Xúc tác quang hóa ti o2 pha tạp vanadi ứng dụng xử lý nước thải

TRƯỜNG ĐẠI HỌC BÁCH KHOA HÀ NỘI LUẬN VĂN THẠC SĨ Xúc tác quang hóa TiO2 pha tạp vanadi ứng dụng xử lý nước thải NGUYỄN ĐỨC MẠNH Manh.ND202421M@sis.hust.edu.vn Ngành Hóa học Giảng viên hướng dẫn: PGS TS Nghiêm Thị Thương Viện Kỹ thuật Hóa học, ĐHBKHN Chữ ký GVHD TS Esteban Mejia Viện LIKAT, ĐH Rostock Chữ ký GVHD HÀ NỘI, 10/2022 HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY MASTER THESIS Visible light-driven photocatalysts based on V-doped TiO2 for wastewater treatment NGUYEN DUC MANH Manh.ND202421M@sis.hust.edu.vn Master of Science in Chemistry Supervior: Assoc Prof Nghiem Thi Thuong School of Chemical Engineering, HUST Dr Esteban Mejia Leibniz Institute for Catalysis, UR Hanoi, 10/2022 Signature Signature ĐỀ TÀI LUẬN VĂN Tên đề tài: Xúc tác quang hóa TiO2 pha tạp vanađi ứng dụng xử lý nước thải Giảng viên hướng dẫn Giảng viên hướng dẫn phụ PGS TS Nghiêm Thị Thương TS Esteban Mejia Acknowledgments First of all, I would like to express my deepest gratitude towards Dr Esteban Mejia, Department of Biocatalysis & Polymer Chemistry, Leibniz Institute for Catalysis, for giving me the great opportunity to work in his group and for his guidance as well as his outstanding support during my master’s research I would like to thank Dr Nguyen Van Anh, School of Chemical Engineering, Hanoi University of Science and Technology, for giving me the concept of my thesis and her helpful guidance I am also thankful to Assoc Prof Nghiem Thi Thuong, School of Chemical Engineering, Hanoi University of Science and Technology, for her all advice and her much support throughout the master’s program I am also indebted to M.Sc Paul Hünemörder, M.Sc Gustavo Alvarez and M.Sc Shuoping Ding at Leibniz Institute for Catalysis, who supported my research with a great deal of valuable discussions Special thanks go to my Vietnamese friends, namely Hung, Thuyen, Tuan, M.Sc Phong Dam, as well as the Vietnamese pioneers, namely Dr Huyen Vuong, Dr Hieu Do, Dr Binh Ngo, M.Sc Trang Pham, M.Sc Quyen Phung, M.Sc Vien Che, for their understanding and for giving me the warm atmosphere in Rostock I would like to thank the RoHan project for giving me the excellent opportunity to visit Germany for studying with fully financial support Last but not least, I am wholeheartedly grateful to my family, especially my parents, my sister, my uncle and his wife for their unconditional love, their encouragement and never-ending support Abstract Trichloroethylene (TCE) is a volatile chlorinated organic compound (VCOC) commonly used as a solvent in automotive, metal, finishing, and textile industries Wastewaters contaminated with TCE are a pollutant of serious concern in groundwaters, as it is harmful to aquatic and surface ecosystems and to the human health There are various reported methods for the degradation of TCE in aqueous media, including the “air stripping method”, where the volatilization is often incomplete, and consequently a residual amount of TCE can still be found in the treated water Moreover, gas-phase degradation of TCE produces toxic by-products such as phosgene and dichloroacetyl chloride Photocatalytic degradation of VCOCs in the aqueous phase using semiconductors such as TiO2 is well known and offers a promising alternative owing to its costeffectiveness and nontoxicity However, pure TiO2 can only absorb UV light, which accounts for  5% of the solar spectrum, thus restricting its practical application Furthermore, the rapid recombination of photogenerated electron-hole pairs kinetically impedes many desired routes to complete pollutant mineralization In this regard, vanadium ions doping has been considerably investigated to improve the optical properties of TiO2 as well as to promote separation of electron-hole pairs In this work, vanadium-doped TiO2 photocatalysts were prepared via a simple onestep hydrothermal method for photocatalytic degradation of TCE at a high concentration in aqueous phase Different characterization methods were employed to reveal the role of vanadium in the enhancement of visible light absorption as well as charge separation, which lead to an improved photoactivity of catalysts In addition, the study also provides evidence for