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Synergic effect of oxygen vacancy defect and shape on the photocatalytic performance of nanostructured TiO2 coating
Journal Pre-proofs Synergic effect of oxygen vacancy defect and shape on the photocatalytic performance of nanostructured TiO2 coating Yan Cao, Liang Huang, Yu Bai, Kittisak Jermsittiparsert, Reza Hosseinzadeh, Hossein Rasoulnezhad, Ghader Hosseinzadeh PII: DOI: Reference:
S0277-5387(19)30659-X https://doi.org/10.1016/j.poly.2019.114214 POLY 114214
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
20 August 2019 5 October 2019 25 October 2019
Please cite this article as: Y. Cao, L. Huang, Y. Bai, K. Jermsittiparsert, R. Hosseinzadeh, H. Rasoulnezhad, G. Hosseinzadeh, Synergic effect of oxygen vacancy defect and shape on the photocatalytic performance of nanostructured TiO2 coating, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.114214
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.
© 2019 Elsevier Ltd. All rights reserved.
Synergic effect of oxygen vacancy defect and shape on the photocatalytic performance of nanostructured TiO2 coating
Yan Caoa, Liang Huanga, Yu Baia, Kittisak Jermsittiparsertb,c**, Reza Hosseinzadehd, Hossein Rasoulnezhade, Ghader Hosseinzadehf*
a. School of Mechatronic Engineering, Xi’an Technological University, Xi’an, 710021 China b. Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Vietnam c. Faculty of Social Sciences and Humanities, Ton Duc Thang University, Ho Chi Minh City, Vietnam d. Medical Laser Research Group, Medical Laser Research Center, ACECR, Tehran, Iran e. Department of Electrical & Electronics Engineering, Standard Research Institute (SRI), Karaj, Iran f. Department of Chemical Engineering, University of Bonab, Bonab, Iran * Corresponding author: Ghader Hosseinzadeh; E-mail: [email protected] ** Co-Corresponding author: Kittisak Jermsittiparsert; E-mail: [email protected]
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Abstract In the present work TiO2 coatings with different level of oxygen vacancy defect and different shapes of nanosphere, nanocube, and nanopyramid were deposited on glass substrate by modified ultrasonic spray pyrolysis method. The prepared coatings were successfully used for the photocatalytic degradation of paraoxon pesticide under visible light irradiation. Based on the obtained results, among the papered coatings, the TiO2 coating with nanopyramidal shape and oxygen vacancy defect has the highest photocatalytic activity, and the lowest photocatalytic activity was observed in the case of the TiO2 coating with nanospherical shape and without oxygen vacancy defect. The reason for these results is the sharp edges and corners of nanopyramid which decreases the electron-hole recombination rate. Furthermore, presence of oxygen vacancy defect improve the visible light photocatalytic activity via narrowing the band gap energy and separation of the electron-hole.
Key words: oxygen vacancy defect; TiO2; photocatalyst; shape; nanostructure
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1. Introduction Water demands are increasing worldwide because of the increasing of the population of the earth. However, the available water resources are limited, and following the rapid industrial growth the water resources are continually contaminated with organic pollutants such as pesticides. In recent decades, various methods such as photocatalysis [1, 2], microbial degradation [3, 4], hydrolytic degradation [5], chemical decontamination [6], and elimination of pollutants using adsorbents [7-9] have developed for decontamination of water from pollutants. Among these methods, photocatalytic degradation of pollutants using semiconductors has received much attention because of its potential advantages such as low-cost, sustainable treatment technology, and environmentally friendly nature [10]. TiO2 is one of the more studied semiconductor for photocatalytic purification and remediation of contaminated water [11, 12]. However, because of fast recombination of photo-generated electron-hole charge carriers on TiO2 and due to its wide band gap (about 3.2 eV for anatase TiO2 [13]), TiO2 has low photoactivity under visible light radiation. According to the literature, presence of the oxygen vacancy defect in the structure of semiconductors could improve their photoactivity [14-17]. The main reasons for the photoactivity improvement are (i) trapping of electron, which in turn, promote the separation of electron-hole [18], (ii) improving the production of superoxide radicals and hydroxyl radicals [19, 20], (iii) narrowing the band gap of semiconductor by creation of a new energy level [14, 17]. Recent studies have shown that the shape of nanomaterial has a significant effect on its photocatalytic activity [21, 22]. Based on the literature, the photocatalytic activity of TiO2, especially in nanometer length scale, is strongly dependent on the surface atomic structure (surface atomic arrangement and coordination) which differ by changing the crystal facets [23, 24]. Wu et al. [25] investigated the photocatalytic activity of TiO2 nanomaterials with different shapes and they found that anatase TiO2 nanobelt has higher photocatalytic activity than TiO2 nanosphere, and they relate this to greater charge mobility in the nanobelts and enhanced charge separation on the (101) facet. In a similar work Camposeco et al. [26] compared the photocatalytic activity of TiO2 nanomaterials with various shapes i.e. nanotubes, nanofibers, nanowires and nanoparticles, and showed that TiO2 nanotubes have the highest photocatalytic activity.
