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(1)AKADEMIA GÓRNICZO-HUTNICZA IM. STANISŁAWA STASZICA W KRAKOWIE WYDZIAŁ INŻYNIERII MATERIAŁOWEJ I CERAMIKI Katedra Chemii Analitycznej. Ph.D. Thesis Katarzyna Anna Michałów. Flame spray synthesis and characterisation of doped TiO2 nanoparticles for photoelectric and photocatalytic applications. Supervisors: Prof. Dr. hab. Mieczysław Rękas Dr. Andre Heel. Kraków 2009.

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(3) Dedicated to my Family.

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(5) ACKNOWLEDGEMENTS In the beginning, I would like to thank to my supervisors, their support and attention was very helpful during carrying out the experimental part and guiding me through this thesis. I am very thankful to Prof. Mieczysław Rękas, who was supporting me since the beginning of my PhD study with a lot of passion and big trust in my ideas and me. I am very grateful to Dr. Andre Heel – my second supervisor, who supported me in all my decisions and allowed to have a freedom in my research activities. But he was also my friend in hard times. Great thanks to Dr. Andri Vital for his supervision in the beginning of my work, for introducing me to EMPA and to Switzerland. I would like to acknowledge Prof. Dr. Thomas Graule, Prof. Władysław W. Kubiak, Prof. Elżbieta Godlewska, Prof. Jerzy Lis, Prof. Dariusz Kata, and Prof. Jolanta Janczak-Rusch for giving me the opportunity to do my PhD study in the International PhD School Switzerland-Poland. Many thanks to all the people who helped me to handle experiments and gave me access to their equipment as well as support and discussion of obtained results. At first, thanks to Prof. Marta Radecka for photoelectrochemical measurements, to Prof. Katarzyna Zakrzewska for support with optical measurements, to Prof. Mirosław M. Bućko for XRD measurements and fruitful discussions, to Dr. Kazimierz Kowalski for help with XPS data interpretation, to Dr. Witold Reczyński for AAS measurements, to Dr. Ewa Niewiara for support. with. laboratory. equipment,. to. Anita. Trenczek-Zając. for. help. with. photoelectrochemical measurements, to Martin Amber for the access to DRS device, to Dr. Guiseppino Fortunato for XPS measurements, to Dr. Davide Ferrie for scientific discussions, to Dr. Dmitry Longvinovich for help with ammonolysis experiments, to Dr. Kranthi K. Akurati for interesting discussion about our common passion – nanosized photocatalysts, to Dorota Flak for help with a part of photocatalytic measurements, to Dr. Renata Solarska, Nikola Castillo and Dr. Agnieszka Gorzkowska-Sobaś for very inspiring scientific discussions. Thanks to my office colleagues Elizabeth Barna and Dr. Sophie Duval as well as to Dr. Paweł Sobaś and especially to Jan Wyrwa for the very nice working atmosphere and great help. In the end, I would like to thanks to David Mauke for his great help for the graphic layout in this thesis as well for his support and belief in me. I am also grateful to all people helping me during the PhD period and which are not mentioned by name.. I.

(6) Table of contents List of abbreviations ..................................................................................................... IV List of tables................................................................................................................ VIII List of figures ................................................................................................................. IX 1. Introduction – purpose of work .....................................................................- 1 -. 2. Titanium dioxide – basic properties...............................................................- 4 -. 2.1. Crystallographic structure and polymorphic forms ...........................................- 4 -. 2.2 Semiconducting properties ................................................................................- 6 2.2.1 Semiconducting properties and defect structure of rutile ..................................- 6 2.2.2 Semiconducting properties and defect structure of anatase...............................- 9 2.3. Electronic structure and optical properties ......................................................- 11 -. 2.4. Application of TiO2 .........................................................................................- 14 -. 3. Principles of photoelectrolysis and photocatalysis .....................................- 16 -. 3.1. Photoelectrochemical mechanism ...................................................................- 16 -. 3.2. Photoelectrochemistry at semiconductor/liquid surface interface ...................- 18 -. 3.3 Photoelectrolysis of water ...............................................................................- 22 3.3.1 Photoelectrochemical cell (PEC) efficiency ....................................................- 24 3.4 Heterogeneous photocatalysis .........................................................................- 26 3.4.1 Photocatalytic decomposition of methylene blue (MB) ..................................- 29 3.4.2 Photocatalytic decomposition of methyl orange (MO) ...................................- 33 3.5. Material requirements for the photoelectrochemical application ....................- 36 -. 4. Modification of TiO2 properties ...................................................................- 39 -. 4.1. Effect of the particle size .................................................................................- 41 -. 4.2. Donor type of doping – tungsten .....................................................................- 44 -. 4.3. Acceptor type of doping – chromium ..............................................................- 46 -. 4.4. Anionic doping – nitrogen ...............................................................................- 48 -. 5. Nanoparticle processing ................................................................................- 53 -. 5.1 Flame spray synthesis (FSS)............................................................................- 54 5.1.1 FSS of TiO2 based nanoparticles .....................................................................- 57 5.1.2 Ammonolysis of FSS-made TiO2 ....................................................................- 61 6. Applied characterization methods ...............................................................- 64 -. 6.1. Determination of the structure and morphology properties (BET, XRD, TEM)- 64 -. 6.2. Surface state (XPS, DSC, DTA- TGA) ...........................................................- 65 II.

(7) 6.3. Band gap and optical properties determination (DRS).................................... - 66 -. 6.4. Evaluation of the photocatalytic activity ......................................................... - 69 -. 6.5. Determination of the photoelectrochemical properties (Layer deposition, SEM, PEC, EIS) ........................................................................................................ - 75 -. 7. Results and discussion ................................................................................... - 78 -. 7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.1.5 7.1.6. Titanium dioxide doped with tungsten ............................................................ - 78 Structure and morphology ............................................................................... - 78 Chemical composition and surface state ......................................................... - 85 Effect of tungsten doping and the particle size on the optical properties ........ - 89 Effect of tungsten doping on the photoactivity under UV and Vis ................. - 95 Effect of thermal treatment ............................................................................ - 104 Effect of tungsten doping on photocurrent .................................................... - 108 -. 7.2 7.2.1 7.2.2 7.2.3 7.2.4. Titanium dioxide doped with chromium ....................................................... - 112 Structure and morphology ............................................................................. - 112 Chemical composition and surface state ....................................................... - 118 Effect of chromium doping on the optical properties .................................... - 119 Effect of chromium doping on photoactivity in UV and Vis ........................ - 123 -. 7.3 7.3.1 7.3.2 7.3.3 7.3.4. Titanium dioxide doped with nitrogen .......................................................... - 124 Structure and morphology ............................................................................. - 124 Chemical composition and surface state ....................................................... - 127 Effect of nitrogen doping on the optical properties ....................................... - 129 Effect of nitrogen doping on photoactivity in UV and Vis ........................... - 133 -. 8. Summary and conclusions .......................................................................... - 136 -. 9. List of literature ........................................................................................... - 140 -. III.

(8) List of abbreviations A A′ a, c ART BET BZ c C0 Cpollutant CB CHAA CN D D• dBET, dXRD, dTEM DRdiff dH dG dsc dXRD,A, dXRD,A DFT DRS DSC DTA-TGA e e′ Eb Eg Eloss Ep E0 ECM EIS ESR EtOH FAS FSP FSS FRdopant FRTTIP FRtotal GC/MS h. surface area of electrode acceptor lattice parameters anatase to rutile transformation Brunauer-Emmett-Teller adsorption isotherm Brillouin zone velocity of light initial concentration concentration of pollutant conduction band chromium acetylacetonate coordination number diffusion coefficient of e' and h• donor particle size obtained from BET, XRD and TEM measurements first derivative of the diffuse reflectance spectrum electrolytic Helmholtz double layer diffuse Gouy–Chapman he interface the space charge layer on the semiconductor side crystallite size of the anatase and rutile, respectively density functional theory diffuse reflectance spectroscopy differential scanning calorimetry differential thermal analysis and thermogravimetric analysis electron charge electron binding energy band gap energy the energy loss photon energy converging energy emulsion combustion method electrochemical impedance spectroscopy electron spin resonance ethanol flame aerosol synthesis flame spray pyrolysis flame spray synthesis feed flow rate of “dopant” precursor feed flow rate of “main” precursor titanium tetraisopropoxide – (TTIP) total feed flow rate gas chromatography with mass spectrometry detector Planck constant IV.

