 74V) in a NH4F and ethylene glycol based electrolyte. Two sets of TiO2 NT arrays have been fabricated with and without using cooling systems. Without cooling system, the solution temperature is increased from 25C (room temperature) to 63C with increase of anodization potential from 34 to 74V. The temperature has been maintained to 25C by using water based cooling system. In both sets of samples, the pore diameter of TiO2 NT arrays increases with increase of anodization potential. The maximum pore diameter has been (outer: 224nm, inner: 172nm) achieved with TiO2 NT arrays (at 74V) without using cooling system. The photocatalytic activity of the TiO2 NT arrays is evaluated by the decomposition of methanol by the use of Fourier transform infrared spectrophotometer. The TiO2 NT (at 44V) sample has been fabricated without using cooling system, which shows the higher photocatalytic activity than the sample at other potentials. Using cooling system, the TiO2 NT (at 54V) sample shows the higher photo-decomposing property than the sample at other potentials. The TiO2 NT (at 54V) sample with cooling system shows the higher rate (photocatalytic activity) constant (k=0.0216) than the TiO2 NT (at 44V) sample without using cooling system (k=0.0186). The photocatalytic activity has been correlated with the nanostructural variation of TiO2 NT arrays at different potentials without (self-grown solution temperature) and with water based cooling systems.
Keywords: Titania nanotube, cooling system, anodization method, photocatalytic activity.
1. Introduction Nowadays, environment pollution and energy crisis are world widely concerned. For a sustainable society, it is an urgent task to develop environment-benign purification technologies and manufacturing technologies of alternative clean energies. Photocatalysis is one of the most promising method for both subjects. Heterogeneous photocatalysis is an advanced oxidation process to destroy unwanted and harmful organic compound in contaminated in water or air[1, 2, 3],and it has been extensively studied since the 1972 [4]. Among the several semiconductor materials tested for photocatalysis, TiO2 is the promising photocatalyst[5, 6] due to its high oxidative power, low operation temperature, non-toxicity, low cost, chemical inertness and high photostability[7-10]. Over the past few decades, the colloidal or powder TiO2 were widely used to photodegrade the pollutants in both the liquid and gaseous phases, it shows highly photocatalytic activity due to its large surface area[11-13].But, the conventional suspended system has a serious limitation, such as it needs post-treatment of separation in a slurry system after photocatalytic reaction[13- 15].This problem can be overcome by immobilized TiO2 particles as thin films on solid support substrate[13, 16]. The various methods have been developed to prepare immobilized TiO2 films such as sol-gel, reactive magnetron and facing target sputtering[14, 16], chemical vapor and liquid-phase deposition [15]. The formation of TiO2 films on the solid substrate significantly reduced the specific surface area of TiO2 photocatalysis, resulting in a decrease of photocatalytic activity compared to corresponding slurries [17]. Therefore it is necessary to develop novel synthesis approach to prepare mesoporous TiO2 photocatalysis, which not only has high photocatalytic activity but also can be steadily separated after photocatalytic reaction. TiO2 nanotube (NT) arrays are expected to be a promising photocatalyst to overcome above mentioned drawbacks due to their special chemical and physical characteristics, high specific area and high pore volume due to their open mesoporous nature[11,13, 17]. TiO2 NT array has excellent charge separation ability due to the absence of gain boundaries [13].Moreover, the TiO2 NT array possesses good chemical and mechanical stability because it grows directly on the titanium substrate, and excellent corrosion resistance.
As
additionally the conductive substrate is able to exhibit some interesting properties of photoelectrocatalysis and photo-electrochemistry [11, 18, 19].
TiO2 NT arrays have been fabricated by several methods, e.g., anodic oxidation [20],sol-gel [21], microwave irradiation [22],seeded growth [23],hydrothermal process[24] and aluminum template synthesis [25]. Among these, electrochemical anodization is relatively simple and efficient process for the fabrication of aligned TiO2 NT array. The first generation of TiO2 NT array via anodic oxidation was grown in an hydrofluoric (HF) aqueous electrolyte or acidic HF mixtures[26-28].These TiO2 NT arrays showed a limited thickness that would not exceed 500-600nm. This technique has been further developed to obtain secondgeneration of highly ordered TiO2 NT array with a high aspect ratio by using buffered neutral electrolyte containing NaF or NH4F instead of HF[29– 32]
and taking importance in the pH
gradient. Grimes and his coworkers achieved 6.4 m long TiO2 NT array by varying pH and electrolyte concentration[33]. To further improve the morphology, third-generation of smoother layers and ordered TiO2 NT array was prepared using almost water free fluoride based electrolyte [34]. The length of TiO2 NT array was greatly extended by using various polar
organic
electrolytes
including
formamide[35-37],dimethyl
sulfoxide[35,
37],
glycerol[38]and ethylene glycol[35, 37]. TiO2 NT array membranes up to 720m thick composed of two nanotube arrays approximately 360 m thick was reported by Prakasam and his coworkers[39]using NH4F and ethylene glycol based electrolyte. However, completely different growth morphology can be obtained, if fluoride ions are present in the electrolyte and suitable anodization conditions are used. The electrolyte composition and applied anodic potential primarily determine the morphology and structure of oxide layers, resulting from an anodization. The growth of TiO2 NT array with some anodization conditions has been investigated by Prakasam and coworkers [39]. However, nobody reported about the effect of self heating solution temperature due to the anodization potential on the growth rate, pore diameter of TiO2 NT array for the application of photocatalysis. In this work, TiO2 NT arrays were fabricated by electrochemical anode oxidation method at different potentials (34-74V) in NH4F and ethylene glycol based electrolyte. This work presents a comparative analysis of the photocatalytic activity of TiO2 NT without (self-heating solution temperature due to potential) and with water based cooling systems (to maintain 25C temperature). The nanostructural and surface morphological property of TiO2 NT array at different anodization potentials and their photocatalytic activity have been investigated and discussed.
2. Experimental details 2.1. Fabrication of TiO2 NT arrays
We used the electrochemical anodization method to fabricate the TiO2 NT arrays. The schematic illustration of electrochemical anodic oxidation experimental set-up with water based cooling system for TiO2 NT array growth is given in Fig. 1. The titanium (Ti) sheet of 99.6% purity, 0.2mm thickness was bought from Nilaco Co., Ltd., Tokyo, Japan. Prior to anodization, the Ti sheet was first mechanically polished (Musashino Denshi MA-150) with different abrasive papers and rinsed in an ultrasonic bath of cold distilled water for 10 min. Then the cleaned Ti sheet was soaked in a mixture of HF, HNO3 acids and H2O (the mixing ratio of HF:HNO3:H2O is 1:4:5 in volume) for 30 sec. After rinsed with acetone and distilled water for 20 min, the Ti sheet was dried in a flow of pure nitrogen stream. The TiO2 NTs were fabricated in a cylindrical electrochemical reactor (the radius is 45 mm and height is about 60 mm). The water based cooling system was used to maintain 25C with temperature controller, heater and thermocouple (TC). A conventional two-electrode system was employed with Ti sheet (2.5x1.5 cm2) as the anode and Pt foil (2.5x1.5 cm2) as the cathode and the distance between these electrodes was maintained 2 cm. The time-dependent current behavior under constant potential was recorded using a computer-controlled Keithley 2400 sourcemeter. All the anodization experiments were carried out in an electrolyte of ethylene glycol (EG), 0.3wt% NH4F and 3vol% of water. The exposed area of Ti sheet was 2x1.5cm2 to the electrolyte. Two-sets of samples were prepared without and with water based cooling systems. Each set of sample was prepared at different potentials of 34, 44, 54, 64 and 74V. After anodization, the sample was cleaned in EG and then distilled water, and was dried with N2 steam flow. The TiO2 NT arrays were subsequently crystallized by annealing in dry oxygen at 450C for 30 min with a heating and cooling rate of 2C/min.
2.2. Characterization of TiO2 NT arrays The crystal structures of the TiO2 NT arrays were examined by grazing incident X-ray diffraction (GIXRD) analysis with data collected from SHIMADZU XRD-6000 using Cu-Kα (λ=1.54060 nm) line with 40 kV–20 mA. The data was recorded from 2 values of 10 to 80 with a step of 0.02. For GIXRD measurement, the incident angle was fixed at 0.5. The optical property of the TiO2 NT arrays ws measured by Diffuse reflection mode spectrophotometer (SHIMADZU UV-VIS NIR 3100) at room temperature within the wave length range 300-900nm. The surface morphologies were studied by using field emission scanning electron microscope (FE-SEM) by JEOL, FE-SEM 6700F and transmission electron microscopy (TEM) by EM-002B (TOPCON Co. Ltd.).
2.3. Measurements of photocatalytic activity The photocatalytic decomposition of methanol (5 μL) was evaluated by FTIR spectrophotometer (JASCO 480 plus) measuring its concentration decay in a cylindrical glass cell of 9.8cm length and 3.6cm, which contained TiO2 NT array of surface area 2.5×1.5cm2, [as given in Fig. 9(b)]. The two ends of the glass cell were fitted with KBr crystals to allow the detection of infrared waves. IR-anti symmetric stretching band of CO2 at 2340 cm−1and H2O (at 3464cm-1) were assigned for the confirmation of photo-decomposition of methanol. Photocatalytic degradation of methanol on the TiO2 NT arrays was carried out under irradiation of an artificial sunlight simulator, consisting of a SOLAX lamp (model: SET-140F, SERIC Ltd.) with danger ultraviolet-blocking (<280nm) filter. The intensity of light was 100mW/cm2. The distance between solar simulator and sample is maintained to 30 cm.
3. Results and discussion 3.1. Mechanism and behavior of anodization process TiO2 NT arrays were fabricated by using self organized electrochemical anodization method in NH4F and ethylene glycol based electrolyte. Figure 1 shows the experimental setup of anodization method. The details discussion (about Fig. 1) is given in experimental section. The overall reaction of the anodic oxidation of titanium can be represented as:
Ti 2H 2 O TiO2 4H 2e
(1)
At the early stages of anodization process, a thin oxide layer forms on the surface of titanium sheet according to the equation (1). This compact oxide layer on the titanium surface leads to a rapid reduction in the current density due to its poor electrical conductivity, which at constant potential brings a decrease in current in the circuit [40]. In the absence of Fions,the resistance will increase to a point where the anodic reactions stop. However, in the presence of F- ions, the oxide layer partially dissolves to form soluble fluoride complexes according to the following reaction:
TiO2 6F 4H [TiF6 ]2 2H 2 O
(2)
The F- ions in electrolyte have two effects including the ability to form water-soluble [TiF6]2- and the small ionic radius that makes them suitable to enter the growing TiO2 lattice and to be transported through the oxide by the applied field (thus competing with O2-). Under the sufficient applied voltage magnitude (growth factor g 1-5 nm/V) is reached then the field-assisted oxidation occurs at the TiO2/Ti interface, where the oxygen ions (O2-) are transported from the solution to the oxide layer. At the same time, a chemical attack (dissolution) occurs to form TiO2 and prevent Ti(OH)xOy precipitation as titanium ions (Ti4+)
are transported from the titanium to the oxide/solution interface and are dissolve into the solution[34], leading to a continuous increase in the depth of the porous structure and thus the formation of ordered nanotubes oriented vertically to the substrate[34, 41]. Normally in anodization process, the chemical dissolution is the key for the formation of self-organized TiO2 NT arrays, which reduces the thickness of the oxide layer (barrier layer) keeping the electrochemical etching (field assisted oxidation and dissolution) process active. No nanotubes can be formed if the chemical dissolution is too high or too low. The electrochemical etching rate depends on the anodization potential as well as electrolyte concentrations. The length of TiO2 NT array increases until the electrochemical etching rate equals to the chemical dissolution rate of the top surface of TiO2 NT array. After this point is reached, the length of TiO2 NT array will be independent of the anodization duration, as determined for a given electrolyte concentration and anodization potential. The chemical dissolution rate is determined by the F- concentration and anodization potential. The anodic potential at which nanotubes are formed is related to the F- concentration, with higher potentials requiring higher F- concentrations. Figure 2(a) and (b) show the typical current transient behavior, recorded during anodization of Ti metal sheet in the NH4F and ethylene glycol based electrolyte without (W/o) and with cooling system. The recording of anodization current starts with the starting of voltage ramp. Initially, with the increase of anodization potential, the current increases almost linearly and reaches to maximum value within 6 sec. After application of the potential (1st stage), within 60-70 sec the measured current density reduces 90% from the maximum value, which is due to the formation of an oxide barrier layer and induces a voltage drop between the working electrode, Ti sheet, and electrolyte. In this region, the electronic conduction decreases and the ionic conduction through the TiO2 increases. The current again increases (2nd stage) with a time lag, the higher the fluoride concentration represents the enhanced rate of dissolution. Then the current reaches a quasi solid-steady state, where the current increases with increasing fluoride concentration [42]. The Beyond this point (3rd stage) the current gradually drops due to a corresponding increase in porous structure depth. After the current increases very slowly and reaches to maximum value. At this stage, the dissolution and oxidation of titanium reach a kind of equilibrium which leads to maximize of formation of nanotubes. Beyond this point, the current drastically reduces, finally (4th stage) dropping to zero as the metal film becomes completely discontinuous [43]. In both cases (without and with cooling systems), the current increases with the increase of anodization potential. Comparing to these figures [Fig. 2(a) and (b)], it is clearly indicated that without cooling
system, the current increases at 3rd stage due to increase of solution temperature (self heating). Solution temperature increases due to increase the anodization potential, because of higher potential requiring higher F- ions to attack Ti surface. So, molecules move faster in electrolyte as well as electron transfer become faster, which in turns increase of solution temperature. In 3rd stage, the current for 74V increases exponentially. But, using cooling system the overall current increases with increase of the anodization potential, and the current is in steady state in 3rd stage. Figure 2(c) and (d) shows the self-heating solution temperature and deposition time, respectively. The solution temperature increases due to increase the anodization potential without using cooling system. The temperature using 34, 44, 54, 64 and 74V is 25, 30, 35, 46 and 63C, respectively. By using water-flow based cooling system, the solution temperature maintains to 25C. The deposition rate increases with increase of anodization potential. The deposition rate without cooling system is higher than the deposition rate of using cooling system. The deposition rates for 74V are 514 and 319nm/min without and with cooling systems, respectively. The deposition rate increases with anodization potential due to the F ions move very fast in electrolyte.
