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Inverse Opal TiO2 photocatalysts for environmental applications
Roberto Fiorenza,*a Marianna Bellardita,b Salvatore Sciré,a Leonardo Palmisano,b Bao-Lian Su.c a Dip. Scienze Chimiche, Università di Catania, Italy b DEIM, Università di Palermo, Italy c Laboratory of Inorganic Chemistry CMI, University of Namur, Belgium
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INTRODUCTION Photocatalysys: Environmental Applications
AOP (Advanced Oxidation Processes) for water purification Eco-compatibility Low temperature Mild Oxidation condition Applicability to a wide selection of contaminants Photocatalysis is one of the most attractive advanced oxidation technologies for thorough decompositions of organic pollutions in water or air [19]. Several photocatalysts have been developed [20]; Titania (TiO2) is the most studied and Titanium dioxide (TiO2) has become a dominant photocatalyst since Fujishima and Honda’s pioneering work in the photoelectrocatalytic oxidation of water in 1972 It is well established that the photocatalytic oxidation process is initiated through the photogeneration of electron/hole pairs under UV illumination (Fig. 1). Illumination of TiO2 with a photon whose energy is equal to or greater than the band-gap energy (i.e., Eg = 3.2 eV, excitation light wavelength range shorter than ca. 400 nm, Eg = hc/λ=1240/ λ) will lead to the promotion of an electron from the valence band (VB) to the conduction band (CB) (Eq. (1)), resulting in free photoelectrons in the conduction band (ecb−) and photoholes in the valence band (hvb+). Owing to its powerful oxidation capability the photohole is able to mineralize almost all types of organic compounds.
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TiO2 High Band-gap Charge recombination
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AIM OF WORK Synthesis of a highly ordered porous TiO2 with an Inverse Opal Structure How different chemical agents influence: Chemico-physical properties Photocatalytic perfomance in the degradation of dye (RhB) in water under Vis irradiation CeO2 (1-25 wt.%) N, W and Hf (0.2-2 at.%) with the aim to synthesize, an active catalyst for both reactions.
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INVERSE OPAL MATERIALS
Inverse opals are photonic crystals consisting of fcc packed voids embedded in a high refractive index material (opal) matrix Semiconductor/electrons Periodic Potential Band-gap Photonic cristal/photons Periodic index of refraction Photonic band-gap λ irradiation A crystal is a periodic arrangement of atoms or molecules. The pattern with which the atoms or molecules are repeated in the space is the crystal lattice. There may be gaps in the energy band structures of the crystal, meaning that electrons are forbidden to propagate with certain energies in certain directions. If the lattice potential is strong enough, the gap can extend to cover all possible propagation directions, resulting in a complete band-gap, as the semiconductors, where there is a complete gap between the valence and the conduction energy bands. The optical analogue is the photonic crystal, in which the atoms or molecules are replaced by macroscopic media with differing dielectric constant and the periodic potential is replaced by a periodic dielectric constant (or, equivalently, a periodic index of refraction) [2]. If the dielectric constants of the constituent media are different enough, Bragg scattering off the dielectric interfaces can produce many of the same phenomena for photons as the atomic potential does for electrons [3]. Thus a photonic crystal could be design to possess a photonic band-gap: a range of frequencies for which light is forbidden to exist within the interior of the crystal. I t is easy to make a parallelism between the couples photonic crystals/photons and semiconductor/electrons. Indeed, the photonic band gap is similar to the electronic band gap and this similarity allows us to imagine a transposition of electronic technologies in