Supporting Information

Transcription

1 Supporting Information Improved Stability of Atomic Layer Deposited Amorphous TiO 2 Photoelectrode Coatings by Thermally Induced Oxygen Defects Markku Hannula, Harri Ali-Löytty, Kimmo Lahtonen, Essi Sarlin, Jesse Saari, and Mika Valden, Surface Science Group, Laboratory of Photonics, Tampere University of Technology, P.O. Box 692, FI Tampere, Finland Materials Science, Tampere University of Technology, P.O. Box 589, FI Tampere, Finland mika.valden@tut. 1

2 Cleanness of the substrate and the as-deposited TiO 2 lm Survey spectrum XPS signal intensity (a.u.) Ti 2p N 1s C 1s Si 2s Si 2p Ti 3s Ti 3p O 1s Si LMM Ti LMM O KLL As-deposited Ti 2s UHV cleaned Binding energy (ev) Figure S1: XPS survey scan spectra of UHV cleaned n-si(100) substrate and ALD grown asdeposited TiO 2 lm. The substrate cleaning was performed with a combination of annealing and atomic hydrogen treatments, as described in Ref. 1 The TiO 2 lm was 30 nm thick and the deposition was carried out at 200 C temperature. 2

3 Eect of ALD parameters on the thermally induced reduction Ti 2p ALD 100 C Ti 2p ALD 200 C XPS signal intensity (a.u.) 700 C As deposited Ti 4+ Ti 3+ Ti C As deposited Ti 4+ Ti 3+ Ti Binding energy (ev) Figure S2: Comparison of Ti 2p XPS spectra measured on TiO 2 lms deposited at 100 C (left) and 200 C (right). The 100 C lm is much closer to the stoichiometric TiO 2 composition (Ti almost entirely in 4+ oxidation state) in the as-deposited condition. Similar dierence can be seen after 10 min annealing at 700 C. Reduction in the 100 C deposited lm is much weaker compared to the 200 C deposited lm. 3

4 Currentvoltage curves 00 Current density (µa/cm 2 ) As deposited 400 C 500 C H treated Light Potential vs. RHE (V) Dark 60 Figure S3: Currentvoltage characteristics measured in dark and under simulated sunlight from the open circuit voltage to 2 V vs. RHE at 50 mv/s in 1 M NaOH. Black: as-deposited lm without post-treatment, blue: annealed at 400 C, red: annealed at 500 C, green: hydrogen treated for 10 min at 300 C. The OCP voltages in dark and under light were determined after 30 min immersion time before applying any bias voltage to the sample. Then three potential scans were performed in the following order: 1. under light, 2. in dark, and 3. under light. Only second scans measured under light are presented in the gure. 4

5 C 1s and N 1s XP spectra of the thermally and hydrogen treated samples (a) C 1s N 1s Anneling in UHV (b) C 1s N 1s H exposure at 300 C 900 C 50 min 800 C XPS signal intensity (a.u.) 700 C 600 C 500 C XPS signal intensity (a.u.) 10 min 5 min 1 min 400 C As deposited As deposited Binding energy (ev) Binding energy (ev) Figure S4: C 1s and N 1s XP spectra for (a) the thermal treatment and (b) the hydrogen treatment series. The intensity has been scaled up by a factor of 40 compared to the O 1s spectra in Figure 2 in the paper. 5

6 Si 2p XP spectra of the thermally treated samples Si 2p 900 C SiO x Anneling in UHV XPS signal intensity (a.u.) 800 C 700 C 600 C 500 C 400 C As deposited Binding energy (ev) Figure S5: Si 2p XP spectra for the thermal treatment series. The intensity has been scaled up by a factor of 40 compared to the O 1s spectra in Figure 2 in the paper. 6

7 XPS tting parameters Table S1: Fitting parameters (line shapes, atomic concentrations (c), binding energy values (Eb) and component full width at half maximum (FWHM) values) for XPS measurements of representative as-deposited, thermally treated and hydrogen treated samples. Transition Ti 2p 3/2 O 1s C 1s N 1s Si 2p Chemical state Ti 4+ Ti 3+ Ti 2+ O Si OH O O 2 Line shape GL(60)T(1.8) GL(50)T(1.8) GL(50)T(1.8) SGL(13)T(1.4) SGL(13)T(1.4) SGL(13)T(1.4) SGL(13)T(1.4) As-dep. c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) Thermal treatment 800 C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) min c (at. %) E b (ev) FWHM (ev) min c (at. %) E b (ev) FWHM (ev) min c (at. %) E b (ev) FWHM (ev) Hydrogen treatment 50 min c (at. %) E b (ev) FWHM (ev)

