Atomic layer-deposited tunnel oxide stabilizes silicon photoanodes for water oxidation Yi Wei Chen 1, Jonathan D. Prange 2, Simon Dühnen 2, Yohan Park 1, Marika Gunji 1, Christopher E. D. Chidsey 2, and Paul C. McIntyre 1* 1 Department of Materials Science & Engineering, Stanford University 2 Department of Chemistry, Stanford University These authors contributed equally to this work. * Corresponding author: pcm1@stanford.edu Supplementary Materials Experimental Methods Materials. All chemicals used in this study were purchased from commercial sources and used as received. Degenerately doped p + -type Si (100) wafers (0.001-0.002 Ωcm, 500 m thickness) and n-type Si (100) wafers (0.1-0.2 Ωcm, 500 m thickness) were purchased from El-Cat. All electrochemical measurements were performed in electrolyte solutions prepared with DI water. A ferri/ferrocyanide solution was made to be 10 mm of both K 3 Fe(CN) 6 and K 4 Fe(CN) 6 3H 2 O in 1 M aqueous KCl. Experiments for water electrolysis were performed in either acidic (1 M H 2 SO 4 ), neutral (1 M phosphate-buffered, ph 7) or basic (1 M NaOH) solutions. Sample Preparation. Atomic layer deposition (ALD) of TiO 2 was performed at 200 C on silicon wafers that were coated with a thin (< 2 nm) SiO 2 layer as prepared by the wafer vendor.. Tetrakisdmethylamido titanium (TDMAT) was used as the titanium source and water vapor as oxygen source. Each source was heated to a line temperature of 80 C. The system pressure was maintained at 1.1 Torr nominally, and nitrogen was used as the carrier gas. The pulse and purge durations of the titanium and water sources were 5 s and 0.75 s respectively. Unless stated otherwise, a total of 24 cycles of ALD NATURE MATERIALS www.nature.com/naturematerials 1
was performed to obtain a TiO 2 film of approximately 2 nm thickness. The noble metal layer was then deposited by e-beam evaporation with a quartz crystal balance used to monitor the thickness of material deposited to obtain the nanocomposite electrode. For all samples, a thin layer (3 nm) of metal was e-beam evaporated onto the backside of the silicon wafer (platinum for p + -Si samples and aluminum for n-si samples). This backside metal forms the electrical contact and eliminates any Schottky junctions that could be formed at the back of the substrate. Samples were annealed in forming gas (95% N 2, 5% H 2 ) at 400 C for 30 minutes in a quartz tube furnace before experiments were performed. Electronic tunneling mediated by the metallic overlayer. Efficient direct tunneling from the Si substrate through a thin TiO 2 layer requires sufficient density of states in the layer above TiO 2. A metal layer (e.g. Pt, Ir, Ru, etc ) has abundant density of states, and therefore, could sustain large tunneling current. The liquid electrolytes, however, have limited density of states, resulting in the low tunneling current when they are in direct contact with TiO 2. Therefore, the metal layer effectively mediates the charge transfer from Si and the reaction with the electrolyte. Other conductive catalysts, such as IrO 2, are also capable of serving the same role as the demonstrated metal catalyst layers, with details to be reported in future publications. Electrochemical Methods. All electrochemical experiments were performed on either a WaveNow or WaveNano potentiostat (Pine Research Instrumentation) in air at room temperature. A Pt wire was used as the counter electrode and a glass frit-isolated Ag/AgCl/sat. KCl electrode as the reference electrode. A 5 mm bored Teflon cone (area 0.196 cm 2 ) was pressed against the DI water-rinsed nanocomposite working electrode. All measured potentials were converted to the NHE reference scale using E(NHE) = 2 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION E(Ag/AgCl/KCl) + 0.197 V. The ph values for the electrolyte solutions used in this study were determined by utilizing a reversible hydrogen electrode and measuring the open circuit potential for each solution and adjusting the ph dependent water oxidation potentials accordingly. Hydrogen gas was sparged into the electrolyte solutions for ten minutes, with a platinum rotating disc electrode as the working electrode and platinum mesh as the counter electrode. Cyclic voltammetry (CV). All CVs were measured, unless stated otherwise, at 100 mv/s in the indicated electrolyte solution. The