Electronic properties of photocatalytic improved Degussa P25 titanium dioxide powder

Accepted Manuscript Title: Electronic properties of photocatalytic improved Degussa P25 titanium dioxide powder Author: M. Hantusch V. Bessergenev M.C. Mateus M. Knupfer E. Burkel PII: DOI: Reference:

S0920-5861(17)30757-5 https://doi.org/doi:10.1016/j.cattod.2017.11.005 CATTOD 11119

To appear in:

Catalysis Today

Received date: Revised date: Accepted date:

28-11-2016 24-10-2017 4-11-2017

Please cite this article as: M. Hantusch, V. Bessergenev, M.C. Mateus, M. Knupfer, E. Burkel, Electronic properties of photocatalytic improved Degussa P25 titanium dioxide powder, (2017), https://doi.org/10.1016/j.cattod.2017.11.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

*Highlights (for review)

Highlights: Redistributing oxygen defects in TiO2 during mild heat treatment up to 400°C. Resistive switching in low-defected TiO2 powder can be observed in vacuum atmosphere. During annealing oxygen atmosphere suppress an enhanced charge carrier transport.

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Conductive state is correlated with surface/interface charge carrier trapping sites.

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*Graphical Abstract (for review)

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Electronic properties of photocatalytic improved Degussa P25 titanium dioxide powder M. Hantusch1 , V. Bessergenev2 , M.C. Mateus3 , M.Knupfer4 , E. Burkel1 1 University

of Rostock, Institute of Physics, Rostock, Germany do Algarve, FCT and Centre of Marine Sciences (CCMAR), Campus de Gambelas, 8005-117 Faro, Portugal 3 Universidade do Algarve, FCT and Centro de Investiga¸ c˜ ao em Qu´ımica do Algarve, Campus de Gambelas, 8005-117 Faro, Portugal 4 IFW Dresden P.O. Box 270116, D- 01171 Dresden, Germany

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2 Universidade

Keywords: Titanium dioxide, Resistive switching, P25, Oxygen defects, Defect physics

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Abstract

1

Introduction

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The defect structures of photocatalytic improved Degussa P25 powder were evaluated by X-Ray photoelectron spectroscopy (XPS), paramagnetic resonance spectroscopy (EPR) and in-situ studies of the electrical resistivity in order to understand the charge carrier transport inside the material. Annealing the titanium dioxide in vacuum at temperatures below the phase transition temperature did not increase the oxygen defect concentration drastically. However, annealing Degussa P25 powder in vacuum resulted in a redistribution of Ti3+ -lattice states and lattice oxygen vacancies. The diffusion of these lattice oxygen vacancies from the bulk to the surface of the material forms charge carrier trapping sites. Such transformation of the defect structure of TiO2 leads to switching from an insulating state to a conducting state.

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Several reviews focus on the enhancement of photocatalytic reactions [1–3]. It has been shown that titanium dioxide Degussa P25 powder is one of the most efficient current photocatalysts for a wide spectrum of applications [4–6]. A detailed discussion of the photocatalytic process can be found in [7]. In principle, the photocatalytic efficiency can be improved by one of the following three parameters of the material: Firstly, by doping titanium dioxide with impurities the bandgap energy can be lowered to access a wider range of the solar spectrum [2, 8–10]. Secondly, the external specific surface area of the material can be increased to raise the possibility of pre-adsorption of organic compounds on the surface of the photocatalysts [11]. And thirdly, the charge carrier separation process inside the semiconductor can be enhanced to suppress the electron-hole recombination by changing the defect structure to trap electrons and holes on the surface [12–14]. Hereby, the idea is to create localized diffusion centres to strictly separate electrons and holes. These influences are widely studied separately as well as in their synergy for various titanium dioxide systems [15–18]. For example, detailed information on crystal structure and electronic properties of TiO2 photocatalysts is a base for so called surface science approach in understanding heterogeneous photocatalysis. Such approach was developed in some research papers and review articles [1–3, 19, 20]. This approach allows the researcher to understand and control for many variables (such as coverage, surface structure, temperature, etc.) while examining in detail various fundamental aspects of photon-initiated events [20]. However, due to complexity and many steps character of 1 Page 3 of 22

