HUE UNIVERSITY HUE UNIVERSITY OF SCIENCESLE THI THANH TUYEN A RESEARCH OF CeO2/TiO2 NANOTUBES PREPARATION AND THEIR PHOTOCATALYTIC DEGRADATION UNDER VISIBLE LIGHT IRRADIATION Major: The
Trang 1HUE UNIVERSITY HUE UNIVERSITY OF SCIENCES
LE THI THANH TUYEN
A RESEARCH OF CeO2/TiO2 NANOTUBES
PREPARATION AND THEIR
PHOTOCATALYTIC DEGRADATION UNDER VISIBLE LIGHT IRRADIATION
Major: Theoretical Chemistry and Physical Chemistry
Code: 62.44.01.19
PhD DISSERTATION ABSTRACT
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Trang 2HUE, 2019
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Trang 3The thesis has been completed at Department of Chemistry, Hue University of Sciences, Hue University.
Supervisors:
1 Prof Dr Tran Thai Hoa
2 Dr Truong Quy Tung
Examiner
1 :
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Examiner 2 :
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Examiner 3 : :
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The dissertation will be defended at .
Time: date month year 2019 The dissertation could be found at: .
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Trang 4Being one of rare-earth metal oxides, CeO2 has attracted agreat deal of attention due to its special electron orbital structure, theunique optical, Ce3+/Ce4+ redox behavior, high thermal stability andlarge oxygen-storage capability Surface defects such as oxygenvacancies working as electron traps can impede e–/h+ recombinationand the 4f electron configuration can enhance the electron transferprocess from the adsorbed dye to oxygen species The oxygenstorage capacity makes Ce suitable to many applications as animportant component of an automatic three-dimensional catalyst or
an oxidation catalyst CeO2 is also used in many sensors, in fuel-celltechnology as a solid-state electrolyte, and even in comesticchemistry The abilities to store and liberate oxygen in Ce areseemingly facilitated by the structure being similar to that of fluorite.The oxygen in the crystals lies in parallel planes, allowing theoxygen atoms to diffuse effectively in order to form a network ofoxygen-empty holes This is favourable for the oxidation of the solid.Therefore, CeO2 has special properties in the transfer of electronsand the rise of absorb-light ability
TiO2 is classified as a semiconductor widely used in chemical techniques to decompose numerous kinds of toxic organiccontaminants because of its outstanding features TiO2 is a low-costnon-toxic compound with high chemical durability, high photo-chemical stability and biological inertness However, its photo-chemical activity only activates under UV light irradiation due to itswide band gap (3.2 eV for anatase) and fast recombination of thephoto-generated electron/hole pairs (10–9 to 10–12 s) Thus, variousapproaches have been made to further improve the photo-catalytic
photo-1
Trang 5performance of TiO2 including innovating physical properties of TiO2like morphology, dimension and crystallite phase or doping/couplingTiO2 with other metallic elements or oxides In comparison withnanoparticles, TiO2–NTs possess photo-catalytic features Depending
on the synthesizing method utilized, those preeminent features aremassive surface (up to 478 m2/g), great volume of capillary (up to1,25 cm3/g), capacity of transferring electrons from long distances,capacity of ion exchange, and noticeable capacity of absorbing light
as a result of the considerable proportion between the length and thediameter of the tube
The combination of TiO2 with CeO2 is expected tosignificantly improve the catalytic activity that derives from the role
of TiO2 as a mechanical, thermal and chemical stabilizer with thegood dispersion of CeO2 nanoparticles The adjoining surface of thisoxide system is referred as the unique center (Sui generis) whichexhibits unique chemical properties Thus, TiO2 is becoming animportant supporting substance which is investigated to elucidate therelationship between the structure and catalytic activity of the oxide-mixture system
Although the structure and properties of the CeO2/TiO2system have attracted strongly the development of the scientificresearch projects in recent years, scientific research projects on thismaterial in Vietnam is quite limited There is no project toadequately study the structural characteristics as well as theapplications in photochemical catalysis that relates to the publishedCeO2/TiO2-NTs In view of the needs and current status of research
in the country and in the world as a whole, as well as the researchconditions in Vietnam, we chose a scientific research project titled:
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Trang 6“A study of the synthesis CeO 2 /TiO 2 nanotubes and its photocatalytic activity in the visible spectrum”.
