Non-agglomerated iron oxyhydroxide akaganeite nanocrystals incorporating extraordinary high amounts of different dopants

Transcription

1 Supporting Information Non-agglomerated iron oxyhydroxide akaganeite nanocrystals incorporating extraordinary high amounts of different dopants Ksenia Fominykh 1, Daniel Böhm 1, Siyuan Zhang 2, Alena Folger 2, Markus Döblinger 1, Thomas Bein 1, Christina Scheu 2 and Dina Fattakhova-Rohlfing 1,3,4* 1 Department of Chemistry and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München (LMU Munich), Butenandtstrasse 5-13 (E), Munich, Germany 2 Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany 3 Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research (IEK-1) Materials Synthesis and Processing, Wilhelm-Johnen-Strasse, Jülich, Germany 4 Faculty of Engineering and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstraße 1, Duisburg, Germany These authors contributed equally Figure S1 Influence of different iron precursors and the reaction conditions on the formation of akaganeite. (a) XRD patterns of powders obtained at the same reaction conditions using Fe(NO 3 ) 3, Fe(acac) 3 (black) and FeCl 3 (pink) as precursors. (b) XRD patterns of samples obtained using FeCl 3 6H 2 O at different reaction conditions. The numbers indicate the microwave power in W, temperature in C and reaction time in hours or minutes. -FeOOH pattern ICDD card number (grey lines), FeCl 2 4H 2 O pattern ICDD card number (black lines). 1

2 The Fe compounds Fe(NO 3 ) 3, Fe(acac) 3 and FeCl 3 were investigated as precursors for the akaganeite synthesis. The XRD patterns in Figure S1a show the formation of β-feooh only in case of FeCl 3. Iron nitrate results in an amorphous phase and Fe(acac) 3 did not react at the relatively low temperature of 120 C, probably because the solubility of iron acetylacetonate in tert-butanol is rather low. The sharp reflections in the XRD pattern correspond to the precursor signals. Figure S1b shows XRD patterns of selected powders obtained at different reaction conditions. The initial synthesis was performed at 1200 W microwave power, 120 C for 1 h and resulted in phase-pure 8 nm sized β-feooh particles (calculated from the (301) peak using the Scherrer equation). To obtain smaller crystals, the reaction temperature was reduced to 80 C, resulting in particle sizes of around 5 nm. The further decreased temperature of 60 C was too low to obtain phase-pure akaganeite, as a second phase corresponding to FeCl 2 4H 2 O is visible in the XRD pattern (Figure S1b, black reference pattern). Further reactions were carried out at 80 C with reduced microwave radiation power. The smallest particle dimensions of around 5 nm were obtained upon heating with 120 W for 1 h (Figure S1b, pink). Further decrease in the reaction time to 40 min and 30 min again resulted in a mixture of β-feooh and FeCl 2 4H 2 O. The reaction conditions 120 W, 80 C and 1 h were therefore applied in all following reactions. Figure S2 Time-resolved Raman investigation of the undoped -FeOOH powder. (a) Raman spectra recorded every 5 min with a filter (optical density of 0.6) to reduce the intensity of the laser beam (grey). The orange spectrum was taken after exposure of the sample to the laser with full intensity for 1 s. Micrographs of the sample with a 10-fold magnification (b) before, (c) during and (d) after the Raman measurement, showing a dark spot caused by the laser beam. Figure S2 shows an in situ time-resolved Raman study of the undoped β-feooh. The spectra were taken with a time step of 5 min using an intensity-reducing laser filter (Figure S2a) with an optical density of 0.6. The signals at 221 cm 1 (A 1g ), 237 cm 1 (E g ), 284 cm 1 (E g ), 398 cm 1 (E g ), 489 cm 1 (A 1g ), 601 cm 1 (E g ) and 649 cm 1 (E u ) can be assigned to hematite (α-fe 2 O 3 ) bands. Noticeably, neither β- FeOOH nor any other phases are visible below 10 min irradiation. The hematite spectrum starts to appear after 15 min and becomes gradually more pronounced with extended exposure time. To compare, the unfiltered laser beam immediately leads to the formation of hematite (Figure S2a, orange line). Corresponding micrographs recorded before and after the Raman measurement in Figure S2b d show a dark spot occurring at the position where the laser beam was focused. This additionally confirms the structural change. 2

3 Figure S3 Low laser intensity (filter with optical density 2.0) Raman spectra of undoped (a) Ni(II) doped (b) V(III) doped and (c) Sn(IV) doped akaganeite nanocrystals. Figure S3 shows Raman spectra of β-feooh with selected doping elements ( (a) Ni, (b) V and (c) Sn) with oxidation states +II to +IV in various concentrations acquired by low laser intensity (filter with optical density 2.0) to avoid heat induced phase transformation to the hematite phase. The Rama spectra confirm the formation of akaganeite phase in the microwave assisted solvothermal synthesis, indicated by the presence of the phase specific bands at 310 cm -1 and 390 cm -1 as well as by the absence of the hematite specific bands at 223 cm -1 and 289 cm -1. Higher doping concentration reduces the crystallinity in the shown cases, indicated by a relative loss in the signal intensity of the specific bands and causes the formation of side products above the solubility limit of the respective doping elements in the β- FeOOH structure. An analysis of the shift of the akaganeite specific Raman bands in dependence of the doping ion concentration is not possible due to broad peaks in general and a low signal level caused by the reduced laser intensity. 3