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Previous study

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photochemical reactions direct relations between these variables and the efficiency of photocatalysts is still not well established. Probably, the main reason is the absence of experiments on determination of above mentioned variables and photocatalytic activity simultaneously, using the same samples. So, the purpose of this paper is to study electronic properties of the Degussa P25 TiO2 samples treated at different temperatures in vacuum and in air and for which photocatalytic properties have been established. The central idea is to characterise the charge carrier transfer inside Degussa P25 by determining the resistivity of the material using a new in-situ measurement in mild temperature regime. Additionally, well established methods for defect analysis, X-Ray photoelectron spectroscopy (XPS) and paramagnetic resonance spectroscopy (EPR), are used to provide informations for samples showing mild temperature anomalies.

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Figure 1.1: overall photocatalytic constant k of titanium dioxide Degussa P25 powders annealed in different atmospheres at various temperatures for 4 h [5] calculated by equation 1.1 Former studies on Degussa P25 powder from this group focused on annealing Degussa P25 powder in different atmospheres at temperatures up to 700 ◦ C. In figure 1.1 the results for the photocatalytic activity of this powders represented as the velocity constant k of a first order kinetic reaction are shown. k was calculated by:   [C(t)] =k·t (1.1) ln [C0 ] with the concentration [C(t)] and initial concentration [C0 ] of Fenarimol in water. A maximum for high vacuum treated samples was observed at 400 ◦ C. Annealing titanium dioxide at 400 ◦ C in vacuum improves the photocatalytic activity by 75 % compared to the original powder. Above this global maximum, a photocatalytic activity similar to the original powder is measured at 550 ◦ C. It could be concluded that at this temperature a neutralisation took place and initial values are restored [5]. The powders annealed in air are showing a constant photocatalytic efficiency up to 620 ◦ C. The crystallographic analysis reveals no significant changes in the anatase-rutile phase 2 Page 4 of 22

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composition and in the crystallite sizes for all powders annealed up to 400 ◦ C independently of the surrounding atmosphere. Only Degussa P25 powders treated at 550 ◦ C or higher show an increased rutile fraction compared to the original powder [5]. Because of this, the heat treatment up to the beginning of the phase transition at 550 ◦ C will be called a heat treatment in mild temperature range.

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Figure 1.2: specific surface area of titanium dioxide Degussa P25 powders annealed in different atmospheres at various temperatures for 4 h [5]

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The results of nitrogen adsorption measurements to determine the specific surface area from the previous study are shown in figure 1.2. Focussing on the temperature range from room temperature to 550 ◦ C, an inverse behaviour compared to the photocatalytic activity is observed. The powders annealed in vacuum are showing a minimum at 400 ◦ C while the powders annealed in air are showing a maximum at this very temperature. But a neutralisation of the morphology changing process is observed at 550 ◦ C for both atmospheres [5]. Degussa P25 titanium dioxide powder forms particles based on van-der-Waals interactions which is called aggregation [21,22]. During heat treatment in a mild temperature range, there are the following processes to influence the surface area: Firstly, an evaporation of organic compounds leading to a pore formation process and an increase in surface area [23, 24]. Secondly, lowering the particle sizes leading to an increase in surface area. Thirdly, an agglomeration of the material. During this process covalent bonds are formed instead of the van-der-Waals interaction and rigid grains are created. This leads to a decrease in the surface area [21, 22]. From the observed specific surface area behaviour for annealed Degussa P25 powder it is concluded that the grain forming process is preferred in vacuum atmosphere [5]. The anatase to rutile phase transition is an irreversible process. The transition begins with breaking covalent bonds of the anatase structure, especially close to line defects, such as grain boundaries. After that new covalent bonds of rutile structures are formed [25]. This characteristics of the phase transition leads to a decrease of the grain size and an increase of the surface area. The observed neutralisation at 550 ◦ C is connected with the phase transition.