photonics technologies for many applications In particular this effect can be used to enhance the light absorption only if a specific condition is verified: the wavelength of slow photons (at red or blue edge) overlaps with the electronic excitation wavelength (electronic band-gap frequency) of semiconducting materials and the applied irradiation light wavelength. When these three wavelengths fall at the same wavelength zone, i.e. the condition cited above is satisfied, an enhancement of the light absorption by these slow photons can be expected and these slow photons will excite electrons of semiconducting materials from valence band to conduction band with the generation of a large number of electron-hole pairs resulting in an enhanced photocatalytic activity [24-26 INVERSE OPAL TiO2 λ Slow Photon Effect λ excitation
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Inverse Opal TiO2 : Synthesis
The first mass loss, between 20 and 300 °C, is related to the evaporation of solvents: ethanol, water and 2-propanol (formed during the synthesis of the TiO2 sol), it is marked by a broad endothermic peak. Between 300 and 350 °C, significant loss of mass is observed, accompanied by a strong exothermic peak corresponding to the degradation of polystyrene coke. The third mass loss, lower, is between 350 and 500 °C, characterized by a major exothermic peak. The coke oxidation and condensation of the Ti-O-H hydroxides in Ti-O-Ti take place. Preparation of polystyrene (PS) Opal Spheres Infiltration by sol-gel method using a solution of titanium isopropoxide Aging and calcination to obtain an inverse opal structure
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SAMPLES MORPHOLOGY Inverse Opal TiO2
They exhibit a homogeneous close-packed structure. The PS sphere size was around 300 nm, with a low polydispersity (<5%). The porous TiO2obtained after calcination and consequently removal of PS templatehad aordered inverse opal structure with interconnected pores (Figure 11). No substantial variation in the morphology of TiO2 was related due to the presence of doping agents.
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Phase Composition TiO2-25%CeO2 Cr. Size = 34 nm TiO2 Cr. Size = 27 nm
Only anatase phase are present after the calcination at 550°C In consideration of the low amount of doping agent (0.2-2 at.%) no substantial variations In the TiO2- N -Metal ions systems respect to the bare TiO2 pattern and in the crystallite size are detected The lack of signals of the other oxide with an amount lower of 25 wt.% is presumably due to a combination of the low content and high dispersion of the other component on TiO2 composites Bare BiVO4 nanoparticles correspond to the monoclinic scheelite phase In the TiO2-BiVO4 system ,indeed, the crystal size varies progressively with the amount of bismuth vanadate from 23 to 29 nm. In this case only a low amount of the second oxide (1-5% wt.) prevents the formation of bigger crystal domains.
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OPTICAL PROPERTIES: UV-Vis DRS SPECTRA
Catalysts Eg(eV) TiO2 3.23 TiO2-1%CeO2 TiO2-2%CeO2 3.24 TiO2-3%CeO2 3.21 TiO2-5%CeO2 3.22 TiO2-10%CeO2 3.18 TiO2-25%CeO2 3.17 CeO2 2.72 TiO2-0.2%N TiO2-0.5%N TiO2-1%N 3.27 TiO2-2%N TiO2-0.2%W 3.26 TiO2-0.5%W TiO2-1%W TiO2-2%W TiO2-0.2%Hf TiO2-0.5%Hf 3.29 TiO2-0.5%Hf-1%N the Kubelka-Munk method [45, 46]: αhν= A(hν−Eg)1/2 , where α, h, ν, Eg, and A are the absorption coefficient, Planck's constant, light frequency, band-gap energy, and a constant, respectively.
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XPS ANALYSIS: TiO2-CeO2 A mixture of Ce3+/Ce4+ states exists on the surface of TiO2-CeO2 catalysts The presence of Ce3+ species, is attributed to the occurrence of a strong interaction between TiO2 support and CeO2 nanolayer Catalysts Ti (at %) O (at%) Ce3+/Ce4+ ratio TiO2 22.5 55.7 - TiO2- 3%CeO2 22.9 57.1 0.46 TiO2- 25%CeO2 20.2 53.1 0.15 causing a mutual influence between the Ti and Ce metal cations that could change their oxidation states more easily on the surface.