(9) h• HPLC hν I Ir IEP IPCE Jg Jλ k K kB K-M kapp Kλ LD LMB L-H LC/MS m* me*, mh* mo mx m/z MB MO MXylene n N nads ND Ni Ns Neff npsd Ntot n0 NHE p P Pads pO2 PA PC PEC PZC. hole high performance liquid chromatography energy current of the cell incidence of solar irradiance isoelectric point incident photon-to-current conversion efficiency the flux density of absorbed photons photocurrent density wave vector adsorption coefficient Boltzman constant Kubelka-Munka function apparent first-order photodegradation kinetic rate constant absorption coefficient from K-M Debye length leuco methylene blue Langmuir-Hinshelwood mechanism liquid chromatography with mass spectrometry detector effective mass effective mass for the electrons and holes electron rest mass power coefficient informs about the nature of the defect structure mass-to-charge ratio methylene blue methyl orange 1,3-dimethylobenzene range of the electronic conductivity number of monometric unit total number of adsorbed species donor density interstitial nitrogen substitutional nitrogen number of effective incidents leading to the photoinduction of electron-hole pairs particle size distribution the total number of absorbed photons total number of adsorption sites normal hydrogen electrode range of the hole conductivity pollutant pollutant adsorbed on the semiconductor surface oxygen partial pressure photocatalytic activity photocatalysis photoelectrochemical cell point o zero charge V.

(10) P25 Q QSE R rdeg Rdiff Rc Rref R&D R(H2) S Sirr Sλ SC SCE SED SEM SMR SP SPSD SSA T Tdiff tirr TEM THC THF TTIP UV Vfb Vd VB VESP Vis W Wλ x XPS XRD y α α0 γ ∆G0 ε εf. commercial TiO2 (Degussa) composed of ca. 20 wt.% rutile and 80 wt. % anatase the charge density quantum size effect particle radius degradation rate of the reactant diffuse reflectance critical particle size reference diffuse reflectance research and development hydrogen evolution rate (mol/s) surface sites of the semiconductor (photocatalyst) irradiated surface area of the photoanode scattering coefficients from K-M semiconductor calomel electrode sacrificial electron donor scanning electron microscope steam methane reformation spray pyrolysis self-preserving size distribution specific surface area absolute temperature diffuse transmittance the irradiation time transmission electron microscope tungsten hexacarbonyl tetrahydrofuran titanium tetraisopropoxide ultraviolet radiation flat band potential volume fraction of dopant in form of oxide valence band spray pyrolysis using a vapour flame reactor visible irradiation depletion layer power of the monochromatic light flux deviation from stoichiometry X-ray photoelectron spectroscopy X-ray diffraction the distance perpendicular to the surface of the semiconductor Absorption coefficient Material constant power coefficient (Tauc plot) standard free enthalpy dielectric constant Fermi energy of electrons in the solid VI.

(11) ε0 ηc ηg ηch ηQE θ λ λFSS µh µe ρ ρA, ρR, ρCr, ρW σ/kT σg τd Φ. vacuum permittivity solar conversion efficiency fraction of the incident solar irradiance with photoenergy ≥ Eg chemical efficiency quantum efficiency coverage of the surface of photocatalyst by adsorbate wavelength the oxygen to fuel ratio hole mobility electron mobility quantum yield of the photocatalytic process density of anatase and rutile TiO2 polymorph, and dopant in form of oxide, respectively slop from Urbach law spread of particle distribution average transit time radiation flux. VII.

(12) List of tables Table 2.1 Basic crystallographic and physical properties of anatase and rutile [13, 14]...................... - 6 Table 2.2 Selected application of TiO2 in respect to its photoelectrochemical properties [20, 42, 43].- 15 Table 3.1 Time characteristics of processes involved in the photochemical mechanism in the TiO2 applied as a photocatalyst [43]............................................................................................................................... - 17 Table 3.2 Cost and performance characteristics of selected hydrogen production processes [48]. ..... - 24 Table 3.3 General group classification of organic pollutants decomposed by photocatalysis systematized by Herrmann in 1999 [7]. .......................................................................................................................... - 28 Table 5.1 The flame spray synthesis parameters of TiO2, TiO2:W and TiO2:Cr nanopowders. ............ - 60 Table 5.2 The ammonolysis parameters. ............................................................................................... - 63 Table 7.1 Main crystallographic and structural properties of undoped and W-doped TiO2 flame in comparison to TiO2-P25 (Degussa) used as a reference. ...................................................................... - 84 Table 7.2 Results of the XPS analysis of TiO2 and TiO2:W nanopowders. ............................................ - 87 Table 7.3 The values of the Eg obtained from the DRS measurements. ................................................. - 90 Table 7.4 Structural parameters of P25, Ti-13 and Ti:0.7W-11 before and after thermal treatment.. - 106 Table 7.5 Main crystallographic and structural properties of TiO2:Cr FSS-made nanopowders, Ti-1.4 is gaven for comparison. ......................................................................................................................... - 116 Table 7.6 Binding energies of Cr 2p3/2 and Ti 2p3/2 XPS peaks for TiO2:Cr nanopowders ................. - 119 Table 7.7 Optical properties of TiO2 and TiO2:Cr FSS-made nanopowders....................................... - 120 Table 7.8 Summary of data obtained by X-ray diffraction measurements of pure and N-doped TiO2 nanopowders (Fig. 7.42). .................................................................................................................... - 127 Table 7.9 Location of N1s and nitrogen concentration in TiO2 and TiO2:N. ...................................... - 129 Table 7.10 Summary of band gap Eg and steepness parameter of Urbach law calculation for pure and Ndoped TiO2 nanopowders. ................................................................................................................... - 131 -. VIII.

(13) List of figures Figure 1.1 Hydrogen product ion path , after [3]. .................................................................................. - 1 Figure 2.1 Application of TiO2. ............................................................................................................... - 4 Figure 2.2 The oxygen-titanium phase diagram after [11]. .................................................................... - 5 Figure 2.3 Unit cell of anatase (a), with Ti interstitial (b), with Ti vacancy (c), with O interstitial (d) and with O vacancy (e). The magenta spheres indicate Ti atoms and the green spheres indicate O atoms [24].- 10 Figure 2.4 Scheme of the direct and indirect optical transition in a semiconductor. ............................ - 12 Figure 2.5 Calculated band structures of TiO2 in rutile (left) and anatase (right) form, after [34]...... - 13 Figure 2.6 A schema of the particle size dependency of the absorption and scattering coefficient [40].- 14 Figure 3.1 Schematic photoexcitation in a semiconductor photocatalyst (1) followed by the deexcitation pathways (2), after [38, 43]. .................................................................................................................. - 17 Figure 3.2 Schematic diagram showing the possible types of the space layers in an n-type semiconductor and an electrolyte containing redox couple, accumulation layer (a), depletion layer (b) inversion layer (c), deep depletion layer (d), adapted from Radecka et al. [20]........................................................................... - 19 Figure 3.3 Energy band bending and space charge layer formation for a) bulk type of semiconductor, b) and nano-sized semiconductor photocatalyst in equilibrium with redox level in solution [46, 47]. ............ - 21 Figure 3.4 Photoelectrochemical cell (PEC) for water splitting. .......................................................... - 23 Figure 3.5 The definition and the schema of catalysed photolysis (left) and photogenerated catalysis (right). ............................................................................................................................................................... - 26 Figure 3.6 Chemical formula (inset) and absorption spectrum of methylene blue (MB) with marked range of irradiation applied by author for photodecomposition of MB, near UV with λmax = 355 nm (violet) and Vis with λmax = 435 nm (blue). ..................................................................................................................... - 30 Figure 3.7 The pathway of the photocatalytic degradation of MB were detected by a) GC/MS, and LC/MS b) [66]. ....................................................................................................................................................... - 31 Figure 3.8 Chemical formula (inset) and absorption spectrum of methyl orange (MO) with marked range of irradiation applied by author for the photodecomposition of MO, near UV with λmax = 355 nm (violet) and Vis with λmax = 435 nm (blue). ............................................................................................................... - 33 Figure 3.9 The pathway of the photocatalytic degradation were detected by HPLC-MS after Baiocchi et al. [77], where m/z indicates mass-to-charge ratio. ................................................................................... - 35 Figure 3.10 Band positions of several semiconductors in contact with an electrolyte at pH 1, (energy scales is eV, normal hydrogen electrode (NHE) or vacuum levels as a reference). The green marker indicates the valence band (VB), and the red one the conductivity band (CB). On the right hand side the standard potentials of typical redox couples are presented [1]............................................................................ - 37 Figure 3.11 The solar irradiance vs. wavelength for AM 1.5 [80] and UV-Vis range of solar spectrum with indicated absorption range of TiO2. ...................................................................................................... - 38 Figure 4.1 Energy levels of transition metal ions incorporated in TiO2 (rutile) lattice [2, 106] (a) and the diagram showing the possible effect of donor dopant on electronic structure of TiO2 (anatase) (b). ... - 40 Figure 4.2 Model of change in the electronic structure of a semiconductor with the change of the number of monometric units (N), after Hoffmann et al. [43]. ................................................................................. - 43 Figure 4.3 The TiO2 – WO3 phase diagram, after Chang et al. [145]. .................................................. - 45 Figure 4.4 The TiO2 – Cr2O3 phase diagram, where C indicates Cr2O3, T indicates TiO2 (in rutile form), n – a high temperature solid state solution and E is E-phase (Cr2Ti2O7), after Somiya et al. [160]. .......... - 47 Figure 4.5 Schemes of substitutional (left) and interstitial (right) position of nitrogen in TiO2 of anatase structure, adapted from Livraghi at al. [178]........................................................................................ - 49 -. IX.