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As mentioned in the previous paragraphs, shape of nanostructures and presence of oxygen vacancy defect have considerable effect on the photocatalytic activity of photoactive semiconductors. In the current study, we want to investigate the synergic effects of shape of nanostructures and presence of oxygen vacancy defect on photocatalytic performance of TiO2 coatings. In our previous works we reported the preparation of transparent and nanostructured TiO2 coatings doped with various metal and nonmetal elements on glass substrate by using ultrasonic spray pyrolysis technique [27-30]. In this work, it is proposed to prepare the nanostructured TiO2 coatings on glass substrate with different shapes and different level of oxygen vacancy defect with some modifications in our technique. In this regard, before the deposition process, whole of the reaction chamber was evacuated and then the depositions was started under N2 or O2 or a mixture of these gases. 2. Material and methods 2.1 Materials In this study, the following chemicals were used: Titanium (IV) n-butoxide (≥97.0%) from Sigma-Aldrich, absolute ethanol (99.5% v/v) from Merck, concentrated hydrochloric acid (37 wt %) from Merck and acetone from Merck. 2.2 Preparation of TiO2 coatings A schematic diagram of the deposition set-up is shown in Fig. 1. TiO2 sol solution was prepared by addition of 20 ml the HCl ethanolic solution (0.15M) into the 30 ml of ethanolic solution of Titanium (IV) n-butoxide (0.5M) in dropwise manner to reach the pH of 2.1. After about 2 hour stirring, the obtained solution was poured into the flask 1 and was sprayed with aid of a floated piezoelectric ultrasonic transducer. For better spraying, the ultrasonic transducer was floated on the surface of the solution and the solution thickness on transducer was about 5 mm. In order to the preparation of the TiO2 coating with oxygen vacancy defect, whole of the deposition setup was evacuated by using rotary vacuum pump (RVP) and the sprayed solution was carried by N2 or O2 or a mixture of these gases with overall flow rate of 200 ml/min. The large particles were sedimentated in flask 2 to improve the monodispersity of the final coatings. The migrated droplets were thermally decomposed at the surface of the hot glass substrate at 400°C in flask 3 for 30 min. The samples
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prepared under the atmospheres of N2 or O2 or a mixture of these gases were labeled as N2-TiO2, O2-TiO2, and N2-O2-TiO2, respectively.
Fig. 1. Schematic illustration of the ultrasonic-assisted spray pyrolysis technique.
2.3. Characterization XRD experiments of all the products were conducted on a Philips PW-3710 (Netherlands) X-ray Diffractometer with Cu-Kα irradiation (λ=1.54018 Å). Surface morphology, shape and size all the prepared samples were investigated by VEGA3 TESCAN scanning electron microscopy (SEM) (Czech Republic) operated at 15 kV. The UV–visible absorption spectra of the prepared coatings were acquired using Cary 100 Bio spectrophotometer (Varian, Inc., USA). XPS spectra of the samples were obtained by using a GammadataScienta Esca 200 (Uppsala, Sweden) hemispherical analyzer equipped with an Al Kα (1486.6 eV) X-ray source. All binding energy values were calibrated by using the value of the C 1s peak at 284.6 eV as a reference. Photoluminescence (PL) emission spectra of the prepared coatings were recorded using Cary Eclipse fluorescence spectrophotometer (Varian, Inc., USA) at room temperature, with an excitation wavelength of 320 nm. 2.4. Evaluation of the visible light photocatalytic activity of the coatings Paraoxon pesticide was used as a contaminant for investigation the photocatalytic performance of the prepared coatings. A 570 W Xenon lamp (OSRAM Co) with a L41 UV cut-off filter (Kenko Co.) was used as a visible light source. 4 ml of pesticide solution with an initial concentration of 60 mg/L was added to a cuvette containing the coated glass sample with an active surface area of 2 cm2. Before irradiation, the reaction 5
solution was placed in the dark for 24 hours with stirring. Afterward, the solution was irradiated with visible light source at room temperature. At given time intervals, the concentration of paraoxon in the solution as analyzed using a Cary 100 Bio spectrophotometer (Varian). 3. Results and discussion 3.1 Surface morphology and roughness Fig. 2 represents the surface morphologies of the prepared coatings. As Fig. 2(a) indicates the O2-TiO2 coating contains irregular spherical nanoparticles with sizes of about 60 to 90 nm. SEM images of the N2-O2TiO2 coating, at different magnifications, have been shown in Fig. 2(b) and Fig. S1. As can be seen in this figure, this sample is composed of cubic nanoparticles with dimension of 40 to almost 45 nm in its structure. Therefore, with change of the deposition atmosphere, the morphology of nanostructures changes. The N2-O2TiO2 sample has high monodispersity and quality. Fig. 2(c) shows SEM images of the N2-TiO2 coating sample. As this figure demonstrates this sample is composed from nanopyramidal shaped nanostructures with different sizes and has high monodispersity.
