Supporting information for: Light-induced cation exchange for copper sulfide

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1 Supporting information for: Light-induced cation exchange for copper sulfide based CO 2 reduction Aurora Manzi,, Thomas Simon,, Clemens Sonnleitner,, Markus Döblinger,, Regina Wyrwich, Omar Stern, Jacek K. Stolarczyk,,, and Jochen Feldmann, Photonics and Optoelectronics Group, Department of Physics and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität München, Amalienstr. 54, Munich, Germany, Nanosystems Initiative Munich (NIM), Schellingstr. 4, Munich, Germany, Department of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstr (E), Munich, Germany, and General Electric (GE) Global Research, Freisinger Landstr. 50, Garching bei München jacek.stolarczyk@physik.uni-muenchen.de Preparation of the photocatalyst CdS nanorods with lengths around nm and diameters of 5 nm were synthesized according to previously published procedures. S1 The obtained CdS nanorods were transferred To whom correspondence should be addressed Ludwig-Maximilians-Universität (LMU) Munich Nanosystems Initiative Munich (NIM) Ludwig-Maximilians-Universität (LMU) Munich General Electric (GE) Global Research S1

2 to aqueous environment by changing the ligands to hydrophilic ones. L-cysteine was used as stabilizing ligand. The decoration with Pt clusters randomly distributed on the CdS nanorod surface was obtained through a photo-deposition technique, as reported from Dukovic et al. S2 The CdS nanorods, already transferred to water and stabilized with L-cysteine, were dispersed in a aqueous solution of 0.67 M Hexachloroplatinic Acid (H 2 PtCl 6 (H 2 O) 6 ), 0.29 M Triethanolamine (TEOA) and 24.7 mm Ascorbic acid. The amount of CdS nanorods was adjusted to an optical density of the sample at the first excitonic peak of The solution was then illuminated with an UV lamp upon stirring for 1 hour. The site-selective growth of Pt clusters on the CdS tips was performed according to previously published procedures. S3 25 mg of Pt(II) Acetylacetonate were used as Pt precursors and the decorated nanorods were stored in toluene. Mercaptoundecanoic acid (MUA) was used to stabilize the CdS nanorods decorated with Pt on tips after the transfer to aqueous environment. To perform the light-induced cation exchange of CdS to Cu 2 S, the nanorods, already transferred to water, were dispersed in a 3 ml aqueous solution of 0.63 mm Copper(II) chloride (CuCl 2 ) and 0.29 M TEOA. The amount of CdS nanorods was adjusted to an optical density of 1.97 at the first excitonic peak. The solution was then illuminated with a 447 nm laser (incident power at the sample position 30 mw) or a 450 W Xe lamp upon stirring under normal atmosphere. Photocatalytic reactions The photocatalyst was tested performing in situ photocatalytic experiments. The starting solution was made of CdS nanorods already decorated with Pt clusters and redispersed in an aqueous solution containing 0.1 M Na 2 SO 3 as hole scavenger, 1 M Na 2 CO 3 as CO 2 source and 0.63 mm CuCl 2 as copper precursor. Note that we selected a carbon free hole scavenger S2

3 to avoid undesired carbon contaminations. The amount of CdS nanorods was adjusted to an optical density of the sample at the first excitonic peak of The ph of the prepared solution was adjusted to 11 with the addition of ultra-pure NaOH. Two different illumination sources were used in this work: a 447 nm blue diode laser, whose wavelength matches the excitonic absorption of CdS, and a 450 W Xe lamp, whose broad spectrum was used to excite Cu 2 S. In both cases, the illumination was carried in custom-built quartz cuvettes with a 1 cm 2 illumination area, 3 cm illumination path length and 3 ml of sample volume. Their special geometry is appropriate to let the solution and the gaseous phases coexist in the same environment. The samples were flushed with inert gas (either Ar or He) depending on which gas chromatograph (GC) was used to analyze them, in order to increase the sensitivity of the system for the detection of the evolved gaseous phase. The gas chromatograph Shimadzu GC-2014 was used for H 2 detection and the gas chromatograph Bruker 456-GC equipped with a flame ionization detector and coupled to a Scion SQ mass spectrometer (MS) was used to detect CO, CH 4 and heavier compounds. The measurements on the MS have shown the presence of water and CO 2 in the samples, as expected. A volume of 0.8 ml was taken from the cuvette head-space for every measurement and the same amount of carrier gas (He) was afterwards re-injected to avoid pressure drops. Photocatalyst characterization For the spectroscopic measurements an absorption spectrometer Varian Cary 5000 UV-VIS- NIR was used. Photoluminescence (PL) spectra were taken using a Fluorolog-3 FL3-22 (Horiba Jobin Yvon GmbH) spectrometer equipped with a water-cooled R928 PMT photomultiplier tube mounted at 90 C angle. A monochromated Xe-lamp was used as light source. A ph meter ph540glp with Schott BlueLine18pH electrode was used to measure the ph of the analysed solutions. The morphology of the samples was investigated using a JEOL JEM-1011 TEM operating S3