3.2. Structural analysis of TiO2 NT arrays The grazing incident X-ray diffraction (GIXRD) patterns (incidence angle =0.5) of the TiO2 NT arrays using different anodization potentials without and with water based cooling systems are given in Fig. 3(a) and (b), respectively. All the diffraction peaks are indexed on anatase (JCPDS card: 21-1272) and rutile (JCPDS card: 21-1276) phase. It is cleared that the TiO2 NT array is in polycrystalline nature with almost anatase phase including two rutile phases. From both figures, the anatase diffraction peaks are enhanced by increase of anodization potential but decrease at 74V potential. Comparing with other potentials, the rutile peak R(111) appears strongly at 64V as shown in Fig. 3(a). In Fig. 3(b), the anatase peak A(101) is also become strong with raise of anodization potential up to 64 V and reduces at 74V potential. The rutile peaks appears for using 44 to 74V potential and become strong with 54V. In both cases, the crystallite size has been calculated using A (101) peak by Debye– Scherrer’s equation [44], as follows:
D
0.94 cos
(3)
where, D is the crystallite size, λ is the wavelength of the X-ray radiation (Cu K = 0.15406 nm), θ is the diffraction angle and β is the full width half maxima. Without cooling system,
the crystallite size of TiO2 NT arrays at different potentials of 34, 44, 54, 64 and 74V is 15.6, 16.8, 18.9, 20.4, and 22.2nm, respectively. Using water based cooling system, the crystallite size of TiO2 NT array at different potentials of 34, 44, 54, 64, and 74V is 15.6, 18.5, 19.3, 21.1, 21.5, and 23.1nm, respectively. It is necessary to note that if the rutile peak appears, which means that band gap become low, so it is reasonable for enhancing photocatalytic activity.
3.3. Optical property of TiO2 NT arrays Figures 4(a) and (b) show the absorbance spectra as a function of wavelength in the wavelength range 300-900nm for TiO2 NT arrays, prepared without and with cooling systems, respectively. In both figures, it has been observed that the absorption edge shows little bit red shift (382~400 nm) with the increase of anodization potential. It is may be due to observed the different crystallinity and presence of rutile phase within the sample. In both sets of samples, the TiO2 NT array (at 34V) shows the minimum absorbance, whereas TiO2 NT array (at 74V) exhibits the maximum absorbance within 300-500 nm range. We assume direct transition between the top of the valence band and the bottom of the conduction band in order to estimate the optical band gap (Eg) of the TiO2 NT arrays using the relation [45]:
h Ah E g 1 / 2
(4)
Where, is absorption coefficient, A is the edge width parameter and hν is the photon energy. The optical band gap of the TiO2 NT array was determined from the extrapolation of the linear plots of (αhν)2 versus hν at α=0. Figures 5(a) and (b) show the plots of (αhν)2versus the photon energy of the TiO2 NT array grown without and with cooling systems, respectively. From Fig. 5(a), the optical band gap of the TiO2 NT array without cooling system decreases form 3.20-3.01eV with the increase of anodization potential form 34 to 64V, respectively, may be due to the presence of rutile phase and self heating temperature. But TiO2 NT array (at 74V) shows higher band gap of 3.07eV than the TiO2 NT array (at 64V). It is cleared from the Fig. 5(b) that the TiO2 NT array with cooling system also decreases from 3.20 to 3.06eV with the increase of anodization potential. Band gap shows less than the 3.2eV due to presence of rutile phase and little bit nitrogen (N) doping in TiO2 NT arrays [46]. The summarized optical band gap data are given in Table 1. Comparing Fig. 5(a) and (b), a slight ~0.01eV variation was observed with TiO2 NT array (at 74V) using cooling system, which can be attributed to the quantum size effect of bigger crystallite size, as supported by GIXRD results, and the
highly smooth and ordered structure, which promoted separate efficiencies of photogenerated charges and extends the range of excited spectrum.
3.4. Morphological property of TiO2 NT arrays The surface morphological property of TiO2 NT array is investigated by field emission electron microscope (FE-SEM). Figures 6(a)-(e) show the FE-SEM images of TiO2 NT array without cooling system. The surface of the TiO2 NT array has great influence by the anodization potential as well as self-heating solution temperature. The outer and inner diameter of TiO2 NT array increases with increase of anodization potential. And the wall thickness also varies from 20-24nm with increase of the anodization potential. The maximum outer (224 nm) and inner (172 nm) diameter exhibits with TiO2 NT array (at 74V). The Fconcentration increases with increase of anodization potential and also increase of self heating solution temperature (25 to 63C), which attacks very fast to Ti surface. For that reason, the pore diameter and deposition rate increases with increase of anodization potential. Figures 7(a)-(d) show the surface morphological property of TiO2 NT array with cooling system. In this case, the outer and inner diameter also increases with anodization potential. The wall thickness also varies 20-22nm with increase of anodization potential. With cooling system, the pore diameter and deposition rate vary only with the anodization potential, because of eliminating the self-heating due to use cooling system.
Table 1 also shows the summarized data from FESEM images (Fig.6 and 7) including average pore diameter of TiO2 NT without and with water based cooling systems. In both cases, this phenomenon can be ascribed from FESEM images to the increase in the dissolution rate, which increases by the increase of the F- ions stimulated by increasing the anodization potential, leading to increase in the inner and outer diameter as well as wall thickness of the TiO2 NT. The films can be well simulated by a regular network of identical, equally spaced and vertically ordered tubes, their porosity (P) that controls the diffusion of the pollutant molecule inside the catalyst can be estimated by purely geometrical considerations. In fact, P is the complement of the surface solid fraction factor, which expresses the ratio of the TiO 2 surface area forming the nanotubes with respect to the total surface area of thesample [47, 48]. Thus, when the tubes are close packed, as in our case, the porosity equals to:
P 1
2w( w D) 3 ( 2 w D) 2
(5)
Where, where D, w, and (2w+D) are the respective pore diameter, wall thickness, and centerto-center distance between NTs. The surface roughness factor calculated from the Eq. 4.1 [47].
rf
2(1 P) 4 ( w D) w 3 ( 2 w D) 2
(6)
Relying on the same geometric model, the root mean square roughness, Rrms can be also predicted by the following expression:
Rrms P(1 P) L
(7)
Where L is the length of TiO2 NTs. Table 1 shows the porosity and Rrms values. In both cases, the porosity increases with the increase of anodization potential. The porosity of TiO2 NT with cooling system exhibits better porosity. In both cases, the Rrms value of TiO2 NT with 44V shows higher value compared to other potentials. After 44V, the Rrms value decreases with the increase of anodization potential.
Transmission electron microscope (TEM) was also used to examine morphology of TiO2 NT arrays (at 54 and 74V) without and with cooling systems, as shown in Fig. 8(a)–(h). Figures 8(a)-(b) show TEM and high resolution TEM (HRTEM) images of TiO2 NT array (at 54V) without cooling system. Here, it is cleared that the outer and inner diameter is ~1652 and 1232nm [from inset of Fig. 8(a)], respectively, which is similar to measure from the FESEM images [in Fig. 6(c) and Table 1]. The fringe of the TiO2 NT array (at 54V) without cooling system is 0.324nm corresponding to anatase lattice (101) [49]. In Fig. 8(c)-(d), the pore diameter and wall thickness of TiO2 NT array (at 54V) with cooling system is also similar to measure from FESEM image [in Fig. 7(b) and Table 1]. In that case, the fringe of anatase lattice (004) is d=0.332 nm [46]. Figure 8(e) and (f) show the TEM and HRTEM images of TiO2 NT array (at 74V) without cooling system. The outer, inner, and wall thickness are 2205, 1705, and 222 nm, respectively. The fringe is 0.313 nm corresponding to anatase (101) lattice [49]. Lastly, Fig. 8(g) and (h) exhibit the TEM and HRTEM images of TiO2 NT array (at 74V) with cooling system. The inner, outer and wall thickness is similar to measure in FESEM image [in Fig. 7(d) and Table 1]. The fringe of anatase lattice is 0.324 nm for A(200) [50].
3.5. Mechanism of Photocatalytic degradation of methanol on TiO2 NT arrays In this study, we also discuss the mechanism of methanol decomposition on the surface of TiO2 NT array. Figure 9(a) shows the schematic diagram of mechanism of
photocatalytic methanol gas-phase decomposion on the surface of TiO2 NT array. The electron of the valence band of TiO2 NT array is excited by illuminated light with energy equals to or greater than the band gap of TiO2. The excess energy of this excited electron promoted the electron to the conduction band of TiO2, therefore creating the negative-electron (e-) and positive-hole (h+) pair, while the holes are remained in the valance band. We knew that TiO2 is more effective photocatalyst due to recombine more slowly.[10] The percentage of carrier recombination has a major effect on the photocatalytic efficiency. One of the notable features of titanium oxide is the strong oxidative decomposing power of positive holes,[7] which is greater than the reducing power of electrons excited to the conduction band.[5] The rate of O2 reduction by electron is an important matter to prevent the recombination during photocatalytic process [51, 52]. Firstly, the oxygen reacts with a conduction band electron, ecb, to form a superoxide radical ion O2-, follows the reaction (8):
ecb O2 O2
(8)
Secondly, the oxygen combines with an organic radical generated by a photohole, or an OH radical reaction to produce an organoperoxy radical, HOCH2(OO) , as shown in reactions (9)- (13):
O2 H OOH
(9)
CH 3OH h CH 2 OH H
(10)
H 2 O h OH H
(11)
CH 3OH OH CH 2 OH H 2 O
(12)
or
CH 2 OH O2 HOCH2 (OO)
(13)
It is further suggested that the superoxide diffuses on the surface, or desorbs and diffuses in solution in either case eventually finding and reacting with the organoperoxide radical on the surface to form an organotetroxide by reactions (14) and (15):
HOCH2 (OO) OOH HOCH2 OOOOH
(14)
HOCH2 (OO) O2 HOCH2 OOOO
(15)
The organotetroxide decomposes in a reaction sequence similar to that occurring in Russel reactions, as discussed by Schwitzgebel et al. [51] with the product formation analogous to that shown in equation (16):
HOCH2 OOOOH HCOOH O2 H 2 O
(16)
which is evident from the increase of C=O. Hence, it can be concluded that the methanol decomposition proceeds via the HCOOH rather than other intermediate compound formation.
HCOOH CH 3OH HCOOCH3 H 2 O
(17)
Reaction (12), utilizing a second CH3OH molecule shows the formation of methylformate in the oxidative photocatalysis of CH3OH.[52,
53]
As stated above, one of key steps
proposed is the surface diffusion, or desorption plus solution diffusion, of O2 or OOH to a surface site occupied by HOCH 2 (OO) . It is now proposed that overall reaction efficiency
might be further enhanced if the radical intermediates leading to the formation of the organotetraoxide, as shown in reaction steps (11) and (12), were formed in close proximity on the catalyst surface [54].