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Fig. 2. SEM images of (a) O2-TiO2, (b) N2-O2-TiO2 and (c) N2-TiO2 samples. 3.2 XRD The XRD patterns of the prepared coatings have been represented in Fig. 3. In XRD patterns of the all coating samples, the peaks appeared at 2θ values of 25.2°, 37.8°, 47.9°, 54.2°, 55.2°, and 62.7° can be assigned to the diffractions from the (101), (004), (200), (105), (211), and (204) crystallographic planes of anatase TiO2 (JCPDS no. 01-083-2243). Therefore, the prepared coatings have the pure anatase crystal structure and there is no peak related to the other crystal structure of TiO2. The broadness of diffraction peaks indicates the nanostructure nature of the coatings and the sharpness of the diffraction peaks is related to good crystallinity of the coatings. Because of the texture effect, in nanostructures with the anisotropic morphology such as nanorods or nanowires, the relative intensities of the X-ray diffraction peaks from different lattice planes are different from the reported standard XRD patterns [31]. Except for the O2-TiO2 coating which has normal relative intensities of the diffraction peaks, in N2-TiO2 and N2-O2-TiO2 samples which have nanocube and nanopyramid morphologies the relative intensity of diffraction peaks have been changed [32]. 7
Fig. 3. XRD patterns of (a) O2-TiO2, (b) N2-O2-TiO2 and (c) N2-TiO2 samples.
3.3 XPS In order to further investigate the presence of oxygen vacancy defect in the prepared coatings, XPS spectra were acquired from the samples. Fig. 4 represents High-resolution XPS spectra of the samples in the Ti 2p region. Two peeks are seen in the XPS spectrum of the O2-TiO2 coating which could be related to the Ti4+2p3/2 (458.7 eV) and Ti4+2p3/2 (464.5 eV), these peaks are symmetric, suggesting Ti in 4+ oxidation state and there is no oxygen vacancy defect in this sample. XPS peaks of N2-O2-TiO2 and N2-TiO2 coatings are broad and asymmetric, and in comparison to the XPS peaks of O2-TiO2 sample, there is a shift to lower binding energy at the maximum of these peaks, therefore both of the 3+ and 4+ oxidation states of Ti are present in these samples. Deconvolution of the XPS peaks of N2-O2-TiO2 and N2-TiO2 coatings demonstrates two extra peaks at 457.6 eV and 463.4 eV which are related to the Ti3+2p3/2 and Ti3+2p3/2, respectively [18, 20, 33]. Furthermore, the concentration of oxygen vacancy defects in N2-TiO2 coating is higher than that of N2-O2TiO2 coating.
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Fig. 4. High-resolution XPS spectra in the Ti 2p region for (a) O2-TiO2, (b) N2-O2-TiO2 and (c) N2-TiO2 samples.
3.4 Optical properties Fig. 5 indicates the optical absorbance spectra of the prepared coatings. There is no absorption in visible light region for the O2-TiO2 coating. However, N2-TiO2 and N2-O2-TiO2 coatings show absorption in visible light region and a considerable red shift in their absorption edges. The visible light absorption in these samples could be related to the presence of oxygen vacancy defects and Ti3+ species in their structures. TiO2 base semiconductors (as an indirect-band-gap semiconductor), through extrapolation of the tangent line in plots of (𝛼ℎ𝑣)1/2 vs ℎ𝑣, where α is absorption coefficient and ℎ𝑣 is photon energy [34, 35], as presented in the inset of Fig. 5. The band-gap energy (Eg) of the O2-TiO2, N2-O2-TiO2 and N2-TiO2 are 3.21, 2.82 and 2.74 eV, respectively. Therefore, because of the formation of the defect states in their structures, the band-gap energy of samples with nanocube and nanopyramid morphology have reduced [36, 37].