(14) Figure 4.6 The charges pathway in the N-doped TiO2 in the O2 atmosphere and visible light irradiation, after Livraghi et al. [178]. ............................................................................................................................. - 50 Figure 4.7 The possible changes in the electronic stracture of anatase, which may origin from the N-doping: localized impurity levels above VB and below CB [182] (a), narrowing of the band gap coused by broadening of VB [65] (b), localized impurity levels and electronic transition to CB [173] (c) and electronic transition from the localized levels above VB to their corresponding excited states Ti3+ and F+ centres (d), after Serpone [175]. .............................................................................................................................. - 51 Figure 5.1 Flame in FSS (left), and scheme of particles formation and growth by liquid-to-particle conversion (right). ................................................................................................................................. - 55 Figure 5.2 Set-up of the flame spray synthesis plant (left), and an image of the FSS used at EMPA (right).- 58 Figure 5.3 The specific surface area (SSA) as function sum of precursor’s mole number (Σnprecursors) for TiO2:0.4at.%W and TiO2:0.7 at.%W nanopowders (left) and TiO2:Cr with varying dopant concentration (right)..................................................................................................................................................... - 59 Figure 5.4 Mechanism of N-doping of TiO2 by the decomposition of NH3 proposed by Kuroda et al. [218]. 62 Figure 5.5 Set-up for the ammonolysis of the TiO2 nanopowder. Scheme provided by D. Longvinovich.- 62 Figure 6.1 Schematic diagram for the spectrophotometric measurement with the integrating sphere and sample position for diffuse transmittance (a) and diffuse reflectance measurement (b), after Zakrzewska [14]. ............................................................................................................................................................... - 67 Figure 6.2 Diffuse reflectance spectrum (a) and its first derivative DRdiff (b) as a function of wavelength λ and photon energy hν for TiO2-P25E nanopowder. ...................................................................................... - 67 Figure 6.3 The Tauc plots of TiO2-P25E nanopowder as a function of photon energy hν calculated for different value of the power coefficient γ: γ = 1/2 direct allowed (a), and γ = 2 indirect allowed (b) electronic transition................................................................................................................................................ - 69 Figure 6.4 Semi-logarithmic plot of the absorption coefficient α vs. hν of TiO2:NE (2TiNA) nanopowder and linear fit which meet the Urbach rule. ................................................................................................... - 69 Figure 6.5 Set-up of photocatalytic reactor. Detailed description of the single parts, denoted by a-f is described in the text. .............................................................................................................................. - 71 Figure 6.6 Bulb for UVA (Sylvania Blacklight Blue F8W/BLB T5 lamp, at the top), and Vis irradiation (LCD Lightening Inc, M2-65-01 Hg lamp, at the bottom) irradiation (left). The intensity distribution of the particular lamps vs. wavelength (right). ................................................................................................ - 72 Figure 6.7 Evolution of the optical absorbance peak of methylene blue (MB) in presence of Ti-3 (TiO2-FSS, SSA = 54 m2/g a), and methyl orange (MO) in presence P25 (TiO2, Degussa) b) under UVA irradiation.- 73 Figure 6.8 The flat band evaluation from photocurrent-voltage characteristic for Ti-3 photoanode under white light irradiation (a) and from Mott-Schottky polot obtained from EIS measurement (b). ........... - 76 Figure 7.1 Specific surface area (SSA) of TiO2 and TiO2:W nanoparticles with various concentration of W as a function of λFSS. ................................................................................................................................... - 79 Figure 7.2 TEM images of the Ti:1W-7 (a, b), Ti-3 (c, d) and Ti-1.4 (e, f) nanoparticles (notation like in Table 7.1)............................................................................................................................................... - 80 Figure 7.3 Particle size distribution of Ti:3 (a) and Ti:1W-7 (b) nanopowder calculated from TEM image.81 Figure 7.4 X-ray diffraction pattern of the TiO2 synthesized with λFSS = 13 (Ti-13) and TiO2:W nanopowders synthesized with λFSS = 11 for W concentration (Ti:0.4W-11, Ti:0.7W-11, Ti:1W-11) (Table 7.1) (a) and of the TiO2:0.4 at.%W with increasing value of λFSS (b) . Symbols A and R indicate anatase and rutile reflections, respectively. ........................................................................................................................ - 83 X.