NEW CONTRIBUTIONS OF THESIS
1 This is the first time in Vietnam, we have been studiedsystematically about the synthesis of the CeO2-doped TiO2 nanotubes(CeO2/TiO2-NTs) and their visible light photocatalytic degradationbehavior
2 This is the first time, the mechanism of free radicalformation of MB photocatalytic degradation over CeO2/TiO2-NTshas been studied by using the fluorescence technique withterephthalic acid as probe and using tert-butanol as a hydroxylradical scavenger
3 This is the first time using the Arrhenuis and Eyringequations to study the photocatalytic degradation of MB byCeO2/TiO2-NTs in visible light The results showed that thephotocatalytic degradation of MB by CeO2/TiO2-NTs is controlled
by diffusion and free hydroxyl radical reaction
4 This is the first time the Box Behnken design of theresponse surface methodology was employed to optimize synthesisconditions for the photocatalytic degradation of MB over thesynthesized CeO2/TiO2-NTs, including four experimental parametersnamely; hydrothermal temperature, hydrothermal time, CeO2/TiO2molar ratio and calcination temperature
Chapter 1 LITERATURE REVIEW
1.1 Overview of photocatalytic reaction
1.2 Overview of TiO2
1.3 CeO2-doped TiO2 nanotubes (CeO2/TiO2-NTs)
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Trang 71.4 Overview of response surface methodology (RSM) in
optimization
Chapter 2 AIMS, CONTENTS AND EXPERIMENTAL
METHODS 2.1 Aims
Synthesis of TiO2 nanotubes and CeO2/TiO2 nanotubes for enhancing the photocatalytic degradation in the visible light
2.2 Contents
2.2.1 Synthesis of TiO2 nanotubes
2.2.2 Synthesis of CeO2-doped TiO2 nanotubes (CeO2/TiO2-NTs)
2.2.3 Study on the application of CeO2/TiO2-NTs in visible light photocatalytic degradation of dyes
2.3 Research methods
2.4 Experimental
Chapter 3 RESULTS AND DISCUSSION
3.1 SYNTHESIS OF CeO 2 /TiO 2 NANOTUBES (CeO 2 /TiO 2 -NTs) 3.1.1 Synthesis of TiO 2 nanotubes (TiO 2 -NTs)
3.1.1.1 Effects of hydrothermal temperature
From the SEM image (Figure 3.1), it can be seen that theobtained TiO2 is shaped like an singe fiber of nanoparticles with theaverage size of about 240 nm at 140 °C Simultaneously, there is theaccumulation of many rough surfaces on the fibers When rising to
160 °C, the tube structure of TiO2 appears more prominently thanwith the average size of 290 nm The tube structure is not onlyunclear but also appears the aggregation of several rods at 180 °C.Additionally, thin nanoplates and nanoparticles form large and long
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Trang 8bars At 200 °C, TiO2 likes thin, long and similar rods, which cannotsee the agglomeration of nanoparticles, nanorods and nanoplates as
1,291,040,140,06
17,5616,8414,7318,53
IVIVIVIV
H3H3H3H3
The Table 3.1 shows that the increase in hydrothermaltemperature leads to the decrease in the specific surface area and thevolume of capillary as well as the change in capillary diameters
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Trang 9When the hydrothermal temperature was 140 °C and 160 °C, theTiO2-NTs had a very large surface area, much larger than the P25 (50
m2/g) At 180 °C, the SBET of the TiO2-NTs declines by nearly sixtimes, compared to that of hydrothermals at 160 °C Meanwhile,samples of the hydrothermal at 200 °C had SBET less than 16 timesthat of 160 °C More importantly, at over 160 °C, the SBET of thematerial is smaller than that of P25
3.1.1.2 Effects of hydrothermal time
From the Figure 3.2, it can be seen that the main phasecomposition of the obtained TiO2 samples is in amorphous formalong with the presence of Ti9O17 and Na2Ti3O7 crystals in lowdiffraction intensity, which shows the poor crystallinity Thepresence of Na2Ti3O7 in hydrothermally synthesized TiO2 nanotubeshas been reported in numerous published studies Anatase and rutilecrystals appear in all samples at various temperatures, but thediffraction intensity is very weak The XRD result illustrates that thehydrothermal time does not significantly affect the composition, thecrystal structure and the crystallinity of the composite material.There was a sharp increase in the surface area of SBET when thehydrothermal time increased from 18 hours to 22 hours (from 107
