Summary of chemistry doctoral thesis: Synthesis and characterization of doped Mn, Ce and C to ZnO nanoparticles and evaluation of their photo-oxidation potentiability

The objectives of the thesis: The thesis focused on the synthesis of Mn, Ce, C doped ZnO nanoparticles materials and doped ZnO with multi-layered carbon nanotube nanocomposite materials and assessed their photo-oxidative potentiality through photocatalytic reactions of methylene blue (MB) decomposition in aqueous solution under visible light.

MINISTRY OF EDUCATION AND TRAINING VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY SCIENCE AND TECHNOLOGY - LUU THI VIET HA SYNTHESIS AND CHARACTERIZATION OF NANOMATERIALS Mn, Ce AND C DOPED ZnO AND EVALUATION OF THEIR PHOTO-OXIDATION POTENTIAL Major: Inorganic chemistry Code: 9.44.01.13 SUMMARY OF CHEMISTRY DOCTORAL THESIS Hanoi - 2018 This thesis was completed at: Graduate University Science and Technology - Vietnam Academy of Science and Technology Adviser 1: Assoc Prof Dr Luu Minh Đai Adviser 2: Assoc Prof Dr Dao Ngoc Nhiem 1st Reviewer: 2nd Reviewer: 3rd Reviewer: The thesis will be defended at Graduate University of Science and Technology - Vietnam Academy of Science and Technology, at hour date month 2018 Thesis can be found in The library of the Graduate University of Science and Technology, Vietnam Academy of Science and Technology INTRODUCTION The urgency of thesis Today, the rapid growth of industries is in parallel with the level of serious environmental pollution, especially water pollution The textile industry is one of the industries that causes bad water pollution due to direct discharge of wastewater into rivers The World Bank estimated that from17 % to 20 % of industrial water pollution came from dyeing and fabric processing plants which cause alarming number for textile manufacturers, as well as environmental managers and scientists So far, the methods of treating textile wastewater are used such as coagulation, sintering, biodegradation, adsorption by activated carbon, oxidation methods Among these methods, the biodegradation is widely applied to treat textile wastewater on a large scale However, under anaerobic condition, azo dye can be reduced to byproducts as the very toxic aromatic amines Recently, we have found out advanced oxidation method which is a new and promising method for treating textile and dyeing wastewater This method usually uses a catalyst as photocatalysts to generate OH radicals , under illumination which have strong oxidizing ability and can breakdown most organic chemicals Photocatalysts are oxides such as TiO2, ZnO, SnO2, WO2 and CeO2, which are abundant in nature and are widely used by heterogeneuos processes Among them, ZnO is considered to be a promising catalyst for decomposition of organic pigments as well as for water disinfection The photocatalytic ability of ZnO is higher than that of TiO2 and some other semiconductor oxides on the basis of absorption of solar radiation energy However, ZnO has a relatively large (3,27eV) bandgap energy, which corresponds to the ultraviolet light zone for optimum photocatalytic efficiency Meanwhile, ultraviolet light accounts about % radiation of solar Therefore, practical application of ZnO has limited In order to improve photocatalytic activity and expand application field, it is necessary to transform the electron properties in ZnO nanostructure and reduce bandgap energy and electron recombination dynamics and optical hole properties DopedMetallic or non-metallic or co-doped metallic and non-metalic to the ZnO is