(15) Figure 7.5 Comparison of dBET, dXRD and rutile fraction as a function of λFSS of TiO2 (a) and TiO2:0.7W (b) nanopowders. ......................................................................................................................................... - 84 Figure 7.6 XPS and fitted spectra of Ti 2p (a) and W 4f (b) of Ti:0.7W-11 nanopowder. ..................... - 85 Figure 7.7 XPS and fitted spectra of O 1s of TiO2:0.7W-11 nanopowder. ............................................ - 86 Figure 7.8 Comparison of DSC curves of P25, Ti-13 and Ti:0.7W-11 powders (a) and DSC curves of Ti:0.7W-11 from first cycle and after second cycle of the DSC experiment (b) where M1, M2, M3, M4 indicated the exothermic peaks at 255°C, 393°C, 267°C 385°C, respectively. ..................................................... - 87 Figure 7.9 Simultaneously measured DTA (right axis) and TGA (left axis) curves of Ti:0.7W-11. M1, M2, M5 indicate the exothermic peaks at 277°C, 383°C, 850°C, respectively. .................................................. - 88 Figure 7.10 Optical diffuse reflectance Rdiff as a function of wavelength λ for flame made TiO2A with different SSA in comparison to P25A. Inset describes the first derivative (a) and first derivative of the diffuse reflectance spectra of Ti-13 E, with fitted Gaussian plots (b). ................................................................ - 89 Figure 7.11 The comparison of the Tauc plots of flame made TiO2 with different SSA and P25 for γ = 1/2 for direct allowed transition (a) and γ = 2 for indirect allowed transition (b)............................................ - 90 Figure 7.12 Optical diffuse reflectance Rdiff as a function of wavelength λ for flame made TiO2:WE in comparison to Ti-13E and P25E (a), first derivative of the diffuse reflectance spectra of Ti:1W-7E, with fitted Gaussian plots (b). ................................................................................................................................. - 93 Figure 7.13 The comparison of the Tauc plots of for flame made TiO2:WE with increasing tungsten concentration in comparison to Ti-13E and P25E for γ = 1/2 for direct allowed transition (a) and γ = 2 for indirect allowed transition (b). .............................................................................................................. - 93 Figure 7.14 The semi-logarithmic plot of absorption coefficient vs. photon energy hν for TiO2A nanopowder with increasing SSA in comparison to P25A (a) and TiO2:WE nanopowder with increasing W concentration in comparison to T-13E and P25E (b). ....................................................................................................... - 94 Figure 7.15 Degradation of MB and MO with and without Ti:0.7W-11 photocatalyst under UVA irradiation. Negative time (-10 min) indicates the period of MB and MO adsorption on the photocatalyst surface without irradiation. ............................................................................................................................................. - 96 Figure 7.16 MB photodegradation kinetics under UVA a) and the calculated apparent first-order photodegradation kinetic rate constant (kapp) b) for flame made TiO2:W compared to undoped TiO2-FSS and P25. ........................................................................................................................................................ - 97 Figure 7.17 MO photodegradation kinetics under UVA (a) and the calculated apparent first-order photodegradation kinetic rate constant (kapp) (b) for flame made TiO2:W compared to undoped TiO2-FSS and P25. ........................................................................................................................................................ - 97 Figure 7.18 MB photodegradation kinetics under UVA (a) and calculated apparent first-order photodegradation kinetic rate constant (kapp) (b) for TiO2:0.7 at.%W nanopowder as function of SSA.- 99 Figure 7.19 MO photodegradation kinetics under UVA (a) and the calculated apparent first-order photodegradation kinetic rate constant (kapp) (b) for TiO2:0.7 at.%W nanopowder as function of SSA.- 99 Figure 7.20 Comparison of the theoretical and experimental apparent first-order kinetic rate constant (kapp) as a function of the particle size (dBET). ............................................................................................... - 100 Figure 7.21 MB photodegradation kinetics under Vis (a) and the calculated apparent first-order photodegradation kinetic rate constant (kapp) (b) for flame made TiO2:W with compared to undoped TiO2FSS and P25. ....................................................................................................................................... - 101 Figure 7.22 MO photodegradation kinetics under Vis for flame made Ti:0.7W in comparison to undoped Ti-3 and P25. ............................................................................................................................................... - 102 Figure 7.23 XRD pattern of the as-prepared Ti:0.7W-11 and after heat-treatment at 400oC, 600oC, 700oC and 1000oC. A and R indicates anatase and rutile phase. ................................................................... - 105 -. XI.

(16) Figure 7.24 MB photodegradation kinetics under Vis irradiation for flame made TiO2:W with increasing tungsten concentration before and after thermal treatment in comparison to undoped TiO2-FSS and P25. 107 Figure 7.25 Calculated apparent first-order photodegradation kinetic rate constant of flame made TiO2:W with increasing tungsten concentration before and after thermal treatment in comparison to undoped TiO2FSS and P25. The calculation made for MB-Vis irradiation set-up. ................................................... - 107 Figure 7.26 SEM pictures of the TiO2-0.4at.% W nanopowder derived layer at two different magnifications.109 Figure 7.27 Photocurrent (I) corresponding to white (190 – 700 nm) and dark light irradiation as a function of U vs. SCE (a) and the flat band potential vs. W concentration (b) for undoped and W-doped TiO2 flame made nanopowders in comparison to TiO2-P25. ................................................................................. - 109 Figure 7.28 Mott-Shottky plots (1/C2 vs. Vb) for 100 – 3500 Hz frequency range (a) and calculated flat band potential (Vfb) as a function of frequency (b) for TiO2 – P25. ............................................................. - 110 Figure 7.29 Photocurrent (I) corresponding to the monochromatic light irradiation in the range of 300 – 700 nm as a function of Vb vs. SCE for Ti:0.7-W-1.3 layer (a) and solar energy conversion efficiency (ηc) as a function of Vb for undoped and W-doped TiO2 layers out of FSS-made nanopowders and TiO2-P25 layer. 110 Figure 7.30 IPCE as a function of wavelength for Ti:0.4W-1.3 and Ti:1W-1.3. ................................. - 111 Figure 7.31 TEM images of Ti:0.1Cr (a, b), Ti:1Cr (c, d) and Ti:15Cr (e, f) nanopowders prepared by FSS. 113 Figure 7.32 Comparison of dBET, dTEM, dXRD,A, dXRD,R and rutile fraction as a function Cr concentration in TiO2:Cr nanopowders (A and R indicates anatase and rutile phase, respectively). ............................ - 114 Figure 7.33 HR TEM images of Ti:0.1Cr (a), Ti:1Cr (b) and Ti:15Cr (c) nanopowders. ................. - 115 Figure 7.34 X-ray diffraction patterns of TiO2 and TiO2:Cr nanopowders prepared by FSS. The pattern for Cr2O3 nanopowder with SSA=32.4 m2/g is given for comparison, A and R indicates anatase and rutile phase, respectively. ......................................................................................................................................... - 115 Figure 7.35 Raman spectra of TiO2:Cr FSS-made nanopowders, A – indicates anatase.................... - 117 Figure 7.36 XPS results for TiO2 (a), Cr2O3 (b), TiO2:0.5at.%Cr (c) and TiO2:10at.%Cr (d) nanopowders prepared by FSS over the range of binding energy in which Cr 2p and Ti 2p lines occur. ................. - 118 Figure 7.37 Optical diffuse reflectance Rdiff as a function of wavelength λ and photon energy hν for flame made TiO2:Cr A nanopowders, with indicated colours of powders (right),A. ....................................... - 120 Figure 7.38 Energy of the optical transition Eg as a function of Cr concentration in TiO2. EgI and EgII correspond to the fundamental band gap of anatase and rutile, respectively. EgIII is the transition energy from the band related to Cr dopant, formed within the TiO2 band gap. ...................................................... - 121 Figure 7.39 The semi-logarithmic plot of absorption coefficient vs. photon energy hν for TiO2:Cr A nanopowder. The slope from the linear fitting of Urbach tails (σ/kT) as a function of Cr concentration and particles size, dBET (inset). ................................................................................................................... - 122 Figure 7.40 Photodecomposition of methylene blue under UV irradiation by TiO2:Cr and P25 (reference). 123 Figure 7.41 TEM image of Ti-3 – the starting material a) and after ammonolysis and annealing 2TNAa b) and c). .................................................................................................................................................. - 125 Figure 7.42 XRD pattern of the P-TiO2, P-TiO2-xNx, F-TiO2 and F-TiO2-xNx. Symbols A and R indicate anatase and rutile phases. ................................................................................................................... - 125 Figure 7.43 Fraction of anatase a), anatase (dXRD,A) and rutile (dXRD,R) average crystallite size b) for TiO2 and TiO2:N nanoparticles. .................................................................................................................. - 126 Figure 7.44 Anatase lattice parameter a and c vs. process parameters for undoped and N-doped TiO2 nanopowders........................................................................................................................................ - 126 XII.

(17) Figure 7.45 N1s XPS spectra of Ti-3 (staring powder) a), TiN (after ammonolysis) b), TiNA (after ammonolysis and annealing) c), 2TiNA (after ammonolysis and annealing under optimal conditions) d).- 128 Figure 7.46 Photographs of Ti-3 a), TiN b), TiNA c) and 2TiNA d) nanopowders. ............................ - 130 Figure 7.47 Diffuse reflectance spectrum of undoped and N-doped TiO2 nanopowders a) and differential reflectance of Ti-3, TiN, TiNA and 2TiNA (b) as a function of wavelength λ. EgI, EgII, EgIII refers to anatase, rutile and N-dopant band gap. ............................................................................................................. - 131 Figure 7.48 Diffuse reflectance spectrum of P-TiO2, P-TiO2-xNx, F-TiO2 and F-TiO2-xNx (a) and F-TiO2 and F-TiO2-xNx first derivative (b) as a function of wavelength λ. EgI, EgIII, EgII refers to anatase, rutile and Ndoping band gap. ................................................................................................................................. - 132 Figure 7.49 Photodecomposition of methylene blue under UVA (a) and Vis (b) light irradiation by pure and N-doped TiO2 nanopowder. ................................................................................................................. - 133 -. XIII.