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Figure 1.3: photocatalytic constant normalised on the specific surface area kAS of titanium dioxide Degussa P25 powders annealed in different atmospheres at various temperatures for 4 h [5]

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A highlight in the previous study is the new comparison method which normalizes the overall photocatalytic activity over the external surface area of the photocatalysts to characterize the influence of the charge carrier transport and the electron-holeseparation ability of the material. The results are shown in figure 1.3 and they evaluate that the normalized photocatalytic efficiency is increased by more than 100%. Furthermore, it was shown that increasing the external surface area has minor influence on improving the photocatalytic efficiency [5]. According to the results of the previous study, a closer look on the defect structure is necessary. Therefore, a detailed investigation of the powder annealed at 400 ◦ C in vacuum will be performed since this powder shows the highest overall photocatalytic activity and the same anatase-to-rutile-ratio as the original powder. Though the amount of the photocatalytic active phase (anatase) and the crystal structure is similar to the original Degussa P25 powder and the specific surface area is already analysed.

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oxygen defects

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Figure 1.4: Schematic draw of defects in a 2D titanium dioxide lattice, the defects(square) between titanium interstitials, Ti3+ and Ti4+ , and oxygen vacancies, VO . The charge neutrality of the material leads to different influences on the binding states of the lattice titanium(red) [13]

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The defects in TiO2 can be distinguished into lattice point defects such as Ti3+ states or oxygen vacancies ,VO , and line defects such as grain boundaries or crystallographic shear planes [19]. Figure 1.4 shows a schematic representation of the possible point defects for self-doped titanium dioxide. Oxygen vacancies, VO , are defined as structural oxygen defects on crystal lattice points [13] and can be assumed as quasi-free-electrons. Ti3+ -states are structural defects which are connected with titanium interstitials leading to a local reduction of the binding state of a lattice titanium ion. Generally, lattice oxygen vacancies are formed by a thermal treatment in an oxygenpoor atmosphere at elevated temperatures. Using the standard Kr¨oger-Vink notation [26], this process can be described by the following equilibrium [27, 28]: 1 OO ←→ VO + O2 + 2e 2

(1.2)

with the lattice oxygen OO and lattice oxygen vacancy VO . The transformation of the equilibrium constant of this reaction K leads to the concentration of lattice oxygen vacancies inside the TiO2 powder: [VO ] = Kn−2 p (O2 )−1/2

(1.3)

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with the surrounding oxygen pressure p (O2 ) and the concentration of electrons n. It can be deduced from equation 1.3 that the formation of lattice oxygen vacancies is enhanced under vacuum conditions.

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Figure 1.5: Schematic representation of conduction band traps in a defective semiconductor with the conduction band CB, the valence band V B, the intrinsic Fermi-energy Ef i , the Fermi-energy Ef and the barrier energy of the traps EB . Every trap is considered to include one electron of each spin and there is no interaction of traps. A higher amount of defects leads to a lower energy barrier and a larger trap density [29, 30]

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A special interest lies in defects that can act as charge carrier traps. These are special modifications of the oxygen vacancies as well as of the Ti3+ -states. They change the band structure of the semiconductor material by creating new energy levels inside the band gap. Figure 1.5 shows a sketch of two possible band structures for a low defective (left) material and a high defective one (right). A lower amount of defects leads to a higher trap energy EB and a smaller trap density [30]. The presence of charge carrier traps can be observed by measuring a trapping current. This trapping current is connected with the change of the resistivity of the semiconductor with changing the temperature [29]: 1

EB BT

−k

σ ∼ T −2 e

(1.4)

with the conductivity σ, the temperature T , energy barrier EB and the Boltzmann constant kB . In this study, the focus is on the characteristics of the oxygen defects in Degussa P25 titanium dioxide powder. Therefore, the original powder and the powder annealed at 400 ◦ C in high vacuum (≈ 2.0 · 10−5 mbar) for 4 h are analysed by XPS and EPR. Additionally, the original powder is measured by quasi-in-situ XPS and by an in-situ resistivity measurement to determine the dynamics of the oxygen defects and their changes. 2 2.1

