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Article

Structural and Photoelectrochemical Properties of DC Magnetron-Sputtered TiO-Layers on FTO 2

Angela Kruth, Sandra Peglow, Antje Quade, Marga-Martina Pohl, Rüdiger Foest, Volker Bruser, and Klaus-Dieter Weltmann J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507487c • Publication Date (Web): 06 Oct 2014 Downloaded from http://pubs.acs.org on October 17, 2014

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The Journal of Physical Chemistry

Structural and Photoelectrochemical Properties of DC Magnetron-Sputtered TiO2-Layers on FTO Angela Kruth 1*, Sandra Peglow 1, Antje Quade 1, Marga-Martina Pohl 2, Rüdiger Foest 1, Volker Brüser 1 and Klaus-Dieter Weltmann 1 1

Leibniz Institute for Plasma Science and Technology (INP), Felix-Hausdorff-Str. 2, 17489 Greifswald, Germany 2

Leibniz Institute for Catalysis (LIKAT), Albert-Einstein-Str. 29a, 18059 Rostock, Germany

Abstract TiO2-based photocatalysts layers are still highly favoured materials for photocatalysis on immobilised semiconductor electrodes because of their favourable electronic band positions, high corrosion stability, cost efficiency and readily availability of titanium. Since chemical synthesis of nanocrystalline TiO2 layers from alkoxide precursors is usually complex and of low reproducibility, new methods are required to enable largescale production of immobilised titania catalyst layers for practical applications. The synthesis of TiO2-layers on fluorine doped tin oxide (FTO) by means of a reactive sputtering process is reported here, with stable operation being implied by the means of a λ-sensor. Structural, electrochemical and photocatalytic properties were investigated

for

different

layer

thicknesses

and

post-deposition

annealing

temperature of the deposited TiO2 by means of SEM, TEM, XRD, XPS, UV/Vis spectroscopy and a range of photochemical measurements including iE curves, acIS, IMVS and IMPS and IPCE.

Keywords:

photocatalysis, titania, catalyst immobilisation, reactive magnetron sputtering, photoefficiency

* Corresponding author: [email protected], tel: ++49 3834 554 3960 1

ACS Paragon Plus Environment


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Introduction Solar energy is the world’s largest renewable energy source. To utilise solar energy, semiconductor photocatalysis has received increasing attention due to its capability to convert light into chemical energy by photo-induced redox reactions. Although many systems undergo photo-induced charge separation, only few enable electronexchange with a donor or acceptor because charge recombination is usually highly favoured 1. Titanium dioxide has widely been proven to be an effective material for photocatalysis

2,3

, dye-sensitised solar cells

antifogging surface coatings photocorrosion

7

6

4,5

and self-cleaning, antibacterial and

with high chemical stability and resistance towards

and is traditionally employed as nanopowders. However, the high

agglomeration affinity of nanopowders as well as difficulties to separate TiO2 nanoparticles from an aqueous phase after the photo-induced reaction have lead to approaches involving immobilisation of the catalyst. Deposition of the semiconductor layer onto conductive substrates such as transparent conductive oxides, e.g. fluorineor indium-doped tin oxide, FTO or ITO, gives also rise to the possibility to enhance photoefficiency by electronic band alignment through application of an external bias 8,9

. The general consensus places the band gap energy, EG, for the three naturally

occurring crystallographic phases anatase, brookite and rutile at EG = 3.2 eV, 3.1 eV and 3.0 eV, respectively, although the reported value for the band gap energy varies greatly in the literature, depending on the stoichiometry and the impurity content. For instance, the reported literature values for anatase vary from 2.86 to 3.34 eV

10-12

.

The photocatalytic activity decreases in the order anatase > brookite > rutile, despite the decreasing electronic band gap conduction band

14

13

, as a consequence of different positions of the

. Depending on the synthesis method, amorphous titania is often

observed as a grain boundary or minor phase and was calculated to exhibit a local electronic structure that is similar to the electronic structure of the crystalline phases, with optical properties resembling these of anatase 15. Various methods have been employed to prepare titania thin films, e.g. thermal oxidation of titanium sheets 16, sol-gel methods involving a hydrolytic route 17-19, spray pyrolysis methods deposition

23,24

12,20

but also chemical vapour deposition

, filtered arc deposition

25

21,22

, pulsed laser

and magnetron sputtering techniques

26-33

.

2


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Although chemical synthesis is still the main method used for catalyst production on an industrial scale, increasing environmental limitations to the use of solvents and other toxic products necessitate the development of new and environmentally friendly technologies. Magnetron sputtering is an easily available method that can be used for coating large-area substrates at a high rate and low cost. New reaction pathways can be created in the gas phase and at the surface since reactive species such as ion or neutral radicals and excited species are created in the plasma environment

34

.

