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Photocatalytic Properties of TiO: Evidence of the Key Role of Surface Active Sites in Water Oxidation Tadeusz Bak, Wenxian Li, Janusz Nowotny, Armand Jason Atanacio, and Joel Davis J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 21 Aug 2015 Downloaded from http://pubs.acs.org on August 21, 2015

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The Journal of Physical Chemistry

Photocatalytic Properties of TiO2: Evidence of the Key Role of Surface Active Sites in Water Oxidation Tadeusz Baka, Wenxian Li,a, Janusz Nowotny,a,* Armand J. Atanacio,b and Joel Davisc a

Solar Energy Technologies, University of Western Sydney, Penrith NSW 2751, Australia

b

Australian Nuclear Science & Technology Organisation, Institute for Environmental Research, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia c

Australian Nuclear Science & Technology Organisation, Materials Institute, Locked Bag 2001, Kirrawee DC NSW 2232, Australia ABSTRACT Photocatalytic activity of oxide semiconductors is commonly considered in terms of the effect of the band gap on the light-induced performance. The present work considers a combined effect of several key performance-related properties (KPPs) on photocatalytic activity of TiO2 (rutile), including the chemical potential of electrons (Fermi level), the concentration of surface active sites and charge transport, in addition to the band gap. The KPPs have been modified using defect engineering. This approach led to imposition of different defect disorders and the associated KPPs, which are defect-related. This work shows, for the first time, a competitive influence of different KPPs on photocatalytic activity that was tested using oxidation of methylene blue (MB). It is shown that the increase of oxygen activity in the TiO2 lattice from 1012 Pa to 105 Pa results in: (i) increase in the band gap from 2.42 eV to 2.91 eV (direct transitions) or 2.88 eV to 3 eV (indirect transitions), (ii) increase in the population of surface active sites, (iii) decrease of the Fermi level, and (iv) decrease of the charge transport. It is shown that the observed changes in the photocatalytic activity are determined by two dominant KPPs: the concentration of active surface sites and the Fermi level, while the band gap and charge transport have a minor effect on the photocatalytic performance. The effect of the defect-related properties on photoreactivity of TiO2 with water is considered in terms of a theoretical model offering molecular-level insight into the process. KEYWORDS: Rutile; oxygen activity; performance-related properties; Fermi level; photocatalysis. Corresponding Author * E-mail: [email protected]

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1. INTRODUCTION Intensive research aims at the development of oxide materials for solar-to-chemical energy conversion in water oxidation. The research is based on a common perception that photocatalytic and photoelectrochemical performance of oxide semiconductors, such as TiO2, are determined by the band gap1-20. which is considered as the key performance-related property (KPP). It has been recently documented that photocatalytic properties of oxide materials are profoundly influenced by the KPPs that, in addition to the band gap, include a range of alternative properties, which are closely related to lattice imperfections20,21. Therefore, the research strategy in the development of photocatalysts with enhanced performance should take into account the effect of defect disorder and the related semiconducting properties in addition to the band gap. The present work studied the effect of several defect-related properties, including the concentration of surface active sites, the Fermi level, the charge transport and the band gap of pure TiO2 on photocatalytic performance tested by oxidation of methylene blue (MB). This work shows for the first time that the photocatalytic activity is determined by the population of surface active sites and the Fermi level rather than the band gap width. Surprisingly, this study shows that the observed changes of the band gap has a minor effect on photocatalytic performance. 2. KEY PERFORMANCE-RELATED PROPERTIES At present the common perception in TiO2 photocatalysis is that the band gap has a crucial effect on light absorption and the conversion of light energy into the chemical energy required for water oxidation.1-20 Awareness is growing, however, that the photocatalytic performance must be considered in terms of a range of KPPs, in addition to the band gap.20, 21 The KPPs that should be taken into account include the chemical potential of electrons, the concentration of surface active sites (needed for effective charge transfer), the charge transport, the flat band potential as well as the electronic structure. • Band Gap (KPP-1). This KPP is associated with a range of photons of different energies that can be absorbed by a semiconductor when photon energy is equal to, or larger than, the band gap. Therefore, the studies on the formation of TiO2 with enhanced performance with respect to this KPP are concentrated on reduction of the band gap from 3 eV (rutile) to a lower value, which better matches the solar spectrum. While this KPP must always be taken into account in considering the photocatalytic activity, it appears that the KPPs alternative to the band gap may have equally large effect on the performance. • Chemical potential of electrons (KPP-2). The charge transfer between the surface of a semiconductor and the adsorbed molecules is determined by the difference between the chemical potential of electrons of semiconductor (Fermi level) and the electronic affinity of the molecule involved in the reaction. Therefore, the charge transfer may be modified by changing the Fermi level. • Concentration of surface active sites (KPP-3). The reactivity of oxides with adsorbed molecules, such as water and oxygen, is profoundly influenced by the population of surface active sites. Titanium vacancies have been identified as surface active sites for water oxidation by TiO220. Such defects, which are strong acceptors, have an outstanding ability to remove electrons from the adsorbed water molecules.