Materials and methods Sample preparation

The titanium dioxide Degussa P25 powder is annealed for 4 h in a high vacuum of ≈ 2.5 · 10−5 mbar. Further details concerning the powder and the sample preparation 6 Page 8 of 22

are published in [5]. The structural or defective changes during the heat treatment of the powder were studied in quasi-in-situ XPS studies and in in-situ resistivity studies. Therefore, titanium dioxide P25 disks of 10 mm in diameter were prepared by cold pressing approximately 0.5 g powder with an hydraulic press applying a load of about 4 t for 5 min. Resistivity measurement

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Figure 2.1: Schematic draw of the circuit diagram for the resistivity measurement setup, with reference resistance Rr , sample resistance Rs

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For the charge carrier transport inside Degussa P25 powder studies the resistivity was analysed. Cold pressed pellets were used with silver electrodes attached symmetrically in the centre of both sites of the titanium dioxide disks. The electrodes had a diameter of around 1.5 mm and were connected to the setup by gold wires. Figure 2.1 shows the applied resistive setup. Ensuring a high resolution during the annealing, the resistance of the P25 sample, Rs , was measured indirectly via a reference resistance RR . The sample resistance, Rs , was calculated by: U0 − U r Ur

(2.1)

Rs = 2 ∗ Rc + Ri

(2.2)

Rs = Rr

with the contact resistance Rc , intrinsic sample resistance Ri , original voltage U0 and voltage over the reference resistance Ur . Assuming that the contact resistance between the silver electrodes and the titanium dioxide is negligible compared to the intrinsic sample resistance and using equation 2.1, the resistivity ρ is: Rs ∗ A (2.3) l with the thickness of a titanium dioxide disk l and the area of an electrode A. The reference resistance was chosen as Rr = 1 M Ω which resulted in a maximum current of 0.1 mA by applying a voltage of 100 V to insure that the samples were not ρ=

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2.3

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influenced by ohmic heating. The titanium dioxide P25 disks were placed in a copper furnace inside a vacuum chamber of ≈ 5.0 · 10−6 mmbar. They were dielectricly screened by a quartz glass and a mica foil. Inside the furnace, two halogen lamps (1 kW) are installed as external heat sources. There is a linear correlation between the resistivity and the temperature. Considering that there is no thermoelectric effect because of an homogeneous temperature distribution inside the material, a trap filling current can be measured during annealing. With equation 1.1 the barrier energy of the charge carrier traps EB can be calculated by [29]: d(log(ρ)) (2.4) EB = d( (kB1T ) ) X-Ray photoelectron spectroscopy (XPS)

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The chemical composition as well as the relation of the Ti3+ /Ti4+ -states for the two sample systems were analysed by using a XPS setup with a PHI 5600ci spectrometer at the IFW Dresden. A monochromated Al-Kα X-Ray source operating at 1486.7 eV was used in combination with an analyser at 20 eV pass energy and a 0.5 eV step width for the survey spectra and an analyser at 5.85 eV pass energy and a 0.05 eV step width for the detailed spectra. Two kinds of XPS experiments were performed. Firstly, the pure titanium dioxide P25 powder and the powder treated in vacuum at 400◦ C, were studied. They were kept in a floodgate at ≈ 10−7 mbar for 2 h at room temperature before the measurement in order to clean the surface from physisorbed organic compounds. Secondly, a quasi-in-situ study was done with P25 disks. In a preparation chamber directly connected with the measurement chamber, the pellets were heated in ultra high vacuum (≈ 10−9 mmbar) for 2 h at temperatures between 100◦ C and 350◦ C. After this, they cooled down and they were transferred into the measurement chamber without any oxygen contact in order to analyse the defect concentrations inside the pellet. The Calculation of the chemical composition of the samples is based on the area ratios of the element specific band edges multiplied by the sensitivity factors σ with σT i = 1.798 for Ti 2p, σO = 0.711 for O 1s and σC = 0.296 C 1s [31] from of the survey spectra.

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Figure 2.2: XPS detailed spectrum of the Ti 2p3/2 -peak after background subtraction including the experimental curve and the theoretical fitting curve based on Ti4+ -species and Ti3+ -species of a Degussa P25-pellet after heat treatment in the preparation chamber at 350◦ C for 2 h.