one of the effective methods to increase photocatalytic activity of ZnO Thus, the research topic of the thesis "Synthesis and characterization of doped Mn, Ce and C to ZnO nanoparticles and evaluation of their photo- oxidation potentiability" was selected with the following objectives and contents: The objectives of the thesis: The thesis focused on the synthesis of Mn, Ce, C doped ZnO nanoparticles materials and doped ZnO with multi-layered carbon nanotube nanocomposite materials and assessed their photo-oxidative potentiality through photocatalytic reactions of methylene blue (MB) decomposition in aqueous solution under visible light The content of the thesis: 2.1 The material synthesis: - Synthesis of Mn doped ZnO and Ce doped ZnO nanoparticle materials by combustion and hydrothermal method; - Synthesis of C, Mn and C, Ce co-doped ZnO nanoparticles by hydrothermal method; - Synthesis of C, Ce co-doped ZnO combined with multi-layer carbon nanotube composite materials 2.2 Physical properties and characteristic studies of the synthesized materials: Synthesized materials were investigated using thermo-gravimetric and differential thermal analysis (DTA-TG), X-ray diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FT-IR), UV-Vis diffuse reflectance spectroscopy (UV-VIS), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS), Brunauer-Emmett-Teller (BET)analysis and energy –dispersive x-ray spectroscopy (EDX) 2.3 Study of photo-oxidative potentiality of the materials: Synthesized materials were evaluated for photo-oxidative potentiality through photocatalytic reactions of methylene blue (MB) decomposition in aqueous solution under visible light CHAPTER 1: OVERVIEW 1.1 ZnO materials 1.1.1 Introduction of ZnO 1.1.2 Application of ZnO 1.2 Methods of synthesizing ZnO materials 1.2.1 Hydrothermal method 1.2.2 Combustion method 1.3 Doped ZnO materials 1.3.1 Doped ZnO materials 1.3.2 Reseaches of ZnO and doped ZnO photocatalytic materials 1.4 Photocatalyst 1.4.1 ZnO photocatalysis 1.4.2 Doped ZnO photocatalysis CHAPTER 2: EXPERIMENTS AND RESEARCH METHODS 2.1 Synthesis of materials 2.1.1 Synthesis of Mn doped ZnO and Ce doped ZnO 1.1.1.1 Synthesis of Mn doped ZnO and Ce doped ZnO by combustion method (Mn-ZnOĐC) a> Synthesis of Mn doped ZnO by combustion method 1,32 grams of PVA were dissolved completely in 40 ml of distilled water at 50oC, then cooled to room temperature and added slowly 9,9 ml of Zn(NO3)2 1M solution and 0,1 ml of Mn(CH3COO)21M solution (molar ratio Mn2+/ (Zn2+) = 1:100) The solution was stirred on the magnetic stirrer Distilled water was added and adjusted pH = by ammonium acetate buffer The solution was stirred continuously on the magnetic stirrer for about hours at 80°C until the water is evaporated completely The gel obtained is clear, transparent and viscous The gel was aged about 12 hours at room temperature anddred at 100oC and heated at 500oC to obtain gray powder The surveys on the conditions of Mn-ZnOĐC materials were synthesized - Effects of temperature: Experimental process was synthesized as above, the samples were calcited at different temperatures, from 300 to 800oC in the air with the heating ramp of 8oC/min - Effects of pH: The experiments werethe same conditions with different pH The pHs of the reaction solution were changed 3, 4, and respectively - Effects of doped manganese concentration: The molar ratios of Mn2+/ Zn2+ were 1%; 3%; 5% and 8% respectively 𝑃𝑉𝐴 - Effect of PVA content: The molar ratios of the synthesized 𝑍𝑛2+ +𝑀𝑛2+ of samples were 1/1; 2/1; 3/1 and 4/1 respectively - Effect of temperature of gel formation: The temperatures of gel formation were 50°C, 60°C, 80°C and 95°C, respectively b> Synthesis of ZnO doped by Ce by combustion method The synthesis of Ce doped ZnO(Ce-ZnOĐC) was the same as that of the Mn-ZnOĐC synthesis Salt Mn(CH3COO)2was changed by salt Ce(NO3)3 2.1.1.2 Synthesis of Mn doped ZnO and Ce doped ZnO by hydrothermal method a>Synthesis of Mn doped ZnO doped (Mn-ZnO) 0,664 g of Zn(CH3COO)2.2H2O and an amount of Mn(CH3COO)2.4H2O salt (the molar ratio of Mn2+/Zn2+ 2%) were dissolved in 75 ml of C2H5OH to obtain the solution A 0,4g of NaOH were dissolved in 75ml of distilled water to obtain solution B Solution A was added gradually to solution B and stirred for 1,5 hours The whole mixture was transferred to the autoclave and heat in the oven at 150°Cfor 24 hours cooled to room temperature, filtered and rinsed twice with distilled water and ethanol and dried the final product at 80°C for 10 hours to obtain a solid powder The survey on the conditions of material was synthesized - Effects of the temperature: The temperatures of hydrothermal were changed from 110°C to 170°C - Effects of NaOH concentration: The molar ratios of NaOH/Zn2+were changed 1.5; 3.0 and 6.0 - Effect of solvent: The volume ratio (ml) of water/ethanol (H/R) solution were 150/0, 70/80, 110/40 and 40/110 - Effect of doped manganese content: The molar ratios were Mn2+/Zn2+ were 1%, 2% and 6%, respectively - Influence of PVA content: Experiments carried out were 7%, 10% and 15%, respectively b> Synthesis of Ce doped ZnO by hydrothermal method The synthesis of Ce-ZnO was prepared similarly to the Mn-ZnO synthesis Salt Mn(CH3COO)2 was replaced by salt Ce(NO3)3 2.1.1.3 Synthesis of C, Mnco- doped ZnO and C,Ce co-doped ZnO by hydrothermal method a> Synthesis of C and Mn co-doped ZnO by hydrothermal method (C, MnZnO) 0,664 g of Zn(CH3COO)2.2H2O and 0,015 g of Mn(CH3COO)2.4H2O salt (the molar ratio of Mn2+/Zn2+ 2%) were dissolved in 60 ml of C2H5OH to obtain the solution A 0,0132 g of PVA (the molar ratio of PVA/Zn2+ =10%) were dissolved in 30 ml of distilled water to obtain solution B 0,4 g of NaOH (the molar ratio of NaOH/Zn2+ = 3) were dissolved in 60 ml of C2H5OH to obtain solution C Solution A was added gradually to solution B and stirred for 0,5 hours Then, solution C was added gradually and stirred for hours This mixture was transferred to the autoclave and heat in the oven at 150°C for 24 hours After, the autoclave was cooled to room temperature The thermal products were filtered and rinsed twice with distilled water and ethanol and dried the final product at 80°C for 10 hours to obtain a solid powder The survey on the conditions for material was synthesized - Effects of the temperature: The temperatures of hydrothermal were changed from 110°C to 170°C - Effects of NaOH concentration: The molar ratios of NaOH/Zn2+were changed 1.5; 3.0 and 6.0 - Effect of solvent: The volume ratio (ml) of water/ethanol (H/R) solution were 150/0, 70/80, 110/40 and 40/110 - Effect of doped Manganese content: The molar ratios were Mn2+/Zn2+ were 1%, 2% and 6%, respectively - Influence of PVA content: Experiments carried out were 7%, 10% and 15%, respectively b> Synthesis of C,Ce co-doped ZnO (C, Ce-ZnO) The synthesis of C,Ce-ZnO was prepared similarly to the C,Mn-ZnO synthesis Salt Mn(CH3COO) was replaced by salt Ce(NO3)3 2.1.3 Synthesis of C, Ce co-doped ZnO combined with multi-layer carbon nanotube composite materials (C,Ce-ZnO/MWCNTs) a>Preperation and treatment of multi-layer carbon nanotubes (MWCNTs) The mixture