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Fig. 5. UV-Vis absorption spectra of the prepared coatings samples (inset: band gap energy determination from Tauc plot).
3.5 Photoluminescence (PL) In order to get insight into the recombination rate of photo-generated electron and holes, PL spectra of the prepared coatings were obtained (Fig. 6). In this spectroscopy, the PL intensity of a photocatalyst sample is directly proportional to the electron–hole recombination rate on its surface, that is, the faster the recombination rate, the greater the PL intensity, and vice versa [38]. As the results of this experiment in Fig. 8 indicate, among the samples, the O2-TiO2 coating has the highest PL intensity and consequently the highest electron– hole recombination rate, and the N2-TiO2 coating has the lowest PL intensity and consequently the lowest electron–hole recombination rate. Because of its special pyramidal shape and greater charge mobility and enhanced charge separation on its sharp edges and corners [25], the N2-TiO2 coating has the lowest PL intensity (Fig. 7).
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Fig. 6. PL spectra of the prepared coating samples.
Fig. 7. Schematic representation for enhancement the PL spectra of the prepared coating samples with pyramidal and cubic shapes.
3.6 Photocatalytic activity The photocatalytic performances of the prepared coatings were assessed in degradation of paraoxon as seen in the Ct/C0 versus irradiation time plots (Fig. 8 (a)), where Ct is the paraoxon concentration following irradiation time of t and C0 is the initial concentration of paraoxon. As this figure indicates the visible light degradation of paraoxon in absence of any photocatalyst is almost negligible and presences of the prepared coatings, especially the N2-TiO2 sample, remarkably enhance the paraoxon photocatalytic degradation. Fig. 8 (b) indicates the linear correlation between ln(C0/Ct) and irradiation time (t), therefore the paraoxon photocatalytic degradation on the prepared coatings follows pseudo-first-order kinetics ln(C0/Ct)=kt. The calculated value of the reaction rate constants (k) for visible light photocatalytic degradation of paraoxon pesticide on O2-TiO2, N2-O2-TiO2 and N2-TiO2 coatings are 0.0019, 0.0036 and 0.0049 min-1, respectively. 11
Based on these results it can be concluded that among the prepared coatings, the coating with pyramidal morphology (i.e. N2-TiO2 sample) and highest oxygen vacancy defect has the highest photocatalytic activity and the coating with spherical morphology (i.e. O2-TiO2 sample) and lowest oxygen vacancy defect has the lowest photocatalytic activity. Because of the low electron-hole recombination rate on the sharp corner and edges of nanostructures with cubic and pyramidal shape, and trapping of electron in oxygen vacancy defect sites of N2-O2-TiO2 and N2-TiO2 coatings, these samples show an improvement in photocatalytic activity. In a similar work by Prasad et al. [39] they reported the k value of 0.0069 min-1 for the photocatalytic degradation of paraoxon on TiO2 coating under UV light irradiation, however, in our work due to the synergic effect of oxygen vacancy defect and shape of nanostructures the k value of 0.0049 min-1 was obtained for the visible light photocatalytic degradation of paraoxon on N2-TiO2 coating.
Fig. 8. (a) The visible light photocatalytic activity of the different coatings in degradation of paraoxon, and (b) calculation of the reaction rate constants (k) based on the pseudo-first order kinetic model.
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5. Conclusion In summary, at the present work TiO2 coatings with different level of oxygen vacancy defect and with different shapes of nanosphere, nanocube, and nanopyramid were deposited on glass substrate by modified ultrasonic spray pyrolysis method, for the first time. As the results of XRD experiment indicate, all of these prepared coatings have anatase crystal structure. Based on the PL results, among the prepared samples, the coating with nanopyramidal shape and highest oxygen vacancy defect has the lowest electron-hole recombination rate, which can be related to the enhanced charge separation on its sharp corners and edges and trapping of electrons in the oxygen vacancy defect. Furthermore because of the presence of oxygen vacancy defect in its structure, this samples also has the smallest band gap energy. For these reason the nanopyramidal shaped coating, has the highest photocatalytic activity for degradation of paraoxon pesticide under visible light irradiation. The approach of this work can be useful for researchers in development of TiO2 based photocatalyst coatings with enhanced photocatalytic activity for water purification from organic poisonous pollutants.
Acknowledgments: The authors would like to express their sincere thanks to the Deputy of Research of University of Bonab for the financial support “Grant no: 97/I/ER/2318”.