4 at an accelerating voltage of kv. High-resolution images in TEM mode and high-angle annular dark field scanning TEM mode (HAADF-STEM) were recorded with a Titan at an accelerating voltage of 300 kv. For elemental analysis with this instrument, energydispersive X-ray (EDX) analysis was performed with a Si(Li) detector. Nickel grids covered with a holey carbon film were used as support for these measurements. The chemical composition of the samples was measured using a X-ray photoelectron spectrometer equipped with a VSW TA10 X-ray source and a VSW HA100 hemispherical analyser. A VSW AS10 argon ion gun was used to remove the superficial layers of the sample. The composition of the supernatants and precipitates in the system was checked using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Cu and Cd amounts were measured looking at the characteristic electromagnetic radiation wavelengths emitted after the interaction with the coupled plasma. The emission lines analysed were nm and nm for Cd, nm and nm for Cu, nm and nm for S. The results obtained from the first line of each series are plotted afterwards. In the case of Cd and Cu the measurements of the two different emission lines were almost identical, whereas for S a small deviation was observed due to the low sensitivity of the spectrometer for S detection. Control experiments Several control experiments were carried out to prove the description of the system that we propose in this work. The measurements in the absence of Cu 2+ cations have shown a typical behavior of Pt tip-decorated CdS nanorods, that is the photocatalytic generation of hydrogen. The experiments without the CdS, but in the presence of Cu 2+, Na 2 SO 3 and Na 2 CO 3, have shown a slow formation of sedimenting dark particles (presumably of CuO) leaving a colorless solution. The formation of such precipitate was not observed when CdS S4

5 was present suggesting a very different reaction progress. To verify the importance of light illumination for the cation exchange process we performed dark experiments, leaving the solution of CdS, TEOA and CuCl 2 in the dark upon stirring. No modifications in the absorption spectra were observed after a total reaction time of 76 hours. Another control experiment was made excluding the hole scavenger from the reaction. The system evolved small amounts of CO under illumination with a Xe lamp in the first 2.5 hours and the CO detected decreased significantly after 25 hours, showing photo-degradation of the system. To measure the importance of carbon contaminations coming not from the used C-source, we performed a photocatalytic experiment without Na 2 CO 3. The results showed a small evolution of CH 4 and CO in the first 2.5 hours that abruptly decreased after 25 hours of illumination, meaning that minor C contaminations in the system do not lead to significant contributions for long illumination times. To verify the broad absorption of the Cu 2 S formed in the system, photocatalytic experiments with filters were performed using the Xe lamp as illumination source. The experiments were divided in two phases: in the first one, only the region below 480 nm was illuminated for 2.5 hours to ensure light absorption from the CdS nanorods and subsequent activation of the light-induced cation exchange reaction. After that time, the illumination was shifted only to the region above 570 nm, to exclude the contribution from the remaining CdS in the system and allow the absorption only from Cu 2 S. The results showed that both CH 4 and CO were evolved in the system and that the production increased also during the second illumination step, indicating the importance of the broad absorption from Cu 2 S. To observe the effect of Cu 2+ in the photocatalytic system, we performed control experiments preparing two identical photocatalytic solutions of CdS, Na 2 SO 3, Na 2 CO 3 and CuCl 2 and introducing in one of them a 6.4 mm Cu 2+ excess after a certain illumination time, while observing the gas production of both samples over the time. We have observed that the photo-activated reaction that leads to the formation of Cu + from Cu 2+ slows down the S5

production of other compounds such as CO and CH 4 in the photocatalytic system, which results on the smaller formation rate of the solution with Cu 2+ excess.