3.6. Photocatalytic activity of TiO2 NT arrays The schematic diagram of photocatalytic gas cell (reactor) is given in Fig. 9(b). The details parameter of photocatalytic reactor is discussed in experimental section. The photocatalytic activity of TiO2 NT arrays (at different potentials) without and with cooling systems is evaluated from the decomposition of methanol. Fig. 10(a) shows the Fourier transform infrared (FTIR) spectra of methanol decomposition on the surface of TiO2 NT array (at 74V) without cooling system, as a function of irradiation time. The undissociated methanol owns the bands at 1350, 1055, 1455, 2864, 2952 and 3680cm−1 when the irradiation time is zero. The double bands at 1013 and 1055 cm-1 are detected due to the C–O and C–H stretching contribution regions. The bands of 2864 and 2952cm−1 correspond to the symmetric and asymmetric CH3 vibrations, respectively [56, 57]. The band observed at 3680cm−1 could be assigned to the OH stretching mode of intermediate species probably present at room temperature [58]. On the basis of experimental data, we illustrate the variation of peaks height of different bands in Fig. 10 (b) and (c). From Fig. 10(b), It is cleared that the bands C-OH (1350cm-1), OH (3680cm-1), C-O (1013cm-1), CH (1055 and 2864cm-1) start to decrease in intensity with increase of irradiation time. Some new peaks appear at C-O (1200 and 2075cm1
), C=O (1750cm-1), and CH3 (1455cm-1) band with the increase of irradiation time, as shown
in Fig. 10(c). The band of 1200 and 1750cm-1 can be assigned to the C–O and C=O stretching vibration modes, respectively. It is noteworthy that the absorption bands C-OH (1350cm-1), CO (1013cm-1), CH (1055, 2864, and 3680cm-1), and CH3 (1455cm-1) are almost disappeared after 3 h of irradiation, while the bands at 2864 and 2952cm−1 still exist at a decreasing intensity. The gaseous CO2 is a linear molecule with two infrared active absorption bands at 2340cm−1 (antisymmetric stretching mode) and 669cm−1 (bending mode) [59, 60]. The two
peaks of H2O (1635 and 3460 cm-1) bands increases with increase of irradiation time up to 2h, after that decreases with increase of irradiation time. Hence, we can conclude that when methanol is decomposed then the primary two peaks of CO2 (at 2340cm−1) and H2O (3464cm−1)are formed [52]. From Fig. 2-8, it is cleared that the crystallinity, band gap, pore diameter and deposition rate of TiO2 NT array varies with anodization potential. Figure 11(a) shows the variation of pore diameter, deposition rate and surface area with respect to anodization potential. The pore diameter of TiO2 NT array increases with increase of potential. The surface area decreases with increase of anodization potential, may be due to the less number of NTs remain in same area. The TiO2 NT array at low potential (small pore diameter) contains large number of NTs in an area, compared to TiO2 NT array at high potential (large pore diameter) which has less number of NTs in the same area [in Fig. 11(a)]. So, TiO 2 NT array (at low potential) enhances the surface area as well as improve in electron-hole separation for photocatalysis property. Figures 11(b) and (c) show the CO2 (at 2340cm−1) and H2O (at 3464cm-1) transmittance peak height, respectively for TiO2 NT arrays at different potentials without cooling system. From Figure 11(b) and (c), it is observed that the variation of photocatalytic (CO2 and H2O peak heights) activities of TiO2 NT array depend on the anodization potential as well as on the self heating solution temperature. In Fig. 11(a), TiO2 NT array (at 44V) shows the better photocatalytic activity than the sample at other anodization potentials. The TiO2 NT array (at 64V) shows also good photocatalytic activity due to presence the strong rutile phase, compared to other samples, except at 44V. TiO2 NT (at 44V) arrays may be has high surface area i.e. large number of NTs remain in same area compared to other potentials, as shown in Fig. 11(a), which leads to more contact with methanol gas. So, the oxygen can easily combine with an organic radical generated by a photohole, or an OH radical. Similarly, Figures 11(d) and (e) show the CO2 and H2O peak height, respectively for TiO2 NT array with water based cooling system. Here, the photocatalytic activity only depends on the variation of anodization potential, because of maintaining constant solution temperature (around 25C) by using cooling system. From both figures, it is cleared that the TiO2 NT array (at 54V) shows the highest photocatalytic activity (CO2 and H2O peak heights), compared to other samples. The photocatalytic activity decreases when the potential becomes to lower and higher than 54V. Here also, The TiO2 NT (at 34V) array shows the less photocatalytic activity. The improvement of photocatalytic activity of TiO2 NT (at 54V) array may be due to its high strong rutile phase as indicated by GIXRD analysis, lessen band gap
and has large number of TiO2 NTs in same area compared to other samples [given in Fig. 11(a)]. From Fig. 11(b)-(d), there is no photodecomposition occurs (CO2 and H2O peak) without TiO2 NTs under irradiation. The experimental data of Fig. 11(b) and (d) is found to fit approximately a pseudofirst-order kinetic model [50] by the linear transforms:
f (t ) f 1 e kt
(18)
where, (t) is peak height as the function of time, is peak height at infinite time, and k is rate constant. The values of the rate constant, k and regression coefficient (R) are listed in Table 2. The TiO2 NT (at 44V) sample without cooling system shows the highest rate constant value, k=0.0186 i.e. highest photocatalytic activity, compared to other samples. Using cooling system, TiO2 NT (at 54V) array shows the better photocatalytic activity (k=0.0216) than the samples at other potentials. The TiO2 NT (at 54V) array with cooling system shows the higher photodecomposition rate than the TiO2 NT (at 44V) array without cooling system, which means that the TiO2 NT (at 54V) array with cooling system enhances the photocatalytic activity may be due to strong rutile phase and lessen band gap (3.1eV).
4. Conclusion In summary, TiO2 NT arrays have been successfully fabricated by using a selforganized anodization method at different potentials. Two sets of TiO2 NT arrays were prepared using without and with cooling systems to evaluate the ideal anodization potential. It has been observed that the variation of anodization potential largely modifies the crystallinity as well as surface morphology of TiO2 NT arrays, which affects on the photocatalytic degradation. In both cases, the pore diameter and deposition rate increase with increase of anodization potential. The TiO2 NT array without cooling system shows the larger pore diameter than the TiO2 NT with cooling system. The maximum pore diameter (outer: 224nm, and inner: 172nm) has been exhibited by TiO2 NT (at 74V) without cooling system, may be due to self-heating solution temperature and fast electron transfer. The TiO2 NT arrays (at 44V) sample without cooling system shows the maximum photocatalytic activity and rate constant (k=0.186), compared to TiO2 NT samples at other potentials, may be due to large number of NTs remain in same area. Using cooling system, the TiO2 NT (at 54V) array shows the highest photo-decomposing property and rate constant (k=0.216) than the sample at other potentials, may be owing to it strong rutile phase and also has large number of NTs in same area. Moreover, the TiO2 NT (at 54V) with cooling system shows the higher rate constant than the TiO2 NT (at 44V) without cooling system, due to have strong rutile phase and lessen
band gap (3.1 eV). The variation of photocatalytic activity with various anodization potentials and self-heating temperatures due to increase potential is tried to be explained in terms of surface morphology and crystallinity of the TiO2 NT arrays.
Acknowledgements Authors would like to thank to Mr. T. Kawabata, Materials Science and Engineering, University of Toyama, Japan for measuring of TEM image.
References [1] D. Chen, J. Ye, Adv. Funct. Mater.18(2008) 922. [2] D. F. Ollis, H. Al-Ekabi, Elsevier, Amsterdam, 1993. [3] M. Kalvacova, J.M. Macak, F. S. Stein, C.T. Mierke, P. Schmuki, Phys. Stat. Sol. (RRL), 2(2008) 194. [4] A. Fujishima, K. Honda, Nature,238(1972) 37. [5] X. Li, T. Fan, H. Zhou, S. K. Chow, W. Zhang, D. Zhang, Q. Guo, H. Ogawa, Adv. Funct. Mater.19, (2009)45. [6] H. Choi, A. C. Sofranko, D. D. Dionysiou, Adv. Funct. Mater.16(2006) 1067. [7] G. Wu, J. Wen, S. Nigro, A. Chen, Nanotechnology, 21(2010) 085701. [8] X. Zhang, L. Lei, J. Zhang, Q. Chen, J. Bao, Bo Fang, Separ. Puri. Technol.66(2009) 417. [9] K. H. Park, C. K. Hong, Electrochem. Commun. 10, (2008)1187. [10] W. Zhao, W. Ma, C. Chen, J. C. Zhao, Z. G. Shuai, J. Am. Chem. Soc.126(2004) 4782. [11] H. F. Zhuang , C. J. Lin, Y. K. Lai, L. Sun, J. Li, Environ. Sci. Technol. 41(2007) 47354740. [12] X. Hu, T, Zhang, Z. Jin, J. Zhang, W. Xu, J. Yan, J. Zhang, L. Zhang, Y. Wu, Mater. Letts.62(2008)4579. [13] A. Kar, Y. R. Smith, V. Ravi Subramanian, Environ. Sci. Technol. 43(2009)3260. [14] M. F. Hossain, S. Biswas, T. Takahashi, Y. Kubota, A. Fujishima, Thin Solid Films, , 517 (2008) 1091. [15] I. M. Arabatzis, S. Antonaraki, T. Stergiopoulos, A. Hiskia, E. Papaconstantinou, M. C. Bernard, P. Falaras, J. Photochem. Photobiol. A 149 (2002) 237. [16] M. F. Hossain, S. Biswas, T. Takahashi, Y. Kubota, A. Fujishima, Phys. Stat. Sol. (a), 205(2008) 2018. [17] H. C. Liang, X. Z. Li, Y. H. Yang, K. H. Sze, Chemosphere, 73 (2008) 805.
[18] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater.11(1999) 1307. [19] H. Yu, J. Yu, B. Cheng, J. Lin, J. Hazard. Mater.147(2007) 581. [20] G. K. Mor, O. K. Varghese, M. Paulose, C. A.Grimes, Adv. Funct. Mater. 15(2005) 1291 [21] S. G. Yang, Y. Z. Liu, C. Sun, Appl. Catal. A, 301, (2006) 284. [22] X. Wu, Q. Z. Jiang, Z. F. Ma, M. Fu, W. F. Shangguan, Solid State Sci. 136(2005)513. [23] Z.R.R. Tian, J.A. Voigt, J. Liu, B. McKenzie, H.F. Xu, J. Am. Chem. Soc.125 (2003) 12384. [24] H.H. Ou, S.L. Lo, Sep. Purif. Methods.58(2007)179. [25] S. I. Na, S. S. Kim, W. K. Hong, J. W. Park, J. Jo, Y.C. Nah, T. Lee, D. Y. Kim, Electrochim. Acta. 53(2007) 2560. [26] V. Zwilling, E. D. Ceretti, A. B. Forveille, D. David, M.Y. Perrin, M. Aucouturier, Surf. Interface. Anal.27(1999)629. [27] D. Gong, C. A. Grimes, O. K. Varghese, Z. Chen, E. C. Dickey. J. Mater. Res.16(2001)3331. [28] R. Beranek, H. Hildebrand, P. Schmuki, Electrochem Solid-State Lett. 6(2003) B12. [29] J. M. Macak, K. Sirotna, P. Schmuki, Electrochim Acta, 50(2005)3679. [30] J. M. Macak, H. Tsuchiya, P. Schmuki, Angew Chem.44(2005)2100. [31] L. V. Taveira, J. M. Macak, H Tsuchiya, L. F. P. Dick, P. Schmuki, J. Electrochem. Soc. 152(2005)B405. [32] A. Ghicov, H. Tsuchiya, J. M. Macak, P. Schmuki, Electrochem. Commun.7(2005) 505. [33] Q. Cai, M. Paulose, O.K. Varghese, C.A. Grimes, J. Mater. Res. 20(2005) 230. [34] J. M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer and P. Schmuki, Curr. Opin. Solid State Mater. Sci.11(2007) 3. [35] M. Paulose, K. Shankar, S. Yoriya, H. E. Prakasam, O. K. Varghese, G. K. Mor, J. Phys. Chem. B, 110(2006) 16179. [36] K. Shankar, G. K. Mor, A. Fitzgerald, C.A.Grimes, J. Phys. Chem. C, 111(2007) 21. [37] S. Yoriya, H. E. Prakasam, O. K. Varghese, K. Shankar, M. Paulose, G. K. Mor, T. J. Latempa, C. A. Grimes, Sens. Lett.4(2006)334. _[38] J. M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. Int. Ed.44(2005)7463. [39] H. E. Prakasam, K. Shankar, M. Paulose, O. K. Varghese, C. A. Grimes,J. Phys. Chem. C,111(2007) 7235. [40] A. Elsanousi,J. Zhang, H. M. H. Fadlalla, F. Zhang, H. Wang, X. Ding, Z. Huang, C. Tang, J. Mater. Sci.43(2008) 7219.