A further significant advantage of sputtering techniques for depositions of nanoparticulate layers is the low deposition temperature. Unique nanostructures may be synthesised and retained in low temperature methods, as well as enabling deposition onto novel substrates such as flexible organic substrates that are not stable at higher temperature. Magnetron sputtered layers are typically uniform coatings with strong adhesion and high packing density

27

. For deposition of metal

oxides, reactive radio frequency magnetron sputtering, RF-MS, is commonly employed

26,27

. Direct current magnetron sputtering, DC-MS, is a more simple

sputtering technique that is convenient for industrial applications since it avoids the complexity of RF systems and it exhibits higher deposition rates compared to that for RF-MS

27

. TiO2 films are either deposited by sputtering from a TiO2 target or by

reactive sputtering from a metallic Ti target with addition of oxygen to the process gas. These films are generally amorphous. Deposition of crystalline TiO2 by DC-MS is, however, challenging and the deposition process must usually be accompanied by subsequent thermal annealing

28-30

. Previous studies investigated the influence of

sputtering parameters such as oxygen partial pressure, working pressure, sputtering time and DC power on film morphology and crystal phase formation. For deposition of TiO2 on a Si substrate, it was reported that magnetron powers above 150 W as well as the establishment of an optimum oxygen content of 23 % O2 in Ar at a working pressure of 0.27 Pa are required to ensure formation of the crystalline anatase phase under given conditions

31

. In another study, TiO2 was deposited onto

FTO by DC-MS at an oxygen content of 30 % in Ar, with the working pressure being varied from 1.2 Pa to 2.0 Pa 42. The films contained both, rutile and anatase, with the crystallinity observed to decrease with increasing sputtering pressure. Although using a metallic sputtering target allows for relatively high deposition rates, reactive sputtering processes, e.g. of oxide layers, require precaution during operation in the intermediate region between the metallic and oxidised state of the target. Here, the 3



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so-called hysteresis effect occurs, with immediate transition of the target state between the metallic state and an oxidised state when the deposition is largely influenced because of poisoning of the target. DC sputtering of a metal target in a fully poisoned state is generally associated with a lower deposition rate, but also with significant arcing, poor process stability and poor quality of coatings. For deposition of high quality films, operation must therefore be carried out in the transition region. The target voltage, optical emission from the plasma but also partial pressure of the reactive gas are well-established parameters used to observe and control the state of the sputtering process 43-45. In this work, TiO2 films on FTO have been prepared by DC-MS using a Ti rotating target. To ensure stability of the deposition process, a closed loop process control system was employed, maintaining the balance of the reactive gas and ensuring stable operation within the transition state. An in situ oxygen content feedback system was incorporated into the process control loop, consisting of an oxygen partial pressure determining λ-sensor. The structural and photoelectrocatalytic properties of the deposited layer for different deposition thicknesses and postprocess high temperature annealing treatments was investigated by means of GIXRD, SEM, UV/Vis spectroscopy and photoelectrochemical measurements.

Experimental TiO2-layer deposition and annealing treatment TiO2 layers were deposited onto fluorine-doped tin oxide (FTO, SOLARONIX, TCO 22-7) by reactive DC-MS. The substrates had a geometry of 25 mm x 25 mm. Prior to the TiO2-layer deposition process, they were thoroughly cleaned by ultrasonic treatments, first, in ethanol and then in acetone, followed by soaking of the FTO substrates in 30% HNO3 with rinsing in double deionised (DD) water (Milli-Q Integral 3 System, 18.3 MΩ) after each cleaning step. The substrates were subsequently dried in a drying furnace at 60°C. Cellulose acetate was applied as a removable mask to parts of the FTO substrate to ensure electrical contact for electrochemical measurements. The geometric area of the remaining array for TiO2 deposition was 5 cm2. After drying of the masked substrates at 60°C, samples were immediately 4


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transferred into the vacuum chamber of the sputtering reactor.