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Graphical abstract
Because of their sharp edges and corners, and the presence of oxygen vacancy defect in the structure of TiO2 nanopyramids (prepared under N2 atmosphere), it has higher photocatalytic activity than TiO2 nanosphere (prepare under O2 atmosphere).
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S0277-5387(19)30659-X https://doi.org/10.1016/j.poly.2019.114214 POLY 114214
To appear in:
Polyhedron
Received Date: Revised Date: Accepted Date:
20 August 2019 5 October 2019 25 October 2019
Please cite this article as: Y. Cao, L. Huang, Y. Bai, K. Jermsittiparsert, R. Hosseinzadeh, H. Rasoulnezhad, G. Hosseinzadeh, Synergic effect of oxygen vacancy defect and shape on the photocatalytic performance of nanostructured TiO2 coating, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.114214
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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.
© 2019 Elsevier Ltd. All rights reserved.
Synergic effect of oxygen vacancy defect and shape on the photocatalytic performance of nanostructured TiO2 coating
Yan Caoa, Liang Huanga, Yu Baia, Kittisak Jermsittiparsertb,c**, Reza Hosseinzadehd, Hossein Rasoulnezhade, Ghader Hosseinzadehf*
a. School of Mechatronic Engineering, Xi’an Technological University, Xi’an, 710021 China b. Department for Management of Science and Technology Development, Ton Duc Thang University, Ho Chi Minh City, Vietnam c. Faculty of Social Sciences and Humanities, Ton Duc Thang University, Ho Chi Minh City, Vietnam d. Medical Laser Research Group, Medical Laser Research Center, ACECR, Tehran, Iran e. Department of Electrical & Electronics Engineering, Standard Research Institute (SRI), Karaj, Iran f. Department of Chemical Engineering, University of Bonab, Bonab, Iran * Corresponding author: Ghader Hosseinzadeh; E-mail: [email protected] ** Co-Corresponding author: Kittisak Jermsittiparsert; E-mail: [email protected]
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Abstract In the present work TiO2 coatings with different level of oxygen vacancy defect and different shapes of nanosphere, nanocube, and nanopyramid were deposited on glass substrate by modified ultrasonic spray pyrolysis method. The prepared coatings were successfully used for the photocatalytic degradation of paraoxon pesticide under visible light irradiation. Based on the obtained results, among the papered coatings, the TiO2 coating with nanopyramidal shape and oxygen vacancy defect has the highest photocatalytic activity, and the lowest photocatalytic activity was observed in the case of the TiO2 coating with nanospherical shape and without oxygen vacancy defect. The reason for these results is the sharp edges and corners of nanopyramid which decreases the electron-hole recombination rate. Furthermore, presence of oxygen vacancy defect improve the visible light photocatalytic activity via narrowing the band gap energy and separation of the electron-hole.
Key words: oxygen vacancy defect; TiO2; photocatalyst; shape; nanostructure
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1. Introduction Water demands are increasing worldwide because of the increasing of the population of the earth. However, the available water resources are limited, and following the rapid industrial growth the water resources are continually contaminated with organic pollutants such as pesticides. In recent decades, various methods such as photocatalysis [1, 2], microbial degradation [3, 4], hydrolytic degradation [5], chemical decontamination [6], and elimination of pollutants using adsorbents [7-9] have developed for decontamination of water from pollutants. Among these methods, photocatalytic degradation of pollutants using semiconductors has received much attention because of its potential advantages such as low-cost, sustainable treatment technology, and environmentally friendly nature [10]. TiO2 is one of the more studied semiconductor for photocatalytic purification and remediation of contaminated water [11, 12]. However, because of fast recombination of photo-generated electron-hole charge carriers on TiO2 and due to its wide band gap (about 3.2 eV for anatase TiO2 [13]), TiO2 has low photoactivity under visible light radiation. According to the literature, presence of the oxygen vacancy defect in the structure of semiconductors could improve their photoactivity [14-17]. The main reasons for the photoactivity improvement are (i) trapping of electron, which in turn, promote the separation of electron-hole [18], (ii) improving the production of superoxide radicals and hydroxyl radicals [19, 20], (iii) narrowing the band gap of semiconductor by creation of a new energy level [14, 17]. Recent studies have shown that the shape of nanomaterial has a significant effect on its photocatalytic activity [21, 22]. Based on the literature, the photocatalytic activity of TiO2, especially in nanometer length scale, is strongly dependent on the surface atomic structure (surface atomic arrangement and coordination) which differ by changing the crystal facets [23, 24]. Wu et al. [25] investigated the photocatalytic activity of TiO2 nanomaterials with different shapes and they found that anatase TiO2 nanobelt has higher photocatalytic activity than TiO2 nanosphere, and they relate this to greater charge mobility in the nanobelts and enhanced charge separation on the (101) facet. In a similar work Camposeco et al. [26] compared the photocatalytic activity of TiO2 nanomaterials with various shapes i.e. nanotubes, nanofibers, nanowires and nanoparticles, and showed that TiO2 nanotubes have the highest photocatalytic activity.