8 UPS tting parameters Table S2: Fitting parameters (line shapes, atomic concentrations (c), binding energy values (E b ) and component full width at half maximum (FWHM) values) for UPS measurements of representative as-deposited, thermally treated and hydrogen treated samples. As-dep. Thermal treatment Hydrogen treatment Component Γ σ π O 2p n/b Ti 3+/2+ 3d Line shape GL(0) GL(0) GL(0) GL(0) LF(0.1,6,10,80) c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) C c (at. %) E b (ev) FWHM (ev) min c (at. %) E b (ev) FWHM (ev) min c (at. %) E b (ev) FWHM (ev) min c (at. %) E b (ev) FWHM (ev) min c (at. %) E b (ev) FWHM (ev)

9 Monte Carlo simulations The XPS results of the Ti oxidation state evolution during the thermal and hydrogen treatment series strongly suggested that the underlying reduction mechanisms could be dissimilar for these two treatments (Figure 4 in the paper). The starting point for the simulation was a predened set of Ti 2 O 4 and Ti 2 O 3 structural units, and in each time step a certain number of the bonds was broken depending on the chosen reduction mechanism and selected probabilities. One should note that the simulation was purely mathematical without any quantitative physical information (e.g. no interaction energies were used) and the idea was to provide further insight into the interpretation of the XPS results regarding the reduction mechanisms. Dierent reduction mechanisms and simulation parameters were tested for both the thermal and hydrogen treatment series and parameters were modied until the best possible agreement with the experimental data was achieved. The initial conguration for the simulation consisted of a group of Ti 4+ 2 O 2 4 and Ti 3+ 2 O 2 3 structural units. The initial values of the Ti 4+ :Ti 3+ ratios for both the thermal and hydrogen treatment series were obtained from the relative concentration of Ti oxidation states measured by XPS. For thermal treatment series the Ti 4+ :Ti 3+ ratio was set to 57:43 corresponding to the starting point of the Ti 2+ formation at 400 C. For hydrogen treatment series the initial ratio was set to 74:26 which corresponds to the relative concentration of these cations in the as-deposited lm. The dierent initial concentrations are justied because in the thermal treatment series a threshold temperature of 400 C had to be exceeded for Ti 2+ formation whereas in hydrogen treatment series the eect of H atoms begins instantaneously. For thermal treatment series the best agreement between the simulated and measured results was obtained if in each time step either one or two reduction events can take place simultaneously. This means that a Ti 4+ cation can be reduced to either a Ti 3+ or Ti 2+ cation in one step. The probability of the double reduction was set to 73% of all reduction events. As seen in Figure 4(a) this direct reduction mechanism related to the thermal treatment series (solid blue lines) is in excellent agreement with the experimental results (squares). 9

10 Furthermore, the possibility of direct reduction from Ti 4+ state to Ti 2+ state leads to linear increase of both Ti 3+ and Ti 2+ states. Clearly the direct reduction mechanism does not describe the experimental data from the hydrogen treatment series (Figure 4(b)). In the hydrogen exposure, one H atom at a time encounters the titanium oxide surface and removes one O atom. Thus, the simulation had to be changed to a reduction mechanism where the reduction steps are either from Ti 4+ to Ti 3+ or from Ti 3+ to Ti 2+. Additionally, the reduction probability from Ti 3+ to Ti 2+ was set to 19% of the probability for the reduction step from Ti 4+ to Ti 3+. This gave the best agreement with the XPS results and also describes the fact that the removal of the second O from the same structural unit is less likely than the removal of the rst O atom. References (1) Hannula, M.; Lahtonen, K.; Ali-Löytty, H.; Zakharov, A.; Isotalo, T.; Saari, J.; Valden, M. Fabrication of Topographically Microstructured Titanium Silicide Interface for Advanced Photonic Applications. Scr. Mater. 2016, 119,