of grams of carbon (MWCNTs) was boiled with 500ml by concentrated HNO3 solution in the circulation system about hours MWCNTs were rinsed by deionized water to pH = and dried at 90oC for 12 hours and continued to boil it by 500ml NaOH 0.5M for hours in the circulation system and cooled and rinsed several times with deionized water to pH approximately and dried at 90oC for 12h b> Synthesis of C,Ce-ZnO/MWCNTs nanocomposite materials The condition to prepare C,Ce-ZnO/MWCNTs materials was the same as that to prepare C, Ce-ZnO, The difference in reaction condition is the of weight percentages of MWCNTs/materials of approximately 16.94%; 22.22%; 37.97%; 50.5% and 62.01%, respectively denoted CZCT1, CZCT2, CZCT3, CZCT4 and CZCT5 were added after solution was stirred 60 minutes 2.2 The methods to study materials Characteristic properties of materials were investigated using thermogravimetric and diferential themal analysis (DTA-TG), X-ray diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FTIR), UV-Vis diffuse reflectance spectroscopy (UV-VIS), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS), Brunauer-Emmett-Teller (BET)analysis and energy –dispersive x-ray spectroscopy (EDX) 2.3.Evaluation of photocatalytic activities of materials: 2.3.1.Photocatalytic reactions of methylene blue (MB) decomposition of materials under visible light a >Caribration curve results of MB solution The caribration curve was created and identified by linear interval of MB solution b> Perform photocatalytic reactions 0.1 g of material were reacted in 100 ml of MB solution under visible light (from Osram and Ace glassphotochemical equipment- sunlight simulated photocatalytic device) First, the mixture was stirred in the dark until the solution was in absorptive balance or ml of MB solution was centrifuged to extract the solids and measure the optical density (time t = 0, optical density Ao) The solution was illuminated and stirredcontinuosly For each period of time, ml of sample were centrifuged and measured density (At) until the solution was discolored Decompositon efficiency MB was calculated with the formula: 𝐶 −𝐶 𝐴 −𝐴 𝐻 = 𝑜 𝑡 × 100 = 𝑡 × 100 𝐶𝑜 𝐴0 In which: Ao: initial optical density, At: optical density at that time, H: decomposition efficiency of MB solution 2.3.2 Determination of photodynamic kinetics Langmuir - Hinshelwood model were used to test the photocatalytic kinetic reactions The kinetic equation is ln (Co/Ct) = kapp.t Where Co and Ct are the reactant concentrations at times t = and t = t respectively, kappis constant of speed of reactions 2.3.3 Measurement method of chemical oxygen demand (COD) TCVN 6491: 1999 COD was determined by accordance with TCVN 6491: 1999 (ISO 6060: 1989) and SMEWW 5220 standardized methods for analysis water and waste water CHAPTER 3: RESULTS AND DISCUSSIONS 3.1 The synthesis of Mn doped ZnO and Ce doped ZnO materials In this section, Mn doped ZnO and Ce doped ZnO materials were synthesized by two different methods: combustion and hydrothermal method The effects of factors on the structure, phase composition and average crystal size of the materials were investigated by using X-ray diffraction (XRD) method The characterized properties and photocatalytic activity of representative materials (materials that had been synthesized at the best conditions within the scope of the thesis review) were studied by using modern physic-chemical methods 3.1.1 X-ray diffraction (XRD) results (a) (b) (c) (d) Fig 3.1 The XRD paterns of (a) Mn-ZnOĐC, (b) Ce-ZnO ĐC, (c) Mn-ZnOTN and (d) Ce-ZnOTN with the different