To ensure good

homogeneity of the deposited layer as well as high reproducibility of the reactive sputtering process, a rotating cylindrical Ti target, Ti-133, Bekaert Advanced Coatings NV, Belgium, was employed as cathode, at a rotating speed of 15 rpm. Since oxygen pressure was reported to be highly critical for crystal phase formation in TiO2

35

, the DC-MS process parameters were controlled by means of a λ-sensor-

type solid state zirconia oxygen sensor, ZIROX Sensoren und Elektronik GmbH, Greifswald, Germany, which was integrated into the parameter control in order to adjust the magnetron power as a function of oxygen partial pressure during the reactive sputtering process. The typical response time of the sensor was 100 to 200 ms. The sample was transported on a movable sample transport band underneath the rotating target at 17 mm/s during the deposition process, Figure 1. Prior to layer deposition, a target conditioning step was carried out in pure Ar at 8 kW magnetron power over a period of 5 min, in order to remove the passivating oxide layer from the Ti target. The sample substrate was kept under a protective cover during target conditioning. The system was then equilibrated in the reactive gas mixture consisting of 8.7 % O2, 4.3 % N2 and 87 % of Ar, at a total pressure of 3 Pa for 8 min. During the deposition step, the magnetron power was controlled by means of the pO2 sensor output. The initial value was PDC = 5.3 kW at a DC voltage of EDC = 453 V. During the sputtering process, the DC power increased to a higher value, for instance PDC = 8.2 kW after 24 loops. In this study, the number of sample transport loops was varied from 3 to 120, with an increase of nominal thickness of the as-deposited TiO2 layer by 11 nm per sample transport loop, as determined by profilometry using a Dektak 3ST Profilometer. For profilometry, TiO2-layers were deposited onto flat glass substrates under the same experimental conditions as for deposition on FTO and nominal increase of layer thicknesses per loop calculated by extrapolation, Figure 2. To increase the crystallinity of the as-deposited TiO2 layers, samples were subsequently annealed in a tube furnace at different temperatures, varying from 250 to 600 °C, over the duration of 1h, at a heating rate of 10°C/min and oxygen flow rate of 30 ml/min. Grazing Incidence X-ray Diffraction (GIXRD) and X-ray Photon Spectroscopy (XPS) GIXRD was carried out in order to study the crystallographic structure of the sputtered and tempered TiO2 layers on FTO, using a Siemens D5000 AXS 5



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Diffractometer with Cu Kα radiation at 40 kV and 40 mA. Measurements were performed at a constant incident angle ω = 0.5 ° relative to the sample surface, over a range of 2θ from 10 to 60 °, with a step width of 0.02 ° and data collection time of 3 s per step. The crystallite size of the TiO2 anatase phase was calculated from the (101) reflection using the Scherrer formula 49,50. The chemical composition of surface of the TiO2/FTO samples, as well as the oxidation state of Ti at different thicknesses of the TiO2 layer was determined by XPS using a Kratos Axis Ultra DLD instrument. Spectra were recorded by means of monochromatic Al Kα excitation (1,486.6 eV) with a medium magnification (field of view 2) lens mode and by selecting the slot mode, providing an analysis area of approximately 250 µm in diameter. A pass energy of 80 eV was used for estimating the chemical elemental composition and 10 eV for the high energy resolution of the Ti 2p region. Charge neutralisation was implemented by low energy electron injected into the magnetic field of the lens from a filament located directly atop the sample. Data acquisition and processing were carried out using CASAXPS software, version 2.14dev29 (Casa Software Ltd., UK).

Transition electron microscopy (TEM) and scanning electron microscopy (SEM) TEM measurements were performed at 200kV with an aberration-corrected JEMARM200F, JEOL, with CEOS corrector. The microscope was equipped with a JEOL JED-2300 energy-dispersive X-ray-spectrometer (EDXS) for chemical analysis. The aberration corrected STEM imaging (High-Angle Annular Dark Field (HAADF) and Annular Bright Field (ABF) were carried out using a spot size of approximately 0.13nm, a convergence angle of 30-36 ° and collection semi-angles for HAADF and ABF of 90-170 mrad and 11-22mrad respectively. For TEM investigations, commercial Si3N4 membranes of 50 nm thickness (Plano GmbH) were used as substrate. 30 nm thick TiO2 layers were deposited into Si3N4 membranes using similar PVD process and post-deposition annealing parameters as for TiO2 deposition onto FTO glass. The microstructures of TiO2 layers were investigated for different thicknesses of deposited TiO2 on FTO glass using a JEOL JSM 5800 Low Vacuum Scanning electron Microscope, with a SEI detector and an acceleration voltage of 1 to 5 kV and typical working distances of 3 to 6 mm.

6


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UV/Vis Spectroscopy UV/Vis transmission spectra of the TiO2/FTO samples were obtained using a dual beam PerkinElmer Lambda UV/Vis 850 spectrophotometer with a PbS detector port. Sample direct transmittances were measured under normal incidence with air as reference. The extinction, ε, was calculated from the transmittance values, using ε = - log T. For the characterisation of the direct and indirect electronic transitions, Tauc plots were calculated according to (εhν)2 vs E (eV) and (εhν)1/2 vs E (eV), respectively. The value of the band gap was obtained by extrapolating the linear part of the Tauc plot to the axis of the abscissa.

