Supporting Information

Transcription

1 Supporting Information Carlos G. Morales-Guio, Kerstin Thorwarth, Bjoern Niesen, Laurent Liardet, Jörg Patscheider, Christophe Ballif, and Xile Hu*, Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne CH-1015, Switzerland. Laboratory for Nanoscale Materials Science, Swiss Federal Laboratories for Materials Science and Technology (EMPA), Dübendorf CH-8600, Switzerland. Ecole Polytechnique Fe de rale de Lausanne (EPFL), Institute of Microengineering (IMT), Photovoltaics and Thin- Film Electronics Laboratory, Neuchâtel CH-2000, Switzerland. S1

2 Experimental Procedures All manipulations were carried out under atmospheric conditions, unless otherwise mentioned. All reagents were purchased from commercial sources and used without further purification. Sources and purity of reagents are reported below. Millipore deionized water 18.2 M was used to prepare the solutions used in the electrochemical and photoelectrochemical measurements. Mo 2 C Deposition Samples were prepared using unbalanced DC magnetron co-sputter deposition from elemental targets (a C target with a purity of 5N and a Mo target with a purity of 3N5) in confocal arrangement in a UHV chamber at room temperature. The deposition chamber used is an AJA ORION -8-UHV system from AJA international Inc. (North Scituate, MA, USA). The base pressure before deposition was <5 x 10-7 Pa; during deposition the pressure was kept at 2.2 Pa using a constant Ar-flow of 15 sccm. Carbon was sputtered at a power density of 10 W/cm 2, while the power density on the Molybdenum target was 2.5 W/cm 2. Prior to deposition the samples were cleaned using an rf-bias on the substrate of -250 V in flowing Ar (15 sccm) at a pressure of 0.5 Pa. Samples were grown to a thickness of 5 nm and 10 nm, as determined using a Tencor P10 surface profilometer equipped with a diamond stylus. a-si/mo 2 C Photocathode Preparation The experimental procedure for the preparation of similar a-si cells has been previously reported. 1 In brief, a boron-doped ZnO (ZnO:B) front electrode with a thickness of 2 m was deposited on a clean glass substrate by means of low-pressure chemical vapor deposition (LP-CVD), using diethylzinc (Akzo Nobel), diborane (Messer), and water as precursors. The amorphous silicon (a-si) cell was deposited on top of the ZnO:B front electrode by plasma-enhanced CVD (PE-CVD) in a reactor with a parallel plate configuration. This cell comprised a p-type hydrogenated microcrystalline Si (µc-si:h) layer, a p-type a- SiC:H window layer, an intrinsic a-si:h layer with a thickness of 230 nm, and n-type a-si:h, µc-sio x :H, and µc-si:h layers. The precursor gases used for Si layer deposition had purities of at least %. The surface-protective layers were deposited by atomic layer deposition according to a previously reported protocol. 2 In brief, a thin layer (20 nm) of aluminium-doped zinc oxide (Al:ZnO, AZO) was deposited using an ALD system (Savannah 100, Cambridge Nanotech) with a substrate temperature of 120 C. The precursors were diethyl zinc (ABCR, 95%), trimethyl aluminium (ABCR, 98%) and H 2 O kept at room temperature as the Zn, Al and O precursors, respectively. TiO 2 (100 nm) was deposited at a substrate temperature of 150 C using tetrakis-(dimethylamino)titanium (99.99%, Aldrich), at a precursor temperature of 75 C, and hydrogen peroxide (50% in water, stabilized, Aldrich) at room temperature. The amorphous silicon cell was rinsed with deionized water previous to the surface-protective layers deposition. The photovoltage for the AZO/TiO 2 protected a-si photocathodes were determined to be mv for the various samples reported here. The photovoltage was determined by measuring the stable voltage necessary to keep a small cathodic current (~-0.2 ma cm -2 ) under light and in the dark S2