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XPS ANALYSIS: TiO2-N-W-Hf
No substantial shift in the Ti 2p and in the O 1s region The binding energy peak of N 1s was at about eV for all the samples. The signal can be attributed to a nitrogen atom in the environment O-Ti-N The hafnium had a surface concentration lower of the nominal one. This is probably due a more homogeneous dispersion of hafnium on the surface and inside the pore of TiO2 Catalysts Ti (at %) O (at%) N (at%) Hf (at%) TiO2 22.5 55.7 - TiO2- 0.5%N 23.4 57.8 0.1 TiO2- 2%N 22.7 56.5 0.2 TiO2- 0.5%Hf 29.7 51.5 0.08 TiO2- 0.5%Hf-1%N 22.4 55.9 0.3 0.07 Catalysts Ti (at %) O (at%) N (at%) Hf (at%) TiO2 22.5 55.7 - TiO2- 0.5%N 23.4 57.8 0.1 TiO2- 2%N 22.7 56.5 0.2 TiO2- 0.5%Hf 29.7 51.5 0.08 TiO2- 0.5%Hf-1%N 22.4 55.9 0.3 0.07
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XPS ANALYSIS: TiO2-N-W-Hf
No substantial shift in the Ti 2p and in the O 1s region The signals of tungsten are attributed to the formation of WO3, whereas the fitting at 35.2 eV and 35.5 eV are consistent with the W4+ oxidation state The surface of TiO2-0.5% W was richer in tungsten respect to the nominal concentration. Probably a lower amount of tungsten leads facilitate the ionic exchange between W and Ti ions favoring a surface enrichment of tungsten oxide Also in this case, as reported in the literature, the stoichiometric ion exchange between W4+ and Ti4+ might occur. In fact, W4+ can substitute Ti4+ in the lattice of TiO2 because of the similarity in the ionic radius of W4+ and Ti4+, the bond lengths to W–O and Ti–O and the crystal structures of WO2 and TiO2. So, non-stiochiometric solid solution of WxTi1−xO2 would form, with possible producion a tungsten impurity energy level Catalysts Ti (at %) O (at%) W (at%) TiO2 22.5 55.7 - TiO2- 0.5%W 21.8 53.4 2.2 TiO2- 2%W 21.7 54.7 2.3
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Experimental Setup: Photocatalytic oxidation
The reactor was illuminated using 6 neon under visible light irradiation ( nm). The intensity of UV light emitted by the neon lamps was very low and can be neglected, and hence no UV filter was used to cut the UV light. The fan located on the surface of the cylindrical reactor maintained the reaction temperature at room temperature.
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PHOTOCATALYTIC ACTIVITY
Compared to commercial TiO2 anatase the Inverse Opal TiO2 exhibited higher activity It has been shown that the major absorption peaks of RhB, located at around 554 nm, diminished gradually under visiblelight irradiation in the presence of the TiO2 porous photocatalyst A very low degradation rate of RhB,similar to that reported in the literature [27, 90], was observed and can be neglected. It can clearly be observed that for some samples thedegradation of RhB increases gradually with increasing irradiation time. It is possible to note that the single porous TiO2 material (red curve, Figure 27a) exhibit a good catalytic performance; namely 60% of RhB degradation after 120 minutes of irradiation; it is interesting becausedue toits wide band-gap energy in the UV range, TiO2 could not absorbin the visible range. Thus theoretically, no possible degradationof dye molecules by TiO2 can be expected The photocatalytic degradation process of RhB follows first-order kinetics
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PHOTOCATALYTIC ACTIVITY Inverse Opal TiO2-CeO2 systems
Only a low amount of CeO2 (1, 2 and 3 wt.%) had a positive effect, whereas an higher amount had a negative effect It has been shown that the major absorption peaks of RhB, located at around 554 nm, diminished gradually under visiblelight irradiation in the presence of the TiO2 porous photocatalyst A very low degradation rate of RhB,similar to that reported in the literature [27, 90], was observed and can be neglected. It can clearly be observed that for some samples thedegradation of RhB increases gradually with increasing irradiation time. It is possible to note that the single porous TiO2 material (red curve, Figure 27a) exhibit a good catalytic performance; namely 60% of RhB degradation after 120 minutes of irradiation; it is interesting becausedue toits wide band-gap energy in the UV range, TiO2 could not absorbin the visible range. Thus theoretically, no possible degradationof dye molecules by TiO2 can be expected
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PHOTOCATALYTIC ACTIVITY Inverse Opal TiO2-N-Metal ion systems