Photoelectrochemical and ac impedance measurements Electrochemical measurements were carried out using a Zahner controlled intensity modulated

photo

spectroscopy

system

(CIMPS),

consisting

of

an

Im6e

electrochemical workstation and an XPOT external potentiostat (Zahner Elektrik GmbH), a high intensity UV light LED (Zahner UV375, λ = 375 ± 15 nm) providing dc light intensities of up to 60 W/m2 and a calibrated silicon diode photosensor of the incident light power, with the sensor output feeding into the XPOT control loop for stability control of light intensity and modulation

36

. A Teflon-based gastight single

cell compartment with a quartz window was employed for containment of the photoelectrochemical cell (PEC), with a three electrode configuration, employing TiO2/FTO as working electrode, a Pt-ring as counter electrode and an Ag/AgCl reference electrode

37

. Measurements were carried out in a 0.1 M KCl electrolyte

solution prepared with Milli-Q water, with a pH of 5.7. No corrections to the measured light power were made for light absorption and reflection in the quartz window and the electrolyte solution. Current density-voltage, iE, measurements were carried out under dark conditions and under 375 nm illumination, with a measured light power of 55 Wm-2. The sweep rate was 20 mV/s, starting at the open cell potential the working electrode (EOCV,dark = 0.049 V and EOCV,375nm = -0.377V vs NHE) to a maximal potential of 1.2 V and a lower potential limit of -1.0 V vs NHE. Electrochemical ac impedance measurements were also carried out under both, dark conditions and under 375 nm 55 Wm-2 illumination, with an applied ac amplitude, Eac, of 20 mV over 7



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the frequency range of 0.1 Hz ≤ ƒ ≤ 400 kHz, with different applied dc bias, Edc, for the

working

electrode.

Intensity

modulated

photocurrent

and

photovoltage

spectroscopy measurements (IMPS/IMVS) were performed at 375 nm illumination. For IMPS/IMVS, the LED provided both dc and ac components of the illumination, with the ac component of the current set to produce a small modulation (< 10%) of the dc light intensity. For IMPS (short circuit), the potential of the working electrode was fixed at the value of the dark potential and the ac frequency of the illumination varied from 0.1 Hz to 10 kHz. For IMVS (open circuit) measurements, a typical ac frequency range of 20 mHz to 1 kHz was used for the illumination

39,41

. The electron

lifetime, τe, was calculated from the semicircle minimum in the IMVS response, which is located at ωmin = 2πƒmin = 1/τe, where ω is the angular frequency of the light modulation. Correspondingly, the electronic transit time, τD, was defined from the IMPS response, using τD = 1/ωmin = 1/(2πƒmin). Incident photon to current efficiencies, IPCE values, were extracted from the real components of the low frequency limit in the IMPS plot

41

. Data were analysed using the Thales Software package (Zahner

Elektrik GmbH) and calculated values were corrected for sample geometry and voltage offset. Since measured currents were low (below 0.5 mAcm-2), no correction for the voltage drop was required.

Results and Discussion Profilometry data, Figure 2, show a close to linear relation between layer thickness and the number of sample transport loops. This indicates that the DC sputtering process is fairly stable under given conditions as instability of the deposition process is expected to significantly influence the sputtering rate, i.e. as pO2 increases, the sputtering rate decreases. XRD patterns in Figure 3 show the presence of crystalline phases for different TiO2/FTO samples. In Figure 3a, XRD patterns are shown for different thicknesses of TiO2 on FTO, all annealed at 500 °C in O2. In Figure 3b, data are compared for 266 nm TiO2 on FTO at different annealing temperatures, ranging from 300 to 600° C. Up to an annealing temperature of 300 °C, the TiO2 layer was Xray amorphous since the peaks that are present are all attributable to F-doped SnO2, space group P 42/m n m, JCPDS 41-1445, with a crystallographic preferred orientation along (h00). At annealing temperatures above 300 °C, additional peaks 8