[41] D. J. Yang, H. G. Kim, S. J. Cho, W. Y. Choi, Mater. Letts.62(2008) 775. [42] R. Beranek, H. Hildebrand, P. Schmuki, Electrochem. Solid State Lett.6(2003)B12. [43] G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar, C. A. Grimes, Sol. Energy Mater. Sol. Cells, 90(2006)2011. [44] B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, USA, 1978. [45] J. I. Pankove, Optical Processes in Semiconductors, Dover Publications Inc., New York, 1970. [46] O. K. Varghese, M. Paulose, T. J. LaTempa, C. A. Grimes, Nano Letts.9(2009) 731. [47] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [48] A.G. Kontos, A. Katsanaki, T. Maggos, V. Likodimos, A. Ghicov, D. Kim, J. Kunze, C. Vasilakos, P. Schmuki, P. Falaras, Chem. Phys. Letts. 490 (2010) 58. [49] L. Miao, P. Jin, K. Kaneko, A. Terai, N. Nabatova-Gabain, S. Tanemura, Appl. Surf. Sci.212–213 (2003) 255. [50] Y. Gao, S.A. Elder, Mater. Letts.44 (2000) 228 [51] C. M. Wang, A. Heller, H. Gerisher, J. Am. Chem. Soc.114(1992) 5230. [52] Z. H. Zhang, M. F. Hossain, T. Arakawa, T. Takahashi, J. Hazard. Mater.176(2010) 973. [53] J. Schwitzgebel, J. G. Ekerdt, H. Gerischer, A.Heller, J. Phys. Chem.99(1995) 5633. [54] M. Sadeghi, W. Liu, T.G. Zhang, P. Stavropoulos and B. Levy, J. Phys. Chem.100(1996) 19466. [55] N. N. Lichtin, M. Avudaithai, E. Berman, J. Dong, Res. Chem. Intermed.20(1994)755. [56] C. Binet, M. Daturi, Catal. Today, 70(2001) 155. [57] K. Prabakar, T. Takahashi, T. Nakashima, Y. Kubota, A. Fujishima, J. Vac. Sci. Technol. A,24(2006) 1613. [58] K. Mudalige, S. Warren, M. Trenary, J. Phys. Chem. B, 104(2000) 2448. [59] F. Ouyang, S. Yao, K. Tabata, E. Suzuki, Appl. Surf. Sci.158(2000) 28. [60] A. L. Goodman, L. M. Campus, K. T. Schroeder, Energy Fuels, 19(2005) 471.
Figure Captions
Figure 1. Schematic illustration of computer controlled electrochemical anodic oxidation experimental set-up for TiO2 NT arrays; TC means thermocouple, Ti is titanium sheet and Pt is platinum foils.
Figure 2. Characteristics of current transient for Ti anodization (a) without (w/o) and (b) with cooling system in NH4F and ethylene glycol based electrolyte. (c) Self-heating solution temperature and (d) deposition rate of TiO2 NT array without and with water based cooling systems.
Figure 3. The GIXRD pattern of TiO2 NT arrays at different potentials (34 to 74V): (a) without and (b) with water based cooling systems.
Figure 4. Absorption spectra of TiO2 NTs at different potentials: (a) without and (b) with water based cooling systems. Figure 5. The (αhυ)2 versus energy of TiO2 NTs at different potentials: (a) without and (b) with water cooling systems.
Figure 6. FE-SEM images of TiO2 NT array at different potentials of (a) 34, (b) 44, (c) 54, (d) 64 and (e) 74V, without using cooling system.
Figure 7. FE-SEM images of TiO2 NT arrays at different potentials of (a) 44, (b) 54, (c) 64, and (d) 74V with water based cooling system.
Figure 8. Surface morphology of TiO2 NT array: (a) TEM and (b) HRTEM images at 54V without cooling system; (c) TEM and (d) HRTEM images at 54V with cooling system; (e) TEM and (f) HRTEM images at 74V without cooling system; and (g) TEM and (h) HRTEM images at 74V with cooling system.
Figure 9. (a) The schematic diagram of methanol gas-phase decomposition mechanism on surface of TiO2 NT array; (b) A Schematic diagram of photocatalytic reactor for photocatalysis process. Figure 10. (a) The FTIR spectra for the decomposition of methanol on the surface of TiO2 NT array at 74V without using cooling system as the function of irradiation time. (b) Variation of
the peaks height of CO2 (669 and 2340 cm-1), H2O (1635 and 3460 cm-1), C-OH (1350 cm-1) and OH (3680 cm-1) bands with the time. (c) Variation of the peaks height of C-O (1013, 1200, and 2075 cm-1), C=O (1750 cm-1), CH (1055, 2864, and 3680 cm-1), and CH3 (1455 cm1
) bands with the time.
Figure 11.(a) Schematic diagram of TiO2 NT array with variation of anodization potential; Variation of (b) the CO2 peak (2340cm-1)and (c) H2O peak (3464cm-1), height of TiO2 NT array at different potentials without cooling system; Variation of (d) the CO2 peak(2340cm1
)and (e) H2O peak (3464cm-1),height of TiO2 NT array at different potentials with cooling
system.
Figure 1. Hossain et al.
(+)
Temp. controller
(-)
PV SV
Cold water (~10°C) Heater
TC
Water out Ti
Pt
50
(a) 34 V 44 V 54 V 64 V 74 V
40 30 20
Anodization current (mA/cm2)
60 W/o cooling
10 0
0
5
10 15 20 25 Anodization time (min)
30
60 50
(b)
Cooling 34 V 44 V 54 V 64 V 74 V
40 30 20 10 0
0
5
10 15 20 25 Anodization time (min)
0.6
(c)
Deposition rate (µm/min)
Anodization current (mA/cm2)
Figure 2. Hossain et al.
0.5
(d)
0.4
W/o Cooling Cooling
0.3 0.2 0.1 0.0
34
44 54 64 74 Anodization potential (V)
30
Figure 3. Hossain et al.
(a)
A(204) A(116) A(220) A(215)
A(200) A(105) A(211)
R(101) A(004) R(111)
74 V
A(101)
Intensity (a.u.)
W/o cooling
64 V 54 V 44 V 34 V 10
20
30 40 50 60 Diffraction angle, 2θ (deg.)
80
A(204) A(116) A(220) A(215)
A(200) A(105) A(211)
Cooling R(101) A(004) R(111)
Intensity (a.u.)
74 V
A(101)
(b)
70
64 V 54 V 44 V 34 V 10
20
30 40 50 60 Diffraction angle, 2θ (deg.)
70
80
Figure 4. Hossain et al.
(a)
0.7 34 V 44 V 54 V 64 V 74 V
Absorbance (a.u.)
0.6 0.5 0.4 0.3 0.2 0.1 0.0 300
(b)
W/o cooling 400
500 600 700 Wavelength, λ (nm)
900
0.7 34 V 44 V 54 V 64 V 74 V
0.6
Absorbance (a.u.)
800
0.5 0.4 0.3 0.2 0.1 0.0 300
Cooling 400
500 600 700 Wavelength, λ (nm)
800
900
Figure 5. Hossain et al.
(a)
25 34 V 44 V 54 V 64 V 74 V
(αhν)2x1012 (eV2m-2)
20 15 10 5 0 2.6
(αhν)2x1012 (eV2m-2)
(b)
W/o Cooling 2.8
3.0 3.2 3.4 Photon energy (eV)
3.6
3.8
25 20
34 V 44 V 54 V 64 V 74 V
15 10 5 0 2.6
Cooling 2.8
3.0 3.2 3.4 Photon energy (eV)
3.6
3.8
Figure 6. Hossain et al.
Figure 7. Hossain et al. (a)
(b)
100 nm
100 nm
(c)
(d)
100 nm
100 nm
Figure 8. Hossain et al.
(b)
(a) 100nm
2 nm
50nm
(c)
d=0.324 10 nm
(d)
500nm
2 nm d=0.332 100nm
10 nm
(f)
(e) 100nm
2 nm d=0.313 100nm
10 nm
(h)
(g)
2 nm d=0.324 50 nm
10 nm
Figure 9. Hossain et al.
Detachment of Oxygen
(a) Uv-visible light
Mn+ Reduction
eCB
Activated •VB Oxygen (O2 ) h+
M CH3OH Oxidation
• Hydroxy radical (•OH)
H2O
(b)
H2O
CO2
Artificial Solar simulator
Gas cell KBr FT-IR Light
CH3OH gas
TiO2 NT array
Detector
CO2, H2O, C-OH, OH peak hieght (a.u.) C-O, C=O, CH, CH3 peak hieght (a.u.)
(c)
OH
1.5 h 2.0 h 2.5 h 3.0 h 3.5 h 4.0 h 500
(b)
H2O
CO2
CH CH
0h 0.5 h 1.0 h
Transmittance (a.u.)
CO2
(a)
C-O CH C-O C-OH CH3 H2O C=O
Figure 10. Hossain et al.
1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm-1)
70 60
CO2
50
CO2
40 H2O
30
H2O
20
OH
10
C-OH 0 0.0
0.5
1.0
1.5 2.0 2.5 3.0 Irradiation time (hour)
70
3.5
4.0
C-O CH C-O CH3 C=O C-O CH
60 50 40 30 20 10 0 0.0
0.5
1.0
1.5 2.0 2.5 3.0 Irradiation time (hour)
3.5
4.0
Figure 11. Hossain et al.
(a)
Pore diameter and deposition rate increase Surface area increases
Surface area increases
30 20 10
W/o TiO2 NTs 34 V 44 V 54 V 64 V 74 V
0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour)
CO2 peak hieght at 2340 cm-1(a.u.)
70 W/o cooling 60 50 (b) W/o TiO2 NTs 40 34 V 30 34 V 34 V 20 34 V 34 V 10 0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour) 50 W/o Cooling 40 (c)
H2O peak hieght at 3460 cm-1(a.u.)
H2O peak hieght at 3460 cm-1(a.u.)
CO2 peak hieght at 2340 cm-1(a.u.)
Anodization potential increases
70 Cooling 60 50 (d) W/o TiO2 NTs 40 34 V 30 44 V 54 V 20 64 V 74 V 10 0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour) 50 Cooling 40 (e) 30 20 10
W/o TiO2 NTs 34 V 44 V 54 V 64 V 74 V
0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour)
Table 1. Pore diameter, porosity and rms roughness and band-gap of TiO2 NT at different potentials without and with water based cooling systems. Potential (V)
Without cooling system Outer
Inner
(nm)
(nm)
Porosity
With cooling system
rms
Band
Outer
Inner
Roughness
gap
(nm)
(nm)
(nm)
(eV)
Porosity
rms
Band
Roughness
gap
(nm)
(eV)
34
92
52
0.38
1311.9
3.20
92
52
0.38
1311.9
3.20
44
128
88
0.52
1348.8
3.18
132
90
0.51
1349.5
3.15
54
167
125
0.60
1322.2
3.12
160
117
0.58
1333.7
3.1
64
198
151
0.62
1310.5
3.01
188
144
0.62
1307.3
3.09
74
224
172
0.63
1305.5
3.07eV
196
152
0.64
1297.9
3.06
Table 2.The values of rate constant kand regression coefficients (R) of methanol decomposition on TiO2 NT arrays at different potentials without and with water based cooling systems. Anodization
Without cooling system
With cooling system
potential
k(min-1)
R
k(min-1)
R
34
0.0155
0.9919
0.0155
0.9919
44
0.0186
0.9924
0.0166
0.9925
54
0.0150
0.9862
0.0216
0.9896
64
0.0147
0.996
0.0128
0.9934
74
0.0134
0.9840
0.00109
0.9904
S1010-6030(16)30752-3 http://dx.doi.org/doi:10.1016/j.jphotochem.2016.11.019 JPC 10443
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date: Revised date: Accepted date:
5-9-2016 9-11-2016 18-11-2016
Please cite this article as: Ibrahim Mustafa Mehedi, M.F.Hossain, H.Okada, Md.Shofiqul Islam, Nano-structural variation of highly aligned anodic Titania nanotube arrays for gas phase photocatalytic application, Journal of Photochemistry and Photobiology A: Chemistry http://dx.doi.org/10.1016/j.jphotochem.2016.11.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nano-structural variation of highly aligned anodic Titania nanotube arrays for gas phase photocatalytic application
Ibrahim Mustafa Mehedi1, M.F. Hossain2,*, H. Okada3,Md. Shofiqul Islam1 1
Department of Electrical & Computer Engineering Engineering, King Abdulaziz University,
Jeddah 21589, Saudi Arabia 2
Department of Electrical & Electronic Engineering, Rajshahi University of Engineering &
Technology, Rajshahi-6204, Bangladesh. 3
Graduate School of Science and Engineering for Research, University of Toyama, 3190
Gofuku, Toyama 930-8555, Japan
*Corresponding Author: E-mail: [email protected] Or [email protected] Or [email protected] Tel./Fax: +88-01778-400-600
Highlights
Highly aligned TiO2 nanotube (NT) arrays have been grown on titanium sheet by using a self organized anodic oxidation method at different potentials (34 to 74V) in a NH4F and ethylene glycol based electrolyte.