m2/g to 275 m2/g), and the surface area decreased almost a half as thehydrothermal time increased to 24 hours
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Trang 1020 30 40 50 60 70 80
Na 2 Ti 3 O 7
Ti9O17Anatase Rutile
a sharp diffraction peak at 25.3° corresponding to the characteristicanatase (101) crystal plane appeared in the TiO2–NTs 550 samplewith a more noticeable intensity compared to that of the CeO2 dopedTiO2 sample Diffraction peaks corresponding to the (004), (200),(105), (211), and (204) planes were also present in the XRD pattern
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Trang 11In addition, a quite unclear diffraction peak near the position of 43°which can be assigned to the rutile (210) crystal plane was detected,revealing the remaining of the rutile phase in TiO2–NTs This factmight serve as an evidence of the transformation of rutile to anataseduring calcination Therefore, it is undeniable that, after beingcalcined at high temperature, there is an improvement in the anatasecrystallinity and the anatase to rutile phase transformation.
The face centered cubic CeO2 found in the synthesized CeO2/TiO2–NTs sample is based on the presence of (101) and (200)characteristic peaks The existence of the anatase phase inCeO2/TiO2–NTs at the same position as in TiO2–NTs 550 and theface centered cubic CeO2 phase is a firm proof for the separation ofthese oxides during the synthesis process; in other words, thesynthesized materials exist as CeO2–TiO2 composites As a result,ceria could disperse mainly on the surface of the TiO2 nanotubes inthe form of grains creating the boundaries in the synthesizedcomposites (Figure 3.4b)
Figure 3.4 TEM images of (a) TiO 2 –NTs, (b) CeO 2 /TiO 2 –NTs, (c)
HRTEM image
of CeO 2 /TiO 2 –NTs and (d) EDX spectrum of CeO 2 //TiO 2 –NTs
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Trang 12As shown in Figure 3.4a and 3.4b, the aggregated CeO2nanoparticles ranging from 5 to 10 nm in size on the surface of theTiO2 nanotubes could be clearly observed The TiO2 nanotubes werehollow and open-ended with an average inner diameter of 4 nm,average outer diameter of 10 nm, and about 200 nm in length Ceriadispersed mainly on the surface of TiO2 nanotubes and, therefore,formed a boundary between those particles and TiO2 nanotubes This
was confirmed by the measured lattice spacing, d of 0.27 nm due to
(200) planes of CeO2 and d of 0.35 nm due to (101) of TiO2 obtainedfrom the high-resolution HR–TEM image (Figure 3.4c).Furthermore, EDX spectra for the sample CeO2/TiO2–NTs in Figure3.4d confirmed the presence of titanium, cerium and oxygen in thedoped material
The results of XPS analysis of Ce 3d showed that a mixture
of Ce4+/Ce3+ oxidation states exists on the surface of the synthesizedCeO2/TiO2-NTs catalyst and the presence of CeO2 does not changethe binding energies of Ti 2p, and Ti exists as ion Ti4+ in the dopedmaterials The textural properties of resulting materials wereinvestigated by means of nitrogen adsorption/desorption isotherms(see Figure 3.5) It can be seen from the Figure 3.5 that the curves ofthe N2 adsorption-desorption isotherms for all samples were similar,
in which TiO2-NTs, TiO2-NTs 550 and CeO2/TiO2-NTs exhibited the
type IV isotherm according to IUPAC classification indicating the mesoporous structure of materials, and the type H3 hysteresis loop
suggesting the presence of slit-shaped pores The synthesized TiO2NTs presents a higher BET surface area (247 m2/g) than both TiO2-NTs 550 (64 m2/g) and CeO2/TiO2-NTs (66 m2/g)
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Trang 13Relative pressure (P/Po)
CeO2/TiO2-NTs
Figure 3.5 Nitrogen adsorption/desorption isotherms of TiO 2 –NTs,
TiO 2 –NTs 550 and CeO 2 /TiO 2 -NTs@0,1.