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• •

Charge transport (KPP-4). This KPP determines the ability of electronic charge carriers to be relocated from the site of their generation (beneath the surface) to the surface site where the chemical reaction takes place. Flat band potential (KPP-5). This KPP, which is reflective of the electric field within the surface layer, is responsible for separation of the light-induced electrons and holes. This KPP may be modified by imposition of a chemically-induced electric field21, 22.

So far, little is known on the effect of the individual KPPs on performance, except the case of the band gap. Therefore, the aim of the present work is to assess the effect of several KPPs (defined above) on the photocatalytic performance. Since all KPPs studied in this work are related to the concentration of lattice imperfections20,21, the performance of TiO2 in solar energy conversion may be modified by defect engineering. In the present work we show that defect disorder and the related KPPs can be modified by imposition of variable oxygen activity. The competition between these KPPs, affecting the photocatalytic activity of TiO2, will be tested by oxidation of MB used as an indicator of the reaction progress. In this work we also show that the change of the band gap has a minor effect on the photocatalytic performance, which is determined by two other KPPs: the concentration of surface active sites and the Fermi level. The experimental part of this work is preceded by definition of basic terms describing the defect disorder and the related semiconducting properties of TiO2, including the chemical potential of electrons, the concentration of surface active sites, the charge transport and the reactivity with water. It is shown that the change of oxygen activity in the oxide lattice of TiO2 leads to modifications of the KPPs and the associated photoreactivity of TiO2 with water. 3. DEFINITION OF TERMS Band Gap. The band gap, Eg, is the smallest energy difference between the valence band and the conduction band. The Eg of pure TiO2 is 3.0 eV and 3.2 eV for rutile and anatase1, respectively. It is interesting to note that despite lower band gap of rutile, anatase exhibits better photocatalytic properties1. This effect indicates that other properties, alternative to the band gap, must have an effect on the photocatalytic performance. The reports consider either direct22 or indirect23 transitions in rutile. Energy Levels. Spontaneous charge transfer within the electrochemical chain for water splitting (involving anodic and cathodic reaction sites) requires that the bottom of the conduction band is above the H+/H2 energy level and the valence band is below the O2/H2O energy level. Since the electronic structure is sensitive to defect disorder, the charge transfer may be modified by appropriate change of the energy level of bands using defect engineering. Chemical Potential of Electrons. The chemical potential of electrons (Fermi level), which is a collective property of the entire surface, has a profound influence on the reactivity and the related charge transfer. It has been documented that there is a close relationship between oxygen activity in oxide semiconductors, such as TiO2, and the chemical potential of electrons, which can be expressed as: µ n = µ no + RT ln n (1)