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(19) 1 Introduction – purpose of work. 1. Introduction – purpose of work Since the discovery of the photoelectric effect by Edmond Becquerel in 1839,. conversion of the solar energy to electric energy and chemical fuel become not only dream but also real aim and challenge for generations of the scientists. The energy supplied by the sun into the Earth is about 3×1024 J/year, in the other words it is 10 000 times more than the global population currently consumes. If we would be able to cover 0.1 % of the Earth’s surface by solar cells with efficiency of 10 % we would cover the needs of the present population [1]. The global oil crisis in the 1970’s stimulated a worldwide research in the direction of alternative source of energy. Since this time, the hydrogen is considered as the fuel of the future. The present concerns about the climate change due to use of the fossil fuel drives the development of the environmentally friendly and renewable fuel. Hydrogen has many potential applications like powering of vehicles and aircrafts or domestic heating [2]. Hydrogen can be generated by several methods as can be seen in Fig. 1.1. Among them, the direct photoelectrochemical water splitting carried out under solar irradiation is the safest and the cleanest way of obtaining this fuel.. Figure 1.1 Hydrogen product ion path , after [3].. Since 1972 when Fujishima and Honda [4] proposed the TiO2 single crystal as a photoanode in a first photoelectrochemical cell (PEC) and Frank and Bard in the 1977 [5] -1-.

(20) 1 Introduction – purpose of work. applied TiO2 for photocatalytic oxidation of cyanide ions, a strong interest and development of TiO2 based materials began. Heterogeneous photocatalysis became the next application of TiO2 and it is the most promising method for the water and air pollutants removal. Many of the organic and inorganic type of toxic contaminates as well as bacteria [6] can be totally mineralized and hence neutralized by photooxidation. The main advantages of this method when the so-called first generation TiO2 photocatalyst is used are: •. chemical stability of TiO2 in the aqueous media in the large range of pH (0≤ pH ≤ 14). •. low production cost. •. no additives required – only oxygen in the air. •. system applicable under both high and low contaminant concentration. •. total mineralization of wide group of organic pollutants or oxidation to non-toxic compounds. •. great deposition capacity for noble metal recovery. •. possible combination with other decontamination method (e.g. biological) [7]. However, TiO2 due to its wide band gap (3.0 – 3.2 eV) is able to absorb only in the UV range, which is 3 – 5% of the solar irradiation [8]. In order to increase the efficiency of photoreaction in both type of processes, photoelectrochemical water splitting and photocatalytic pollutants decomposition, TiO2 have to be modified in direction of better absorption under visible irradiation (400 – 700 nm). There are several ways to overcome these drawbacks like doping of cationic sublattice of TiO2 with either donor-type ions such as W6+, Mo6+ and Nb5+ or acceptor-type ions such as. Cr3+, Fe3+. These so-called. second-generation photocatalysts are particularly interesting to increase the electrical conductivity, resulting in a decrease of the recombination losses, as well as to modify the photocatalyst’s surface properties leading to improvement of the optical properties and relating electronic structure. The third generation of photocatalysts – the anion-doped TiO2 has attracted considerable attention due to its enhanced photocatalytic activity in the visible light. In this case, N-doped TiO2 seems to be the most interesting due to the noticeable red-shift of light absorption. Additionally, new physical and chemical properties emerge when the size of the material becomes smaller down to the nanometre scale. Well crystalline anatase particles with a size of about 25 – 40 nm are most suitable for a photocatalytic application.. -2-.

(21) 1 Introduction – purpose of work. The purpose of the work is to synthesize TiO2-based nanoparticles with enhanced photocatalytic activity by increasing electrical conductivity and charge separation, to inhibit the electron-hole recombination and to enhance their visible light absorption, using a flame spray process (FSS). Further on, to elaborate scientific understanding and correlations between the flame process parameters, the resulting particle characteristics, and the functional properties of these particles. The dissertation is composed of theoretical and experimental parts. The theoretical part covers description of basic properties of TiO2 and its role in the photoelectrochemical phenomenon in context of application in photoelectrochemical water splitting and photocatalytic pollutants decomposition in aqueous system. The methods of improving the photocatalytic properties are also described. In the experimental part, the flame spray synthesis (FSS) of undoped and W, Cr or N doped TiO2 is presented. Following by the characterization of the basic properties like: particle size, shape and morphology (BET, TEM, XRD), the chemical composition and surface state (XPS, DSC, DTA-TGA), and optical as well as electronic properties (DRS). In the next step, the verification of the photocatalytic (decomposition of MB and MO) and photoelectrochemical (layers deposition, SEM, PEC) properties are discussed. The obtained results allowed to find correlation between the structural and optical properties and the photocatalytic and photoelectric performance of the flame made nanopowders.. -3-.

(22) 2 Titanium dioxide – basic properties. 2. Titanium dioxide – basic properties Titanium dioxide is one of the most widely applied metal oxide thanks to its unique. properties. Due to its high refractive index it is used as a pigment in the paint industry [9]. Its non-toxicity and stability makes it possible to apply it in the pure form as a food additive, in pharmaceuticals, and cosmetic products. TiO2 plays also an important role as a biocompatible material for bone implants etc. (Fig. 2.1).. Figure 2.1 Application of TiO2.. Among all properties of TiO2, in the recent years the photocatalytic property is the most investigated for various applications e.g.: disinfection of the operating hospitals rooms, self-cleaning surfaces. Among them, the photodecomposition of the organic pollutants in aqueous environment and the photoelectrochemical water splitting are of the main interest of this work [10].. 2.1. Crystallographic structure and polymorphic forms The oxygen – titanium system has been deeply studied and resulted in proposition of. several O-Ti phase diagrams.. -4-.

(23) 2 Titanium dioxide – basic properties. Figure 2.2 The oxygen-titanium phase diagram after [11].. The most up-to-date one, reported by Murray and Wriedt [11], is shown in Fig. 2.2 and includes the condensed phases in the composition range between pure Ti and TiO2. Titanium dioxide (TiO2) occurs in the nature in three polymorphic forms: brookite, anatase, and rutile where the two last are commonly used as photocatalysts. Anatase and brookite as metastable phases transform to rutile in the range 973K – 1173K. The anatase to rutile transformation (ART) temperature depends on purity, type of impurities, particle shape/size, atmosphere and reaction conditions. Reidy et al. [12] discussed the role of the critical particle size (Rc) i.e. about 45 nm as a main parameter of ART irrespective of the purity and the type of impurity in the anatase. The size dependence of the ATR is related to the surface free energy, which is lower for anatase than that of rutile, what is reversed to the bulk situation [10, 12]. Rutile due to its thermodynamical stability is the most extensively studied among all TiO2 forms. In the anatase and rutile structures each titanium ion is in the centre of an oxygen octahedron, but in rutile the oxygen ions form a slightly distorted hexagonal compact lattice and in anatase form a cfc lattice (Fig. 2.3).. -5-.

(24) 2 Titanium dioxide – basic properties. Table 2.1 Basic crystallographic and physical properties of anatase and rutile [13, 14].. Property. Anatase. Rutile. Crystallographic structure. tetragonal. tetragonal. Space group. I41/amd. P42/mmm. Lattice parameters [nm]. a = 0.3784 b = 0.9515. a = 0.4594 c = 0.2959. Volume of the cell/molecule [10-3 nm3]. unit 34.172. 31.216. Density [g /cm3], T = 298 K. 3.894. 4.250. Band gap energy (Eg) [eV]. 3.2. 3.03. Electron effective mass (m*). 1mo. 20mo. Hall mobility of electron [cm2 /Vs], T = 298 K. 4. 0.1. In both structures, each oxygen ion has coplanar near neighbour titanium cations. The three Ti-O-Ti angles are roughly equal to 120o in rutile. On the other hand, in anatase one Ti-O-Ti angle is about 180o while the two other are close to 90o. The various Ti-O-Ti angle values result in a significant widening of the d bands with an accompanying decrease in effective mass and increase in mobility. Basic crystallographic properties of two polymorphic forms of titanium dioxide are listed in the Table 2.1 [13, 14].. 2.2. Semiconducting properties. 2.2.1. Semiconducting properties and defect structure of rutile. Undoped and stoichiometric titanium dioxide is an insulator at the room temperature. Under reducing atmosphere and higher temperature is oxygen deficient (eq. 2.1). TiO2-x is a semiconductor showing either n or p conductivity related to the defect structure, which depends on the oxygen partial pressure and temperature, as well as preparation conditions and history e.g. sintering conditions [15]. TiO2 ↔ TiO2− x + 2x O2 (2.1) The range of homogeneity of TiO2-x is limited and the value of the deviation from the stoichiometry (x) is sill under discussion. Some researchers placed x in the range 0.1 ÷ 0.008 [11, 16, 17], where according to present results the lowest value of x is the most correct. Low range of the homogeneity of the nonstoichiometry in TiO2 is related to -6-.

