Photocatalytic reduction of CO 2 with H 2 O over modified TiO 2 nanofibers: Understanding the reduction pathway

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1 Electronic Supplementary Material Photocatalytic reduction of CO 2 with H 2 O over modified TiO 2 nanofibers: Understanding the reduction pathway Anjana Sarkar 1 ( ), Eduardo Gracia-Espino 2, Thomas Wågberg 2, Andrey Shchukarev 1, Melinda Mohl 3, Anne-Riikka Rautio 3, Olli Pitkänen 3, Tiva Sharifi 2, Krisztian Kordas 3, and Jyri-Pekka Mikkola 1,4 ( ) 1 Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå University, SE Umeå, Sweden 2 Department of Physics, Umeå University, SE Umeå, Sweden 3 Microelectronics and Materials Physics Laboratories, Department of Electrical Engineering, University of Oulu, P.O. Box 4500, Oulu FI-90014, Finland 4 Laboratory of Industrial Chemistry and Reaction Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, Åbo-Turku FI-20500, Finland Supporting information to DOI /s S1 Characterization S1.1 Inductively coupled plasma- optical emission spectrometry (ICP-OES) The determination of Cd, Na, Pd, Pt, Se and Ti in the samples were carried out by a Perkin Elmer Optima 5300 DV ICP-OES by which either an axial or radial mode of viewing of the plasma is possible. The apparatus is equipped with an AS-93 plus auto sampler, a Ryton double-pass Scott-type spray chamber and the Gem Tip Gross-flow pneumatic nebuliser. RF power 1.3 kw, nebuliser gas flow 0.8 L min 1, auxiliary gas flow 0.2 L min 1, plasma gas flow 15 L min 1 and sample uptake 1.5 ml min 1 were maintained during the analysis. The preliminary characterisation of the digestion residue was carried out using a Siemens D5000 diffractometer equipped with a 2θ/θ goniometer and using nickel-filtered Cu Kα radiation (40 kv, 30 ma). Diffraction patterns were recorded in the 2θ-range For the determination of Ti, Na, Cd and Se present in the samples were digested as follows. To teflon beaker containing carefully weighed sample, 10 ml of conc. HCl was added and was heated to dryness at 50 C. 1 ml of concentrated H 2 SO 4 and 5 ml of HF were added to the dried residue and was heated to dryness on a sand bath at 200 C. Then, the residue was dissolved to 1 M HCl and finally diluted to 100 ml using 1 M HCl. For the determination of Pt and Pd samples were digested in microwave oven (CEM MARS 5X, CEM Corp.) equipped with XP-1500 plus high pressure Teflon TFM vessels (CEM Corp., 100 ml, maximum pressure 10 MPa and temperature 300 C). Carefully weighed 100 mg sample was added in the microwave digestion vessel. After adding, 2 ml of conc. HNO 3 and 6 ml of HCl, the vessels were closed and the samples were heated in the microwave oven with the heating rate at 15 min to 200 C and holding at 200 C for 10 min. Address correspondence to Anjana Sarkar, sarkar.anjana@gmail.com; Jyri-Pekka Mikkola, jyri-pekka.mikkola@umu.se

2 Nano Res. S2 TEM image Figure S1 TEM images of (a) 1%Pt TiO2 NFs, (b) 1%CdSe 1%Pt TiO2 NFs, (c) 1%Pd TiO2 NFs, and (d) 1%CdSe 1%Pd TiO2 NFs. S3 Raman spectra Figure S2 Raman Spectrum of modified TiO2 nanofibers. Labels A and B refer to anatase and TiO2 ( ) phases of TiO2 NFs, respectively.

3 S4 The reactor setup Figure S3 (a) Schematic diagram of the reactor setup. (b) Photograph of the reactor. S5 Theoretical methods for rutile TiO 2 The bulk rutile TiO 2 exhibit lattice parameters equal to a = b = Å, and c = Å, resulting in a c/a ratio of 0.644, these values are consistent with experimental lattice parameters reported elsewhere [S1]. We calculated an electronic band gap equal to 1.85 ev, which is consistent with previously reported values (1.88 ev) using similar DFT methods [S2]. Rutile TiO 2 (110) surface was built using a (2 2) super cell containing 4 TiO 2 -slabs, where the bottom two layers as well as the lattice parameters are kept fix during the calculations, while the top two slabs and the molecular adsorbates are freely to move until the internal forces are less than 0.03 ev Å 1. All other simulations parameters are similar to those used for anatase TiO 2 as stated in the main manuscript. In the case of rutile TiO 2 (110) surface, the reaction was initiated by the adsorption of a CO 2 molecule on a five-fold coordinated Ti atom (Ti 5f ), see Fig. S4.2, then six H atoms are added one-by-one, according to the chemical reactions stated in main manuscript. Figure S4.1 (a) Anatase-TiO 2 (101) supercell. (b) Rutile-TiO 2 (110) supercell. In both surfaces the 5-fold coordinated Ti atom was used as a catalytic site for the CO 2 adsorption and subsequent reduction. Nano Research

4 Nano Res. Figure S4.2 CO2 reduction on rutile TiO2 (110) surface. (a) Molecular model of the CO2 reduction pathway. (b) Energy change (ΔER) during the CO2 reduction. The rate limiting step (largest change in energy) is related with the hydrogenation of formic acid (HCOOH + H* H2COOH), similar results are obtained for anatase TiO2 (101) surface. Figure S4.3 Adsorption energy (ΔEads) of formic acid at different surface charge on (a) anatase TiO2 (101) and (b) rutile TiO2 (110). The electronic charge of the systems was varied in the range of 0.4 to +0.4 e. The ΔEads on anatase surface is less sensitive to a variation in electronic charge, the ΔEads is just modified by ±0.11 ev, than on rutile surface where a larger change (±0.28 ev) in ΔEads is observed. S6 SEM Images Figure S5 SEM images of (a) 1%Pd TiO2 NFs, (b) 1%Pt TiO2 NFs, (c) 1%CdSe 1%Pd TiO2 NFs and (d) 1% CdSe 1%Pt TiO2 NFs.

5 References [S1] Swope, R. J.; Smyth, J. R.; Larson, A. C. H in rutile-type compounds: I. Single crystal neutron and X-ray diffraction study of H in rutile. Amer. Mineral. 1995, 80, [S2] Landmann, M.; Rauls, E.; Schmidt, W. G. The electronic structure and optical response of rutile, anatase and brookite TiO 2. J. Phys.: Condens. Mat. 2012, 24, Nano Research