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where n is the concentration of electrons and µno is the standard term. The changes of the quantity µn may be determined using the defect disorder diagram plotting the effect of oxygen activity on the concentration of electronic defects.21 The chemical potential of electrons represents the ability of semiconductors to donate or accept electrons. The increase of the Fermi level leads to an increased tendency of semiconductor to donate electrons to the reacting molecules, such as oxygen molecules. In contrast, the decrease of Fermi level increases the tendency of the semiconductor to accept electrons from the adsorbed molecules, such as water molecules. Knowledge of the charge transfer is essential in understanding the electronic mechanism of the photocatalytic process. The commonly applied procedure in the modification of semiconducting properties of metal oxides, including TiO2, is doping with aliovalent ions resulting in the formation of acceptor- or donor-type levels. While the research on doping is mainly consider for extrinsic ions, it appears that oxygen may also be used as an intrinsic dopant in the modification of semiconducting properties.24 In this work we show that the content of oxygen in the oxide lattice, and the related defect disorder of TiO2, may be modified by annealing in the gas phase of controlled oxygen activity. The effect of oxygen activity on the defect disorder diagram for pure TiO2, the related Fermi level and the associated changes in the electrical conductivity, is shown in Figures 1a, b and c, respectively (the Kröger-Vink notation used to describe the point defects is shown in Table 125).


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Figure 1. Effect of oxygen activity on the electrical conductivity (a), Fermi level (b) and the concentration of electronic and ionic defects (c) for pure TiO2 at 1273 K26.


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Table 1. The Kröger-Vink and the traditional notations of defects for TiO225 Description Kröger-Vink notation ସା Ti୶୘୧ Ti ion in the titanium lattice site Tiଷା ion in the titanium lattice site (quasi-free electron) eᇱ ᇱᇱᇱᇱ Titanium vacancy V୘୧ Ti••• Tiଷା in the interstitial site ୧ Ti•••• Tiସା in the interstitial site ୧ O୶୓ Oଶି ion in the oxygen lattice site Oxygen vacancy V୓•• Oି ion in oxygen lattice site (quasi-free electron hole) h•

At this stage it is important to note that most of the reported studies on photocatalytic properties use TiO2 of unknown oxygen content and the related defect disorder. In this work we show that reproducible properties of TiO2 may be achieved only for the specimens of the same oxygen activity.

Population of surface active sites. The prerequisite of the reactivity of TiO2 with water is adsorption of the water molecule on the TiO2 surface. While there is a wide range of surface sites available for adsorption, such as surface lattice ions and lattice imperfections, it has been shown that the most active surface sites for water oxidation by TiO2 are titanium vacancies.20,27 Charge Transport. The effect of oxygen activity on charge transport can be determined using the measurements of the electrical conductivity at elevated temperatures as a function of oxygen activity in the gas/solid equilibrium. The effect of oxygen activity on the electrical conductivity is represented in Figure 1a. Summary. The change of oxygen activity in the TiO2 lattice may be used in the modification of several KPPs, such as band gap, charge transport, Fermi level and the concentration of surface active sites. So far, little is known on the effect of the individual KPPs on the photocatalytic performance. 4. REACTIVITY OF TiO2 WITH WATER The performance of TiO2 in solar-to-chemical energy conversion is closely related to its photoreactivity with water leading either to total or partial oxidation that may be expressed by the following respective anodic reactions: 2H 2 O ↔ O 2 +4H + +4e' (2) 2H 2 O ↔ 2HO* +2H + +2e'

(3)

where e ' denotes a quasi-free electron in the oxide lattice. The charge compensation requires that these reactions are accompanied by the respective cathodic reactions: 4H + +e' ↔ H 2 (4) O 2 +e' ↔ O-2

(5)


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The reactions (2) and (4) represent the cycle of water splitting. The reactions (3) and (5) represent the formation of active radicals, which could be involved in subsequent reactions with organic molecules in water. As seen from Equations (2) and (3), water oxidation requires removal of electrons from the water molecules. Total water oxidation even requires multi-electron transfer. This process is enhanced by a downward shift of the Fermi level that is induced by an increase of oxygen activity in the oxide lattice. A change of oxygen activity also results in the modification of the concentration of titanium vacancies, which are formed according to the reaction: (6) O 2 ↔ 2O Ox +VTi'''' +4h • Titanium vacancies are the active sites for adsorption of water molecules leading to the formation of an active complex, which may be represented by the following reaction27: (7) 2H 2 O+VTi ↔ {2H 2 O 2+ -VTi'''' }* Decomposition of this complex leads to the formation of hydroxyl radical species and regeneration of titanium vacancies: {2H 2 O 2 + − VTi'''' } ↔ VTi'''' + 2HO* + 2h • (8) 27 The formation and the transport of titanium vacancies is an extremely slow process . It is essential to note that an increase of oxygen activity, leading to the formation of titanium vacancies (relation 6) results, at the same time, in a decrease of the Fermi level and the conversion of n-type TiO2 to p-type TiO2.