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appeared that were assigned to TiO2 anatase, space group I 41/a m d, in agreement with JCPDS 21-1272, with a (00l) preferred orientation. The calculated crystallite size of the anatase phase was observed to increase slightly with layer thickness, from a crystallite size of 33 nm at a small layer thickness of 134 nm, to 51 nm for the layer with a thickness of 1361 nm, Table 1. The unit cell parameters are shown in Figure 4a for TiO2 anatase for samples with different thicknesses of the TiO2 layer. There was only very little variation of unit cell parameters and no appearance of additional phases or formation of superstructures apparent, suggesting that no significant structural or compositional changes occurred with increasing layer thickness in TiO2. If the sputtering process was instable, a progressive variation of the oxidation state of titanium would occur, with charge compensation taking place by elimination or formation of oxygen vacancies. Formation of oxygen vacancies, for instance, can be expected to be accompanied by a change in unit cell parameters or appearance of superstructures or new crystal phases such as a Ti4O7 Magnelli phase

46

. None of

these was, however, observed here. On the other hand, the unit cell calculated for the F-doped SnO2 substrate was observed to vary significantly depending on the thickness of the TiO2 layer, i.e. both, a and c parameters were found to increase continuously with increasing thickness of the layer, Figure 4b. This might suggest that the FTO substrate is not stable during the deposition process. Oxygen or fluorine ions might be removed from the (Sn,F)O2 structure under the conditions of the sputtering process and/or during thermal annealing, causing an increase of the unit cell due to anion vacancy formation and subsequent cation repulsion. Another possibility is the progressive formation of interfacial mixed Ti-Sn-O phases, caused by diffusion of Sn into the TiO2 layer over the duration of the sputtering process. Such quantitative reaction of Sn with TiO2 is commonly observed at the TiO2/FTO interface at elevated temperatures

47

.

Alternately it may be discussed that the

progressive shift in unit cell parameters is attributable to the variation of information depth with increasing layer thickness of TiO2. The stronger damage might therefore occur directly at the substrate/layer interface. Anatase crystal phase formation after annealing was also confirmed by HRTEM, carried out on a 30 nm sputtered TiO2 layer on a silicon nitride membrane before and after annealing at 400 °C, Figure 5. The as-deposited TiO2 layer was observed to be largely amorphous, containing few small crystalline domains of less than 1 nm in 9



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size.

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After annealing of the TEM sample, the size of the crystalline domains

increased to 4-5 nm. The atomic layer distance found in the crystals was 0.35 nm, agreeing with the (101) crystallographic plane in the TiO2 anatase structure. The crystallite size observed from HRTEM was, however, one order of magnitude smaller than the values calculated from GIXRD data. This may be a result of the fact that the crystallite size calculated from XRD measurements is an average value of the entire sample array analysed whereas TEM analysis is on discrete particles. Deviations are also likely to be due to different surface properties of the Si3N4 and FTO substrates used for HRTEM und XRD measurements, respectively, as well as differences in layer thicknesses deposited. Also, poor suitability of the Debye-Scherrer model for crystal size calculation will lead to deviations from the real crystallite size

49,50

.

Nevertheless, indications are that as-deposited layers contain largely amorphous material with small crystalline anatase domains and that after sintering at 400 °C, amorphous titania is still present in the grain boundaries.

The amorpous titania,

however, still shows a few atomic layers with a distance of 0.35 nm, indicating a short-range anatase structural ordering. XPS measurements were carried out on samples that were prepared with numbers of transport loops varying from 12 to 120. The surfaces composition of the samples was found to consist of 20-24 % Ti, 1-3% N, 50-55 % O and 20-25 % C, with no or little variation of the elemental ratio for different layer thicknesses of TiO2, Figure 6. The carbon content as well as a proportion of the oxygen is typically due to surface contamination during handling of the sample in ambient laboratory air. Such rather substantial amount of carbon contamination of around 20 at% was, for instance, also reported for atomically cleaned metal surfaces on short exposure to air. Longer term containment in ambient packaging media may result in carbon contaminations as high as 60 at%.48 A small amount of Si of < 0.5% was also observed which is attributable to the glass substrate. The oxidation state of Ti was observed from the Ti 2p highres, Figure 7, and found to be +4.0, corresponding to a bond energy of 459.0 eV, with no deviations apparent for any of the samples. There were no indications of the presence of lower valency Ti species over the entire range of layer thicknesses. Although only surface states rather than bulk properties are investigated during XPS measurements, results suggest that the DC-MS process is highly stable over the investigated period, with numbers of sample transport loops varied up to 120, with all 10


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samples being reproducibly deposited and no phases containing metallic or lower valency Ti are detectable at the surface at any layer thickness investigated. SEM images of different sputtered TiO2 films on FTO in plan view in Figure 8 suggest that the morphology of the TiO2 layer depends on the post-deposition annealing but also on the layer thickness. Generally, deposited layers consist of nanosized grains that arrange in vertical columns on top of the FTO substrate. This is consistent with previous work on sputtered TiO2 and MgO films

51-53

. Since thin film growth by

magnetron sputtering is usually dominated by kinetic effects, insular grain growth is therefore the preferred film growth mechanism, leading to formation of columns

38

.