Figure 2.3: XPS detailed spectrum of the O 1s-peak after background subtraction including the experimental curve and the theoretical fitting curve based on structural and non-structural oxygen of a Degussa P25-pellet after heat treatment in the preparation chamber at 350◦ C for 2 h. Examples of the detailed spectra are shown in figure 2.2 for the Ti 2p3/2 -peak regime and 2.3 the O 1s-peak regime. The Shirley background is subtracted for each single XPS-spectrum [32, 33] before the fitting procedure. In the graphics of the spectra the intensity is plotted in squared scale to achieve same statistical derivation on the maxima and on the background [34]. The determination of the amount of Ti3+ -states was based on the area ratio of these 9 Page 11 of 22

titanium species in the Ti 2p3/2 -peak of each detailed spectrum. The Ti3+ -state is located in the low energy shoulder of the Ti 2p3/2 -peak as it is shown in figure 2.2. The areas of both titanium species, Ti3+ and Ti4+ , are calculated by using a PseudoVoigt line shape in product form with the following parameters for the full-width-halfmaximum (fwhm) and the binding energy of the peak position: f whmT i3+ = 0.95 eV (±0.02 eV )

BET i4+ ,centre = 459.4 eV (±0.2 eV )

f whmT i4+ = 0.95 eV (±0.02 eV )

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BET i3+ ,centre = 457.8 eV (±0.2 eV )

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for each detailed spectrum. Additionally, the detailed spectrum of the oxygen O 1s-peak, figure 2.3, is analysed to determine the structural oxygen or the lattice oxygen in the titanium dioxide system. The area of the structural oxygen peak is calculated by using the parameters: f whmOstructural = 1.08 eV (±0.02 eV )

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BEOstructural ,centre = 530.5 eV (±0.1 eV )

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for each detailed spectrum. The full-width-half-maximum of the non-structural oxygen peak is varying because it includes different oxygen binding states such as C-O, O-O or O-H in different ratios. Paramagnetic resonance spectroscopy (EPR)

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X-Band EPR measurements were performed using a Bruker EMX CW-microspectrometer. Approximately 15 mg powder was analysed in a commercial X-band EPR tube at room temperature and at 100 K using a liquid nitrogen stream. During the measurement with constant microwave frequency the magnetic field was varied between 3100 G and 3600 G. The g-values of the resonance spectra were calculated by:

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with the Planck constant h, the microwave frequency ν, the Bohr magneton µB and the magnetic field B.

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Resistivity

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3.1

Results and discussion

Figure 3.1: Resistivity measurements of cold pressed Degussa P25 pellets in different atmospheres

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The resistivity measurements of the cold pressed titanium dioxide Degussa P25 pellets were characterizing the charge carrier transport inside the materials. The results for the treatment in air and vacuum are shown in figure 3.1. Both heating curves show increases of the resistivity at around 100 ◦ C due to the release of water and of organic residuals which leads to a loss of ions as well as to pore formation. Both are reasons for a higher resistivity. Smaller linear decreases of the resistivity of the samples with higher temperatures up to 180 ◦ C follow caused by a trap filling current. At even higher temperatures, the heating curves show different behaviours. The titanium dioxide treated in air shows a minimum at 255 ◦ C and then a stabilisation at high resistivity values. The sample stayed in an insulating state. The slope of the decrease of the resistivity with higher temperatures (300 ◦ C to 380 ◦ C) changes significantly compared to the slope before the peak (120 ◦ C to 220 ◦ C) as it is represented in table 3.1. The energy barrier of the charge carrier traps inside the band structure of titanium dioxide increase from 0.07 eV to 0.1 eV. This implicated a loss of defects in air at elevated temperatures up to 400 ◦ C since an higher barrier energy implies deeper charge carrier traps and a lower charge carrier trap density [35]. Curve heating vacuum heating vacuum heating air heating air

Temperature range 105 ◦ C 420 ◦ C 120 ◦ C 300 ◦ C

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barrier energy of charge carrier traps