Two sets of TiO2 NTs have been fabricated with and without using cooling systems.
The temperature has been maintained to 25C by using water based cooling system.
The photocatalytic decomposition of methanol (5 μL) has been evaluated by Fourier transform infrared spectrophotometer, measuring its concentration decay in a cylindrical glass cell, which contains the TiO2 NTs.
The TiO2 NT (at 54V) at room temperature shows the highest photocatalytic activity due to its strong rutile phase and large number of TiO2 NTs remain in same area.
Abstract Highly aligned titania (TiO2) nanotube (NT) arrays have been grown on titanium sheet by using a self organized anodic oxidation method at different potentials (34 to 74V) in a NH4F and ethylene glycol based electrolyte. Two sets of TiO2 NT arrays have been fabricated with and without using cooling systems. Without cooling system, the solution temperature is increased from 25C (room temperature) to 63C with increase of anodization potential from 34 to 74V. The temperature has been maintained to 25C by using water based cooling system. In both sets of samples, the pore diameter of TiO2 NT arrays increases with increase of anodization potential. The maximum pore diameter has been (outer: 224nm, inner: 172nm) achieved with TiO2 NT arrays (at 74V) without using cooling system. The photocatalytic activity of the TiO2 NT arrays is evaluated by the decomposition of methanol by the use of Fourier transform infrared spectrophotometer. The TiO2 NT (at 44V) sample has been fabricated without using cooling system, which shows the higher photocatalytic activity than the sample at other potentials. Using cooling system, the TiO2 NT (at 54V) sample shows the higher photo-decomposing property than the sample at other potentials. The TiO2 NT (at 54V) sample with cooling system shows the higher rate (photocatalytic activity) constant (k=0.0216) than the TiO2 NT (at 44V) sample without using cooling system (k=0.0186). The photocatalytic activity has been correlated with the nanostructural variation of TiO2 NT arrays at different potentials without (self-grown solution temperature) and with water based cooling systems.
Keywords: Titania nanotube, cooling system, anodization method, photocatalytic activity.
1. Introduction Nowadays, environment pollution and energy crisis are world widely concerned. For a sustainable society, it is an urgent task to develop environment-benign purification technologies and manufacturing technologies of alternative clean energies. Photocatalysis is one of the most promising method for both subjects. Heterogeneous photocatalysis is an advanced oxidation process to destroy unwanted and harmful organic compound in contaminated in water or air[1, 2, 3],and it has been extensively studied since the 1972 [4]. Among the several semiconductor materials tested for photocatalysis, TiO2 is the promising photocatalyst[5, 6] due to its high oxidative power, low operation temperature, non-toxicity, low cost, chemical inertness and high photostability[7-10]. Over the past few decades, the colloidal or powder TiO2 were widely used to photodegrade the pollutants in both the liquid and gaseous phases, it shows highly photocatalytic activity due to its large surface area[11-13].But, the conventional suspended system has a serious limitation, such as it needs post-treatment of separation in a slurry system after photocatalytic reaction[13- 15].This problem can be overcome by immobilized TiO2 particles as thin films on solid support substrate[13, 16]. The various methods have been developed to prepare immobilized TiO2 films such as sol-gel, reactive magnetron and facing target sputtering[14, 16], chemical vapor and liquid-phase deposition [15]. The formation of TiO2 films on the solid substrate significantly reduced the specific surface area of TiO2 photocatalysis, resulting in a decrease of photocatalytic activity compared to corresponding slurries [17]. Therefore it is necessary to develop novel synthesis approach to prepare mesoporous TiO2 photocatalysis, which not only has high photocatalytic activity but also can be steadily separated after photocatalytic reaction. TiO2 nanotube (NT) arrays are expected to be a promising photocatalyst to overcome above mentioned drawbacks due to their special chemical and physical characteristics, high specific area and high pore volume due to their open mesoporous nature[11,13, 17]. TiO2 NT array has excellent charge separation ability due to the absence of gain boundaries [13].Moreover, the TiO2 NT array possesses good chemical and mechanical stability because it grows directly on the titanium substrate, and excellent corrosion resistance.
As
additionally the conductive substrate is able to exhibit some interesting properties of photoelectrocatalysis and photo-electrochemistry [11, 18, 19].
TiO2 NT arrays have been fabricated by several methods, e.g., anodic oxidation [20],sol-gel [21], microwave irradiation [22],seeded growth [23],hydrothermal process[24] and aluminum template synthesis [25]. Among these, electrochemical anodization is relatively simple and efficient process for the fabrication of aligned TiO2 NT array. The first generation of TiO2 NT array via anodic oxidation was grown in an hydrofluoric (HF) aqueous electrolyte or acidic HF mixtures[26-28].These TiO2 NT arrays showed a limited thickness that would not exceed 500-600nm. This technique has been further developed to obtain secondgeneration of highly ordered TiO2 NT array with a high aspect ratio by using buffered neutral electrolyte containing NaF or NH4F instead of HF[29– 32]
and taking importance in the pH
gradient. Grimes and his coworkers achieved 6.4 m long TiO2 NT array by varying pH and electrolyte concentration[33]. To further improve the morphology, third-generation of smoother layers and ordered TiO2 NT array was prepared using almost water free fluoride based electrolyte [34]. The length of TiO2 NT array was greatly extended by using various polar
organic
electrolytes
including
formamide[35-37],dimethyl
sulfoxide[35,
37],
glycerol[38]and ethylene glycol[35, 37]. TiO2 NT array membranes up to 720m thick composed of two nanotube arrays approximately 360 m thick was reported by Prakasam and his coworkers[39]using NH4F and ethylene glycol based electrolyte. However, completely different growth morphology can be obtained, if fluoride ions are present in the electrolyte and suitable anodization conditions are used. The electrolyte composition and applied anodic potential primarily determine the morphology and structure of oxide layers, resulting from an anodization. The growth of TiO2 NT array with some anodization conditions has been investigated by Prakasam and coworkers [39]. However, nobody reported about the effect of self heating solution temperature due to the anodization potential on the growth rate, pore diameter of TiO2 NT array for the application of photocatalysis. In this work, TiO2 NT arrays were fabricated by electrochemical anode oxidation method at different potentials (34-74V) in NH4F and ethylene glycol based electrolyte. This work presents a comparative analysis of the photocatalytic activity of TiO2 NT without (self-heating solution temperature due to potential) and with water based cooling systems (to maintain 25C temperature). The nanostructural and surface morphological property of TiO2 NT array at different anodization potentials and their photocatalytic activity have been investigated and discussed.
2. Experimental details 2.1. Fabrication of TiO2 NT arrays
We used the electrochemical anodization method to fabricate the TiO2 NT arrays. The schematic illustration of electrochemical anodic oxidation experimental set-up with water based cooling system for TiO2 NT array growth is given in Fig. 1. The titanium (Ti) sheet of 99.6% purity, 0.2mm thickness was bought from Nilaco Co., Ltd., Tokyo, Japan. Prior to anodization, the Ti sheet was first mechanically polished (Musashino Denshi MA-150) with different abrasive papers and rinsed in an ultrasonic bath of cold distilled water for 10 min. Then the cleaned Ti sheet was soaked in a mixture of HF, HNO3 acids and H2O (the mixing ratio of HF:HNO3:H2O is 1:4:5 in volume) for 30 sec. After rinsed with acetone and distilled water for 20 min, the Ti sheet was dried in a flow of pure nitrogen stream. The TiO2 NTs were fabricated in a cylindrical electrochemical reactor (the radius is 45 mm and height is about 60 mm). The water based cooling system was used to maintain 25C with temperature controller, heater and thermocouple (TC). A conventional two-electrode system was employed with Ti sheet (2.5x1.5 cm2) as the anode and Pt foil (2.5x1.5 cm2) as the cathode and the distance between these electrodes was maintained 2 cm. The time-dependent current behavior under constant potential was recorded using a computer-controlled Keithley 2400 sourcemeter. All the anodization experiments were carried out in an electrolyte of ethylene glycol (EG), 0.3wt% NH4F and 3vol% of water. The exposed area of Ti sheet was 2x1.5cm2 to the electrolyte. Two-sets of samples were prepared without and with water based cooling systems. Each set of sample was prepared at different potentials of 34, 44, 54, 64 and 74V. After anodization, the sample was cleaned in EG and then distilled water, and was dried with N2 steam flow. The TiO2 NT arrays were subsequently crystallized by annealing in dry oxygen at 450C for 30 min with a heating and cooling rate of 2C/min.
2.2. Characterization of TiO2 NT arrays The crystal structures of the TiO2 NT arrays were examined by grazing incident X-ray diffraction (GIXRD) analysis with data collected from SHIMADZU XRD-6000 using Cu-Kα (λ=1.54060 nm) line with 40 kV–20 mA. The data was recorded from 2 values of 10 to 80 with a step of 0.02. For GIXRD measurement, the incident angle was fixed at 0.5. The optical property of the TiO2 NT arrays ws measured by Diffuse reflection mode spectrophotometer (SHIMADZU UV-VIS NIR 3100) at room temperature within the wave length range 300-900nm. The surface morphologies were studied by using field emission scanning electron microscope (FE-SEM) by JEOL, FE-SEM 6700F and transmission electron microscopy (TEM) by EM-002B (TOPCON Co. Ltd.).
2.3. Measurements of photocatalytic activity The photocatalytic decomposition of methanol (5 μL) was evaluated by FTIR spectrophotometer (JASCO 480 plus) measuring its concentration decay in a cylindrical glass cell of 9.8cm length and 3.6cm, which contained TiO2 NT array of surface area 2.5×1.5cm2, [as given in Fig. 9(b)]. The two ends of the glass cell were fitted with KBr crystals to allow the detection of infrared waves. IR-anti symmetric stretching band of CO2 at 2340 cm−1and H2O (at 3464cm-1) were assigned for the confirmation of photo-decomposition of methanol. Photocatalytic degradation of methanol on the TiO2 NT arrays was carried out under irradiation of an artificial sunlight simulator, consisting of a SOLAX lamp (model: SET-140F, SERIC Ltd.) with danger ultraviolet-blocking (<280nm) filter. The intensity of light was 100mW/cm2. The distance between solar simulator and sample is maintained to 30 cm.