3 using a 0.2 mm ammonium tetratiomolybdate (ABCR, 99%) solution. The cathodic current corresponds to the electrochemical deposition of MoS 2 on the a-si photocathode which can be driven in the dark and under light. Mo 2 C was deposited on the surface protected a-si photoabsorber by confocal magnetron co-sputtering under identical conditions as those used for the sputtering of Mo 2 C films on FTO described above. Physical and Chemical Characterization The electrodes were characterized by SEM and XPS. SEM images were taken using a Zeiss MERLIN Microscope. Two XPS systems were used for the analysis of the samples. The XPS data for the MoC and Mo 2 C on FTO were collected on a Quantum 2000 (Physical Electronics Inc.) instrument under ultrahigh vacuum (<5 x 10 7 Pa) using monochromic aluminium K X-rays with a photon energy h = ev. Data were recorded at an analyser pass energy of ev and a step size of 0.1 ev. Argon ion and electron neutralizers were used to compensate for surface charging. Samples were sputtered-cleaned (~2 nm removed) in order to eliminate carbon contaminations and the natural molybdenum oxide layer of the molybdenum carbide. The samples investigated after sputter-cleaning were of MoC and Mo 2 C stoichiometry without the presence of other chemical states of Mo.The XPS data for the a-si/mo 2 C before photoelectrochemical measurements and after 1, 2, and 5 hours of stability tests were collected in a PHI VersaProbeII XPS system equipped with automated charge compensation and Ar ion gun under high vacuum using monochromic aluminium K X-rays (h = ev). The deconvolution pass energy was 2.95 ev. The XPS spectra of the a-si/mo 2 C photocathodes before and after ~2nm of sputtering were identical and only the spectra before sputtering is shown here. Both curve fitting of the spectra and quantification were carried out with the CasaXPS software using relative sensitivity factors from the suppliers. Electrochemical and Photoelectrochemical Measurements HER and PEC HER measurements were recorded by an Autolab potentiostat/galvanostat. Electrochemical and PEC measurements were performed in a three-electrode configuration where a Pt wire was used as counter electrode and an Ag/AgCl (KCl sat.) electrode was used as the reference electrode. 1.0 M KOH (Merck) solution and 0.1 M H 2 SO 4 (Merck, 95 97%) were used as electrolytes. 0.1 M H 2 SO 4 was preferred over 1.0 M H 2 SO 4 due to the slower dissolution of the amorphous TiO 2 protective layer in less concentrated acids. The photoresponse was measured under irradiation from a 450 W Xe lamp (Osram) equipped with a KG3 filter (3 mm, Schott, filters ultraviolet and infrared light), calibrated with a Si diode to simulate AM 1.5 G illumination (1 sun) between wavelengths of 300 and 800 nm. Photocurrent stability test were carried out by measuring the photocurrent produced under constant light irradiation at a fixed electrode potential of 0 V versus RHE during various periods of time. The samples further analyzed by XPS were quickly removed from the alkaline solution, rinsed with deionized water and stored under nitrogen to avoid the formation of a molybdenum oxide layer on the catalyst. During linear sweep voltammetry (j V plots) and chronoamperometry (stability plots), the electrolyte was continuously bubbled with N 2 to remove oxygen and thus eliminate erroneous signals arising from oxygen reduction. Solution resistance was determined by impedance measurements and the ir-drop was S3

4 corrected for electrochemical HER. The ir-drop was not corrected during any of the photoelectrochemical measurements. Faradaic efficiencies were measured in a gastight, home made H cell calibrated for pressure and gas quantification previously reported. 2 References 1. Lin, Y.; Battaglia, C.; Boccard, M.; Hettick, M.; Yu, Z.; Ballif, C.; Ager, J. W.; Javey, A. Nano Lett. 2013, 13, Morales-Guio, C. G.; Tilley, S. D.; Vrubel, H.; Gratzel, M.; Hu, X. L. Nat. Commun 2014, 5, S4