180 ◦ C 520 ◦ C 220 ◦ C 380 ◦ C

0.04 0.06 0.07 0.10

(±0.01) eV (±0.01) eV (±0.01) eV (±0.01) eV

Table 3.1: Calculated barrier energy of charge carrier traps due to defects by equation 2.4 11 Page 13 of 22

X-Ray photoelectron spectroscopy

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In contrary, the titanium dioxide P25 pellet in vacuum showed a tremendous drop in resistivity over 4 orders of magnitude at around 200 ◦ C. Followed by a smaller decrease from 220 ◦ C up to 420 ◦ C until a nearly constant value is reached. The titanium dioxide has changed from an insulating state to a conductive state. The decrease in resistivity from 420 ◦ C to 520 ◦ C is due to a trap filling current. The energy barrier of the traps did not differ significantly before (105 ◦ C to 180 ◦ C) and after (420 ◦ C to 520 ◦ C) this resistive switching. There is no significant increase of oxygen defects inside the titanium dioxide explaining the conductive state since the absolute amount of defects being able to trap charge carriers is not changing according to the same energy levels at the beginning and the ending of the heating curve. Hence, a transformation of the defect nature took place during the temperature interval from 180 ◦ C to 420 ◦ C which caused a stable conductive state. The missing of such a state in air suggests that the redistribution of oxygen defects matters. The peak temperature in air is connected with the maximum activation energy for a transformation of defect nature but the presence of oxygen avoided a resistive switching and neutralized the transformation. During the cooling, the titanium dioxide kept its conductive state even below 200 ◦ C. So, there was no reswitching from the conductive state to the insulating state. At 60 ◦ C the valve was opened and the chamber was filled with air. This resulted in an increase of resistivity but only up to several hundreds Ω·cm. That means the conducting channels inside the material were independent of the temperature and the atmosphere after the whole transformation process is finished.

Figure 3.2: Calculated amount of Ti3+ -species of the included titanium(red) and the calculated ratio between the lattice oxygen(OI) and titanium(blue) inside a Degussa P25 cold pressed pellet during a quasi in-situ heat treatment in ultra-high-vacuum. The XPS analysis is done to determine the relative amount of defects inside the sample. Firstly, the Ti3+ -species is analysed, which is located in the low energy shoulder of the Ti 2p band edge. An example is shown in figure 2.2. Secondly, the lattice oxygen inside the titanium dioxide crystal (also called structural oxygen), OI-species [31], in the O1s band edge is analysed, because the OI-species is directly connected with oxygen vacancies in the titanium dioxide lattice, VO , represented by the structural oxygen to titanium ratio: OI AO · σT i = · OI (3.1) Ti σT i · AT i 12 Page 14 of 22

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with the area of the oxygen O 1s peak AO and the area of the titanium Ti 2p peak AT i in the survey measurement of the sample, the sensitivity factors σT i and σO [31] and the relative amount of the structural oxygen,OI, in the O 1s band. The calculated values are not the real stoichiometric ratio of the titanium dioxide because the titanium amounts are overestimated by not distinguishing between structural and interstitial titanium. During the quasi-in-situ measurement, a cold pressed P25 pellet was annealed in ultrahigh-vacuum (≈ 10−8 mmbar) at temperatures up to 350 ◦ C, where the resistive switching finished. The calculated amount for the Ti3+ -species and the structural oxygen to titanium ratio OI/T i are represented in figure 3.2. The number of Ti3+ -sites increases steadily until the final temperature of 350 ◦ C, but there is an anomaly at 220 ◦ C. At this temperature a minimum value of 1.1 % is measured. This value is significantly lower than the amount of Ti3+ measured for 250 ◦ C and 350 ◦ C. The temperature of 220 ◦ C corresponds to the resistive switching temperature in vacuum.