The atomic ratios are calculated and displayed over the illumination time. 30 min 5 h 21h Cu:S 0.17 ± 0.01 0.45 ± 0.03 1.82 ± 0.14 Cd:S 0.94 ± 0.03 1.12 ± 0.06 0.17 ± 0.04 Cd:Cu 5.47 ± 0.38 2.49 ± 0.

6 production of other compounds such as CO and CH 4 in the photocatalytic system, which results on the smaller formation rate of the solution with Cu 2+ excess. Table S1: Wide area EDX measurements of CdS nanorods after 30 min, 5 hours and 21 hours of light-activated cation exchange with Cu, using the 447 nm laser as illumination source. The atomic ratios are calculated and displayed over the illumination time. 30 min 5 h 21h Cu:S 0.17 ± ± ± 0.14 Cd:S 0.94 ± ± ± 0.04 Cd:Cu 5.47 ± ± ± 0.02 a b Figure S1: HAADF STEM pictures of CdS nanorods after 2.5 hours of light-induced cation exchange with Cu, using the 447 nm laser as illumination source. Regions of crystallized CdS are clearly visible indicating the intermediate stage of the cation exchange reaction. Table S2: Wide area EDX measurements of CdS nanorods decorated with Pt on tips after 50 min and 15 hours of light-activated cation exchange with Cu, using the Xe lamp as illumination source. The atomic ratios are calculated and displayed over the illumination time. 50 min 15 h Cu:S 0.60 ± ± 0.02 Cd:S 0.45 ± ± 0.01 Cd:Cu 0.76 ± ± 0.01 S6

7 a b C o u n t s [ a r b. u. ] C u S C d C u E n e r g y ( e V ) Figure S2: (a) HAADF STEM pictures of CdS nanorods decorated with Pt on tips after 50 minutes of light-induced cation exchange with Cu, using the Xe lamp as illumination source. (b) Results of EDX measurements on the areas selected in (a). The atomic ratios relative to the EDX analysis are reported in Table S3. Table S3: Atomic ratios calculated from EDX measurements on selected areas in Figure S2. The labeling of the different areas analyzed refers to the legend reported in the relative Figure. Area 1 Area 2 Area 3 Area 4 Cu:S 1.52 ± ± ± ± 0.11 Cd:S 0.15 ± ± ± ± 0.03 Cd:Cu 0.10 ± ± ± ± 0.02 S7

a b 6 0 C u C o u n t s 4 0 2 0 S C u C d 0 0 1 0 0 0 2 0 0 0 3 0 0 0 8 0 0 0 E n e r g y ( e V ) Figure S3: (a) HAADF STEM pictures of CdS nanorods decorated with Pt on tips after 50 minutes of

8 a b 6 0 C u C o u n t s S C u C d E n e r g y ( e V ) Figure S3: (a) HAADF STEM pictures of CdS nanorods decorated with Pt on tips after 50 minutes of light-induced cation exchange with Cu using the Xe lamp as illumination source. (b) Results of EDX measurements on the area selected in (a). The atomic ratios relative to the EDX analysis are reported in Table S4. Table S4: Atomic ratios calculated from EDX measurements on the selected area in Figure S3. Area 1 Cu:S 1.51 ± 0.15 Cd:S 0.01 ± 0.04 Cd:Cu 0.01 ± 0.03 S8

9 a c C d C u - K b C u - L P t - L S - K Figure S4: (a) HAADF STEM pictures of CdS nanorods decorated with Pt on tips after 50 minutes of light-induced cation exchange with Cu using the Xe lamp as illumination source. (b) The orange box shows the area selected for elemental mapping (a). (c) Results of the elemental mapping showing the localization of Cd, Cu, S and Pt in the sample. S9