The apparent grain size is ca. 20-30 nm, in good agreement with the crystallite size calculated from GIXRD patterns. Interestingly, the grain size does not only appear to be independent off the number of process loops but also remains fairly constant during post-deposition heat-treatment with very little change observable from SEM images. Since non-annealed layers were found to be largely amorphous or smallgrained from GIXRD and TEM investigations, no or only little grain growth is occurring during nucleation and growth of the anatase crystals and the nanocrystallised grain size during post-deposition annealing is defined by the asdeposited amorphous grain size. The column diameter, however, varies greatly for different deposition times. It was found to increase from a diameter of ca. 50-200 nm at small layer thicknesses of 134 and 266 nm to a close to continuous array at a layer thickness of 1361 nm. Post-synthesis annealing was found to have no significant impact on column size for thin layers; in thick layers, however, substantial cracks appeared in the more continuous nanoparticulate arrays, indicative of increasing densification during annealing of thicker layers, possibly associated with elimination of amorphous regions and defects. UV/Vis direct transmittance spectra are shown for different TiO2/FTO samples with different thicknesses of TiO2, Figure 9.

Transmittances are found to generally

decrease across the entire spectral range with increasing nominal thickness of the TiO2 layer. Tauc plots of the extinction, ε, were plotted in Figure 10 for the indirect and the indirect electronic transitions, (εhν)1/2 and (εhν)2 vs E for different thicknesses of TiO2. All samples exhibited indirect band gap energies of around 3.2 eV, Figure 10a. This value is usually discussed in the literature as indicative for the indirect transition in crystalline anatase although values from 2.86 to 3.34 eV have been 11



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reported, depending on synthesis conditions, stoichiometry, impurity content and crystallite size of the semiconductor

56

.

For the direct electronic transition, Figure

10b, high values of above 3.4 eV are observed, with thinner layers exhibiting values above 3.6 eV. Since for an amorphous material, the band gap is usually greater than 3.4 eV, independent on the transition type, it may be concluded that UV/Vis data suggest that all layers consist of crystalline anatase, following an indirect electronic transition type

56

.

Since XPS measurements showed substantial carbon

contamination of ca. 20% but also a small amount of 1-3% of nitrogen at the surface, the presence of additional states in the band gap due to anion doping of TiO2 may be expected. However, no additional absorption shoulder or entire shift of absorption edge could be observed. This suggests that the carbon-containing species and nitrogen are adsorbed at the surface by weak physically interactions that are not sufficient for a chemical modification of the TiO2 and change of band structure. Firstly, electrochemical studies were carried out on a TiO2/FTO sample of 266 nm thickness, tempered at 500 °C in O2. The i-E curve measured under dark conditions, Figure 11, shows that no current is generated in the electrochemical cell over a potential range of 1.1 V, from -0.3 V to +0.8 V vs NHE for this sample. A cathodic current is observed, with onset at -0.3 V vs NHE, which can be attributed to reduction of O2 to OH- 41. At a potential of -0.65 V vs NHE, a strong cathodic peak is observed. This peak can be correlated to electron transfer through a localised inter-band gap TiO2 state, which is discussed in the literature as attributable to surface states or traps at grain boundaries

41,54,55

. At potentials above +0.8 V vs NHE, a positive

current that increases with applied potential is observed, attributable to oxygen evolution. In thin layers of TiO2 on FTO, however, this current may also predominantly arise from the oxygen evolution reaction at the FTO substrate which exhibits a valence band edge position of 0.7 eV

58

. Under UV illumination, a positive

photocurrent is observed within the range of -0.3 to +0.8 V, which is associated with photogeneration of holes. The cathodic peaks at onsets at -0.3 V and -0.65 V are also present and show a small increase under illumination. This suggests that the charge transfer of electrons to the electrolyte solution is not fully dominated by electron-hole recombination. Other studies on colloidal TiO2 layers on FTO showed that at comparably slow charge transfer rates, a decrease of the cathodic peaks due to significant recombination of photogenerated holes with electrons is observed

41

.

