Influence of p-n junction mechanism and alumina overlayer on the photocatalytic performance of TiO₂ nanotubes

Abstract

Modified hybrid structures of TiO2 nanotubes (TONT), p-Al doped TONT/n-TONT with an additional overlayer of alumina, are constructed to achieve 99.57% photodegradation of the stable organic pollutant methylene blue (MB) within 180 min, a degradation rate 17 times higher than pure TONTs. The anodization at three different temperatures 2, 28 and 40 °C followed by impregnation of Al is used for their preparation. The analyses of structure, chemical composition and morphology are completed using x-ray diffraction, x-ray photoelectron spectroscopy (XPS) and high resolution transmission microscopy, respectively, Rutherford back scattering and field emission scanning electron microscopy confirm the formation of the hybrid structure. This structure exhibits the highest photodegradation rate with TONT based catalysts to date for MB blue, by enhancing the electron—hole separation, the absorption of visible photons and the adsorption sites for the pollutant. The optical data coupled with valence band XPS is used for elucidating the energy band structure of the p-n junctions and to gain insight into the effect of the junction mechanism on photoactivity. The rectification ratios of the impregnated p-n junctions, determined by current—voltage measurements, are found to vary from 102 to 106.

Fabrication and Characterization of Type‐II Heterostructure n:In2O3/p:in‐TiO2 for Enhanced Photocatalytic Activity

“…Recently, we reported use of p-Al-doped TONT/n-TONT with an additional layer of alumina to obtain 99.57% degradation of the stable organic pollutant MB within 180 min. [42] In the present case, Figure 11. Photocatalytic mechanism of ITONTs.…”

“…Recently, we reported use of p‐Al‐doped TONT/n‐TONT with an additional layer of alumina to obtain 99.57% degradation of the stable organic pollutant MB within 180 min. [ 42 ] In the present case, the degradation efficiency of 96% in 180 min for the same pollutant is obtained with indium‐doped TiO 2 prepared by a more cost‐effective and time‐saving method. In the case of Al‐doped heterostructure, the catalyst preparation was done by anodization followed by impregnation and the average total time for the process was ≈6.5 h, [ 42 ] whereas for the fabrication of the ITONT in the present work ≈2.5 h is required.…”

“…the degradation efficiency of 96% in 180 min for the same pollutant is obtained with indium-doped TiO 2 prepared by a more cost-effective and time-saving method. In the case of Al-doped heterostructure, the catalyst preparation was done by anodization followed by impregnation and the average total time for the process was %6.5 h, [42] whereas for the fabrication of the ITONT in the present work %2.5 h is required. The ITONTs prepared here by the reproducible, simple, cost-effective, and time-saving twostage anodization preparation method show degradation efficiency in par with any other efficient semiconductor coupling combinations with TiO 2 reported till date.…”

Enhanced photocatalytic activity of nZnO/n+Al:ZnO homojunction with an overlayer of Al2O3 nanoballs

“…Among the three peaks the low XPS spectra of Al 2p and its peak position at 74.4 eV are shown in figure 6(f). This value matches with the binding energy of Al in Al 2 O 3 [57,58] and, hence is a signature of the presence of the binary phase on the surface of AZnO films. The large percentage of composition of Al on the surface of doped samples with respect to that of Zn is due to the contributions from the binary as well as by the substituted Al ions in the lattice.…”

“…These OH * radicals with high oxidation tendency combine with organic compounds which result in the formation of CO Srivatsav et al [66] reported that the formation of homojunctions between semiconductors is one of the strategies to enhance the separation of carriers. Nair et al [57] fabricated Al 2 O 3 /p-Al:TONT/n-TONT hybrid structure and obtained enhanced photocatalytic activity for the prepared structure than the undoped TiO 2 . In the junction, the formation of alumina insulating over-layer (Al 2 O 3 ) was one of the major reasons for the better degradation, as it acted as an adsorption site for the MB solution.…”