3. Results and discussion 3.1. Mechanism and behavior of anodization process TiO2 NT arrays were fabricated by using self organized electrochemical anodization method in NH4F and ethylene glycol based electrolyte. Figure 1 shows the experimental setup of anodization method. The details discussion (about Fig. 1) is given in experimental section. The overall reaction of the anodic oxidation of titanium can be represented as:
Ti 2H 2 O TiO2 4H 2e
(1)
At the early stages of anodization process, a thin oxide layer forms on the surface of titanium sheet according to the equation (1). This compact oxide layer on the titanium surface leads to a rapid reduction in the current density due to its poor electrical conductivity, which at constant potential brings a decrease in current in the circuit [40]. In the absence of Fions,the resistance will increase to a point where the anodic reactions stop. However, in the presence of F- ions, the oxide layer partially dissolves to form soluble fluoride complexes according to the following reaction:
TiO2 6F 4H [TiF6 ]2 2H 2 O
(2)
The F- ions in electrolyte have two effects including the ability to form water-soluble [TiF6]2- and the small ionic radius that makes them suitable to enter the growing TiO2 lattice and to be transported through the oxide by the applied field (thus competing with O2-). Under the sufficient applied voltage magnitude (growth factor g 1-5 nm/V) is reached then the field-assisted oxidation occurs at the TiO2/Ti interface, where the oxygen ions (O2-) are transported from the solution to the oxide layer. At the same time, a chemical attack (dissolution) occurs to form TiO2 and prevent Ti(OH)xOy precipitation as titanium ions (Ti4+)
are transported from the titanium to the oxide/solution interface and are dissolve into the solution[34], leading to a continuous increase in the depth of the porous structure and thus the formation of ordered nanotubes oriented vertically to the substrate[34, 41]. Normally in anodization process, the chemical dissolution is the key for the formation of self-organized TiO2 NT arrays, which reduces the thickness of the oxide layer (barrier layer) keeping the electrochemical etching (field assisted oxidation and dissolution) process active. No nanotubes can be formed if the chemical dissolution is too high or too low. The electrochemical etching rate depends on the anodization potential as well as electrolyte concentrations. The length of TiO2 NT array increases until the electrochemical etching rate equals to the chemical dissolution rate of the top surface of TiO2 NT array. After this point is reached, the length of TiO2 NT array will be independent of the anodization duration, as determined for a given electrolyte concentration and anodization potential. The chemical dissolution rate is determined by the F- concentration and anodization potential. The anodic potential at which nanotubes are formed is related to the F- concentration, with higher potentials requiring higher F- concentrations. Figure 2(a) and (b) show the typical current transient behavior, recorded during anodization of Ti metal sheet in the NH4F and ethylene glycol based electrolyte without (W/o) and with cooling system. The recording of anodization current starts with the starting of voltage ramp. Initially, with the increase of anodization potential, the current increases almost linearly and reaches to maximum value within 6 sec. After application of the potential (1st stage), within 60-70 sec the measured current density reduces 90% from the maximum value, which is due to the formation of an oxide barrier layer and induces a voltage drop between the working electrode, Ti sheet, and electrolyte. In this region, the electronic conduction decreases and the ionic conduction through the TiO2 increases. The current again increases (2nd stage) with a time lag, the higher the fluoride concentration represents the enhanced rate of dissolution. Then the current reaches a quasi solid-steady state, where the current increases with increasing fluoride concentration [42]. The Beyond this point (3rd stage) the current gradually drops due to a corresponding increase in porous structure depth. After the current increases very slowly and reaches to maximum value. At this stage, the dissolution and oxidation of titanium reach a kind of equilibrium which leads to maximize of formation of nanotubes. Beyond this point, the current drastically reduces, finally (4th stage) dropping to zero as the metal film becomes completely discontinuous [43]. In both cases (without and with cooling systems), the current increases with the increase of anodization potential. Comparing to these figures [Fig. 2(a) and (b)], it is clearly indicated that without cooling
system, the current increases at 3rd stage due to increase of solution temperature (self heating). Solution temperature increases due to increase the anodization potential, because of higher potential requiring higher F- ions to attack Ti surface. So, molecules move faster in electrolyte as well as electron transfer become faster, which in turns increase of solution temperature. In 3rd stage, the current for 74V increases exponentially. But, using cooling system the overall current increases with increase of the anodization potential, and the current is in steady state in 3rd stage. Figure 2(c) and (d) shows the self-heating solution temperature and deposition time, respectively. The solution temperature increases due to increase the anodization potential without using cooling system. The temperature using 34, 44, 54, 64 and 74V is 25, 30, 35, 46 and 63C, respectively. By using water-flow based cooling system, the solution temperature maintains to 25C. The deposition rate increases with increase of anodization potential. The deposition rate without cooling system is higher than the deposition rate of using cooling system. The deposition rates for 74V are 514 and 319nm/min without and with cooling systems, respectively. The deposition rate increases with anodization potential due to the F ions move very fast in electrolyte.
3.2. Structural analysis of TiO2 NT arrays The grazing incident X-ray diffraction (GIXRD) patterns (incidence angle =0.5) of the TiO2 NT arrays using different anodization potentials without and with water based cooling systems are given in Fig. 3(a) and (b), respectively. All the diffraction peaks are indexed on anatase (JCPDS card: 21-1272) and rutile (JCPDS card: 21-1276) phase. It is cleared that the TiO2 NT array is in polycrystalline nature with almost anatase phase including two rutile phases. From both figures, the anatase diffraction peaks are enhanced by increase of anodization potential but decrease at 74V potential. Comparing with other potentials, the rutile peak R(111) appears strongly at 64V as shown in Fig. 3(a). In Fig. 3(b), the anatase peak A(101) is also become strong with raise of anodization potential up to 64 V and reduces at 74V potential. The rutile peaks appears for using 44 to 74V potential and become strong with 54V. In both cases, the crystallite size has been calculated using A (101) peak by Debye– Scherrer’s equation [44], as follows:
D
0.94 cos
(3)
where, D is the crystallite size, λ is the wavelength of the X-ray radiation (Cu K = 0.15406 nm), θ is the diffraction angle and β is the full width half maxima. Without cooling system,
the crystallite size of TiO2 NT arrays at different potentials of 34, 44, 54, 64 and 74V is 15.6, 16.8, 18.9, 20.4, and 22.2nm, respectively. Using water based cooling system, the crystallite size of TiO2 NT array at different potentials of 34, 44, 54, 64, and 74V is 15.6, 18.5, 19.3, 21.1, 21.5, and 23.1nm, respectively. It is necessary to note that if the rutile peak appears, which means that band gap become low, so it is reasonable for enhancing photocatalytic activity.
3.3. Optical property of TiO2 NT arrays Figures 4(a) and (b) show the absorbance spectra as a function of wavelength in the wavelength range 300-900nm for TiO2 NT arrays, prepared without and with cooling systems, respectively. In both figures, it has been observed that the absorption edge shows little bit red shift (382~400 nm) with the increase of anodization potential. It is may be due to observed the different crystallinity and presence of rutile phase within the sample. In both sets of samples, the TiO2 NT array (at 34V) shows the minimum absorbance, whereas TiO2 NT array (at 74V) exhibits the maximum absorbance within 300-500 nm range. We assume direct transition between the top of the valence band and the bottom of the conduction band in order to estimate the optical band gap (Eg) of the TiO2 NT arrays using the relation [45]:
h Ah E g 1 / 2
(4)
Where, is absorption coefficient, A is the edge width parameter and hν is the photon energy. The optical band gap of the TiO2 NT array was determined from the extrapolation of the linear plots of (αhν)2 versus hν at α=0. Figures 5(a) and (b) show the plots of (αhν)2versus the photon energy of the TiO2 NT array grown without and with cooling systems, respectively. From Fig. 5(a), the optical band gap of the TiO2 NT array without cooling system decreases form 3.20-3.01eV with the increase of anodization potential form 34 to 64V, respectively, may be due to the presence of rutile phase and self heating temperature. But TiO2 NT array (at 74V) shows higher band gap of 3.07eV than the TiO2 NT array (at 64V). It is cleared from the Fig. 5(b) that the TiO2 NT array with cooling system also decreases from 3.20 to 3.06eV with the increase of anodization potential. Band gap shows less than the 3.2eV due to presence of rutile phase and little bit nitrogen (N) doping in TiO2 NT arrays [46]. The summarized optical band gap data are given in Table 1. Comparing Fig. 5(a) and (b), a slight ~0.01eV variation was observed with TiO2 NT array (at 74V) using cooling system, which can be attributed to the quantum size effect of bigger crystallite size, as supported by GIXRD results, and the
highly smooth and ordered structure, which promoted separate efficiencies of photogenerated charges and extends the range of excited spectrum.
3.4. Morphological property of TiO2 NT arrays The surface morphological property of TiO2 NT array is investigated by field emission electron microscope (FE-SEM). Figures 6(a)-(e) show the FE-SEM images of TiO2 NT array without cooling system. The surface of the TiO2 NT array has great influence by the anodization potential as well as self-heating solution temperature. The outer and inner diameter of TiO2 NT array increases with increase of anodization potential. And the wall thickness also varies from 20-24nm with increase of the anodization potential. The maximum outer (224 nm) and inner (172 nm) diameter exhibits with TiO2 NT array (at 74V). The Fconcentration increases with increase of anodization potential and also increase of self heating solution temperature (25 to 63C), which attacks very fast to Ti surface. For that reason, the pore diameter and deposition rate increases with increase of anodization potential. Figures 7(a)-(d) show the surface morphological property of TiO2 NT array with cooling system. In this case, the outer and inner diameter also increases with anodization potential. The wall thickness also varies 20-22nm with increase of anodization potential. With cooling system, the pore diameter and deposition rate vary only with the anodization potential, because of eliminating the self-heating due to use cooling system.
Table 1 also shows the summarized data from FESEM images (Fig.6 and 7) including average pore diameter of TiO2 NT without and with water based cooling systems. In both cases, this phenomenon can be ascribed from FESEM images to the increase in the dissolution rate, which increases by the increase of the F- ions stimulated by increasing the anodization potential, leading to increase in the inner and outer diameter as well as wall thickness of the TiO2 NT. The films can be well simulated by a regular network of identical, equally spaced and vertically ordered tubes, their porosity (P) that controls the diffusion of the pollutant molecule inside the catalyst can be estimated by purely geometrical considerations. In fact, P is the complement of the surface solid fraction factor, which expresses the ratio of the TiO 2 surface area forming the nanotubes with respect to the total surface area of thesample [47, 48]. Thus, when the tubes are close packed, as in our case, the porosity equals to:
P 1
2w( w D) 3 ( 2 w D) 2
(5)
Where, where D, w, and (2w+D) are the respective pore diameter, wall thickness, and centerto-center distance between NTs. The surface roughness factor calculated from the Eq. 4.1 [47].
rf
2(1 P) 4 ( w D) w 3 ( 2 w D) 2
(6)
Relying on the same geometric model, the root mean square roughness, Rrms can be also predicted by the following expression:
Rrms P(1 P) L
(7)
Where L is the length of TiO2 NTs. Table 1 shows the porosity and Rrms values. In both cases, the porosity increases with the increase of anodization potential. The porosity of TiO2 NT with cooling system exhibits better porosity. In both cases, the Rrms value of TiO2 NT with 44V shows higher value compared to other potentials. After 44V, the Rrms value decreases with the increase of anodization potential.
Transmission electron microscope (TEM) was also used to examine morphology of TiO2 NT arrays (at 54 and 74V) without and with cooling systems, as shown in Fig. 8(a)–(h). Figures 8(a)-(b) show TEM and high resolution TEM (HRTEM) images of TiO2 NT array (at 54V) without cooling system. Here, it is cleared that the outer and inner diameter is ~1652 and 1232nm [from inset of Fig. 8(a)], respectively, which is similar to measure from the FESEM images [in Fig. 6(c) and Table 1]. The fringe of the TiO2 NT array (at 54V) without cooling system is 0.324nm corresponding to anatase lattice (101) [49]. In Fig. 8(c)-(d), the pore diameter and wall thickness of TiO2 NT array (at 54V) with cooling system is also similar to measure from FESEM image [in Fig. 7(b) and Table 1]. In that case, the fringe of anatase lattice (004) is d=0.332 nm [46]. Figure 8(e) and (f) show the TEM and HRTEM images of TiO2 NT array (at 74V) without cooling system. The outer, inner, and wall thickness are 2205, 1705, and 222 nm, respectively. The fringe is 0.313 nm corresponding to anatase (101) lattice [49]. Lastly, Fig. 8(g) and (h) exhibit the TEM and HRTEM images of TiO2 NT array (at 74V) with cooling system. The inner, outer and wall thickness is similar to measure in FESEM image [in Fig. 7(d) and Table 1]. The fringe of anatase lattice is 0.324 nm for A(200) [50].
3.5. Mechanism of Photocatalytic degradation of methanol on TiO2 NT arrays In this study, we also discuss the mechanism of methanol decomposition on the surface of TiO2 NT array. Figure 9(a) shows the schematic diagram of mechanism of
photocatalytic methanol gas-phase decomposion on the surface of TiO2 NT array. The electron of the valence band of TiO2 NT array is excited by illuminated light with energy equals to or greater than the band gap of TiO2. The excess energy of this excited electron promoted the electron to the conduction band of TiO2, therefore creating the negative-electron (e-) and positive-hole (h+) pair, while the holes are remained in the valance band. We knew that TiO2 is more effective photocatalyst due to recombine more slowly.[10] The percentage of carrier recombination has a major effect on the photocatalytic efficiency. One of the notable features of titanium oxide is the strong oxidative decomposing power of positive holes,[7] which is greater than the reducing power of electrons excited to the conduction band.[5] The rate of O2 reduction by electron is an important matter to prevent the recombination during photocatalytic process [51, 52]. Firstly, the oxygen reacts with a conduction band electron, ecb, to form a superoxide radical ion O2-, follows the reaction (8):
ecb O2 O2
(8)
Secondly, the oxygen combines with an organic radical generated by a photohole, or an OH radical reaction to produce an organoperoxy radical, HOCH2(OO) , as shown in reactions (9)- (13):
O2 H OOH
(9)
CH 3OH h CH 2 OH H
(10)
H 2 O h OH H
(11)
CH 3OH OH CH 2 OH H 2 O
(12)
or
CH 2 OH O2 HOCH2 (OO)
(13)
It is further suggested that the superoxide diffuses on the surface, or desorbs and diffuses in solution in either case eventually finding and reacting with the organoperoxide radical on the surface to form an organotetroxide by reactions (14) and (15):
HOCH2 (OO) OOH HOCH2 OOOOH
(14)
HOCH2 (OO) O2 HOCH2 OOOO
(15)
The organotetroxide decomposes in a reaction sequence similar to that occurring in Russel reactions, as discussed by Schwitzgebel et al. [51] with the product formation analogous to that shown in equation (16):
HOCH2 OOOOH HCOOH O2 H 2 O
(16)
which is evident from the increase of C=O. Hence, it can be concluded that the methanol decomposition proceeds via the HCOOH rather than other intermediate compound formation.