12


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For the same sample, impedance spectroscopy was carried out in dark conditions and under UV illumination. At a low negative potential of -0.95 mV vs NHE, the impedance complex plane and the spectroscopic plot in Figure 12 show the onset of an arc at low frequencies with its maximum below the lower limit of the measured frequency range, 0.1 Hz, which corresponds to capacitance values above 10-4 Fcm-1. This response is usually attributed to the chemical capacitance, Cµ, and the charge transfer resistance, Rct. At a low negative potential of -0.95 mV, this arc appears to be independent off the illumination. Within this potential range, the chemical capacitance is therefore determined by the Fermi level of the electrons that strongly depends on the external bias but remaining fairly constant under varying illumination 41

. At more positive potentials, i.e. -600 mV and -400 mV, however, Cµ, and Rct

increasingly show variation with illumination, with the corresponding arc decreasing under UV illumination as compared to the data collected in dark conditions. At high frequencies, a second arc is observed that is intercepting at Z’=5.0 Ωcm or 5.5 Ωcm in dark conditions or under UV illumination, respectively. This arc is likely to arise from the layer resistance with capacitance values smaller than 10-7 Fcm-1. This arc is observed to show a small increase under UV illumination across the entire range of applied potential. This increase of the high frequency arc under illumination maybe due to decrease of electronic conductivity within the sample due to electron-hole recombination that was also observed to occur in colloidal TiO2 on FTO. Increasing of the applied potential to -600 mV vs NHE, gives rise to a 45 ° line at intermediate frequencies as an additional impedance element. This potential coincides with the position of the cathodic peak maximum at 0.65 V in the i-E-curve in Figure 11 that corresponds to a surface state or a trap at the grain boundary. This line is discussed in relation to electron diffusion through the mesoporous layer, corresponding to one third of the transport resistance, RT. Under UV illumination, RT is reduced as compared to the value observed in dark conditions suggesting that more electrons are present then. On further increase of the applied potential to -400 mV vs NHE, the RT is not visible anymore. This phenomenon was previously discussed as indicative for a transition between conducting TiO2 at negative potentials and insulating TiO2 at more positive potentials 41. IMPS/IMVS measurements were carried out for 266 nm TiO2 sintered at different temperatures ranging from 400 to 600 °C. As an example, typical plots are shown in 13



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Page 14 of 39

Figure 13a and b for the sample sintered at 500 °C. IPCE values calculated from the IMPS plots are shown in Figure 14 . A 2 ½ -fold increase of the IPCE from around 1% at 400 °C to 2.5% at 500 °C is observed. This might be due to increasing crystallisation of the anatase phase. On further increase of annealing temperature, the IPCE values are shown to decrease again to around 1% at 600 °C. Electron lifetimes and transit times calculated from the angular frequency of the minimum, ω. in the IMVS and the IMPS plots are shown in Figure 15. Efficient electron collection is usually expected at a difference of more than one order of magnitude between τD and τe. Results suggest that electron lifetime, τe, decreases from 8.6x10-3 s at 400 °C to 3.9x10-3 s at 500 °C with increasing sintering temperature in the lower temperature range. At temperatures above 500 °C, however, lifetimes are observed to increase by almost one order of magnitude to a value of 1.3x10-2 s at 600 °C. In the lower range of the sintering temperature, electron transit times, τD, are generally more than an order of magnitude smaller than τe, remaining fairly constant up to temperatures of 500 °C. At higher sintering temperatures, a drastic increase of transit times over almost two orders of magnitude is observed, leading to a reduction of the difference between τD and τe, even reaching a value that coincides with the electron lifetime at 600 °C. The observed trend of IPCE values at fixed dark potential may therefore be a result of the favourable crystallisation process occurring between 400 ° and 500 °C and electron transition times increasing drastically above this temperature. One possible reason could be commencing densification of the nanostructure of TiO2 and/or interfacial reaction between the TiO2 and FTO at elevated temperatures.

Conclusions A reactive magnetron sputtering process was employed to homogenously and reproducibly deposite TiO2 layers on FTO for photocatalytic applications. Layers are largely amorphous, with small crystalline anatase domains forming. The TiO2 layer deposition is homogenous over the investigated process duration of up to 120 transport loops and thicknesses of 1.3 microns, with no compositional or crystallographic changes of the TiO2 phase occurring. A densification of the nanostructure was, however, observed at thicknesses above 550 °C. During annealing, the size of the anatase domains increases to around 4 nm, with 14


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amorphous material still present at 400 °C. Significant densification of the observed nanocolumn structure with intercolumnar crack formation occurred on heating in samples with higher layer thicknesses. Photoelectrochemical measurements suggest that for a 266 nm TiO2 layer at sintering temperature of 500 °C is most favourable with regard to photocurrent efficiencies, associated with a drastic increase of electron transit times above this temperature and subsequently, increasing charge carrier recombination. This phenomenon is presumably due to the densification of the microstructure at high temperatures.