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Figure 3.3: XPS original spectra of Ti 2p-peaks (left) of a Degussa P25 pellet during quasi-in-situ heat treatment for two different temperatures and the difference of these Ti 2p-spectras (right)

Figure 3.4: XPS original spectra of Ti 2p-peaks (left) of Degussa P25 powder without any treatment and after heat treatment in vacuum at 400 ◦ C and the difference of these Ti 2p-spectras (right) Because of the low amount of Ti3+ -species in P25 and the high experimental error due to the signal-to-background-ratio in the area of the Ti3+ -peak it is necessary 13 Page 15 of 22

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to investigate the significant increase of reduced titanium between 220 ◦ C and 250 ◦ C during the heat treatment in vacuum by an additional qualitative method. An idea is to subtract the normalised spectrum of a pure Ti4+ surface from the normalised spectrum of P25 with surface defects [36]. While the spectrum of a pure Ti4+ surface of titanium dioxide is not available, because TiO2 always includes small amounts of oxygen defects [37], the subtraction is performed between the sample with the lowest amount of Ti3+ calculated (P25 at 220 ◦ C) and the sample with an significant increase of Ti3+ calculated (P25 at 250 ◦ C) compared to the first sample. Figure 3.3 shows the difference of the Ti 2p-signal between these temperatures. A maximum in this curve is located at a binding energy of 457.8 eV which is the energy of Ti3+ -peak. This indicates a significant increase of surface near Ti3+ -species in titanium dioxide during the heat treatment in vacuum above 220 ◦ C. This behaviour could not be measured in P25 powders treated outside the preparation chamber. Figure 3.4 shows the difference between original Degussa P25 powder and the powder treated at 400 ◦ C in vacuum. There is no peak observable. That means there is no significant difference in surface near Ti3+ -defects. It is possible that the defects formed during the heat treatments are healed when the powders gets in oxygen contact. The OI/Ti-ratios decrease from room temperature to 220 ◦ C down to a value of 1.8. A significant difference at 100 ◦ C is observed which is due to a cleaning process where adsorbed and absorbed organic molecules are released. The minimum at 220 ◦ C indicates that the amount of lattice oxygen vacancies reach a maximum followed by a strong decrease of oxygen vacancies for even higher temperatures. Overall, the XPS results show no drastically increase of surface near Ti3+ -defects or oxygen defects, which would explain the significant increase of photocatalytic activity between original powder and powder treated at 400 ◦ C in vacuum. The results for selected powders including the photocatalytic improved powder are shown in table 3.2. The rise in the OI/Ti-ratio and in the amount Ti3+ -states from 220 ◦ C to 400 ◦ C confirm that at 220 ◦ C a minimum of Ti3+ -states and a maximum of oxygen vacancies is reached. It shows that oxygen defects were redistributed during the annealing up to 220 ◦ C in vacuum.

Temperature

Ti3+ /Ti %

OI/O %

OI/Ti

25 ◦ C 220 ◦ C 400 ◦ C

2.01 ± 0.57 1.08 ± 0.57 2.33 ± 0.57

86.6 ± 1.1 87.6 ± 1.1 86.7 ± 1.1

1.93 ± 0.08 1.78 ± 0.08 2.01 ± 0.08

Table 3.2: XPS results of different Degussa P25 powders treated in high vacuum.

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Paramagnetic spin resonance

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Figure 3.5: Electron spin paramagnetic resonance spectra of selected Degussa P25 powders at 100 K

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The Electron spin paramagnetic resonance spectroscopy is well established in case of determining the nature of oxygen defects in different titanium dioxide systems [17,38, 39]. Therefore, EPR measurements were performed to determine the defect structure in the lattice, as well as, on the surface, more carefully. Therefore, as reference, the original pure P25 powder, the pellet treated at 220 ◦ C in ultra high vacuum after the XPS-measurement as the titanium dioxide powder with the highest expected amount of oxygen vacancies (VO ) and the photocatalytic improved powder annealed in high vacuum at 400 ◦ C were measured. Figure 3.5 shows the EPR-spectra measured at 100 K. At this temperature, the Ti3+ lattice states in anatase and rutile can be determined to be g = 1.981 [40–42]. In all these samples Ti3+ -states can be found. Inside the P25 original powder, only Ti3+ -states were detected. The powder annealed at 220 ◦ C showed a slightly increase of Ti3+ -lattice states. This is contrary to the XPS results. Whereas, the surface states are analysed with the XPS experiments, the EPR experiments using microwave radiation are bulk sensitive. Therefore, a more detailed analysis of the lattice states of a Degussa P25 disk is possible. In pure and at 220 ◦ C vacuum annealed P25, Ti3+ -states are corresponding to anatase lattice titanium species [38]. In P25 vacuum annealed at 400 ◦ C a broad signal was detected. This indicates that a mixture of different Ti3+ -species is detected indicating electron trapping sites. Interestingly the sample P25 annealed at 220 ◦ C shows a strong response at g = 2.002 combined with signals at g = 2.012 and g = 2.028 indicating O− 2 -radicals [18, 42]. This special oxygen configuration indicates former lattice oxygen vacancies produced during 15 Page 17 of 22