5. BRIEF LITERATURE OVERVIEW There has been an accumulation of reports on the effect of properties of TiO2-based semiconductors on photocatalytic performance.1-20 Most of these reports consider the effect of doping with aliovalent ions on the band gap. The overview of Carp et al,1 which is one of the most comprehensive reports on TiO2 photocatalysis, considers a wide range of properties, mainly the band gap, and their impact on photocatalytic oxidation of hazardous compounds in water. The overview reports of Nakata and Fujishima2 and Ochiai and Fujishima3 examine the effect of a wide range of parameters, such as light intensity, oxygen activity, humidity and chemical composition of the aqueous solutions on photocatalytic water purification as well as alternative applications, including sterilization and water splitting. Kemp and McIntyre4 determined the effect of a range of ions (Cr, Mn, V and Mo) on photocatalytic activity in photo-degradation of polyvinyl chloride. The related photocatalytic performance was considered in terms of the band gap, the configuration of d electrons, particle size and surface area. However, this report does not specify which quantities have the predominant effect on performance. Yamashita et al5 reported the effect of implantation of a range of cations (V, Mn, Fe) on the rate of degradation of 2-propanol. The XAFS studies show that annealing of the implanted specimens results in cation incorporation into titanium sites. The effect of these cations on enhancement of photocatalytic performance is considered in terms of reduction of the band gap. The indium implantation studies of Nakamura et al.6 have shown that annealing, after implantation, results in indium incorporation in both titanium and interstitial sites. Peng et al7 have observed that beryllium is incorporated into interstitial sites of TiO2, however, the effect of doping on photocatalytic hydrogen production depends on doping procedure. The


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effect of Be on photocatalytic performance is beneficial only when Be is present within the surface layer. Asahi et al8 observed that nitrogen doping leads to narrowing the band gap of TiO2 to 2.5 eV and enhanced degradation of methylene blue (MB). The beneficial effect of nitrogen, that is associated with the interstitial mechanism of incorporation, has been confirmed by Diwald et al9. They observed that only interstitially located nitrogen leads to band gap reduction to 2.4 eV. Wang et al claimed that TiO2 co-doped with bismuth and sulphur results in band gap reduction to 2 eV and enhanced degradation of indigo carmine.10 They also reported that the photocatalytic activity is associated with oxygen vacancies forming acidic active surface sites. It is not clear, however, why the increased concentration of oxygen vacancies lead to enhanced photocatalytic performance. Maeda et al11 reported that the band gap of titanium fluorooxynitrade (TiNxOyFz), which exhibits the structure similar to anatase, is 2.34 eV. They observed that photocatalytic activity of the TiNxOyFz compound in water oxidation to gaseous oxygen is substantially larger than that of TiNxOy and the anatase phase of TiO2. Fang et al observed that nitrogen incorporated into interstitial sites leads to band gap reduction to 2.8 – 2.9 eV.12 They claim that the bronsted acid sites lead to enhanced adsorption of methyl orange. Using first principle density functional theory, Sun et al simulated the structural and electronic properties of TiO2 anatase (101) surface co-doped with lanthanum and nitrogen.13 They reported that both La and N doping results in reduction of the band gap (by approximately 0.17 eV). Sue et al consider this effect in terms of the formation of oxygen vacancies. Khan et al14 reported that carbon incorporation into the TiO2 lattice results in a reduction of the band gap of TiO2 (2.32 eV), leading in consequence to enhanced light energy conversion efficiency in water splitting to 8.35%. It is not clear, however, if the enhanced performance is related carbon incorporation into the TiO2 lattice or a change of defect disorder that is associated with reduced oxygen activity in the TiO2 lattice. It is interesting to note that similar studies of Sakhvitel and Kish15 did not observe any effect of carbon doping on the band gap of TiO2. The apparent conflict between these reports may be considered in terms of the effect of oxygen activity on properties. Rothschild et al16 studied the effect of oxygen chemisorption on the charge transfer at the O2/TiO2 interface using the surface photovoltage spectroscopy. It is shown that the charge transfer results in the formation of chemisorbed oxygen species and depletion of the surface layer in electrons. Chen et al17, 18 observed that annealing of TiO2 in the atmosphere involving hydrogen results in an enhanced photocatalytic performance. The effect of hydrogen on properties of TiO2 has also been extensively studied by Norby et al.19. The work of Norby et al led to derivation of defect disorder model that has been verified by the measurements of electrical properties and thermogravimetry. Most of the reported studies consider the effect of doping or co-doping on the band gap1-19,28. Such approach favours the view that the photocatalytic performance of TiO2 depends mainly on the band gap and the related light absorption. At the same time little is known on the effect of the remaining properties on performance. Analysis of the reported studies indicates a wide range of approaches applied in the modification of the performance-related properties of TiO2. In most cases the related