$Figure S5: Diffraction pattern of CdS nanorods decorated with Pt on tips after 15 hours of light-induced cation exchange with Cu using the$

10 Figure S5: Diffraction pattern of CdS nanorods decorated with Pt on tips after 15 hours of light-induced cation exchange with Cu using the Xe lamp as illumination source. The diffraction patterns can be indexed according to the high-chalcocite phase, in addition to Pt(111). S10

11 a b 1 C o u n t s [ a r b. u. ] C u S C d C u E n e r g y ( e V ) Figure S6: (a) HAADF STEM pictures of CdS nanorods decorated with Pt on tips after 15 hours of light-induced cation exchange with Cu using the Xe lamp as illumination source. (b) Results of EDX measurements on the areas selected in (a). The atomic ratios relative to the EDX analysis are reported in Table S5. Clear lattice planes are visible in the region 2 composed mostly of Cu 2 S. Table S5: Atomic ratios calculated from EDX measurements on selected areas in Figure S6. The labeling corresponds to Figure S6. Area 1 Area 2 Cu:S 1.62 ± ± 0.10 Cd:S 0.12 ± ± 0.04 Cd:Cu 0.07 ± ± 0.03 S11

12 a b Figure S7: ICP-AES measurements on CdS nanorods after different stages (0.5h, 2.5h and 25h) of cation exchange with Cu. The samples were centrifuged and the precipitate and the supernatant were analyzed separately. The compound density of (a) Cd, Cu and (b) S in both the precipitate (continuous line) and the supernatant (dotted line) is plotted with respect to the illumination time. The 447 nm laser was used as illumination source. The amount of Cd in the precipitate (i.e. in the NRs) decreased substantially during the process, while the amount in the solution (i.e. free Cd 2+ ions) increased. The opposite trend was observed for Cu, in line with the expected effect of the cation exchange. The amount of sulfur in both phases remained approximately constant. S12

13 a b I n t e n s i t y [ a r b. u. ] I n t e n s i t y [ a r b. u. ] C u O B i n d i n g e n e r g y ( e V ) B i n d i n g e n e r g y ( e V ) Figure S8: Detailed XPS measurements on CdS nanorods decorated randomly with Pt, after 24 hours of light-assisted cation exchange with Cu using the 447 nm laser as illumination source. (a) Cd 3d 5/2 and 3d 3/2 spectra, (b) Cu 2p 3/2 spectra before and after sputtering. The spectra were fitted using the Doniach-Sunjic profile (orange solid line). The presence of Cd in the spectra before the sputtering can be attributed to the deposition from solution during drying or to the adsorption on the nanorod surface. The content of Cd is significantly diminished by the sputtering process, indicating only few traces of Cd located inside the nanorods. In the Cu spectra a small peak of CuO is found besides Cu, most likely arising from oxidation of the sample before the XPS measurement. The CuO is removed by the sputtering process. The Cu peak could be attributed to either Cu 2 S, Cu 2 O or Cu(0), due to the overlapping of the corresponding binding energies. S13

n o r m. A b s. [ a r b. u. ] 2. 0 1. 5 1. 0 0. 5 0 h 0. 5 h 3 h 2 2 h 7 6 h 0.

nanorods in the presence of Cu(II) precursor (CuCl 2 ) and TEOA as hole scavenger after several hours in

14 n o r m. A b s. [ a r b. u. ] h 0. 5 h 3 h 2 2 h 7 6 h W a v e l e n g t h ( n m ) Figure S9: Absorbance in the UV-Vis region of CdS nanorods in the presence of Cu(II) precursor (CuCl 2 ) and TEOA as hole scavenger after several hours in absence of light irradiation. a b 1 0 n m Figure S10: TEM pictures of CdS nanorods decorated with Pt nanoparticles (a) on the tips or (b) randomly. S14

Figure S11: Typical custom-built quartz glass cuvette used for photocatalytic experiments. A b s. [ a r b. u. ] 1. 6 1. 2 0.