doping content of Mn and Ce The effects of doped Mn and Ce on the structure, crystal phase composition of the synthesized materials were focused The Mn doped ZnO and Ce doped ZnO materials synthesized by the combustion and hydrothermal method were signed by Mn-ZnOĐC,Ce-ZnOĐC and MnZnOTN, Ce-ZnOTN The results of XRD (fig 3.1a, 3.1c, 3.1d) were shown that all of the Mn-ZnOĐC, Mn-ZnO-TN and Ce-ZnOTN materials and the samples with the mole ratio Ce3+/Zn2+ ≤ 2(fig 3.1b) had hexagonal wurtzitesingle phase structure of ZnO However, the XRD patterns of Ce-ZnOĐC with the molar ratio of Ce3+/Zn2+ ≥ (Fig 3.1b) were shown that there were a cubic phase of Ce or CeO2 It was remarkable that the XRD diffraction peaks of these materials were shift slightly towards smaller theta diffraction angle as compared to XRD patern of ZnO synthesized at the same condition It was suggested that Mn and Ce were doped into ZnO due to interstitial incorpration of Ce ions in ZnO matrix or replaced by Zn ions with Mn ions in ZnO crystal lattice Ion Zn2+ and ion Mn2+ had the same charge and their radius were not significantly different (Zn2+: 0.74Ao, Mn2+: 0.8Ao) Therefore, Mn2+ions can be subtituted Zn2+ ions or occupied defects during the synthesis process Radius of Ce ions (Ce4+: 0.92Ao, Ce3+: 1.03Ao) were bigger and more different that of Zn ions so Ce ions can be successfully intergrated into the Zn ions sites.Cerium ions were easily reduced or oxidized to form Ce and CeO2 because of the short reaction time (2 hours) and high heating temperature (550oC)(XRD patern 3.1b) 3.1.2 FT-IR spectra results Table 3.1 Wavenumber and vibration of the bonds Wavenumber (cm-1) Ocillated MnCeMnCeZnO bonding ZnOĐC ZnOĐC ZnOTN ZnOTN 1629-1645 1587 1593 1651 1599 δH-O-H νO-H 3425 3415 3448 3448 3440-3462 νO-C=O 2360 2362 - - 2364-2366 𝛎𝐍𝐎−𝟑 1406 1406 - - 1384 𝛎𝐂𝐎𝐎− - - 1550 1550 - νZn-O-M 445 457 509 524 431-443 14 (a) (b) Fig 3.9 XRD paterns of C,Ce-ZnO at (a) different molar ratios of Ce3+/Zn2+ and (b) different molar ratios of PVA/Zn2+ 3.2.2 Infrared spectrum results of C, Mn-ZnO and C, Ce-ZnO The results of the FT-IR spectra of C, Mn-ZnO and C, Ce-ZnO (Fig 3.10) were shown that the shifts of wavenumber of the Zn-O bonding absorption in Mn-ZnO and C, Mn-ZnO as well as in Ce-ZnO and C, CeZnO towards the large wavenumber compared with ZnO However, these shifts in C, Mn codoped ZnO and C, Ce codoped ZnO were shorter than those in Mn doped ZnO and Ce doped ZnO It was suggested that manganese and carbon as well as cerium and carbon were successfully codoped into ZnO matrix and formed the Zn-O-Mn and Zn-O-Ce bonds (a) (b) Fig 3.10 The FT-IR spectra of (a) C, Mn-ZnO; (b) C, Ce-ZnO 3.2.3 SEM and UV-VIS results of C, Mn-ZnO and C, Ce-ZnO 15 The SEM images of C, Mn-ZnO and C, Ce-ZnO (Figure 3.11) were shown that the particles were ellipsoids, fair uniform, sharpness and nanometer-size Their surfaces were sponge (a) (b) Fig 3.11 SEM images of (a) C,Mn-ZnO; (b) C,Ce-ZnO The UV-VIS spectra of C, Mn-ZnO and C, Ce-ZnO (Figure 3.12) were demonstrated the successful co-doping of Mn and C as well as C and Ce into ZnO by hydrothermal method with their smaller narrow band gap Band gaps of C,Mn-ZnO, and Ce,C-ZnO were determined 2.86 eV and 2.95 eV respectively, corresponded by the wavelengths less than of 434 nm and 421 nm These band gap levels were smaller than Mn-ZnOTN (3.0 eV) and Ce-ZnO (3.03 eV) respectively There, simultaneous co-dopings of carbon and manganese, carbon and cerium enhanced the optical properties of ZnO in the visible light (a) (b) Fig 3.12 Band gap of (a) C,Mn-ZnO; (b) C,Ce-ZnO 3.2.4 The XPS spectra of C,Mn-ZnO and C,Mn-ZnO The XPS spectra of C, Mn-ZnO (Fig 3.13a) confirmed the presence of elements in sample such as Zn, Mn, C and O as well as their chemical states On this spectral image, the peaks at 1022.2 and 1045.38 characterized the bond energies of Zn2p3/2 and Zn2p1/2, respectively In the bonding energy region of the O1s, two peaks were observed one of them was high intensity peak at 531 eV and low intensity peak at 533.01 eV The high intensity peak was corresponded to O-2 in Zn-O bond of the ZnO cryticle lattice The low intensity could be contributed to C-O bond 16 that could be formed by doping carbon into the ZnO matrix Notably, the C1s states were appeared at 285,64 and 289,58 eV corresponded to the COH and O- (C = O) bonds, respectively Here, these peaks were related to the C-O and O- (C = O) bonds in the ZnO matrix This could be predicted that Zn was linked to C-O and C-OH Otherwise, the energy levels at 641.2 and 655.3 eV were corresponded to Mn2p3/2 and Mn2p1/2 This was confirmed that manganeses were existed in the representative sample and the successfully doped into the ZnO crystal lattice Ions of Mn2+ and Zn2+ had the same ion charge and their radius were not much different., Therefore, the Mn2+ ions could be easily replaced the Zn2+ ion sites or occupied into cation holes in ZnO lattice From the XPS results, the atomic percentages of C1s, O1s, Zn2p and Mn2P in C, Mn-ZnO samples were determined 16.4%; 49.63%; 32.49% and 1.48% respectively Therefore, the XPS spectra of C, Mn-ZnO were confirmed the successful doping of carbon and manganese into ZnO matrix (a) (b) Fig 3.13 XPS spectra of (a) C,Mn-ZnO and (b) C,Ce-ZnO 17 The XPS spectrum of C, Ce-ZnO (Fig 3.13b) were shown that the peaks at 1021.3 eV and 1044.2 eV were corresponded to Zn2p3/2 and Zn2p1/2 respectively The wide peak at 530.25 eV was assigned to O1s It was noted that the C1s orbital with bond energy displayed at respective peaks at 282.3 eV, 285.4 and 289.6 The first peak was attributed to carbon in the Zn-C bond due to carbon replacing oxygen in ZnO lattice The second peak was corresponded to referential C of the measurement The third peak was revealed O-(C=O) bond Particularly, there were presence of two spin-orbital sets of Ce3d3/2 and Ce3d5/2 The peaks at 881.9 eV; 885,18 eV; 897,88 eV; 900.2 eV belongedto Ce3d3/2 and the peaks at 903.2 eV; 907.08 eV and 916.28 eV belonged to Ce3d5/2 The peaks at 881.9 eV; 897,88 eV; 900.2 eV; 907.08 eV and 916.28 eV were assigned to +4 oxidation of Ce The two remaining peaks which were weak at 885.18 eV and 903.2 eV were characterised for cerium with +3 oxidation state Therefore, cerium existed with two oxidation states, +3 and +4 The +4 oxidation state was major From the XPS spectrum of C, Ce-ZnO, the atomic percentages of Zn2p, O1s, Ce3d and C1s were determined to be 22.32%; 52.15%; 1.95% and 23.58%, respectively In conclusion, the results of XRD, IR, SEM, XPS and UV-VIS study were shown that C, Mn-ZnO materials were successfully synthesized by hydrothermal method Materials were in the hexagonal wurtzite structure, nanometer size, elliptical shape (nanoelipsoid) C, Mn-ZnO and C, Ce-ZnO materials could be absorbed goodvisible light 3.2.5 Photocatalytic activities of ZnO-Mn, C and ZnO-Ce, C 3.2.5.1 Photocatalytic activities of C, Mn-ZnO and C, Ce-ZnO (a) (b) Fig 3.14 The MB decomposition efficiencies of (a) ZnO, Mn-ZnO and C,Mn-ZnO; (b) ZnO,Ce-ZnO and C,Ce-ZnO Figure 3.14 was shown that the MB decomposition efficiencies increased in the order of ZnO