HCOOH CH 3OH HCOOCH3 H 2 O
(17)
Reaction (12), utilizing a second CH3OH molecule shows the formation of methylformate in the oxidative photocatalysis of CH3OH.[52,
53]
As stated above, one of key steps
proposed is the surface diffusion, or desorption plus solution diffusion, of O2 or OOH to a surface site occupied by HOCH 2 (OO) . It is now proposed that overall reaction efficiency
might be further enhanced if the radical intermediates leading to the formation of the organotetraoxide, as shown in reaction steps (11) and (12), were formed in close proximity on the catalyst surface [54].
3.6. Photocatalytic activity of TiO2 NT arrays The schematic diagram of photocatalytic gas cell (reactor) is given in Fig. 9(b). The details parameter of photocatalytic reactor is discussed in experimental section. The photocatalytic activity of TiO2 NT arrays (at different potentials) without and with cooling systems is evaluated from the decomposition of methanol. Fig. 10(a) shows the Fourier transform infrared (FTIR) spectra of methanol decomposition on the surface of TiO2 NT array (at 74V) without cooling system, as a function of irradiation time. The undissociated methanol owns the bands at 1350, 1055, 1455, 2864, 2952 and 3680cm−1 when the irradiation time is zero. The double bands at 1013 and 1055 cm-1 are detected due to the C–O and C–H stretching contribution regions. The bands of 2864 and 2952cm−1 correspond to the symmetric and asymmetric CH3 vibrations, respectively [56, 57]. The band observed at 3680cm−1 could be assigned to the OH stretching mode of intermediate species probably present at room temperature [58]. On the basis of experimental data, we illustrate the variation of peaks height of different bands in Fig. 10 (b) and (c). From Fig. 10(b), It is cleared that the bands C-OH (1350cm-1), OH (3680cm-1), C-O (1013cm-1), CH (1055 and 2864cm-1) start to decrease in intensity with increase of irradiation time. Some new peaks appear at C-O (1200 and 2075cm1
), C=O (1750cm-1), and CH3 (1455cm-1) band with the increase of irradiation time, as shown
in Fig. 10(c). The band of 1200 and 1750cm-1 can be assigned to the C–O and C=O stretching vibration modes, respectively. It is noteworthy that the absorption bands C-OH (1350cm-1), CO (1013cm-1), CH (1055, 2864, and 3680cm-1), and CH3 (1455cm-1) are almost disappeared after 3 h of irradiation, while the bands at 2864 and 2952cm−1 still exist at a decreasing intensity. The gaseous CO2 is a linear molecule with two infrared active absorption bands at 2340cm−1 (antisymmetric stretching mode) and 669cm−1 (bending mode) [59, 60]. The two
peaks of H2O (1635 and 3460 cm-1) bands increases with increase of irradiation time up to 2h, after that decreases with increase of irradiation time. Hence, we can conclude that when methanol is decomposed then the primary two peaks of CO2 (at 2340cm−1) and H2O (3464cm−1)are formed [52]. From Fig. 2-8, it is cleared that the crystallinity, band gap, pore diameter and deposition rate of TiO2 NT array varies with anodization potential. Figure 11(a) shows the variation of pore diameter, deposition rate and surface area with respect to anodization potential. The pore diameter of TiO2 NT array increases with increase of potential. The surface area decreases with increase of anodization potential, may be due to the less number of NTs remain in same area. The TiO2 NT array at low potential (small pore diameter) contains large number of NTs in an area, compared to TiO2 NT array at high potential (large pore diameter) which has less number of NTs in the same area [in Fig. 11(a)]. So, TiO 2 NT array (at low potential) enhances the surface area as well as improve in electron-hole separation for photocatalysis property. Figures 11(b) and (c) show the CO2 (at 2340cm−1) and H2O (at 3464cm-1) transmittance peak height, respectively for TiO2 NT arrays at different potentials without cooling system. From Figure 11(b) and (c), it is observed that the variation of photocatalytic (CO2 and H2O peak heights) activities of TiO2 NT array depend on the anodization potential as well as on the self heating solution temperature. In Fig. 11(a), TiO2 NT array (at 44V) shows the better photocatalytic activity than the sample at other anodization potentials. The TiO2 NT array (at 64V) shows also good photocatalytic activity due to presence the strong rutile phase, compared to other samples, except at 44V. TiO2 NT (at 44V) arrays may be has high surface area i.e. large number of NTs remain in same area compared to other potentials, as shown in Fig. 11(a), which leads to more contact with methanol gas. So, the oxygen can easily combine with an organic radical generated by a photohole, or an OH radical. Similarly, Figures 11(d) and (e) show the CO2 and H2O peak height, respectively for TiO2 NT array with water based cooling system. Here, the photocatalytic activity only depends on the variation of anodization potential, because of maintaining constant solution temperature (around 25C) by using cooling system. From both figures, it is cleared that the TiO2 NT array (at 54V) shows the highest photocatalytic activity (CO2 and H2O peak heights), compared to other samples. The photocatalytic activity decreases when the potential becomes to lower and higher than 54V. Here also, The TiO2 NT (at 34V) array shows the less photocatalytic activity. The improvement of photocatalytic activity of TiO2 NT (at 54V) array may be due to its high strong rutile phase as indicated by GIXRD analysis, lessen band gap
and has large number of TiO2 NTs in same area compared to other samples [given in Fig. 11(a)]. From Fig. 11(b)-(d), there is no photodecomposition occurs (CO2 and H2O peak) without TiO2 NTs under irradiation. The experimental data of Fig. 11(b) and (d) is found to fit approximately a pseudofirst-order kinetic model [50] by the linear transforms:
f (t ) f 1 e kt
(18)
where, (t) is peak height as the function of time, is peak height at infinite time, and k is rate constant. The values of the rate constant, k and regression coefficient (R) are listed in Table 2. The TiO2 NT (at 44V) sample without cooling system shows the highest rate constant value, k=0.0186 i.e. highest photocatalytic activity, compared to other samples. Using cooling system, TiO2 NT (at 54V) array shows the better photocatalytic activity (k=0.0216) than the samples at other potentials. The TiO2 NT (at 54V) array with cooling system shows the higher photodecomposition rate than the TiO2 NT (at 44V) array without cooling system, which means that the TiO2 NT (at 54V) array with cooling system enhances the photocatalytic activity may be due to strong rutile phase and lessen band gap (3.1eV).
4. Conclusion In summary, TiO2 NT arrays have been successfully fabricated by using a selforganized anodization method at different potentials. Two sets of TiO2 NT arrays were prepared using without and with cooling systems to evaluate the ideal anodization potential. It has been observed that the variation of anodization potential largely modifies the crystallinity as well as surface morphology of TiO2 NT arrays, which affects on the photocatalytic degradation. In both cases, the pore diameter and deposition rate increase with increase of anodization potential. The TiO2 NT array without cooling system shows the larger pore diameter than the TiO2 NT with cooling system. The maximum pore diameter (outer: 224nm, and inner: 172nm) has been exhibited by TiO2 NT (at 74V) without cooling system, may be due to self-heating solution temperature and fast electron transfer. The TiO2 NT arrays (at 44V) sample without cooling system shows the maximum photocatalytic activity and rate constant (k=0.186), compared to TiO2 NT samples at other potentials, may be due to large number of NTs remain in same area. Using cooling system, the TiO2 NT (at 54V) array shows the highest photo-decomposing property and rate constant (k=0.216) than the sample at other potentials, may be owing to it strong rutile phase and also has large number of NTs in same area. Moreover, the TiO2 NT (at 54V) with cooling system shows the higher rate constant than the TiO2 NT (at 44V) without cooling system, due to have strong rutile phase and lessen
band gap (3.1 eV). The variation of photocatalytic activity with various anodization potentials and self-heating temperatures due to increase potential is tried to be explained in terms of surface morphology and crystallinity of the TiO2 NT arrays.
Acknowledgements Authors would like to thank to Mr. T. Kawabata, Materials Science and Engineering, University of Toyama, Japan for measuring of TEM image.
References [1] D. Chen, J. Ye, Adv. Funct. Mater.18(2008) 922. [2] D. F. Ollis, H. Al-Ekabi, Elsevier, Amsterdam, 1993. [3] M. Kalvacova, J.M. Macak, F. S. Stein, C.T. Mierke, P. Schmuki, Phys. Stat. Sol. (RRL), 2(2008) 194. [4] A. Fujishima, K. Honda, Nature,238(1972) 37. [5] X. Li, T. Fan, H. Zhou, S. K. Chow, W. Zhang, D. Zhang, Q. Guo, H. Ogawa, Adv. Funct. Mater.19, (2009)45. [6] H. Choi, A. C. Sofranko, D. D. Dionysiou, Adv. Funct. Mater.16(2006) 1067. [7] G. Wu, J. Wen, S. Nigro, A. Chen, Nanotechnology, 21(2010) 085701. [8] X. Zhang, L. Lei, J. Zhang, Q. Chen, J. Bao, Bo Fang, Separ. Puri. Technol.66(2009) 417. [9] K. H. Park, C. K. Hong, Electrochem. Commun. 10, (2008)1187. [10] W. Zhao, W. Ma, C. Chen, J. C. Zhao, Z. G. Shuai, J. Am. Chem. Soc.126(2004) 4782. [11] H. F. Zhuang , C. J. Lin, Y. K. Lai, L. Sun, J. Li, Environ. Sci. Technol. 41(2007) 47354740. [12] X. Hu, T, Zhang, Z. Jin, J. Zhang, W. Xu, J. Yan, J. Zhang, L. Zhang, Y. Wu, Mater. Letts.62(2008)4579. [13] A. Kar, Y. R. Smith, V. Ravi Subramanian, Environ. Sci. Technol. 43(2009)3260. [14] M. F. Hossain, S. Biswas, T. Takahashi, Y. Kubota, A. Fujishima, Thin Solid Films, , 517 (2008) 1091. [15] I. M. Arabatzis, S. Antonaraki, T. Stergiopoulos, A. Hiskia, E. Papaconstantinou, M. C. Bernard, P. Falaras, J. Photochem. Photobiol. A 149 (2002) 237. [16] M. F. Hossain, S. Biswas, T. Takahashi, Y. Kubota, A. Fujishima, Phys. Stat. Sol. (a), 205(2008) 2018. [17] H. C. Liang, X. Z. Li, Y. H. Yang, K. H. Sze, Chemosphere, 73 (2008) 805.
[18] T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater.11(1999) 1307. [19] H. Yu, J. Yu, B. Cheng, J. Lin, J. Hazard. Mater.147(2007) 581. [20] G. K. Mor, O. K. Varghese, M. Paulose, C. A.Grimes, Adv. Funct. Mater. 15(2005) 1291 [21] S. G. Yang, Y. Z. Liu, C. Sun, Appl. Catal. A, 301, (2006) 284. [22] X. Wu, Q. Z. Jiang, Z. F. Ma, M. Fu, W. F. Shangguan, Solid State Sci. 136(2005)513. [23] Z.R.R. Tian, J.A. Voigt, J. Liu, B. McKenzie, H.F. Xu, J. Am. Chem. Soc.125 (2003) 12384. [24] H.H. Ou, S.L. Lo, Sep. Purif. Methods.58(2007)179. [25] S. I. Na, S. S. Kim, W. K. Hong, J. W. Park, J. Jo, Y.C. Nah, T. Lee, D. Y. Kim, Electrochim. Acta. 53(2007) 2560. [26] V. Zwilling, E. D. Ceretti, A. B. Forveille, D. David, M.Y. Perrin, M. Aucouturier, Surf. Interface. Anal.27(1999)629. [27] D. Gong, C. A. Grimes, O. K. Varghese, Z. Chen, E. C. Dickey. J. Mater. Res.16(2001)3331. [28] R. Beranek, H. Hildebrand, P. Schmuki, Electrochem Solid-State Lett. 6(2003) B12. [29] J. M. Macak, K. Sirotna, P. Schmuki, Electrochim Acta, 50(2005)3679. [30] J. M. Macak, H. Tsuchiya, P. Schmuki, Angew Chem.44(2005)2100. [31] L. V. Taveira, J. M. Macak, H Tsuchiya, L. F. P. Dick, P. Schmuki, J. Electrochem. Soc. 152(2005)B405. [32] A. Ghicov, H. Tsuchiya, J. M. Macak, P. Schmuki, Electrochem. Commun.7(2005) 505. [33] Q. Cai, M. Paulose, O.K. Varghese, C.A. Grimes, J. Mater. Res. 20(2005) 230. [34] J. M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer and P. Schmuki, Curr. Opin. Solid State Mater. Sci.11(2007) 3. [35] M. Paulose, K. Shankar, S. Yoriya, H. E. Prakasam, O. K. Varghese, G. K. Mor, J. Phys. Chem. B, 110(2006) 16179. [36] K. Shankar, G. K. Mor, A. Fitzgerald, C.A.Grimes, J. Phys. Chem. C, 111(2007) 21. [37] S. Yoriya, H. E. Prakasam, O. K. Varghese, K. Shankar, M. Paulose, G. K. Mor, T. J. Latempa, C. A. Grimes, Sens. Lett.4(2006)334. _[38] J. M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angew. Chem. Int. Ed.44(2005)7463. [39] H. E. Prakasam, K. Shankar, M. Paulose, O. K. Varghese, C. A. Grimes,J. Phys. Chem. C,111(2007) 7235. [40] A. Elsanousi,J. Zhang, H. M. H. Fadlalla, F. Zhang, H. Wang, X. Ding, Z. Huang, C. Tang, J. Mater. Sci.43(2008) 7219.
