TiO2 Nanotube Arrays: Synthesis, Properties, and Applications

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Patrik Schmuki: Self-organized TiO nanotube arrays: Formation, properties, applications

Stock photo. Search Results Results 1 -5 of 5. Grimes; Gopal K. Springer, Grimes, Gopal K. In the potential range from 0. This observation is interesting when considering that the NT layers have an inherent superhydrophilic character allowing for easy electrolyte percolation [58]. However, the electrolyte percolation mechanism of the TNTs is complicated, and depends on the synthesis conditions, posttreatments and exposure to UV—vis light [59]. Hence, the enhanced current density could be related to an increased area of the TNTs, which depends directly on the increase of t a.

A constant polarization at 0.

TiO2 Nanotube Arrays: Synthesis, Properties, and Applications

Other authors have stated several observations regarding the geometrical aspects of TNTs, where the nanotube length plays an important role in the overall performance of their specific systems [27,60,61]. For this purpose, the electrodes were illuminated in a cell with quartz window while performing LSV using the same pretreatment of 10 s polarization. The photoinduced processes are dynamic phenomena that are affected by several factors such as the electrochemical parameters and the chemical environment.

For example, high scan rates might lead to mass-transport limitations due to diffusion of the charge carriers within the bulk of the semiconductor. Subsequently, the electron percolation occurs in the TNTs, and crystalline defects and trap states e. Thus, low scan rates are preferred in these experiments for more accurate measurements of the j ph. However, chemical species, such as O 2 , in the vicinity of the electrode are prone to scavenge the photogenerated electrons.

The maximum obtained value of j ph decreased by ca. To evaluate the oxidative contribution of the photogenerated holes, methanol MeOH was added to the electrolyte.

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This refers to a phenomenon known as current-doubling in which a secondary electron is obtained from the oxidation of an organic molecule via surface holes when a semiconductor is excited with suitable photonic irradiation. The maximum of j ph is obtained after reaching a certain potential, overcoming the recombination events, and characterized by a plateau. This potential was achieved near 0. At even lower potentials it is possible to observe the onset potential, E onset. The importance of E onset in these experiments relies on the fact that it practically merges at the flat-band potential, E fb , and allows us to roughly estimate the position of the conduction band in the electrochemical potential scale.

From the LSV the E onset of all samples seems to start as soon as the surface is irradiated. To discard an instrumental artifact, photocurrent transients were performed to discriminate the appearance of the E onset.

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This observation has been associated with electron mobility and charge separation kinetics [63]. As the electrode is illuminated, some current spikes appear along with j ph , which are related to electron—hole recombination in surface states when the polarization potential of the electrode is not strong enough to prevent these events.

At higher applied electrode potentials around 0. When the irradiation is interrupted, a fast decay of the photocurrent was observed due to recombination. If the potential is high enough to compete with the charge-carrier quenching, usually in the region where the current density achieves the plateau, the decay is prolonged and the influence of the nanotube length is distinguished. A longer decay was observed for longer nanotubes, which is also consistent with the idea that the electron conduction in these systems is governed by a diffusional gradient and, thus, the electron transport can be appreciated [64].

Figure 6: Extracted photo-current density, j ph , from potentiodynamic curves for the TNTs grown at different t a in a N 2 -saturated electrolyte, b O 2 -saturated electrolyte, with c 0. Conditions: 0. Figure 6: Extracted photo-current density, j ph , from potentiodynamic curves for the TNTs grown at different t a Similarly, with longer nanotubes the spikes not only disappear, the time to reach the maximum j on also increases. While such behavior is still related to the length of the nanostructures, it could also be associated with an increased number of electron traps since the potential is high enough to overcome recombination [65].

The differences observed in the interfacial PEC behavior suggest that a link between the nanotube length and the chemical composition is established in agreement with the results obtained by XPS and XRD. While the previous results were obtained from potentiodynamic experiments LSV , a comparative analysis can be done by establishing a constant polarization to understand the sweep-rate dependence.

Tio2 Nanotube Arrays: Synthesis, Properties, and Applications

Moreover, a bathochromic shift occurs when t a is varied. This effect has also been observed for semiconductors where the crystalline structure has been changed by chemical doping. Other authors have reported an improved crystalline stability, attributed to the F dopant atoms inducing a higher UV absorption, while N dopant atoms were associated with the promotion of the red-shift [39,47]. Nevertheless, some physical phenomena such as light scattering from the top part of the TNTs and oxygen vacancies could also create distortions in the IPCE spectra.

Then, the intersection between the extrapolated slope of the Tauc curve with the energy axis corresponds to E g for the sample. The E g values obtained for TNTs are lower than those associated with anatase 3. As it was stated in previous sections, a relation between the capacitive processes seems to be associated with the length of the TNTs.

Consequently, electrochemical impedance spectroscopy was performed to access the capacitance of the space-charge region and then plot the Mott—Schottky M—S curves. From the CV curves, an increase in the capacitive current was observed towards lower potential values, which is typical for a transition to the accumulation region of n-type semiconductors.