(25) 2 Titanium dioxide – basic properties. the tendency of highly dispersed point defects to get ordered when their concentration increases. This results in the formation of crystallographic shear planes, which with the increasing number of this type of defects order, form a new homologous compound TinO2n-1, known as higher Magneli phases (Fig. 2.2) [11]. The next disputable point regarding the defect structure of the reduced rutile (n-type conductivity) is the kind of the predominating point defects (oxygen vacancies or interstitial titanium). So far, there is no general agreement in this matter [16]. According to Kofstad [17], the defect structure simultaneously comprises doubly charged oxygen vacancies and interstitial titanium ions with three and four effective charges (eq. 2.2, 2.3). Their formation, according to Kröger – Vink notation can be written as: OO ↔ 12 O2 + VO•• + 2e' (2.2) TiO2 ↔ Tii+ j + O2 + je' (2.3). where j = 3 and/or 4 The electroneutrality condition is given by: 2[VO•• ] + 3[Tii••• ] + 4[Tii•••• ] = [e' ] (2.4). The oxygen vacancies are predominant at near-atmospheric oxygen pressures, while interstitial titanium ions are dominant at the lower limit of the homogeneity range. The concentration of the defects based on the equations (2.2), (2.3) and (2.4) can be expressed as a function of the oxygen partial pressure pO2,. [e' ] ∝ pO−12 / mx (2.5) where mx assumes a value between 4 and 6 depends on the type of defects. The experimental evidence shows that at high oxygen partial pressure pO2 (~ 104 Pa) and below 1200 K rutile exhibit an n-p type transition [18-20]. To discuss the n-p transition and then p-type conductivity the following intrinsic electronic disorder should be taken into account: nil ↔ e'+ h • (2.6). Taking into consideration the presence of impurities acceptor/donor denoted as (A') and (D•), respectively, the electroneutrality condition is: 2[VO•• ] + 3[Tii••• ] + 4[Tii•••• ] + [ h • ] + [ D • ] = [e' ] + [ A' ] (2.7). There are several explanations of p-type of conductivity:. -7-.

(26) 2 Titanium dioxide – basic properties. 1. Model of electronic compensation of charges, where intrinsic electronic disorder dominates over other disorders. Taking into account the negligible contribution of impurities in the condition, i.e., [ A' ] − [ D • ] ≈ 0 (2.8) It can be assumed that [h•] ≥ [e'], therefore p-type of conductivity can occur only when µh >>µe. This means that the mobility of electron holes is much higher than that of electrons. According to Nowotny et al. [21] the mobility of both types of charges in TiO2 at the high temperatures is comparable, what allowed to reject the considered model. 2. Extrinsic disorder model, where p-type of conductivity origins from acceptor type of impurities that are always present in majority over donor-type one in rutile [18]. The electroneutrality condition can be assumed then as: [ A' ] = 2[VO•• ] (2.9). 3. Intrinsic disorder model, assumes formation of titanium vacancies according to either Frenkel TiTi ↔ Tii• •• • + VTi′′′′ (2.10). or Schottky disorder nil ↔ 2VO•• + VTi′′′′ (2.11). where the charge compensation occurs at higher pO2. 2[VTi′′′]′ = [VO•• ] + 2[Tii•••• ] (2.12). Both extrinsic and intrinsic disorder models, give the same charge carrier concentration dependence on pO2:. [e' ] ∝ pO−12 / 4 (2.13) [h• ] ∝ pO1 /24 (2.14) There is still debate among researchers which of these two models is the proper one. The intrinsic disorder model is undermined by the argument that the formation of titanium vacancies is energetically unfavourable. However, Radecka and Rekas [22] investigated the effect of the high-temperature treatment on n-p transition in rutile and postulated that titanium vacancies are able to form during high-temperature treatment (1473 – 1673 K) and are frozen at the measuring temperature (923 – 1323 K), what results in p-type of conductivity of rutile. -8-.

(27) 2 Titanium dioxide – basic properties. To establish the defect structure and related properties of TiO2 the application of electrical conductivity measurements is the most commonly applied method, with the assumption that the studied material is an electronic type of semiconductor. According to Radecka and Rekas [22] this assumption is only correct for strongly reduced and donordoped rutile but not for the undoped materials within the p-type range and under pO2 partial pressure close to n–p transition. Based on the measurements carried out in the temperature range of 923 – 1353K and pO2 range of 10 – 105 Pa, the values of mx were determined. The obtained values differed from those predicted by defect disorder models, what allowed the evaluation of both electronic and ionic components of the electrical conductivity. The ionic conductivity of rutile near to the n-p transition point can be explained by two competitive theories: •. by transport of oxygen ions through oxygen vacancies. •. by transport of titanium ions according to the interstitial mechanism. According to Radecka and Rekas [22] titanium interstitial disorder is the predominant mechanism under the above given conditions. A higher ionic conductivity is also observed in the single crystal rutile in comparison to the polycrystalline one, what stays in the contradiction to the grain boundary theory [23]. 2.2.2. Semiconducting properties and defect structure of anatase. Literature about the defect structure of TiO2 in the anatase polymorph is less numerous [24-26]. Anatase is – as it has been discussed in chapter 2.1 – a metastable polymorph of TiO2, which transfers irreversible to rutile in the temperature, range of 973K – 1173K. In consequence it is difficult to perform electrical measurements in purpose to establish the electrical properties and the type of defects under the influence of temperature and/or oxygen partial pressure [27]. Na-Phattalung et al. [24] took attempt to reveal the anatase native defect structure by using first-principles total-energy calculations. They took Ti vacancy (VTi), O vacancy (VO), Ti interstitial (Tii) and O interstitial (Oi) into consideration (Fig. 2.3). The preference of the interstitial Oi to bind strongly with lattice O results in the formation of a substitutional diatomic molecule (O2) which always occurs on the O side.. -9-.

(28) 2 Titanium dioxide – basic properties. Figure 2.3 Unit cell of anatase (a), with Ti interstitial (b), with Ti vacancy (c), with O interstitial (d) and with O vacancy (e). The magenta spheres indicate Ti atoms and the green spheres indicate O atoms [24].. Based on the formation energy of the particular defects the probability of their ••••. existence in the certain condition was calculated. Tii. has a very low formation energy. and can be the dominating native defects in anatase structure close to equilibrium growth condition. Formation energy of VO•• is as well very low, nevertheless VO•• is annihilated according to following equation: mTiO 2 + 2nVO•• → ( m − n)TiO 2 + nTii•••• + 2ne' (2.15) ••••. On the other hand, the kinetics may not support the creation process of Tii. via eq.. 2.15. To create an oxygen vacancy the three bindings with neighbouring Ti have to be broken but to free one Ti the six bindings with neighbouring O have to get broken (Fig. 2.3 b, e). This let suggest the conclusion that VO•• is the dominating native defect in anatase structure. Sekiya et al. [25] studied the heat-treatment of single crystal anatase in oxidative or reductive atmosphere and established that in most cases VO•• is the dominating type of defect. Measurements on nanocrystalline anatase were carried out by Weibel et al. [27]. Samples were obtained by hot-pressing of previously prepared anatase nanopowder and afterward calcination at 1073 K. The as-prepared material was measured in the temperature range 723 – 973 K (to avoid the grain growth and the anatase-to-rutile phase transformation) and under an oxygen partial pressure between 10-24 – 105 Pa. They established that anatase ceramics are the mostly electronic conducting with a negligible ionic conductivity at high oxygen partial pressure. Taking in to account intrinsic and. - 10 -.