the formation of V2O5 on the surface of TiO2 when doping at high vanadium concentrations and its influence on the photodegradation of TCE under visible light was also discussed Master student NGUYEN DUC MANH Table of Contents LIST OF FIGURES i LIST OF TABLES iv LIST OF ABBREVIATION v CHAPTER I INTRODUCTION I.1 Water Crisis I.2 Trichloroethylene I.2.1 Chemical identity and Properties I.2.2 Applications and Disposal I.2.3 Environmental effects and Human’s health risks I.3 Nanostructured Titanium Dioxide I.3.1 Structural and Crystallographic properties I.3.2 Optical properties 12 I.3.3 Preparation methods 16 I.4 Photocatalysts based on TiO2 materials in wastewater treatment 22 I.5 Research objectives 28 CHAPTER II MATERIALS AND METHODS 29 II.1 Chemicals 29 II.2 Materials characterization 29 II.3 Materials preparation 31 II.4 Photocatalytic activity experiments 32 II.5 Recycling experiments for catalysts 33 CHAPTER III RESULTS AND DISCUSSION 34 III.1 XRD analysis 34 III.2 FT-IR and Raman spectra 36 III.3 ICP-OES analysis 38 III.4 SEM – EDX data 38 III.5 BET surface area and pore distribution 40 III.6 XPS measurements 41 III.7 Morphological properties 43 III.8 Optical properties 44 III.9 Photocatalytic performance 46 III.10 Proposed mechanism 49 CHAPTER IV CONCLUSION 53 REFERENCES 54 APPENDIX 58 LIST OF FIGURES Figure I.1 Families collecting water from water well in Africa (left) and the industrial wastewater disposal in Asia (right) (Source: UNICEF, 2020) Figure I.2 Proportion of population using safely managed drinking water services, 2017 (%) [1] Figure I.3 Applications of Trichloroethylene (all images were taken without permission from public internet sites The rights belong to the corresponding sources) Figure I.4 Illustration of air-stripping technology (Source: Federal Remediation Technologies Roundtable) [12] Figure I.5 Unit cell of TiO2 phases: (a) Anatase, (b) Rutile, and (c) Brookite; blue and red spheres represent titanium and oxygen atoms, respectively [21] Figure I.6 Crystallite structure of TiO2 phases: (a) Anatase, (b) Rutile, and (c) Brookite [22] Figure I.7 the XRD patterns and the ball-and-stick structures of (a) anatase, (b) rutile, and (c) brookite [28] 11 Figure I.8 The ideal structure of TiO2(B) [34] 12 Figure I.9 Comparison of recombination pathways of electron-hole pairs within the direct band-gap semiconductor and the indirect band-gap semiconductors [41] 14 Figure I.10 Photo-induced reactions in the TiO2 photocatalysis versus the corresponding time [42] 16 Figure I.11 Effect of the initial pH on the TiO2 morphology [44] 18 Figure I.12 Different possible growth mechanisms for the formation of TiO2 nanostuctures [28] 19 Figure I.13 The proposed condensation pathway for the nucleation of TiO2 crystals [47] 22 Figure I.14 Photocatalytic activities of different shape-controlled TiO2 nanomaterials for MO degradation and SEM images of TiO2 nanomaterials [50] 23 Figure I.15 Mechanism of photocatalytic reactions of V-doped TiO2 under UV-Vis irradiation 25 Figure I.16 The PL spectra and catalytic activity of V-modified TiO2 samples calcined at 300 oC from [54] 26 i Figure I.17 UV-Vis DRS of V-N co-doped TiO2 samples and photodegradation of PCPNa under visible light irradiation with prepared catalysts from [53] 27 Figure I.18 XRD pattern of V2O5/TiO2 at different V/Ti molar ratio (right) and SEM images of V2O5/TiO2 nanofibers from [56] 28 Figure II.1 Schematic illustration of the one-step hydrothermal process to prepare Vdoped TiO2 photocatalysts 32 Figure II.2 Schematic illustration of photocatalytic activity experiments 33 Figure III.1 XRD patterns of as prepared TiO2 samples and the inset shows anatase (101) peak in detail 34 Figure III.2 Structural properties of as prepared TiO2 samples: Phase content (a), Average crystallite size (b), and Unit cell volume (c) 35 Figure III FT-IR spectra of as prepared TiO2 samples measured at room temperature 36 Figure III.4 Raman spectra of as prepared TiO2 samples with the expanded regions 37 Figure III.5 EDX data of V-doped TiO2 samples 39 Figure III.6 EDX mapping of Ti, O, V elements of 0.10V-TiO2 39 Figure III.7 Nitrogen adsorption/desorption isotherms and the corresponding pore size distribution plot (the inset) of Undoped TiO2 