15 Figure S11: Typical custom-built quartz glass cuvette used for photocatalytic experiments. A b s. [ a r b. u. ] m i n 5 0 m i n - n o r m a l a t m o s p h e r e 5 0 m i n - i n e r t a t m o s p h e r e ( A W a v e l e n g t h ( n m ) Figure S12: Absorbance in the UV-Vis region of CdS nanorods in the presence of Cu(II) precursor (CuCl 2 ) and TEOA as hole scavenger after illumination with a Xe lamp in the presence of normal or inert (Ar) atmosphere. S15

16 C O p r o d u c e d ( n m o l ) C u 2 + P t t i p s C d S : C S u P t t i p s 2 C d S : C S u P t t i p s w 2 i + t h e x C c u e s s T i m e ( h ) Figure S13: Evolution of carbon monoxide (CO) from CdS-Cu 2 S nanorods decorated with Pt on tips after illumination with a broad spectrum Xe lamp. The light-blue trace shows the CO production after the insertion of a 6.4 mm Cu 2+ excess in the system. A b s. [ a r b. u. ] m i n 1 5 m i n 5 0 m i n W a v e l e n g t h ( n m ) Figure S14: Absorbance in the UV-Vis region of CdS nanorods in the presence of Cu(II) precursor (CuCl 2 ) and TEOA as hole scavenger after short illumination time with a Xe lamp in the presence of normal atmosphere. S16

C H 4 p r o d u c e d ( n m o l ) 3. 0 2. 5 2. 0 1. 5 1. 0 0. 5 0.

17 C H 4 p r o d u c e d ( n m o l ) T i m e ( h ) C d S : C u 2 S / P t t i p s Figure S15: Evolution of methane (CH 4 ) from CdS-Cu 2 S nanorods decorated with Pt on tips after illumination with a broad spectrum Xe lamp. Figure S16: HAADF STEM pictures of Pt clusters extracted from the HAASF STEM analysis of CdS nanorods decorated with Pt on tips after 50 minutes of light-induced cation exchange with Cu using the Xe lamp as illumination source. Some clusters show contrasts of atoms of unknown atomic species beyond the lattice fringes of Pt. This could correspond to a very thin layer of copper-based coating on the Pt NPs. The scale bar is 5nm. S17

18 I n t e n s i t y [ a r b. u. B i n d i n g e n e r g y ( e V ) Figure S17: XPS measurements on CdS nanorods decorated randomly with Pt, after 24 hours of light-assisted cation exchange with Cu using the 447 nm laser as illumination source. The Pt 4f 5/2 and 4f 7/2 spectra before and after sputtering are measured. There is no clear peak visible which is probably due to the fact that the Pt does not cover the whole surface but forms conglomerates. S18

19 a C H 4 p r o d u c e d ( n m o l ) b C O p r o d u c e d ( n m o l ) T i m e ( h ) T i m e ( h ) Figure S18: (a) CH 4 and (b) CO evolution from CdS-Cu 2 S nanorods decorated with Pt on tips and illuminated with λ < 480nm for the first 2.5 hours (marked with the blue background) and with λ > 570nm afterwards. The different curves correspond to different batches of Pt decorated CdS nanorods, but all show continued CH 4 and CO production under illumination with wavelengths longer than 570 nm. a C H 4 p r o d u c e d ( n m o l ) C d S : C S u / P t t i p s w / o C - s o u r c e 2 C d S : C S u / P t t i p s w / o H S 2 b C O p r o d u c e d ( n m o l ) C d S : C S u / P t t i p s w / o C - s o u r c e 2 C d S : C S u / P t t i p s w / o H S T i m e ( h ) T i m e ( h ) Figure S19: (a) CH 4 and (b) CO evolution from CdS-Cu 2 S nanorods decorated with Pt on tips without addiction of either carbon source (green traces) or hole scavenger (yellow traces) using the Xe lamp as illumination source. S19

20 References (S1) Saunders, A. E.; Ghezelbash, A.; Sood, P.; Korgel, B. A. Langmuir 2008, 24, (S2) Dukovic, G.; Merkle, M. G.; Nelson, J. H.; Hughes, S. M.; Alivisatos, A. P. Adv. Mater. 2008, 20, (S3) Habas, S. E.; Yang, P.; Mokari, T. J. Am. Chem. Soc. 2008, 130, S20

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