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experimental data are not reproducible. The lack of reproducibility is due to the effect of oxygen on defect disorder and doping mechanism. Defect Disorder Pure TiO2 specimens are compatible when their lattice oxygen activity, and the associated defect disorder, is the same. This requires equilibration of specimens in the gas phase of controlled oxygen activity. However, selection of the annealing conditions, such as time and the temperature, requires knowledge of the related chemical diffusion data. Frequently, the properties are reported for specimens of unknown lattice oxygen activity and the associated defect disorder. Doping Mechanism The effect of doping on properties depends on the doping mechanism, which is well defined when the dopant ions are homogeneously incorporated into the oxygen specimen. This is the case when doping results in the formation of a solid solution that is equilibrated in the gas phase of controlled oxygen activity. Otherwise, doping result in a range of properties that are reflective of the related concentration gradients. In addition to the effect of oxygen activity and dopant concentration, the key performancerelated factors also include particle size29-31, surface area32,33, type of pores, adsorbed species of reactants, percentage of reactive facets34 and surface hydroxylations. While these factors are of key importance for all catalytic reactions, a quantitative analysis of their effect on light energy conversion is awkward. Recently, there has been also an accumulation of papers reporting the effect of morphology and the related geometrical properties, such as surface area, porosity, particle size and shape.35,36 The progress in processing TiO2-based systems for environmental purifications has been reported by Nakata and Fujishima,2 including zero-, one-, two-, and threedimensional structures, such as spheres, nanorodes, fibres, tubes, sheets and interconnected architectures. Nowotny et al37 reported a theoretical model on the effect of titanium vacancies on waster oxidation by TiO2. The objective of the present work is to examine the effect of several defect-related properties modified simultaneously on photocatalytic performance of pure TiO2.

6. EXPERIMENTAL Sample Preparation The specimens were made of the rutile power provided by Ishihara Sangyo Kaisha, Osaka. The cation impurities include iron (77 ppm), calcium (75 ppm), copper (31 ppm), nickel (15 ppm), zinc (16 ppm) and manganese (12 ppm). The specific surface area of the power before sintering is 6.8 m2/g. The average grain size ranges between 0.2 and 0.4 µm. The TiO2 powder was mixed with 3% of paraffin as a binder and pressed into pellets of 1.8-1.9 mm thickness under the load of two tones in a die with 13 mm diameter. The paraffin was removed by heating at 873 K for ten hours in air. Then the pellets were annealed at 1273 K in the gas phase of different oxygen activities. The surface area of the external surface, which is exposed to light, is 1,0011 cm2. Gas Phase - Oxygen Activity


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The TiO2 pellets were annealed in gas mixtures of different oxygen activities, including: • Argon-hydrogen (1%) mixture – this mixture was used for imposition of strongly reducing conditions of approximately p(O2) =10-12 Pa • Pure argon - the oxygen activity in argon was approximately p(O2) =10 Pa • Artificial air {p(O2)=21kPa} was prepared by mixing oxygen and argon • Pure oxygen {p(O2) =105 Pa} The specimens were annealed in the tube furnace connected to the gas flow system (the gas flow velocity was 100 ml/min). The oxygen activities were determined by an electrochemical oxygen sensor based on zirconia. According to the specification of gas supplier, the content of water vapour in argon and oxygen was approximately 0.001%.