3.1.2.1 Effects of calcination temperature
It can be seen from the XRD spectra the calcinationtemperature had a strong effect on crystal structure and phasecomponents of the catalysts The introduction of CeO2 in TiO2-NTsunder proper calcination temperature (below 600 °C) did not changethe nanotubes structure of starting material At 550 °C, thecrystallization of the anatase phase obtained was the most perfectwith the sharpest diffraction peak at 25.3° The CeO2 crystalstructure found in the synthesized products at this temperature wasalso more complete with the clear characteristic peak at 28.5°.Thesurface area SBET decreased as the calcination temperature increased,especially when the temperature changed from 400 °C to 550 °C led
to the decrease in SBET from 149 m2/g to 65 m2/g
Compared to bare TiO2 nanotubes and bare CeO2,CeO2/TiO2-NTs@X clearly exhibits a broader absorption in thevisible region (wavelength = 400-600 nm), displays a slight red shift
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Trang 14in the optical adsorption (Figure 3.6) The band gap values calculated
of CeO2/TiO2-NTs is found to decrease with the increased dopedratio CeO2/TiO2 2.64 eV, lower than that of TiO2-NTs (3.08 eV) andCeO2 (2.93 eV) This red shift of the adsorption edge indicates theenhanced ability of the CeO2/TiO2-NTs hybrid catalyst to adsorbvisible light The red shift in the optical transition as well as the
decreased band gap value (E g) of the CeO2/TiO2-NTs compositecompared to TiO2-NTs and CeO2 were suggested to be relative tothe presence of Ce3+ ions with one electron in the strong localized 4f
orbitals as observed from the XPS spectra
Figure 3.6 a) UV–Vis diffuse reflectance spectra and b) Tauc’ plots
3.2 THE PHOTOCATALYTIC ACTIVITY OF CeO 2 /TiO 2 -NTs 3.2.1 The MB adsorption of CeO 2 /TiO 2 -NTs
-0.50
0.5
f(x) = 0.02 x² − 0.23 x + 0.65R² = 0.99
CeO 2 -TiO 2 -NTs@0,05 CeO 2 -TiO 2 -NTs@0,08 CeO 2 -TiO 2 -NTs@0,5 CeO2-TiO2-NTs@0,2 CeO 2 -TiO 2 -NTs@0,1
0.0 0.5 1.0 1.5 2.0 2.5
CeO2-TiO2-NTs@0,05 CeO2-TiO2-NTs@0,5 CeO2-TiO2-NTs@0,1
Trang 15The point of zero charge (pH PZC) of CeO2/TiO2-NTsestimated by the pH drift method is approximate 3.97 At pH < 3.97,the surface of the CeO2/TiO2-NTs is charged positively due toprotonation and is charged negatively when pH > 3.97
The experimental data of the first order kinetic model with
the high coefficient of determination (R 2 = 0.912-0.963) implies that
the adsorption process of MB on CeO2/TiO2-NTs followed the
pseudo- first order kinetic model This means that the adsorption
process was controlled by a physical adsorption and the rate-limitingstep in this case involved a diffusion of MB to the surface of TiO2nanotubes
The experimental data are analyzed according to the linear form
of Langmuir and Freudlich model The both R2 values are high and
significant p-value < 0.05 which indicates the equilibrium data are fixed
well both isotherm model of Langmuir and Freundlich These resultsimplies a monolayer adsorption and the existence of heterogeneoussurface in the adsorbents, respectively
3.2.2 Photocatalytic degradation of MB over CeO 2 /TiO 2 -NTs in the visible light
Adsorption kinetics and photo-catalytic kinetics over severalcatalysts (TiO2–NTs 550, CeO2, CeO2/TiO2–NTs, and commercialP25 for sake of comparison) are represented in Figure 3.8.Practically, no change in MB concentration was noticed after lightillumination for 150 minutes without any catalyst (blank sample)
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