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Conclusion

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annealing which adsorb an oxygen molecule after being exposed to air. Since the sample annealed at 220 ◦ C is the only powder that shows this type of adsorbed oxygen it could be concluded that it has the highest amount of lattice oxygen vacancies, confirming the XPS measurement. According to [40], the broad minima only detected in sample annealed at 400 ◦ C with g = 2.050 and g = 1.908 mark either surface hole trapping sites for g > 2 or electron trapping sites for g < 2. Since the O2− -states as an indirect proof of lattice oxygen vacancies are not detected in powders treated at 400 ◦ C, it can be concluded that during the annealing between 220 ◦ C and 400 ◦ C the transformation from lattice oxygen defects into surface/interface defects which can trap charge carriers is possible. During the annealing in vacuum the amount of oxygen defects is not proportional to the annealing temperature. Instead, a redistribution process from lattice defects to surface hole trapping defects at mild temperatures may occur. While for titanium dioxide powders annealed in vacuum at 220 ◦ C a high quantity of lattice defects is observed, powders annealed at 400 ◦ C include more surface or interface defects and less lattice defects. This may indicate that the lattice defects diffuse to lattice spots close to structural line defects creating hole trapping sites. Connecting the results from the resistive measurements with XPS and EPR spectra, the switching from the insulating to the conductive state of titanium dioxide at 200 ◦ C may be considered with a redistribution of oxygen lattice defects until they are changing their positions acting as surface/interface hole trapping sites at temperatures around 400 ◦ C. This creates stable conducting channels inside the pellet resulting in not reswitching during cooling or during exposure to oxygen. The ability to create such channels means a high possibility of effective charge separation and could cause the significant improvement of photocatalytic efficiency.

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A redistribution of oxygen defects in self-doped TiO2 P25 powders annealed in vacuum were studied by XPS and EPR methods. As a result of the redistribution lattice oxygen vacancies diffuse to a surface/interface where they create surface/interface hole trapping centres. This process significantly decreases electron-hole recombination rate and acts favourably for the increase of photocatalytic activity. Studies of the electrical resistivity of titanium dioxide P25 powder pellets in air and in vacuum up to 520 ◦ C, close to the phase transition temperature, have been used for the evaluation of the lattice and surface/interface defect structures, as well. The electrical resistivity shows an anomaly in both atmospheres between 200 ◦ C and 300 ◦ C which is neutralized in air and leads to a resistive switching in vacuum. Enhanced photocatalytic activity based on the correlation between an enhancement in the electrical conductivity and in the redistribution of oxygen vacancies which was confirmed by the observation of the transformation of Ti3+ -lattice sites and of lattice oxygen vacancies VO to surface/interface hole trapping sites at temperatures up to 400 ◦ C. Moreover, the hole trapping sites created by vacuum annealing are stable so that the electrical conductivity still works at room temperature and in oxygen atmosphere.

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Acknowledgement This work was supported by FCT, Project PEST-OE/QUI/UI4023/2014 (CIQA 2014), and by Centre of Marine Sciences (CCMAR), Portugal. This research was supported by the DFG Graduate School welisa (No. 1505/2) for CM. The authors are grateful to Dr. Dirk Hollmann of the LIKAT, Rostock.

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