(29) 2 Titanium dioxide – basic properties. extrinsic disorder model, which was discussed in the chapter 2.2.1, it has been concluded that below 853 K the Shottky disorder is predominant. That means that the oxygen vacancies are formatted. On the other hand, above 923 K titanium interstitial formation, indicating the Frenkel cation disorder is predominant in anatase. The transition from VO•• to. Tii•••• predomination has been observed for a temperature between 873 – 893 K and under oxygen partial pressure of 10-2 Pa. Nevertheless, the concentration of both types of defects is comparable and can be easily changed with the change of the experiment condition. The formation of the titanium interstitial appears more favourable in anatase than in rutile due to the 10% lower density and more open crystallographic structure of anatase. However, the formation energy of a titanium interstitial is four times higher than that of the oxygen vacancy if only the charge number will be considered, what is in contradiction to the argumentation of Na-Phattalung [24], but brings the same conclusion: oxygen vacancies are favoured in anatase [27].. 2.3. Electronic structure and optical properties The forbidden band gap (Eg) is defined as energy between the top of the valence band. (VB) and the bottom of the conduction band (CB). The width of the band gap is a measure of the chemical bond strength. Semiconductors, eg. oxides of titanium or niobium, are stable under illumination. They have a wide band gap, what means an absorption edge within UV-range and they are in consequence insensitive to visible light [1]. The charge carriers in the semiconductor can be excited thermally or optically. Under thermal excitation electrons are promoted to the conduction band, and an equal number of holes are produced in the valence band, what means that [e'] = [h•] and the Fermi level is positioned in the middle of the band gap of so-called intrinsic semiconductor. In addition, the light absorption by semiconducting materials leads to the generation of electron-hole pairs. The magnitude and the energy of the absorption process depend on the band structure of the semiconductor. Therefore, we can distinguish two types of electron transition: -. direct band gap transition – when the lowest energy excitation of an electron from VB to CB involves no change of the wave vector k, i.e. the participation of a phonon is not required to conserve momentum (Fig. 2.4).. - 11 -.

(30) 2 Titanium dioxide – basic properties. -. indirect band gap transition – when k at the VB is different from the k at the CB minimum i.e. at least one phonon participate in the absorption or emission of one photon [28, 29].. Figure 2.4 Scheme of the direct and indirect optical transition in a semiconductor.. The important difference between a direct and indirect transition is their probability. Probability of indirect transition is 100 times lower than that of the direct one [30]. The number of charge carriers per unit volume is not affected by irradiation for the intrinsic type of semiconductor – this value is a physical feature of the material. The modification of the concentration of the charge carriers in the semiconductor can be achieved by doping, what is discussed in details in the chapter 4. Electronic structure and optical properties of both rutile and anatase were studied in the theoretical and experimental way. The theoretical studies of rutile properties, were extensive due to numerous experimental data [31-34], where the electronic and optical properties of anatase get more attention in the 90-ies [34-37]. The results of theoretical calculation along the symmetry lines of the Brillouin zone (BZ) for both structures are presented in Fig. 2.5, after Mo and Ching [34]. The calculated direct band gap (Fig. 2.5 left) for rutile is 1.78 eV at Γ, much smaller than the experimentally obtained gap of 3 eV but comparable with other calculations [32]. The upper valence band has a width of 6.22 - 12 -.

(31) 2 Titanium dioxide – basic properties. eV and is composed of O2p orbitals. The width of lower O2s band is 1.94 eV. The separation energy between O2s valence state and the minimum of the conduction band is 17.98 eV. The width of the lowest conduction band is 5.9 eV and is composed of two sets of Ti3d bands. These two sets of bands have their atomic origin from the hybridized states of t2g and eg. There is also a substantial degree of hybridization between O2p and Ti3d in both CB and VB regions, indicating strong interaction between Ti and O atoms in rutile, what results in involving both O2p and Ti3d states in the excitation across the band gap.. Figure 2.5 Calculated band structures of TiO2 in rutile (left) and anatase (right) form, after [34].. The calculated minimum band gap of anatase is an indirect one ((Fig. 2.5, right), and is 2.04 eV, where the experimentally determined one is 3.2 eV. The bottom of the conductive band is at Γ and the top of the valence band is at M where the energy at Γ is 0.18 eV lower than the top of the VB. Therefore, anatase can be still considered as a direct band-gap semiconductor with Eg, which is 0.26 eV larger than that of rutile. The width of the upper VB of anatase is 5.17 eV and it is about 1 eV less than that of rutile. The lower O2s band of 1.76 eV is also narrower than that of rutile, and is located 17.88 eV below the CB minimum. The general features of the VB for anatase are comparable to the rutile ones [34].. - 13 -.

(32) 2 Titanium dioxide – basic properties. It should be noted, that above discussion about the value of Eg of anatase or rutile is based on experimental data obtained from single crystal measurements. The decrease of the size of the polycrystalline material may influence the size of Eg by shifting the fundamental absorption edge to the shorter wavelengths, according to eq. 2.16:. ∆Eg =. h 2π 2  1 1  1.786e 2 0.248e 4 + − − 2 R 2  me* mh*  εR 2ε 2 h 2. 1 1   m* + m*  (2.16) h  e. where R is the particle radius, me* and mh* are effective masses for the electrons and holes, respectively, and ε is the dielectric constant [38, 39]. Light absorption as well as light scattering is particle size dependent (Fig. 2.6). The maximum of the light scattering occurs for the medium sized particles. The minimum of the scattering is observed for very small and for large particles.. Figure 2.6 A schema of the particle size dependency of the absorption and scattering coefficient [40].. Anatase and rutile have a maximum scattering coefficient for λ = 550 nm at a particle size of approximately 220 nm. Rutile in comparison to anatase has a higher scattering coefficient at any particle size and wavelength [40].. 2.4. Application of TiO2 As has been discussed in the provirus chapter titanium dioxide has many interesting. physical, chemical and optical properties. Therefore, TiO2 is widely applied in industry as well as it is a subject of studies focused on new applications. Thanks to its high refractive index, TiO2 is used as a pigment and makes 60% of global pigment production [41]. TiO2 is also used in paper, porcelain, plastic and enamel production. In cosmetic and medical. - 14 -.

(33) 2 Titanium dioxide – basic properties. products is used as a protection against sunlight, and in the food industry as an additive. The number of new TiO2 applications based on its photoelectrochemical properties towards environmental usage is increasing (Table 2.2). Two of them the photocatalytic water purification and photoelectrochemical water decomposition are discussed in details in the following chapter (Table 2.2, grey background). Table 2.2 Selected application of TiO2 in respect to its photoelectrochemical properties [20, 42, 43].. PROPERTY Photocatalytic water purification. CATEGORY. APPLICATION. Drinking water Others. Photoelectrochemical. Hydrogen production. water decomposition Self-cleaning. Materials for residential and office buildings Indoor and outdoor lamps and related systems Materials for roads Others. Air cleaning. Indoor air cleaners Outdoor air purifiers. Antitumor activity Self-sterilizing. Cancer therapy Hospital. Others. river water, ground water, lakes and water storage tanks fish feeding tanks, drainage water and industrial wastewater fuel for automobiles sector, fuel for solid oxide fuel cell, fuel for energetic sector, for pharmaceutical and food industry exterior tiles, kitchen and bathroom components, interior furnishings, plastic surfaces, aluminium siding, building stone and curtains, paper window blinds translucent paper for indoor lamp covers, coatings on fluorescent lamps and highway tunnel lamp cover glass tunnel wall, noise barrier, traffic signs and reflectors tent material, textiles for hospital garments and uniforms and spray coating for cars Room air cleaner, photocatalyst-equipped air conditioners and interior air cleaner for factories concrete for highways, roadways and footpaths, tunnel walls, noise barriers and building walls endoscopic-like instruments tiles to cover the floor and walls of operating rooms, silicone rubber for medical catheters and hospital garments and uniforms public rest rooms, bathrooms and rat breeding rooms. - 15 -.