4 Figure S4 TEM micrographs of (a, b) 5 at% Ti (IV), (c, d) 10 at% V(III) and (e, f) 20 at% Sn(IV) doped -FeOOH with corresponding EDX quantification of doping ion concentrations in the nanocrystals. 4

5 5

6 Figure S5 SEM micrographs of (a) undoped, (b,c,d) 5, 10, 20 at% Sn(IV) doped, (e) 5 at% V(III) and (f) 20 at% Ni(II) -FeOOH with corresponding EDS quantification of doping ion concentrations on the microscale. Figure S6 SEM images of -FeOOH films prepared by spin-coating from H 2 O/EtOH nanoparticle dispersions with the concentrations of (a) 2 mg ml 1 and (b) 10 mg ml 1 on FTO substrate. Figure S7 TGA and DSC curves of the samples Ti10 (pink), Sn10 (black) and Nb10 (grey). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) curves of the samples Ti10, Sn10 and Nb10 in Figure S7 show several processes that are accompanied by changes in mass. The endothermic process with the mass loss of around 5 % at 100 C corresponds to evaporation of adsorbed water and is visible for all samples that are stored in air. In the range between 100 and 400 C all organic species such as tert-butoxide residues are burned off as well as structural bound water and stabilizing chloride ions with the greatest mass loss of around 20 %. The exothermic events with only minor mass losses of 2 3 % between 480 and 600 C are caused by the phase transformation to hematite, as is supported by the XRD results. 6

7 Figure S8 SEM images of a thick β-feooh film grown in situ for 2.5 h on FTO substrate during the microwave reaction. (a1, a2, a3) Different magnifications of the as-prepared β-feooh film. (b1, b2, b3) Different magnifications of the film after annealing at 600 C and phase transformation to -Fe 2 O 3. 7

8 Figure S9 SEM images of as-prepared -FeOOH films grown in situ on FTO during the microwave reaction with a reaction time of (a) 2.5 h and (c) 1 h. (b) SEM image of the film grown in situ for 2.5 h on FTO after annealing at 600 C and phase transformation to -Fe 2 O 3. The scale bars correspond to a length of 10 µm. Powder XRD patterns (material scratched from substrate) of as-prepared -FeOOH films grown in situ during the microwave reaction with a reaction time of 2.5 h (d, grey curve) and XRD pattern of corresponding film (material scratched from substrate) after annealing at 600 C (d, dark-red curve). Corresponding reflections (akaganeite (112), hematite (104)) in insets (d) used for estimation of crystalline domain size according to the Debye-Scherrer equation. In Figure S9, SEM cross section micrographs of in situ prepared akaganeite films by microwave assisted synthesis reveal a homogenous coverage of the substrate over the whole image of ~250 µm. The XRD-based domain size of the in situ prepared akaganeite films is estimated to be ~9 nm according to the Debye-Scherrer equation (Figure S9a, d), indicating that the small akaganeite nanocrystals act as building blocks for larger macro-spheres visible in SEM. The XRD-based domain size of thermally transformed akaganeite films to hematite at 600 C (Figure S9b, d) can be estimated to be about 45 nm according to the Debye-Scherrer equation. Figure S10 (a, b) SEM cross section micrograph and (b, c) EDS elemental distribution maps of a 10 at% Sn doped -Fe 2 O 3 film on FTO substrate prepared by thermal transformation (600 C) of a -FeOOH film grown in situ during microwave reaction (1 h). 8

9 A SEM cross section micrograph (Figure S10a, b) of a 10 at% Sn doped -Fe 2 O 3 film on FTO substrate prepared by thermal transformation (600 C) of a -FeOOH film grown in situ during microwave reaction (1h) was further investigated by EDX to map the Sn atom distribution across the film depth. The quantitative elemental mapping images in Figure S10 c show a homogenous Fe atom distribution in the investigated film depth of ~ 1.5 µm from the FTO substrates upwards. The Sn atom concentration in the FTO substrate can be clearly distinguished from the 10 at% Sn doping in the overlaying -Fe 2 O 3 film and shows a rather homogenous distribution throughout the measured mapping area. Heating of the akaganeite film on the FTO substrate at 600 C does not seem to promote a significant Sn atom migration from the substrate into the -FeOOH film, as no clear gradient in the Sn atom distribution is visible. References 1. Dixit, G.; Singh, J. P.; Srivastava, R.C.; Agrawal, H. M. Chaudhary, R. J. Adv. Mater. Lett. 2012, 3 (1), 21. 9