Optical Properties The optical properties were assessed using the Agilent Cary 5000 UV-Vis-NIR spectrophotometer fitted with a 150 mm diameter integrating sphere (external diffuse reflectance accessory, DRA-2500). The system was warmed up for 30 min before calibration and measurements. The system baseline was calibrated with polytetrafluoroethylene (PTFE) standard plate on the reference port. Then the samples were attached to the sample port to collect the reflectance signals in the wavelength range 300-800 nm with a scan rate of 600 nm/min. Band gap has been determined from reflectance spectra in the Vis-UV range using the Tauc method38,39. Knowledge of electronic transition mode between the valence band and the conduction band is needed in order to select the appropriate formalism for indirect and direct transitions, respectively:

 F ( R∞ ) ⋅ hυ 

1/2

= A ( hυ − Eg )

(9)

 F ( R∞ ) ⋅ hυ  = A ( hυ − Eg ) (10) where F ( R∞ ) is the Kubelka-Munk function, h is the Planck constant, ν is frequency, A is a constant and Eg denotes the band gap. The band gap may be graphically determined from the 2

intersection of extrapolated linear part of the plot of  F ( R∞ ) ⋅ hυ  hν, respectively, with the energy axis.

1/2

vs. hν or  F ( R∞ ) ⋅ hυ  vs. 2

Photocatalytic Activity The photocatalytic performance was estimated by measuring the decomposition ratio of aqueous solution of MB under simulated solar light. The illumination was provided by the Oriel Sol3A solar simulator, model 94043A with 450 W lamp. This class AAA instrument delivers the light closely matching solar spectrum (spectral match better than 98.75%) with intensity of 1 sun at the working distance of 10 cm. The samples were dipped into a 10 ml beaker with a Teflon support and a magnetic stirrer set under the support to make the solution homogeneous all the time. The whole device was standing in dark for two hours to check the absorption of MB by the sample, starting from the concentration of 0.01 mmol/L. Then the solar simulator was turn on to start the photocatalytic decomposition of MB. The samples of MB solution were taken every 15 min to determine the concentration and then poured back to the baker so the total volume of the solution remained unchanged. The changes in the MB concentration were determined spectrophotometrically, comparing the height of the absorbance peak at ~670 nm.


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Electrical Conductivity The electrical conductivity was determined using the four probe technique41. The resistance of the 10 mm square plate (3 mm thick) was approximately 1 kΩ. The specimens annealed at higher oxygen activities, exhibited the resistance that was much larger than 1 GΩ. 8. RESULTS The SEM micrographs of the TiO2 pellets after a range of surface processing, including (i) sintering, (ii) sintering and polishing as well as (iii) sintering followed by polishing and subsequent re-annealing, are shown in Figure 2a, 2b and 2c, respectively.


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Figure 2. SEM photos of (a) TiO2 sintered at 1273 K for 3 hours, (b) TiO2 sintered at 1273 K for 3 hours followed with polishing treatment, and (c) TiO2 sintered at 1273 K for 3 hours followed with polishing treatment and final annealing for 1 h at 1273 K. As seen, the sintered specimen exhibits uneven surface with exposed individual grains (2a). The polished surface is flat and the porosity is apparent (2b). The sintered, polished and reannealed surface exhibits well shaped grain boundaries (Figure 2c). The specimen (before and after annealing) exhibits a substantial degree of porosity. As also seen, the annealing does not have a substantial effect on the grain size that remains between 0.2 and 0.4 µm. The effect of oxygen activity on the band gap is shown in Table 2.