5 a Sputtered Mo 2 C Mo:C = 2.1:1 Mo 3p Mo 3d O KLL O 1s Mo 3s C 1s Mo 4p Regions of interest Background b Mo 3d Mo 3d 5/2 in Mo 2 C ev = 3.19 ev c C 1s C in carbide ev Figure S1. XPS spectra of sputtered Mo 2 C (5 nm) on FTO. (a) Survey spectra. (b) Mo 3d region. (c) C 1s region. The XPS spectra was measured after sputter-cleaning (~2 nm removed) in order to remove carbon contaminations and the molybdenum oxide layer. S5

6 Overpotential (mv) Overpotential (mv) a M H 2 SO Mo 2 C (5 nm) MoC (5 nm) Mo 2 C (10 nm) log(j/a cm -2 ) b M KOH Mo 2 C (5 nm) MoC (5 nm) Mo 2 C (10 nm) log(j/a cm -2 ) Figure S2. Tafel plots of sputtered Mo 2 C and MoC catalyst films on FTO during electrochemical hydrogen evolution in (a) 0.1 M H 2 SO 4 and (b) 1.0 M КОH at 5 mv s -1. S6

7 j (ma cm -2 ) = 310 mv Time (h) 1.0 M KOH 0.1 M H 2 SO 4 Figure S3. Long term stability of Mo 2 C (5 nm)/fto electrodes. S7

8 j (ma cm -2 ) Potential (V vs RHE) Mo 2 C (5 nm) 0.1 M H 2 SO 4 Mo 2 C (5 nm) 1.0 M KOH Figure S4. Comparison of electrochemical activity of Mo 2 C (5 nm) films on FTO in 0.1 M H 2 SO 4 and 1.0 M КОH at 5 mv s -1. S8

9 j (ma cm -2 ) j (ma cm -2 ) a M H 2 SO 4 a-si/mo 2 C Potential (V vs RHE) b M KOH a-si/mo 2 C Potential (V vs RHE) Figure S5. Linear sweep voltammetry curves of (a) a-si/mo 2 C in acidic electrolyte and (b) a-si/mo 2 C in alkaline electrolyte under light and in the dark and during light chopping. S9

10 j (ma cm -2 ) a-si/mo 2 C in 1.0 M KOH 1 hour 2 hours 5 hours Time (h) Figure S6. Long term stability of a-si/mo 2 C photocathodes in alkaline solutions biased at 0 V vs. RHE under simulated AM 1.5G (1 sun) illumination. Fluctuations in the photocurrent are due to hydrogen bubbles attaching to the surface of the photoelectrode reducing the surface area in contact with the electrolyte. S10

11 a O 1s Mo 3d O KLL Mo 3s Mo 3p C 1s Zn 2p Si 2s Zn 3s Si 2p 0 h 1 h 2 h Regions of interest Background h b Mo 3d Mo 3d 5/2 in Mo 2 C ev = 3.19 ev c C 1s C in carbide ev 0 h x12 0 h 1 h 1 h 2 h 2 h 5 h 5 h d Zn 2p Zn 2p 3/2 in ZnO ev = ev e Si 2p Si in SiO ev 1 h 1 h 2 h 2 h 5 h 5 h S11

12 Figure S7. XPS spectra of a-si/mo 2 C photocathodes before and after stability testing during 1, 2 and 5 hours. (a) Survey spectra. (b) Mo 3d region. An oxidized Mo signal (Mo 3d 5/2 at ev) is visible on the photocathode before testing in 1.0 M KOH. Only the reduced Mo signal corresponding to Mo 2 C is observed after testing in 1.0 M KOH. This indicates that surface molybdenum oxides is dissolved in base. (c) C 1s region. (d) Zn 2p region. (e) Si 2p region. Neither Ti, Zn or Si are detected in the as-prepared photocathode indicating conformal coating by the Mo 2 C catalyst. After operation, the intensities due to Mo 2 C decreased and signals due to Zn and Si appeared, indicating a partial corrosion of the photocathode. S12