Acknowledgements The authors would like to thank Daniel Köpp and Karl-Heinz Schmidt for sample synthesis, Jan Schäfer and Uwe Lindemann for support with the SEM work and Uwe Lindemann again for profilometry. We would also like to thank Anja Albrecht and Dr. Harm Wulff at the University of Greifswald for XRD measurements. We are grateful for the BMBF for funding this work as part of the Light2Hydrogen project in the “Spitzenforschung und Innovation in den neuen Ländern” program.

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Table 1: Crystallite size for TiO2 anatase caculated from GIXRD for different nominal thicknesses of sputtered TiO2 layer. All samples were annealed at 400 °C in oxygen.

TiO2 nominal layer thickness / nm TiO2 crystallite size / nm 134

33

266

44

550

44

1361

51

22


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Figure 1

Schematic representation of the reactive sputtering system for TiO2 deposition.

23



1500

layer thickness / nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 39

1000

500

0 0

20

40

60

80

100

120

sample transport loops

Figure 2 Dependence of nominal layer thickness of as-sputtered TiO2 on number of sample transport loops.

24


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(a)

(b)

Figure 3

GIXRD patterns for TiO2 on FTO for (a) different layer thicknesses of TiO2 annealed at 500 °C in O2 and (b) different annealing temperatures

25



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Page 26 of 39

in O2 for 266 nm TiO2. FTO main reflexes are marked with an open circle.

(a)

(b)

Figure 4

Variation of unit cell parameter of (a) TiO2 anatase indexed in I41/amd symmetry and (b) F-doped SnO2 in P42/mnm. Data are plotted as a 26


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function of increasing thickness of TiO2 for different annealing times in oxygen: 450, 500 and 550 °C.

27



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Page 28 of 39

(a)

(b)

Figure 5

TEM Images of 30 nm layer of sputtered TiO2 on silicon nitride membrane (a) as deposited and (b) after annealing at 400 °C.

28


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XPS element fraction [atom%]

Page 29 of 39

60 55 50 45 40 35 30 25 20 15 10 5 0

C

132 nm

Figure 6

N

O

266 nm

Ti

Si

550 nm

1361 nm

Surface composition of TiO2, prepared using 3 to 120 process loops, resulting in layer thicknesses varying from 132 to 1361 nm.

29



3/2 component

11000 10000

Ti

4+

9000 8000 7000

Int [cps]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 39

6000 5000

1/2 component

4000 3000 2000

1361 nm 550 nm 266 nm 132 nm

1000 0

474 472 470 468 466 464 462 460 458 456 454 452 450 448

binding energy [eV]

Figure 7

XPS Ti 2p highres confirming an oxidation state of Ti of 4.0+ at the surface of samples prepared by applying 3 to 120 process loops, with resulting layer thicknesses of 132 to 1361 nm of TiO2.

30


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(a)

(e)

(b)

(f)

(c)

(g)

(d)

(h)

Figure 8

SEM Images at magnification 100,000 for different layer thicknesses before (a)-(d) and after (b)-(h) annealing at 500 °C in oxygen. (a) und (e) 132 nm, (b) und (f) 266 nm, (c) und (g) 550 nm and (d) und (h) 1361 nm TiO2 on FTO. The white bar in the bottom of each image represents a distance of 100 nm.

31



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Figure 9

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UV/Vis direct transmission spectra for different thicknesses of TiO2 on FTO.

32


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(a)

(b)

Figure 10

Tauc plots of extinction, ε, for the (a) indirect electronic transition, (εhν)1/2 vs. E (eV) and (b) direct electronic transition, (εhν)2 vs. E (eV) for different thicknesses of TiO2.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 11

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Comparison of i-E-curves for 266 nm TiO2/FTO, annealed at 550 °C in dark conditions and under 375 nm UV light illumination.

34


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(a)

(b)

Figure 12 (a) acIS complex planes and (b) spectroscopic plot of imaginary part of the impedance at different polarisation potentials, -950, -600 and -400 mV, in dark conditions and under 375 nm UV illumination for 260 nm TiO2 annealed at 500 °C. 35



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Page 36 of 39

(a)

(b)

Figure 13

(a) IMPS und (b) IMVS plots under 55Wm-2 375 nm illumination for 266 nm TiO2, annealed at 500 °C

36


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3 2,5 2 1,5 1 0,5 0 400

Figure 14

450

500

550

600

650

Variation of incident-photon-to-current efficiency values, IPCE, under 375 nm UV illumination for a 260 nm layer of TiO2 with annealing temperature.

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 15

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Variation of calculated electron transit times, τD, and electron life times, τe, with annealing temperature for 260 nm TiO2.

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TOC Graphic

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Layers on FTO - American Chemical Society

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