[41] D. J. Yang, H. G. Kim, S. J. Cho, W. Y. Choi, Mater. Letts.62(2008) 775. [42] R. Beranek, H. Hildebrand, P. Schmuki, Electrochem. Solid State Lett.6(2003)B12. [43] G. K. Mor, O. K. Varghese, M. Paulose, K. Shankar, C. A. Grimes, Sol. Energy Mater. Sol. Cells, 90(2006)2011. [44] B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, USA, 1978. [45] J. I. Pankove, Optical Processes in Semiconductors, Dover Publications Inc., New York, 1970. [46] O. K. Varghese, M. Paulose, T. J. LaTempa, C. A. Grimes, Nano Letts.9(2009) 731. [47] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [48] A.G. Kontos, A. Katsanaki, T. Maggos, V. Likodimos, A. Ghicov, D. Kim, J. Kunze, C. Vasilakos, P. Schmuki, P. Falaras, Chem. Phys. Letts. 490 (2010) 58. [49] L. Miao, P. Jin, K. Kaneko, A. Terai, N. Nabatova-Gabain, S. Tanemura, Appl. Surf. Sci.212–213 (2003) 255. [50] Y. Gao, S.A. Elder, Mater. Letts.44 (2000) 228 [51] C. M. Wang, A. Heller, H. Gerisher, J. Am. Chem. Soc.114(1992) 5230. [52] Z. H. Zhang, M. F. Hossain, T. Arakawa, T. Takahashi, J. Hazard. Mater.176(2010) 973. [53] J. Schwitzgebel, J. G. Ekerdt, H. Gerischer, A.Heller, J. Phys. Chem.99(1995) 5633. [54] M. Sadeghi, W. Liu, T.G. Zhang, P. Stavropoulos and B. Levy, J. Phys. Chem.100(1996) 19466. [55] N. N. Lichtin, M. Avudaithai, E. Berman, J. Dong, Res. Chem. Intermed.20(1994)755. [56] C. Binet, M. Daturi, Catal. Today, 70(2001) 155. [57] K. Prabakar, T. Takahashi, T. Nakashima, Y. Kubota, A. Fujishima, J. Vac. Sci. Technol. A,24(2006) 1613. [58] K. Mudalige, S. Warren, M. Trenary, J. Phys. Chem. B, 104(2000) 2448. [59] F. Ouyang, S. Yao, K. Tabata, E. Suzuki, Appl. Surf. Sci.158(2000) 28. [60] A. L. Goodman, L. M. Campus, K. T. Schroeder, Energy Fuels, 19(2005) 471.
Figure Captions
Figure 1. Schematic illustration of computer controlled electrochemical anodic oxidation experimental set-up for TiO2 NT arrays; TC means thermocouple, Ti is titanium sheet and Pt is platinum foils.
Figure 2. Characteristics of current transient for Ti anodization (a) without (w/o) and (b) with cooling system in NH4F and ethylene glycol based electrolyte. (c) Self-heating solution temperature and (d) deposition rate of TiO2 NT array without and with water based cooling systems.
Figure 3. The GIXRD pattern of TiO2 NT arrays at different potentials (34 to 74V): (a) without and (b) with water based cooling systems.
Figure 4. Absorption spectra of TiO2 NTs at different potentials: (a) without and (b) with water based cooling systems. Figure 5. The (αhυ)2 versus energy of TiO2 NTs at different potentials: (a) without and (b) with water cooling systems.
Figure 6. FE-SEM images of TiO2 NT array at different potentials of (a) 34, (b) 44, (c) 54, (d) 64 and (e) 74V, without using cooling system.
Figure 7. FE-SEM images of TiO2 NT arrays at different potentials of (a) 44, (b) 54, (c) 64, and (d) 74V with water based cooling system.
Figure 8. Surface morphology of TiO2 NT array: (a) TEM and (b) HRTEM images at 54V without cooling system; (c) TEM and (d) HRTEM images at 54V with cooling system; (e) TEM and (f) HRTEM images at 74V without cooling system; and (g) TEM and (h) HRTEM images at 74V with cooling system.
Figure 9. (a) The schematic diagram of methanol gas-phase decomposition mechanism on surface of TiO2 NT array; (b) A Schematic diagram of photocatalytic reactor for photocatalysis process. Figure 10. (a) The FTIR spectra for the decomposition of methanol on the surface of TiO2 NT array at 74V without using cooling system as the function of irradiation time. (b) Variation of
the peaks height of CO2 (669 and 2340 cm-1), H2O (1635 and 3460 cm-1), C-OH (1350 cm-1) and OH (3680 cm-1) bands with the time. (c) Variation of the peaks height of C-O (1013, 1200, and 2075 cm-1), C=O (1750 cm-1), CH (1055, 2864, and 3680 cm-1), and CH3 (1455 cm1
) bands with the time.
Figure 11.(a) Schematic diagram of TiO2 NT array with variation of anodization potential; Variation of (b) the CO2 peak (2340cm-1)and (c) H2O peak (3464cm-1), height of TiO2 NT array at different potentials without cooling system; Variation of (d) the CO2 peak(2340cm1
)and (e) H2O peak (3464cm-1),height of TiO2 NT array at different potentials with cooling
system.
Figure 1. Hossain et al.
(+)
Temp. controller
(-)
PV SV
Cold water (~10°C) Heater
TC
Water out Ti
Pt
50
(a) 34 V 44 V 54 V 64 V 74 V
40 30 20
Anodization current (mA/cm2)
60 W/o cooling
10 0
0
5
10 15 20 25 Anodization time (min)
30
60 50
(b)
Cooling 34 V 44 V 54 V 64 V 74 V
40 30 20 10 0
0
5
10 15 20 25 Anodization time (min)
0.6
(c)
Deposition rate (µm/min)
Anodization current (mA/cm2)
Figure 2. Hossain et al.
0.5
(d)
0.4
W/o Cooling Cooling
0.3 0.2 0.1 0.0
34
44 54 64 74 Anodization potential (V)
30
Figure 3. Hossain et al.
(a)
A(204) A(116) A(220) A(215)
A(200) A(105) A(211)
R(101) A(004) R(111)
74 V
A(101)
Intensity (a.u.)
W/o cooling
64 V 54 V 44 V 34 V 10
20
30 40 50 60 Diffraction angle, 2θ (deg.)
80
A(204) A(116) A(220) A(215)
A(200) A(105) A(211)
Cooling R(101) A(004) R(111)
Intensity (a.u.)
74 V
A(101)
(b)
70
64 V 54 V 44 V 34 V 10
20
30 40 50 60 Diffraction angle, 2θ (deg.)
70
80
Figure 4. Hossain et al.
(a)
0.7 34 V 44 V 54 V 64 V 74 V
Absorbance (a.u.)
0.6 0.5 0.4 0.3 0.2 0.1 0.0 300
(b)
W/o cooling 400
500 600 700 Wavelength, λ (nm)
900
0.7 34 V 44 V 54 V 64 V 74 V
0.6
Absorbance (a.u.)
800
0.5 0.4 0.3 0.2 0.1 0.0 300
Cooling 400
500 600 700 Wavelength, λ (nm)
800
900
Figure 5. Hossain et al.
(a)
25 34 V 44 V 54 V 64 V 74 V
(αhν)2x1012 (eV2m-2)
20 15 10 5 0 2.6
(αhν)2x1012 (eV2m-2)
(b)
W/o Cooling 2.8
3.0 3.2 3.4 Photon energy (eV)
3.6
3.8
25 20
34 V 44 V 54 V 64 V 74 V
15 10 5 0 2.6
Cooling 2.8
3.0 3.2 3.4 Photon energy (eV)
3.6
3.8
Figure 6. Hossain et al.
Figure 7. Hossain et al. (a)
(b)
100 nm
100 nm
(c)
(d)
100 nm
100 nm
Figure 8. Hossain et al.
(b)
(a) 100nm
2 nm
50nm
(c)
d=0.324 10 nm
(d)
500nm
2 nm d=0.332 100nm
10 nm
(f)
(e) 100nm
2 nm d=0.313 100nm
10 nm
(h)
(g)
2 nm d=0.324 50 nm
10 nm
Figure 9. Hossain et al.
Detachment of Oxygen
(a) Uv-visible light
Mn+ Reduction
eCB
Activated •VB Oxygen (O2 ) h+
M CH3OH Oxidation
• Hydroxy radical (•OH)
H2O
(b)
H2O
CO2
Artificial Solar simulator
Gas cell KBr FT-IR Light
CH3OH gas
TiO2 NT array
Detector
CO2, H2O, C-OH, OH peak hieght (a.u.) C-O, C=O, CH, CH3 peak hieght (a.u.)
(c)
OH
1.5 h 2.0 h 2.5 h 3.0 h 3.5 h 4.0 h 500
(b)
H2O
CO2
CH CH
0h 0.5 h 1.0 h
Transmittance (a.u.)
CO2
(a)
C-O CH C-O C-OH CH3 H2O C=O
Figure 10. Hossain et al.
1000 1500 2000 2500 3000 3500 4000 Wavenumber (cm-1)
70 60
CO2
50
CO2
40 H2O
30
H2O
20
OH
10
C-OH 0 0.0
0.5
1.0
1.5 2.0 2.5 3.0 Irradiation time (hour)
70
3.5
4.0
C-O CH C-O CH3 C=O C-O CH
60 50 40 30 20 10 0 0.0
0.5
1.0
1.5 2.0 2.5 3.0 Irradiation time (hour)
3.5
4.0
Figure 11. Hossain et al.
(a)
Pore diameter and deposition rate increase Surface area increases
Surface area increases
30 20 10
W/o TiO2 NTs 34 V 44 V 54 V 64 V 74 V
0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour)
CO2 peak hieght at 2340 cm-1(a.u.)
70 W/o cooling 60 50 (b) W/o TiO2 NTs 40 34 V 30 34 V 34 V 20 34 V 34 V 10 0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour) 50 W/o Cooling 40 (c)
H2O peak hieght at 3460 cm-1(a.u.)
H2O peak hieght at 3460 cm-1(a.u.)
CO2 peak hieght at 2340 cm-1(a.u.)
Anodization potential increases
70 Cooling 60 50 (d) W/o TiO2 NTs 40 34 V 30 44 V 54 V 20 64 V 74 V 10 0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour) 50 Cooling 40 (e) 30 20 10
W/o TiO2 NTs 34 V 44 V 54 V 64 V 74 V
0 -5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Irradiation time (hour)
Table 1. Pore diameter, porosity and rms roughness and band-gap of TiO2 NT at different potentials without and with water based cooling systems. Potential (V)
Without cooling system Outer
Inner
(nm)
(nm)
Porosity
With cooling system
rms
Band
Outer
Inner
Roughness
gap
(nm)
(nm)
(nm)
(eV)
Porosity
rms
Band
Roughness
gap
(nm)
(eV)
34
92
52
0.38
1311.9
3.20
92
52
0.38
1311.9
3.20
44
128
88
0.52
1348.8
3.18
132
90
0.51
1349.5
3.15
54
167
125
0.60
1322.2
3.12
160
117
0.58
1333.7
3.1
64
198
151
0.62
1310.5
3.01
188
144
0.62
1307.3
3.09
74
224
172
0.63
1305.5
3.07eV
196
152
0.64
1297.9
3.06
Table 2.The values of rate constant kand regression coefficients (R) of methanol decomposition on TiO2 NT arrays at different potentials without and with water based cooling systems. Anodization
Without cooling system
With cooling system
potential
k(min-1)
R
k(min-1)
R
34
0.0155
0.9919
0.0155
0.9919
44
0.0186
0.9924
0.0166
0.9925
54
0.0150
0.9862
0.0216
0.9896
64
0.0147
0.996
0.0128
0.9934
74
0.0134
0.9840
0.00109
0.9904