Table 2 The band gap dependence on the oxygen activity of the samples processed at 1273 K for 3 hours in pure oxygen, argon, artificial air, and H2/argon mixture derived by the Kubelka-Munk model with  F ( R∞ ) ⋅ hυ  M-K model

1/2

 F ( R∞ ) ⋅ hυ 

1/2

vs. hυ

 F ( R∞ ) ⋅ hυ  vs. hυ 2

vs. hν or  F ( R∞ ) ⋅ hυ  vs. hν, respectively. O2 (eV) Air (eV) Ar (eV) 2.91 2.91 2.89 2

3.01

3.03

3.01

H2/Ar (eV) 2.42 2.88

As seen in Table 2, the band gap of the samples processed in pure oxygen, argon, and air is about 2.90 eV or 3.00 eV (depending on the assumed). As also seen, imposition of strongly reducing oxygen activity results in reduction of the band gap to the level of 2.46 eV or 2.88 eV, respectively24. The observed effect of oxygen activity on the band gap seems to be associated with strong interactions between the predominant defects (oxygen vacancies and titanium interstitials) and the related structural deformations.

Figure 3 displays the photocatalytic decomposition of MB by the TiO2 samples annealed in pure oxygen (as an example).


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Figure 3. The graphs representing the photocatalytic decomposition of MB under solar light (simulated) with photocatalysts annealed in oxygen. The spectra of the specimens annealed in the gas phase of different oxygen activities are similar in shape. It turned out that the ceramic samples absorb strongly MB from the solution (after two hours in darkness, approximately 40% of MB was absorbed into the pores). All samples gradually decompose MB under the light imposed by solar simulator. However, even in absence of the photocatalyst, light results in MB decomposition. The relative concentrations of MB after different irradiation times, with or without photocatalysts are shown in Figure 4.

Figure 4. Normalized concentrations of MB after irradiation with simulated solar light for photocatalysts annealed under oxygen, argon, and hydrogen/argon mixture. For comparison, the direct decomposition ratio of MB under solar simulator irradiation (without the photocatalyst) is included as well. As seen, the decomposition of MB is accelerated in the presence of the photocatalysts. The sample annealed in pure oxygen shows the best performance in terms of the MB decomposition ratio. As also seen, the sample annealed in H2/Ar mixture exhibits the lowest MB decomposition ratio. The relatively low values of the decomposition ratios observed in this work seem to be related to low surface area exposed to light. The effect of oxygen activity during annealing on the decomposition rates for all specimens is shown in Figure 5. The related experimental data, determined at different times corresponding to different reaction progress, allow the following points to be made:


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Figure 5. The comparison of decomposition rates of MB (expressed as the ratio of the actual concentration to the initial concentration), which is defined as RD, under solar simulator with photocatalysts annealed under different oxygen activities. The numbers on the lines correspond to the time (in minutes) after which the concentration of MB was determined. • • • •

The decomposition ratio, defined as RD, for samples annealed in pure oxygen and pure argon exhibit comparable values. The decomposition ratio for the sample processed in the hydrogen/argon mixture exhibits the lowest value. The above tendency is independent of the time difference between the imposition of light and the determination of the effective concentration of MB. The increase of oxygen activity during the processing results in an increase of photocatalytic activity. This data exhibits good reproducibility in successive experiments.

9. DISCUSSION The experimental data reported in this work can be considered in terms of the KPPs and the associated effect of oxygen activity on photocatalytic performance, including: the band gap, the charge transport, the Fermi level and the concentration of the surface active sites. As seen in Figure 5, the RD ratio increases with the increase of oxygen activity for all experiments. This tendency indicates that the decomposition ratio of MB is determined by the KPPs, which have the enhancing effect on the performance when the oxygen activity increases, including: • The concentration of titanium vacancies


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• The Fermi level On the other hand, the observed effect of oxygen activity on the photocatalytic performance is inconsistent with the assumption that the following KPPs dominate: • Band gap • Charge transport