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As mentioned in the previous paragraphs, shape of nanostructures and presence of oxygen vacancy defect have considerable effect on the photocatalytic activity of photoactive semiconductors. In the current study, we want to investigate the synergic effects of shape of nanostructures and presence of oxygen vacancy defect on photocatalytic performance of TiO2 coatings. In our previous works we reported the preparation of transparent and nanostructured TiO2 coatings doped with various metal and nonmetal elements on glass substrate by using ultrasonic spray pyrolysis technique [27-30]. In this work, it is proposed to prepare the nanostructured TiO2 coatings on glass substrate with different shapes and different level of oxygen vacancy defect with some modifications in our technique. In this regard, before the deposition process, whole of the reaction chamber was evacuated and then the depositions was started under N2 or O2 or a mixture of these gases. 2. Material and methods 2.1 Materials In this study, the following chemicals were used: Titanium (IV) n-butoxide (≥97.0%) from Sigma-Aldrich, absolute ethanol (99.5% v/v) from Merck, concentrated hydrochloric acid (37 wt %) from Merck and acetone from Merck. 2.2 Preparation of TiO2 coatings A schematic diagram of the deposition set-up is shown in Fig. 1. TiO2 sol solution was prepared by addition of 20 ml the HCl ethanolic solution (0.15M) into the 30 ml of ethanolic solution of Titanium (IV) n-butoxide (0.5M) in dropwise manner to reach the pH of 2.1. After about 2 hour stirring, the obtained solution was poured into the flask 1 and was sprayed with aid of a floated piezoelectric ultrasonic transducer. For better spraying, the ultrasonic transducer was floated on the surface of the solution and the solution thickness on transducer was about 5 mm. In order to the preparation of the TiO2 coating with oxygen vacancy defect, whole of the deposition setup was evacuated by using rotary vacuum pump (RVP) and the sprayed solution was carried by N2 or O2 or a mixture of these gases with overall flow rate of 200 ml/min. The large particles were sedimentated in flask 2 to improve the monodispersity of the final coatings. The migrated droplets were thermally decomposed at the surface of the hot glass substrate at 400°C in flask 3 for 30 min. The samples
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prepared under the atmospheres of N2 or O2 or a mixture of these gases were labeled as N2-TiO2, O2-TiO2, and N2-O2-TiO2, respectively.
Fig. 1. Schematic illustration of the ultrasonic-assisted spray pyrolysis technique.
2.3. Characterization XRD experiments of all the products were conducted on a Philips PW-3710 (Netherlands) X-ray Diffractometer with Cu-Kα irradiation (λ=1.54018 Å). Surface morphology, shape and size all the prepared samples were investigated by VEGA3 TESCAN scanning electron microscopy (SEM) (Czech Republic) operated at 15 kV. The UV–visible absorption spectra of the prepared coatings were acquired using Cary 100 Bio spectrophotometer (Varian, Inc., USA). XPS spectra of the samples were obtained by using a GammadataScienta Esca 200 (Uppsala, Sweden) hemispherical analyzer equipped with an Al Kα (1486.6 eV) X-ray source. All binding energy values were calibrated by using the value of the C 1s peak at 284.6 eV as a reference. Photoluminescence (PL) emission spectra of the prepared coatings were recorded using Cary Eclipse fluorescence spectrophotometer (Varian, Inc., USA) at room temperature, with an excitation wavelength of 320 nm. 2.4. Evaluation of the visible light photocatalytic activity of the coatings Paraoxon pesticide was used as a contaminant for investigation the photocatalytic performance of the prepared coatings. A 570 W Xenon lamp (OSRAM Co) with a L41 UV cut-off filter (Kenko Co.) was used as a visible light source. 4 ml of pesticide solution with an initial concentration of 60 mg/L was added to a cuvette containing the coated glass sample with an active surface area of 2 cm2. Before irradiation, the reaction 5
solution was placed in the dark for 24 hours with stirring. Afterward, the solution was irradiated with visible light source at room temperature. At given time intervals, the concentration of paraoxon in the solution as analyzed using a Cary 100 Bio spectrophotometer (Varian). 3. Results and discussion 3.1 Surface morphology and roughness Fig. 2 represents the surface morphologies of the prepared coatings. As Fig. 2(a) indicates the O2-TiO2 coating contains irregular spherical nanoparticles with sizes of about 60 to 90 nm. SEM images of the N2-O2TiO2 coating, at different magnifications, have been shown in Fig. 2(b) and Fig. S1. As can be seen in this figure, this sample is composed of cubic nanoparticles with dimension of 40 to almost 45 nm in its structure. Therefore, with change of the deposition atmosphere, the morphology of nanostructures changes. The N2-O2TiO2 sample has high monodispersity and quality. Fig. 2(c) shows SEM images of the N2-TiO2 coating sample. As this figure demonstrates this sample is composed from nanopyramidal shaped nanostructures with different sizes and has high monodispersity.