The doping with nitrogen and metal ions lead to improve the photocatalytic activity of TiO2, mainly when the doping agent is present at low concentration (0.2, 0.5, 1 at.%) It has been shown that the major absorption peaks of RhB, located at around 554 nm, diminished gradually under visiblelight irradiation in the presence of the TiO2 porous photocatalyst A very low degradation rate of RhB,similar to that reported in the literature [27, 90], was observed and can be neglected. It can clearly be observed that for some samples thedegradation of RhB increases gradually with increasing irradiation time. It is possible to note that the single porous TiO2 material (red curve, Figure 27a) exhibit a good catalytic performance; namely 60% of RhB degradation after 120 minutes of irradiation; it is interesting becausedue toits wide band-gap energy in the UV range, TiO2 could not absorbin the visible range. Thus theoretically, no possible degradationof dye molecules by TiO2 can be expected
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PHOTOCATALYTIC MECHANISM
TiO2 e- VB CB O2 O2- H+ HOO OH RhB RhB Vis. light Moreover in the as-synthesized TiO2 porous system the slow photon effect generate to the inverse opal structure and the intrinsic porosity lead to have a more active surface area, namely more contact area and increased mass transfer. Due to its high accessible porosity, light waves easy enter inside the photocatalyst increasing the path length of light and thus enhancing the light absorption of the material and then improving its photo-reaction efficiency RhBads+ (O2, O2•-, HOO•or•OH) → intermediates→ degradation products Slow photon effect + highly porous structure
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CONCLUSIONS The amount of dopants was low
Catalysts k (min-1) Inverse Opal TiO2 0.006 Com. TiO2Anatase 0.001 TiO2-0.2%N 0.017 TiO2-0.5%N 0.011 TiO2-1%N 0.009 TiO2-1.5%N 0.007 TiO2-0.2%W 0.013 TiO2-0.5%W 0.008 TiO2-1%W TiO2-0.2%Hf TiO2-0.5%Hf 0.012 TiO2-0.5%Hf-1%N TiO2-2%CeO2 TiO2-3%CeO2 A positive effect of chemical agents was verified when: The amount of dopants was low The ionic exchange with Ti4+ ions was favored and/or the substitution of these in the crystal lattice of TiO2 could occur The formation of non-stoichiometric Ti-composites was favored on the surface N atoms act as impurity sensitizers, influencing also the TiO2 surface defects concentrations[ , 113]. Similar results were found by Sato et al. using the wet-method to doping TiO2 samples with nitrogen [83]. A higher amount of nitrogen as “impurities dopants” on the contrary, lead to a decrease of the positive effect regarding the degradation of dye. Similar to the effect of nitrogen also the presence of tungsten and hafnium especially at low concentration (0.2, 0.5 and 1 at.%) enhanced the photo-activity of bare TiO2. Some authors correlate the positive effect of tungsten to the effects of shift of light absorption band from near UV to the visible range, to the presence of tungsten oxides that hinder the recombination rate of excited electrons/holes and to the stoichiometric ion exchange between tungsten and titanium ions. Subsequently W could be oxidised to W(VI) by transferring electrons to adsorbed O2 [86, 114, 115]. In our case XPS analysis revealed the presence of different W species (mostly W6+and W4+). The presence of W4+ions could favour the ion exchange with Ti 4+ and the tungsten ions can substitute the titanium ions in the lattice of TiO2. Moreover the non-stiochiometric solid solution of WxTi1−xO2 would form, leading to have tungsten impurity energy level [87-89]. These effects were more pronounced when the concentration of tungsten was low, with a surface enrichment of tungsten oxide in the surface of TiO2. Also the presence of hafnium, as detected by XPS, favours the formation of non-stiochiometric Hf1-xTixO2 composite with the ions Hf4+ that can easily reply the Ti4+ions [80, 85] and this was beneficial for the photocatalytic performance. The combination of nitrogen with tungsten as doping agents of TiO2 inverse opal structure didn’t improve the performance compared to single atoms doping (Table 12). Only for the catalyst TiO2-0.5%Hf-1%N the kinetic rate constant of degradation of RhB was about two times higher than bare TiO Xu et al. studied the degradation of methylene blu under visible irradiation on titania inverse opal (IO) films co-doped with N and F. They correlated the higher activity of co-doped sample over to slow photon effect to the existence of defects in the titania structure and/or Ti3+ species due to the co-presence of doping agents [84]. Probably the combination of the impurity effect due to nitrogen and the co-presence of hafnium oxide that facilitate the formation of O-H bond on the TiO2 surface and consequently the adsorption of atmospheric oxygen, could explain the higher activity of the N-Hf co-doped sample. This synergistic effect had a minor role when tungsten was present instead of hafnium.