and 0.10V-TiO2 samples 40 Figure III.8 High-resolution XPS spectra of the Ti 2p, V 2p, O 1s, and the C 1s regions of Undoped TiO2 and 0.10V-TiO2 41 Figure III.9 STEM images and expanded regions (a1-a4; b1-b4) of 0.10V-TiO2; (a5) and (b5) are the images of inverse fast Fourier transform of (a4) and (b4), respectively; (a6) and (b6) is the plot profile of IFFT of the selected regions in (a5) and (b5) respectively, obtained by the ImageJ software 43 Figure III.10 UV-VIS DRS spectra (left) and the Tauc plot (right) of as prepared TiO2 samples 44 Figure III.11 Photoluminescence spectra of as prepared TiO2 samples upon 355 nm excitation 45 Figure III.12 The photocatalytic degradation of Trichloroethylene over the as-prepared TiO2 samples under the UV irradiation (left) and the visible irradiation (right) at room temperature 46 ii Where, a is the absorption coefficient, h is Planck’s constant, ν is the photon’s frequency, A is a proportionality constant and Eg is the band gap energy [76] As shown in Figure III.10 (right), the band gap energy decreases from 3.24, 2.39 and 2.33 eV corresponding to the molar ratio V : Ti increasing from 0% (undoped), 5% and 10% According to Nguyen et al., the incorporation of V4+ species into the TiO2 lattice could be a main reason for band gap narrowing [61] Instead of the band-to-band electron transition in the pristine TiO2, the electrons of the doped samples are photoexcited and transfer from the valance band to the t2g level of the V4+ 3d orbital, which is the intermediate state within the TiO2 band gap, then these charge carriers transfer from the V4+ 3d state to the conduction band Moreover, the increasing dopant concentration produces a higher amount of oxygen vacancies, which can generate another intermediate energy state just below the conduction band of TiO2, also resulting in band gap narrowing [53, 63] However, the band gap energy of 0.15V-TiO2 increases slightly to 2.35 eV in comparison with 0.10V-TiO2 This widening could be explained by the Burstein-Moss effect At a particular dopant concentration, the Fermi level is completely filled with electrons, then the following excited electrons can only go to energy states higher than the Fermi level [77-78] Consequently, this situation makes the band gap of 0.15V-TiO2 wider than that of 0.10V-TiO2 Figure III.11 Photoluminescence spectra of as prepared TiO2 samples upon 355 nm excitation Photoluminescence (PL) emissions mainly result from the recombination of photogenerated electrons and holes; thus, PL spectra is commonly used in investigation of electron-hole pairs recombination, trapping and migration processes of semiconductors Figure III.11 depicts the room temperature PL spectra in the range 41045 500 nm of as prepared photocatalysts upon 355 nm excitation In the high energy region, the emission peak located at 421 nm corresponds to the indirect band-to-band recombination of TiO2 [63] Besides, the emission peak that appeared in the longer wavelength region at 485 nm is ascribed to the formation of self-trapped excitons associated with oxygen vacancies or V 3d states under the conduction band of TiO2 [7980] Noticeably, the PL intensity of the V-doped samples decreases significantly, which confirms the limitation of electrons-holes recombination due to the interaction between vanadium and TiO2 at the interface as reported elsewhere [53-54] Particularly, V5+ ([Ar] 3d0) can trap the conduction band electrons and their electron configuration will be [Ar] 3d0 4s1 (V4+), which is unstable Thus, the trapped electrons would be transferred to adsorbed oxygen on the catalyst surface to generate highly oxidative superoxide radicals (V4+ + O2(ads.) → V5+ + O2●─) It shows that vanadium doping can inhibit the electronhole pairs recombination as well as enhance the photocatalytic activity of the V-doped samples III.9 Photocatalytic performance Recently, many research efforts have been made to investigate TiO2 semiconductor for photocatalytic degradation of TCE in the aqueous phase [81-85] Most of them reported that commercial TiO2 in scientific grade shows promising photocatalytic activities It should be noted that the scientific grade catalysts are too expensive to be used in cleaning rivers or lakes Here, the