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Fig. 2. SEM images of (a) O2-TiO2, (b) N2-O2-TiO2 and (c) N2-TiO2 samples. 3.2 XRD The XRD patterns of the prepared coatings have been represented in Fig. 3. In XRD patterns of the all coating samples, the peaks appeared at 2θ values of 25.2°, 37.8°, 47.9°, 54.2°, 55.2°, and 62.7° can be assigned to the diffractions from the (101), (004), (200), (105), (211), and (204) crystallographic planes of anatase TiO2 (JCPDS no. 01-083-2243). Therefore, the prepared coatings have the pure anatase crystal structure and there is no peak related to the other crystal structure of TiO2. The broadness of diffraction peaks indicates the nanostructure nature of the coatings and the sharpness of the diffraction peaks is related to good crystallinity of the coatings. Because of the texture effect, in nanostructures with the anisotropic morphology such as nanorods or nanowires, the relative intensities of the X-ray diffraction peaks from different lattice planes are different from the reported standard XRD patterns [31]. Except for the O2-TiO2 coating which has normal relative intensities of the diffraction peaks, in N2-TiO2 and N2-O2-TiO2 samples which have nanocube and nanopyramid morphologies the relative intensity of diffraction peaks have been changed [32]. 7
Fig. 3. XRD patterns of (a) O2-TiO2, (b) N2-O2-TiO2 and (c) N2-TiO2 samples.
3.3 XPS In order to further investigate the presence of oxygen vacancy defect in the prepared coatings, XPS spectra were acquired from the samples. Fig. 4 represents High-resolution XPS spectra of the samples in the Ti 2p region. Two peeks are seen in the XPS spectrum of the O2-TiO2 coating which could be related to the Ti4+2p3/2 (458.7 eV) and Ti4+2p3/2 (464.5 eV), these peaks are symmetric, suggesting Ti in 4+ oxidation state and there is no oxygen vacancy defect in this sample. XPS peaks of N2-O2-TiO2 and N2-TiO2 coatings are broad and asymmetric, and in comparison to the XPS peaks of O2-TiO2 sample, there is a shift to lower binding energy at the maximum of these peaks, therefore both of the 3+ and 4+ oxidation states of Ti are present in these samples. Deconvolution of the XPS peaks of N2-O2-TiO2 and N2-TiO2 coatings demonstrates two extra peaks at 457.6 eV and 463.4 eV which are related to the Ti3+2p3/2 and Ti3+2p3/2, respectively [18, 20, 33]. Furthermore, the concentration of oxygen vacancy defects in N2-TiO2 coating is higher than that of N2-O2TiO2 coating.
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Fig. 4. High-resolution XPS spectra in the Ti 2p region for (a) O2-TiO2, (b) N2-O2-TiO2 and (c) N2-TiO2 samples.
3.4 Optical properties Fig. 5 indicates the optical absorbance spectra of the prepared coatings. There is no absorption in visible light region for the O2-TiO2 coating. However, N2-TiO2 and N2-O2-TiO2 coatings show absorption in visible light region and a considerable red shift in their absorption edges. The visible light absorption in these samples could be related to the presence of oxygen vacancy defects and Ti3+ species in their structures. TiO2 base semiconductors (as an indirect-band-gap semiconductor), through extrapolation of the tangent line in plots of (𝛼ℎ𝑣)1/2 vs ℎ𝑣, where α is absorption coefficient and ℎ𝑣 is photon energy [34, 35], as presented in the inset of Fig. 5. The band-gap energy (Eg) of the O2-TiO2, N2-O2-TiO2 and N2-TiO2 are 3.21, 2.82 and 2.74 eV, respectively. Therefore, because of the formation of the defect states in their structures, the band-gap energy of samples with nanocube and nanopyramid morphology have reduced [36, 37].
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Fig. 5. UV-Vis absorption spectra of the prepared coatings samples (inset: band gap energy determination from Tauc plot).