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THANK YOU FOR YOUR ATTENTION
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XPS ANALYSIS: TiO2-CeO2 The binding energies of O 1s are located for the TiO2 (red line) at and eV. The lowest binding energy is assigned to lattice oxygen in the metal oxides, whereas the highest binding energy is ascribed to surface oxygen by hydroxyl species and/or weakly adsorbed oxygen species (O2−) on the surface causing a mutual influence between the Ti and Ce metal cations that could change their oxidation states more easily on the surface.
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Textural Properties Inverse opal TiO2
BET surface area of 28 m2/g, narrow pore size distribution centred at 1.2 nm resulting from of the aggregation of TiO2 nanoparticles When the amount of CeO2nanoparticles is increased, the size of nanoparticles becomes slightly bigger and the surface area of the samples decrease Bare BiVO4 nanoparticles correspond to the monoclinic scheelite phase In the TiO2-BiVO4 system ,indeed, the crystal size varies progressively with the amount of bismuth vanadate from 23 to 29 nm. In this case only a low amount of the second oxide (1-5% wt.) prevents the formation of bigger crystal domains. TiO2-25%CeO2 S. Area = 13 m2/gr TiO2 S. Area = 28 m2/gr
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XPS ANALYSIS: TiO2-CeO2 A mixture of Ce3+/Ce4+ states exists on the surface of TiO2-CeO2 catalysts The presence of Ce3+ species, is attributed to the occurrence of a strong interaction between TiO2 support and CeO2 nanolayer Ti4+ in a tetragonal structure such as anatase titania (458.8 eV Ti 2p3/2 and eV Ti 2p1/2) Compared to pure TiO2, a shift of 0.6 eV to a lower binding energy in the peak positions of TiO2 (458.2eV and eV) is observed on TiO2-3%CeO2 (blue line) Catalysts Ti (at %) O (at%) Ce3+/Ce4+ ratio TiO2 22.5 55.7 - TiO2- 3%CeO2 22.9 57.1 0.46 TiO2- 25%CeO2 20.2 53.1 0.15 causing a mutual influence between the Ti and Ce metal cations that could change their oxidation states more easily on the surface.
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PHOTOCATALYTIC MECHANISM
TiO2 h+ e- VB CB UV hν O2 O2- H+ HOO OH H2O RhB OH, R TiO2 e- VB CB O2 O2- H+ HOO OH RhB RhB Vis. light Moreover in the as-synthesized TiO2 porous system the slow photon effect generate to the inverse opal structure and the intrinsic porosity lead to have a more active surface area, namely more contact area and increased mass transfer. Due to its high accessible porosity, light waves easy enter inside the photocatalyst increasing the path length of light and thus enhancing the light absorption of the material and then improving its photo-reaction efficiency RhB or R• + (O2, O2•-, HOO•or•OH) → intermediates→ degradation products RhBads+ (O2, O2•-, HOO•or•OH) → intermediates→ degradation products Slow photon effect + highly porous structure
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Slow photons e- TiO2 Inverse Opal RhB RhB RhB Degraded products
Vis. Light Slow photons RhB O2 e- CB TiO2 Inverse Opal e- O2- Ce3+, Ce4+ RhB RhB CeO2 VB Degraded products
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