modified TiO2 catalysts with vanadium doping for photodegradation of TCE was firstly reported and shown in Figure III.12 Figure III.12 The photocatalytic degradation of Trichloroethylene over the asprepared TiO2 samples under the UV irradiation (left) and the visible irradiation (right) at room temperature 46 The photocatalytic activity of as prepared samples was evaluated by degradation of 1000 ppm TCE under UV light irradiation (λ = 390 nm) and visible light irradiation (λ = 422 ÷ 700 nm) The control tests were performed using only TCE solution (Without (Cat + O2)), TCE solution under the O2 atmosphere (Without Cat.) As shown in Figure III.12, the changes of TCE concentration in these control tests were insignificant, which indicates that the evaporation or photolysis of TCE during the reaction were found to be negligible Besides, it also demonstrates that oxygen did not react with TCE under light irradiation without catalysts However, all experiments to evaluate the photocatalytic activities of as prepared catalysts were carried out under the O2 atmosphere because the presence of oxygen was reported to be essential for the TCE photooxidation to proceed [81-82, 86] Under the UV light, the overall TCE removal efficiency reaches 72% with the 0.10V-TiO2 sample, while the undoped TiO2 sample adsorbed and degraded only 27% of TCE, which reveals that the vanadium doping effectively influenced the overall removal efficiency of TCE because vanadium doped TiO2 samples were above proven to be better than the undoped sample in the texture properties as well as the optical properties Noticeably, the TCE removal efficiency decreases over the TiO2 sample with 15% vanadium It was found that V2O5, which formed on the surface of TiO2 when doping with a high vanadium concentration, may occupy active sites of the catalyst surface and block TiO2 from UV light exposure, leading to a weaker photocatalytic activity of 0.15V-TiO2 under the UV light [56] When conducting experiments under the visible light, the obtained V-doped samples also exhibited promising photocatalytic activities for the TCE removal As shown in Figure III.12 (right), 0.15V-TiO2 can degrade the TCE most effectively, and the overall removal efficiency was about 3.6 times higher than Undoped TiO2 Interestingly, the small amount of V2O5 on the surface of the doped sample could act as an appropriate co-catalyst because V2O5 is mainly photoexcited to generate electrons and holes under the visible light due to its narrow band gap (2.2 eV) [87] These charge carriers would participate in the redox reaction with species present on the catalyst surface to increase the concentration of free active radicals, which results in the higher photocatalytic activities of 0.15V-TiO2 under the visible irradiation In another aspect, both the undoped and doped samples possess a high specific surface area, the absorbed molecules of TCE on the catalyst surface is only around 10% because the TiO2 surface was covered with water predominantly in the aqueous phase Such phenomenon was also observed by other groups [82, 84] To compare 47 the effectiveness of the TCE removal with other photocatalysts, Table III.3 shows a brief comparison of TCE removal efficiency with various materials from other works Table III.3 Comparison of TCE removal efficiency with various photocatalysts Catalyst Reaction condition Effectiveness Ref TCE removal (%) 69 Time (min) 243 [86] P25 TiO2 [TCE] = 0.33 mM, [Cat.] = g L-1, pH = 7, atm (air, O2), 150W Xe-lamp (λ > 290 nm) ST-B11 TiO2 pellets [TCE] = 50 ppm, [Cat.] = g L-1,  4W fluorescence black (λ = 365 nm), K2S2O8 = M, temp = 30 oC 87.7 150 [82] Commercial TiO2 [TCE] = 35 ppm, [Cat.] = 0.7 g L-1, 15W UV lamp (λ = 365 nm) 55 90 [84] Nano-ZnO/ Laponite composite [TCE] = 10 ppm, [Cat.] = 20 g L-1, pH = 7,  8W UV-C lamp (λ = 254 nm), temp = 25 ~ 27 oC [TCE] = 10 ppm, [Cat.] = 10 g L-1, pH = 7,  8W UV-C lamp (λ = 254 nm), temp = 25 ~ 27 oC 90 60 [88] 95 60 [89] LaFeO3 [TCE] = ppm, [Cat.] = g L-1, pH = 11, 250W Xe lamp (λ = 321 ~ 380 nm) 93 60 [90] V-doped TiO2 [TCE] = 1000 ppm, [Cat.] = g L-1, atm (O2), pH = 7, temp = 24 ~ 30 oC, 40W LED lamp (λ = 390 nm) 80 210 This work Nano-ZnO/ Polybutadiene Composite The stability of the photocatalyst was