3.5 Photoluminescence (PL) In order to get insight into the recombination rate of photo-generated electron and holes, PL spectra of the prepared coatings were obtained (Fig. 6). In this spectroscopy, the PL intensity of a photocatalyst sample is directly proportional to the electron–hole recombination rate on its surface, that is, the faster the recombination rate, the greater the PL intensity, and vice versa [38]. As the results of this experiment in Fig. 8 indicate, among the samples, the O2-TiO2 coating has the highest PL intensity and consequently the highest electron– hole recombination rate, and the N2-TiO2 coating has the lowest PL intensity and consequently the lowest electron–hole recombination rate. Because of its special pyramidal shape and greater charge mobility and enhanced charge separation on its sharp edges and corners [25], the N2-TiO2 coating has the lowest PL intensity (Fig. 7).
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Fig. 6. PL spectra of the prepared coating samples.
Fig. 7. Schematic representation for enhancement the PL spectra of the prepared coating samples with pyramidal and cubic shapes.
3.6 Photocatalytic activity The photocatalytic performances of the prepared coatings were assessed in degradation of paraoxon as seen in the Ct/C0 versus irradiation time plots (Fig. 8 (a)), where Ct is the paraoxon concentration following irradiation time of t and C0 is the initial concentration of paraoxon. As this figure indicates the visible light degradation of paraoxon in absence of any photocatalyst is almost negligible and presences of the prepared coatings, especially the N2-TiO2 sample, remarkably enhance the paraoxon photocatalytic degradation. Fig. 8 (b) indicates the linear correlation between ln(C0/Ct) and irradiation time (t), therefore the paraoxon photocatalytic degradation on the prepared coatings follows pseudo-first-order kinetics ln(C0/Ct)=kt. The calculated value of the reaction rate constants (k) for visible light photocatalytic degradation of paraoxon pesticide on O2-TiO2, N2-O2-TiO2 and N2-TiO2 coatings are 0.0019, 0.0036 and 0.0049 min-1, respectively. 11
Based on these results it can be concluded that among the prepared coatings, the coating with pyramidal morphology (i.e. N2-TiO2 sample) and highest oxygen vacancy defect has the highest photocatalytic activity and the coating with spherical morphology (i.e. O2-TiO2 sample) and lowest oxygen vacancy defect has the lowest photocatalytic activity. Because of the low electron-hole recombination rate on the sharp corner and edges of nanostructures with cubic and pyramidal shape, and trapping of electron in oxygen vacancy defect sites of N2-O2-TiO2 and N2-TiO2 coatings, these samples show an improvement in photocatalytic activity. In a similar work by Prasad et al. [39] they reported the k value of 0.0069 min-1 for the photocatalytic degradation of paraoxon on TiO2 coating under UV light irradiation, however, in our work due to the synergic effect of oxygen vacancy defect and shape of nanostructures the k value of 0.0049 min-1 was obtained for the visible light photocatalytic degradation of paraoxon on N2-TiO2 coating.
Fig. 8. (a) The visible light photocatalytic activity of the different coatings in degradation of paraoxon, and (b) calculation of the reaction rate constants (k) based on the pseudo-first order kinetic model.
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5. Conclusion In summary, at the present work TiO2 coatings with different level of oxygen vacancy defect and with different shapes of nanosphere, nanocube, and nanopyramid were deposited on glass substrate by modified ultrasonic spray pyrolysis method, for the first time. As the results of XRD experiment indicate, all of these prepared coatings have anatase crystal structure. Based on the PL results, among the prepared samples, the coating with nanopyramidal shape and highest oxygen vacancy defect has the lowest electron-hole recombination rate, which can be related to the enhanced charge separation on its sharp corners and edges and trapping of electrons in the oxygen vacancy defect. Furthermore because of the presence of oxygen vacancy defect in its structure, this samples also has the smallest band gap energy. For these reason the nanopyramidal shaped coating, has the highest photocatalytic activity for degradation of paraoxon pesticide under visible light irradiation. The approach of this work can be useful for researchers in development of TiO2 based photocatalyst coatings with enhanced photocatalytic activity for water purification from organic poisonous pollutants.
Acknowledgments: The authors would like to express their sincere thanks to the Deputy of Research of University of Bonab for the financial support “Grant no: 97/I/ER/2318”.
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Graphical abstract
Because of their sharp edges and corners, and the presence of oxygen vacancy defect in the structure of TiO2 nanopyramids (prepared under N2 atmosphere), it has higher photocatalytic activity than TiO2 nanosphere (prepare under O2 atmosphere).
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