(34) 3 Principles of photoelectrolysis and photocatalysis. 3. Principles of photoelectrolysis and photocatalysis The beginning of the photoelectrochemistry started in 1839, when Becquerel observed. that an electric current is produced when a silver chloride electrode, connected with a counter electrode is immersed in an electrolyte solution under applied illumination. Until 1955 and work of Brattain and Garret, the “Becquerel effect” was not well understood. First, zinc oxide had attracted considerable attention until it was replaced by titanium dioxide thanks to its corrosion and photocorrosion resistance, and its susceptibility to property modifications [2, 44]. Modern photoelectrochemistry started in 1972 when Fujishima and Honda [4] proposed the first electrochemical cell constructed from TiO2 single crystal as a photoanode where an oxygen get evolved and Pt cathode on which a hydrogen evolution was observed. A 500 W xenon lamp was used for irradiation and the electrolyte with a pH of 4.7. They also improved the efficiency of the cell by applying chemical bias by adding Fe3+ ions to the compartment of the Pt electrode. In the 1977 Frank and Bard [5] were first who examined rutile and anatase polymorph of TiO2 and their non-stoichiometric form as well as a mixture of both polymorphs for decomposition of cyanide ions in 0.1M KOH under different type of irradiation, including unfocused sunlight. After that, heterogeneous photocatalysis became widely investigated in view of environmental applications. This following chapter is about the fundamental understanding of photoelectrolysis and photocatalysis, two processes based on the same phenomena and the same requirements, which have to be fulfilled by the materials applied in both processes.. 3.1. Photoelectrochemical mechanism The first step in both photoelectrolysis and photocatalysis processes is an interaction of. the semiconductor with the light. A photon with an energy (hν) equal or higher than the band gap energy (Eg) of the semiconductor is absorbed, then an electron (e') is promoted into the conduction band (CB) from the valence band (VB) leaving a hole (h•) behind Fig. 3.1, (1). This process can be expressed by the following quasi- reaction: hν TiO2 ←→ e'+ h • (3.1). The generated charges diffuse to the surface where:. - 16 -.

(35) 3 Principles of photoelectrolysis and photocatalysis. •. in a photoelectrolysis process the created holes oxidize water. It results in an oxygen generation on the photocatalyst surface, which is usually applied as an anode. The electrons are transported over an external circuit into a cathode where through a reduction of water a hydrogen is generated.. •. in a photocatalysis process the holes and the electrons react with solution or gas derived species adsorbed on the photocatalyst surface (Fig. 3.1) [43]. Figure 3.1 Schematic photoexcitation in a semiconductor photocatalyst (1) followed by the deexcitation pathways (2), after [38, 43].. This photoexcitation process is always followed by the deexcitation events. While the (c) and (d) pathways are the desirable deexcitation through the redox reaction of the electrons and holes with acceptor or donor type of species. The pathways marked as (a) and (b) illustrate the unwanted recombination of the charges both in the volume (a) and on the surface (b) [38]. According to Table 3.1 there are two main processes which determine the overall quantum efficiency: •. competition between charge carrier recombination and trapping. •. competition between trapped carrier recombination and interfacial charge transfer An improvement of the quantum efficiency and through this an improvement of the. photoactivity can be realised by diminishing of the recombination rate of the charge carriers and increasing the interfacial charge transfer [43]. Table 3.1 Time characteristics of processes involved in the photochemical mechanism in the TiO2 applied as a photocatalyst [43].. - 17 -.

(36) 3 Principles of photoelectrolysis and photocatalysis. Primary process. Characteristic time [ns]. Comment. Charge carrier generation Hole trapping Electron trapping. 10-6 10 10 (shallow trap). Fig 3.1 (1). 10-2 (deep trap) 10-2. • — —. Recombination of the mobile charge carrier Trapped charge carrier recombination with: • mobile hole • mobile electron Interfacial hole transfer Interfacial electron transfer. 3.2. 10 102 102 106. •. dynamic equilibrium irreversible. — —. Photoelectrochemistry at semiconductor/liquid surface interface When the surface of the bulk semiconductor stays in contact with an electrolyte the. charge transfer from semiconductor to electrolyte (or vice versa) may occur. The electrons proceed until the equilibrium is reached i.e. the Fermi energy of electrons in the solid (εf) is equal to the redox potential of the electrolyte. If we assume that there is no ion diffusion through the electrode/electrolyte junction, the electrons are the only charges capable to migrate through this interface in both directions. Such an electric transfer causes a difference charge distribution across the interface, the so called the space-charge layer. On the electrolyte side, it corresponds to the electrolytic Helmholtz double layer (dH) followed by the diffuse Gouy–Chapman (dG) layer. On the semiconductor side of the interface the space charge layer (dsc) is related to the type of the band bending, which depends on the position of the Fermi level in the semiconductor. Therefore, four types of charge layers can be distinguished: accumulation, depletion, inversion, and deep depletion (Fig. 3.2). The accumulation layer is created and the bands bend down when the electrons are accumulated at the semiconductor side (Fig. 3.2 a). When the electrons deplete from the solid into the solution leaving behind a positive excess charge formed by immobile ionized donor states then a depletion layer is formed and the bands bend upward toward the surface (Fig. 3.2 b). The electron depletion in the n-type of semiconductor can go that far, that their concentration at the interface falls below the intrinsic level. In consequence, the semiconductor gets p-type at the surface and n-type in the bulk, corresponding to an inversion layer (Fig. 3.2 c). A deep depletion layer is formed by a strong band bending when the minority carriers are not accumulated at the junction. This can happen for a semiconductor with a wide band gap or when the annihilation of the minority carriers is faster than their accumulation (Fig. 3.2 d).. - 18 -.

(37) 3 Principles of photoelectrolysis and photocatalysis. Figure 3.2 Schematic diagram showing the possible types of the space layers in an n-type semiconductor and an electrolyte containing redox couple, accumulation layer (a), depletion layer (b) inversion layer (c), deep depletion layer (d), adapted from Radecka et al. [20].. To avoid band bending and the creation of the space charge layer, the potential has to be imposed over the electrode/electrolyte interface. Such a potential is known as the flat band potential (Vfb) and it is constant in the electrolyte for given pH. Vfb is a very important parameter of photoelectrochemical process, which allows to establish, the position of VB and CB edges of a given semiconductor. If Vfb is higher than the redox potential of the H+ /H2 couple the water splitting proceed [1, 2, 4, 20]. The electric field, which is present on the semiconductor/electrolyte interface, prevents the light-induced charge recombination by a separation of the holes and electrons. For a semiconductor applied in a PEC, the strength of such an electric field is a characteristic parameter, which depends also on its doping levels. The solution of the Boltzman-Poisson’s equation provides the photoelectrochemical process parameters for a given difference between the CB energy of an n-type semiconductor and the electrochemical potential of the electrolyte [45]:. Q = eN D when 0 ≤ y ≤ W (3.2) Q = 0 when y ≥ W (3.3). - 19 -.

(38) 3 Principles of photoelectrolysis and photocatalysis. where e – is the electron charge, Q – the charge density, W – the depletion layer (which depends on the donor density, applied potential, pH of the electrolyte and illumination intensity), ND – the donor density and y – the distance perpendicular to the surface of the semiconductor.  k T Vsc ( x) =  B 2  2eLD.  ( y − W ) 2 (3.4) .  εε k T  LD =  20 B   e ND . 1/ 2. (3.5).  k T    2εε 0 Vb − V fb − B   e   W =  2  e ND    . 1/ 2. (3.6). where Vb – is the potential drop within the layer, kB – the Boltzman constant , T – the absolute temperature, LD – the Debye length (is the scale over which mobile charge carriers screen out electric fields), ε – the dielectric constant of semiconductor, ε0 – the vacuum permittivity. As already mentioned, the interface of the semiconductor/electrolyte can be described by a three space charge layer model, two related to the electrolyte side of the interface dH and dG, and one related to the space charge in the semiconductor dsc. Such space charge layers can be considered as capacitor layers. The total capacitance (C) is given by: 1 1 1 1 (3.7) = + + C C sc C H CG When the characteristic thicknesses of each layer are taken into account: dH = 0.4 – 0.6 nm, dG = 1-10 nm and dsc = 10 – 100 nm, we can assume that capacitance is determined by the semiconductor space layer, therefore: 1 1 (3.8) ≈ C C sc Then the capacitance of the space charge layer is given by Mott-Schottky equation:.  1  2 kT   V − V fb − =   ≈ B(Vb − V fb ) (3.9) 2 2  b C sc  εε 0 eN D A  e . - 20 -.