Concentration of Titanium Vacancies As seen in Figure 1, the increase of oxygen activity results in an increase in the concentration of titanium vacancies. The increase is substantial in the p(O2) range of 10-8 Pa – 10-5 Pa and less significant at p(O2) >10-5 Pa. Taking into account that titanium vacancies are the active sites for water oxidation, the data on the photocatalytic activity in Figure 5 indicates that the population of these sites has an effect on the performance in water oxidation. It may be considered in terms of the mechanism represented in Figure 6. As seen in Figure 6, the mechanism of partial water oxidation involves the following reactions: • Adsorption of water molecules at surface anodic sites formed of titanium vacancies. Both lead to the formation of an active complex (indicated by the dashed line). • Oxidation of the active complex leading to the formation of hydroxyl radicals (OH*) and protons. The electron holes required for oxidation of the active complex are provided by singly ionized oxygen lattice ions (O-), which are reduced to O2- ions. • Adsorption of molecular oxygen. The adsorption active sites are lattice titanium ions. • Oxidation of oxygen leading to the formation of oxygen superoxide species. The electrons required for oxygen reduction of the adsorbed oxygen species are provided by trivalent titanium lattice ions (Ti3+), which are oxidized to Ti4+ ions. • The protons and superoxide species result in the formation of hydrogen peroxide species. • The species of hydrogen radicals, hydrogen peroxide and superoxide radicals react with an organic molecule, such as microbial cell or alternative organic compound (including MB), leading to their mineralisation42. The model in Figure 6 has been derived assuming that the active radicals formed as a result of water oxidation, include hydrogen peroxide, hydroxyl species and superoxide oxygen species. One may also assume that singly ionized atomic oxygen species are involved as well.


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Figure 6. The model representing the role of structural imperfections in the reactivity of TiO2 with water leading to partial oxidation of water and the related charge transfer. Fermi Level The effect of oxygen activity on the position of the Fermi level (or the chemical potential of electrons), is represented in Figure 1b. As seen, the increase of oxygen activity results in a continuing decline of the Fermi level. This leads to an increased electronic affinity to electrons. Consequently, the observed increase of the photocatalytic activity indicates that the decrease of the Fermi level has positive effect on the photocatalytic performance. Band Gap As seen in Table 2 (Figure 7), the band gap of rutile annealed in oxidising conditions, including both pure oxygen, p(O2)=105 Pa, air (21 kPa) and argon, p(O2)=10 Pa, remains at approximately 3 eV.


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Figure 7. Effect of oxygen activity on the band gap of TiO2 However, the band gap of rutile annealed in strongly reducing conditions is reduced to the level of 2.42 eV (direct transitions) or 2.88 eV (indirect transitions). Therefore, the band gap has the tendency to increase when oxygen activity is elevated. This tendency is inconsistent with the observed effect of oxygen activity on the photocatalytic activity. These facts indicate that the band gap is the KPP, which plays a minor role in the photocatalytic performance, if any.

Charge Transport The electrical conductivity is a certain measure of the charge transport. As seen in Figure 1a, the electrical conductivity at 1273 K decreases within the n-type regime ranging between the strongly reducing conditions and the oxidation conditions imposed by air. The absolute values of the electrical conductivity at room temperature are much lower than those at 1273 K, however, the effect of oxygen activity on the electrical conductivity should be similar. Therefore, the observed effect of oxygen activity on the charge transport indicates that higher value of this KPP might have a negative effect on the photocatalytic performance.

10. CONCLUSIONS This work has examined the effect of oxygen activity in the oxide lattice of the rutile phase, imposed by annealing in the gas phase of controlled oxygen activity, on photocatalytic activity in oxidation of MB. The observed beneficial effect of increased oxygen activity in the TiO2 lattice on the performance may be considered in terms of four KPPs: concentration of surface active sites, Fermi level, band gap and charge transport. The analysis of the effect of oxygen activity on the KPPs indicates that two KPPs have the predominant effect on the performance: • Concentration of surface active sites • Fermi level At this stage it is difficult to assess which of these two KPPs is the dominant one. At the same time the following KPPs have minor effect on the performance: • •

Band gap Charge transport

The outcome of the present work is surprising. This finding indicate that while the light absorption by the photocatalyst is important, the alternative properties related to the conversion of light energy into the chemical energy have more significant effect on the photocatalytic. It appears that the photocatalytic activity must be considered in terms of specific surface sites that are able to effectively remove electrons from the water molecules. In the case of TiO2 photocatalysis, such surface active sites are titanium vacancies. Availability of local sites is essential for effective charge transfer between the water molecule and the rutile surface.


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ACKNOWLEDGMENT The support of the Australian Renewable Energy Agency (ARENA: 4-F001) through fellowships provided to WL is acknowledged.


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Photocatalytic Properties of TiO2 - American Chemical Society

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