SUPPORTING INFORMATION

Transcription

1 SUPPORTING INFORMATION Heat Treatment Effect on Crystalline Structure and Photoelectrochemical Properties of Anodic TiO 2 Nanotube Arrays Formed in Ethylene Glycol and Glycerol Based Electrolytes Magdalena Jarosz 1, Karolina Syrek 1, Joanna Kapusta-Kołodziej 1, Justyna Mech 2, Kamilla Małek 3,4, Katarzyna Hnida 1,5, Tomasz Łojewski 3, Marian Jaskuła 1,Grzegorz D. Sulka 1* 1 Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow, Ingardena 3, Krakow, Poland 2 Faculty of Non-Ferrous Metals, Division of Physical Chemistry and Electrochemistry AGH University of Science and Technology, Al. Mickiewicza 30, Krakow, Poland 3 Faculty of Chemistry, Jagiellonian University, Ingardena 3, Krakow, Poland 4 Jagiellonian Centre for Experimental Therapeutics, Jagiellonian University, Bobrzyńskiego 14, Krakow, Poland 5 AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, A. Mickiewicza 30, Krakow, Poland * Corresponding author. sulka@chemia.uj.edu.pl Department of Physical Chemistry & Electrochemistry, Faculty of Chemistry Jagiellonian University in Krakow Ingardena 3, Krakow, Poland Tel: Fax:

2 1. Raman spectra Figure S1. Raman spectra with their corresponding standard deviation spectra of the anatase and rutile phases and their mixture, used for the determination of the anatase/rutile phase calibration curve. 2

3 2. Grazing incident XRD measurements Figure S2. Grazing incident X-ray patterns of nanoporous TiO 2 formed by anodization in the ethylene glycol based electrolyte at 40 V. The samples were annealed in air for 2 h at 500 C. Enlarged area shows the changes in rutile peak at approximately 2θ = 32 with the increasing ω angle. 3

4 Figure S3. Grazing incident X-ray patterns of nanoporous TiO 2 formed by anodization in the glycerol based electrolyte at 40 V. The samples were annealed in air for 2 h at 500 C. Enlarged areas show the changes in rutile peak at approximately 2θ = 32 and 43 with the increasing ω angle. As it can be seen from diffractograms (Fig. S2 and S3), changes of rutile peaks differ for different electrolytes. For samples anodized in the ethylene glycol based electrolyte, the rutile peak at 32 starts to occur when the incident angle is above 3. Therefore, the rutile phase starts to appear deeper inside the sample, most likely at the bottom of nanotubes. However, when it comes to the glycerol based samples, the rutile peak is present for all applied angles. Moreover, there is another characteristic rutile peak present at approximately 2θ = 43. It means that a small but significant amount of the rutile phase is present throughout the whole sample. Similar to the ethylene glycol samples, with increasing incident angle we are gathering information from deeper parts of the sample. The increasing intensities of rutile peaks indicate that more rutile grains are 4

5 present at the bottom of the sample. Presented results confirm that rutile phase starts to appear at the bottom of the nanotubes. 3. Oxidation of Ti substrate during annealing As it can be seen, annealing at temperatures between 400 and 700 C does not change the mass of the oxide (Fig. S4). The differences are observed when the heat temperature is higher than 700 C. The greatest differences occurs at above 900 C which means that the oxidation of titanium substrate becomes significant. The mass changes are similar for both electrolytes. Figure S4. Gain of the mass of TiO 2 during the thermal treatment for samples anodized in the ethylene glycol and glycerol based electrolytes. Based on the differences between the mass of the sample prior and after the annealing process, the thickness of Ti substrate which underwent oxidation was calculated. The results are shown in Table S1. These results confirm that above 900 C great amount of titanium is oxidized to TiO 2 (over half of the substrate at 1000 C). Table S1. Thickness of Ti layer which underwent oxidation during the annealing process. 5

6 Temperaturę [ C] h [µm] ethylene glycol glycerol Optical properties of ATO The reflectance spectra of the ATO films annealed at different temperatures were measured (typical spectrum is shown in Fig. S5A). In order to calculate the optical band gap of ATO layers, The Kubelka Munk (K-M) function and Tauc plots were used. The reflectance was converted into the equivalent absorption coefficient α KM (eq. S1). αhν = A hν E (S1) where: A is a constant, E g is the optical band gap of ATO, and n can be 0.5 or 2 depending on whether it is direct or indirect transitions, respectively. The optical band gap values were estimated by extrapolating the straight line portion of the αhv 0.5 vs. hv plot (Fig. S5B) and collected in Table S2. With increasing annealing temperature, the decrease in E g was observed. This effect is associated with the increased size of rutile particles in the nanoporous TiO 2 layers. These results are with good agreement with other authors, who tested thin TiO 2 films [1,2]. 6

7 Figure S5. Example of the αhv 0.5 vs. hv plot for ATO layer obtained in glycerol based electrolyte and annealed at 900 C. Table S2. Band gap values calculated from Kubelka Munk equation (eq. S1) for ATO layers obtained in the ethylene glycol and glycerol based electrolytes and annealed at different temperatures. Temperature [ C] Band gap [ev] ethylene glycol glycerol References [1] Sankar, S.; Gopchandran, K.; Effect of Annealing on the Structural, Electrical and Optical Properties of Nanostructured TiO 2 Thin Films. Crystal Res. Tech. 2009, 44,

8 [2] Mathews N. R.; Morales, E. R.; Cortés-Jacome, M. A.; Toledo Antonio, J. A.; TiO 2 thin films Influence of annealing temperature on structural, optical and photocatalytic properties Sol. Energy 2009, 83,