tested by recovering and reusing it times in the photodegradation reaction with 0.10V-TiO2 under UV irradiation Obviously, the degradation of TCE remains relatively stable in the subsequent runs for 12 hours as shown in Figure III.13, indicating the high stability of the photocatalysts, which is due to resistance of catalysts to photo-corrosion effects Additionally, the XRD data shows no change in the crystallinity or the phase states of the catalysts after cycles, which also confirms the stability of the obtained material However, the TCE removal efficiency is only around 50% because the volume of the suspension was upscaled to five times in these experiments, while the condition of the light was kept consistent Thus, it indicates that the volume of the suspension or the light energy also are factors that influence the TCE degradation efficiency 48 Figure III.13 Recycling experiments over 0.10V-TiO2 (Light: 390 nm, 40W; irradiation time: 3h; Catalyst dose: g L-1, CTCE = 1000 ppm, mCat : mTCE = : 1) (left); and XRD pattern of fresh 0.10V-TiO2 and after cycles III.10 Proposed mechanism The reaction mechanism could be revealed in a comprehensive way when intermediates or by-products are investigated Thus, the experiment was conducted with D2O as the solvent to observe the organic products of the TCE degradation process The treated solution was withdrawn from the reactor, filtered and directly analyzed by 1HNMR without extraction to avoid any compound losses The resulting NMR spectra was shown in Figure III.14 49 Figure III.14 1H-NMR spectra (300 MHz, D2O) of TCE degradation over 0.10V-TiO2 photocatalyst under the UV light at 0h and 3h As shown in the spectra, the signal ascribed to TCE decreased significantly, whereas new peaks appear after three hours of reaction The peaks locating at 9.35 ppm and 3.67 ppm are assigned to protons of the aldehyde group and the hydrocarbon group of 2,2dichloroacetaldehyde [91] , respectively However, the weak intensity of these peak suggested that 2,2-dichloroacetaldehyde could be an intermediate product, and it also could be mineralized over time via oxidation processes by highly active radicals Otherwise, this chemical is not believed to cause health effects on people and be less harmful than trichloroethylene [92] The identity of the peak around 8.5 ppm could not be established with certainty In recent decades, a number of research groups have reported the formation and the role of highly active radicals in the aqueous phase photocatalysis process The photocatalytic activity of semiconductors is known to result from the formation of e–CB 50 and h+VB pairs in the bulk when semiconductors absorb the light energy These charge carriers migrate quickly to the surface and generate radical species such as O2●– and OH● when involving in the redox reactions, providing that the CB potential of semiconductor needs to be more negative than the standard redox potential E0(O2/O2●─) ( ̶ 0.33 eV vs NHE, pH =7), and the VB potential of semiconductor is more positive than the standard redox potential E0(H2O/OH●) (+ 2.38 eV vs NHE, pH = 7) Due to oxygen defects, TiO2 is considered as an N-type semiconductor [93], thus, the CB and VB edge potential can be calculated using the following formular: ECB = χ – Ee – 0.5Eg EVB = Eg – ECB Where χ represents for the absolute electronegativity of TiO2 (5.81 eV), Ee and Eg are the free electron energy (4.5 eV) and the bandgap energy of the semiconductor (see the UV-Vis DRS results), respectively [94] Therefore, the CB and VB potentials of 0.10VTiO2 are calculated to be + 0.145 eV and + 2.475 eV, respectively It suggests that the photo-induced electrons on the CB of 0.10V-TiO2 prioritized tranfering to V5+ on the surface instead of directly reducing the adsorbed O2 molecules into O2●─ radicals because the calculated CB potential of 0.10V-TiO2 (+ 0.145 eV) > E0(O2/O2●─) (̶ 0.33 eV) Meanwhile, the photo-induced holes on the VB can oxidize the adsorbed H2O molecular into OH● radicals because the calculated VB potential (+ 2.475 eV) > E0(H2O/OH●) (+ 2.38 eV) [95-96] According to our observation and the literatures [53, 61, 86] , the reaction mechanism was illustrated and shown in Figure III.15 Figure III.15 Proposed mechanism of