In the literature, a wide range of values for E fb have been reported and it has been found that this parameter can be affected by several properties of the material crystallinity, doping level or synthesis conditions as well as the chemical environment ionic force, cation type or pH value. Numerous publications discuss the deviations from the linear behavior of the slope in the M—S curves.

It has been proposed by Dean and Stimming [71] that this behavior arises from a distribution of energy states and the same authors reported an equation to account for such a dispersion. In an attempt to obtain a frequency-independent capacitance, some authors [] have proposed the use of high frequencies, typically 10 kHz, to deal with this phenomenon. However, this criterion fails when porous substrates are evaluated since the ac perturbation cannot reach the interior of the pores [76].

The distribution of these states will not be uniform along the length and the wall thickness of the TNTs due to etching during the growth process. Considering the amorphous-semiconductor theory [78] in the interpretation of the M—S plots of Di Quarto et al. Thence, care should be taken when selecting the frequency for the M—S analysis. The criteria established by Gelderman et al.

TiO2 Nanotube Arrays: Synthesis, Properties, and Applications

For this reason, we estimated the E fb in two ways 1 through extrapolation of the slope where N D was calculated E fb1 , and 2 from the region of 0. The latter represents an estimation of the E fb value at the bottom of the tubes and agrees with those expected for short tubes, while the E fb1 agrees with those values expected for long tubes 0. The values of E fb2 are in agreement with the E onset from LSV , supporting the idea that electron—hole pairs occur initially at the bottom of the tubes via an electron diffusion mechanism. The electrode potential range was fro The data demonstrate good agreement and corroborate the relations between the optical and electrical parameters and t a.

Both valence and conduction band experienced a shift toward higher energies as a consequence of the inherent doping during the anodization process. A similar behavior has been observed when commercial [31] , and prepared samples [32] where modified during their synthesis in a fluoride-rich environment leading to a red-shift in the absorption edge. The energy levels of conduction and valence band show a similar trend, moving towards more positive potentials in the electrochemical scale.

Since the conduction band was assumed to merge with E fb , the calculations for the valence band value were carried out only in terms of the energy gap of the material. The latter observation might suggest that the photogenerated holes for such samples might have higher oxidation power for photocatalytic applications. Ions are found in the waste waters from the textile industry resulting from the dyeing and bleaching processes, thus allowing the use of an electrooxidation process for the degradation of such components.

Chlorides are often present in these waste waters [] , and for that reason a dissolution of 0. The electrolyte was not purged with N 2 or O 2 prior to or during the experiment. The MB dissolution was recirculated using a peristaltic pump to a standard quartz cuvette where the absorbance at nm was measured in situ.

In a typical experiment, a fixed ratio of After this, a marked change in reaction rate is observed indicating a change in the reaction kinetics. The first stage is associated with an adsorption process that takes place independently of the potential bias or light irradiation [35,85]. It is important to underline that the understanding of this process is outside the scope of this study. After the initial phase, by the faster decay is associated with the heterogeneous photocatalytic processes explained by a Langmuir—Hinshelwood model [86].

The oxidation mechanism for organic compounds with irradiated semiconductors has been proposed to proceed via photogenerated holes at the surface of the electrode [87,88]. These peaks become absent in the presence of light, supporting the idea of the oxidation of MB by holes. In the presence of MB, j ph decreases due to its light absorption and a lower I 0 reaches the surface of the electrode.

In the same figure, a dashed line shows the wavelength at which the in situ measurement was performed. Further information on the degradation of MB was obtained with Raman spectroscopy.

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The spectra of the electrolyte and Ti plate were considered as well, to cancel out their corresponding bands that are still visible in the solution spectra. The vibrational bands observed in the recorded spectra agree with those values reported in the literature. From the results we understand that, although a trend is observed for the MB removal with increasing t a , it does not correspond in the same proportion to what it is obtained through the PEC experiments.

In part, this has to be associated to the fact that the PCA was assessed in a continuous-flow regime, while the PEC experiments were not. This would suggest that once the MB has saturated the surface of the electrode nearly 20 min after the experiment started , the real kinetics of the reaction were observed. At this point, the oxidation of MB on the TNTs does not correspond directly to their length, but more to the amount of surface hydroxy groups induced by the fluorination and N dopant atoms observed by XPS, which is related to the anchoring process.

Thus, while the total exposed area inner or outer walls of the tubes remains important for the MB absorption step, the hydrophilicity of the surface is what it leads to the difference in the aforementioned trends, demonstrating the determining the influence of t a on the morphological, surface chemical and opto-electronic properties of the TNTs. The impact of the anodization time, t a , on TNTs was studied. The TNTs were evaluated via different physico-chemical and electrochemical techniques and their photocatalytic activity in the degradation of MB was assessed.

The only parameter that differed during the synthesis of the materials was the anodization time t a. Longer TNTs were obtained for longer t a. This was explained in terms of the exposition time during the anodization process since higher F contents were found for longer t a. The crystalline structure presented a preferential growth in the [] direction. Jump to main content. Jump to site search. Journals Books Databases. Search Advanced. Current Journals.

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Impact of the anodization time on the photocatalytic activity of TiO2 nanotubes

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