photodegradation of TCE over Vanadiumdoped TiO2 photocatalyst 51 The photocatalytic removal mechanism of TCE in solution can be postulated as these steps follows: TCE, H2O and O2 molecules are adsorbed on photocatalysts surface Electrons and holes are produced when irradiating photocatalysts with light sources TiO2 + hν → e–CB + h+VB + TiO2* The holes in the VB band migrate to the surface, then oxidize the adsorbed H2O molecular and hydroxyl group into OH● radicals h+VB + H2O/OH– → OH● + H+ The electrons in the CB band migrate to the surface and is trapped by V5+ sites to form V4+ These V4+ species could be oxidized by the adsorbed O2 to form V5+ and superoxide radicals O2●─ e–CB + V5+ → V4+ V4+ + O2(ads.) → V5+ + O2●─ O2●─ + H+ → HO2● + OH● These highly active radicals can mineralize the TCE compound into less harmful products such as CO2, H2O, HCl OH●, O2●─ + TCE →→ CO2 + H2O + HCl TCE can be reduced by photogenerated electrons under O2 atmosphere to form 2,2dichloroacetaldehyde as an intermediate product Desorption of these product takes place to leave the active sites of photocatalysts 52 CHAPTER IV CONCLUSION This study has demonstrated that an efficient and stable V-doped TiO2 nano catalyst has been successfully synthesized via a simple hydrothermal method toward enhanced photocatalytic degradation of TCE at high concentration As a result, the doped TiO2 samples show higher photocatalytic activities than the undoped TiO2 sample under both UV light and visible light Particularly, 0.10V-TiO2 exhibited 2.7 times promotion toward TCE photodegradation under the UV light, whereas 0.15V-TiO2 showed 3.6 times larger than Undoped TiO2 under visible light This enhancement was found to be due to: i) the presence of V4+ impurity level through the substitution of Ti4+ ions or insertion into TiO2 lattice, which leads to band gap narrowing as well as an effective light absorption, and ii) presence of OH groups and V5+ on the surface of TiO2, which served as charge trapping sites, thus prolonging the lifetime of photoexcited electronsholes pairs according to XPS and PL testing results In our research, the photocatalytic behavior is strongly affected by the change of vanadium concentration Under the UV light irradiation, 0.10V-TiO2 presents the optimal activity for degradation of TCE, while the fastest degradation of TCE was obtained over 0.15V-TiO2 under the visible irradiation The reason for this phenomenon is due to the formation of V2O5 on the surface when doping TiO2 with a high vanadium content It suggested that the excessive V2O5 blocked the active sites of catalyst from UV light exposure, contrastively it supported as an appropriate co-catalyst to obtain the high photocatalytic performance of catalyst under visible light From by-product analysis, 2,2-dichloroacetaldehyde was formed as an intermediate product Although there are not many literatures reporting the toxicity of this chemical, it is believed to be less harmful than trichloroethylene Overall, this work provided a more sustainable alternative to other existing 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[mL] Undoped TiO2 100 0.2 Ti : V molar ratio 0.05V-TiO2 50 0.0105 0.2 : 0.05 140 0.10V-TiO2 100 0.0210 0.2 : 0.10 190 0.15V-TiO2 150 0.0320 0.2 : 0.15 230 Yield [mg] 125 Data of the calibration curve for Trichloroethylene Figure S1 NMR spectra of TCE with different concentrations 58 Table S2 Corresponding peak area of different concentrations of TCE solution Sample Conc of TCE [ppm] 100 250 500 750 900 1000 Peak area 0.05399 0.16065 0.30185 0.44031 0.46907 0.52108 Figure S2 The calibration curve of Trichloroethylene 59 ... Esteban Mejia Leibniz Institute for Catalysis, UR Hanoi, 10/2022 Signature Signature ĐỀ TÀI LUẬN VĂN Tên đề tài: Xúc tác quang hóa TiO2 pha tạp vanađi ứng dụng xử lý nước thải Giảng viên hướng dẫn... oxolation are two main reactions during condensation process: Olation: Ti- OH + Ti- H2O → Ti- OH -Ti + H2O (I.17) Oxolation: Ti- OH + Ti- OH → Ti- O -Ti + H2O (I.18) Under hydrothermal conditions, the change... single-phase crystal formation [27] The crystallographic characteristics such as phase identification, gross structural analysis, phase purity determination, and phase transformation of TiO2 nanostructures