open circuit potential (OCP) was measured before each experiment and used as the starting and ending potential for each CV. A peristaltic pump (Cole Parmer, Norprene tubing, ID = 1.6 mm) was used to continuously circulate electrolyte solution at a flow rate of 1 ml/s impinging on the sample in the bore of the Teflon cone for all dark and illuminated water electrolysis experiments (Figure S1 for dark, Figure S2 for illuminated). CV s obtained with solar illumination on n-si anodes were performed in static solutions without pumping. A modified pumping configuration with the electrolyte flow entering through an angled hole in the side of the bore of the Teflon cone was used for stability tests conducted under solar illumination so as not to block the light with the delivery tube (Figure S2). Table S1 lists the peak-to-peak splitting for the ferri/ferrocyanide solution for different anode structures. Impedance spectroscopy measurements. Impedance spectroscopy was performed on a Princeton Applied Science impedance spectrometer to determine the solution resistance of the electrolytes used in this study. The Nyquist plots for the acidic, neutral and basic electrolyte solutions are shown in Figure S3. Extrapolation of the NATURE MATERIALS www.nature.com/naturematerials 3
Nyquist plots to large frequencies reveals series resistance to be 7.5 Ω, 33.5 Ω and 15 Ω for the acidic, neutral and basic solutions respectively. The obtained series resistance values correspond to theoretical calculations very well. As an example, the resistivity of 1 M NaOH is 5.9 cm (by linear extrapolation). By using a cylindrical geometry of 5 mm height and 5 mm diameter, the calculated resistance is 15. Chronoamperometry. Chronoamperometry experiments were performed by holding the potential at 1.5 V vs. Ag/AgCl/KCl reference electrode and monitoring the current over time in 10 ml of the specified electrolyte solution. The electrolyte solution was circulated with the peristaltic pump as described above. Results for a 18 hour experiment in neutral solution without illumination are shown in Figure S4 below. Chronopotentiometry (CP). All CP experiments were performed by holding the current constant at 1 ma across a 0.196 cm 2 anode surface area while monitoring the potential over time in 10 ml of the specified electrolyte solution. The electrolyte solution was circulated with the peristaltic pump using the procedure described above. Diffusion-limited current. To understand the discrepancy between the expected current density values obtained by extrapolation of the measured Tafel slopes at low current density and the measured values obtained during CP and CA experiments, diffusion-limited currents were estimated by determining the limiting current for ferrocyanide oxidation under identical flow conditions and then adjusting that value for the concentrations and the published diffusion constants of the different species that might limit mass transfer during water oxidation: either transport of the reactants (H 2 O or OH - ) to the surface or transport of the products (O 2 and H + or H 2 O) away from the surface. The analysis showed that with a flow rate of 1 ml/s, mass transfer of the 4 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION reactants to the surface should not be limiting, with achievable current densities beyond the solar flux limitations at 1 sun illumination. However, a major discrepancy was found for mass transport of the sparingly soluble product of water oxidation, O 2, away from the electrode surface. Under the fastest flow rates possible in our cell, 2 ml/s, oxygen bubbles would be expected to form at current densities of a few ma/cm 2. Because the current distribution is not expected to be uniform when mass transport is limiting in this flow arrangement, we expect that bubbles form when the average current density is of the order 1 ma/cm 2. These bubbles should effectively decrease the catalytic surface area and thus reduce the total current at a specific overpotential relative to that expected by extrapolation of the Tafel slope measured at low current density. Temperature dependent tunneling experiments. Temperature dependent tunneling experiments were performed on Ir/TiO 2 /p + -Si electrodes. Three thicknesses (2 nm, 4 nm, and 10 nm) of TiO 2 were chosen to observe the thickness-dependent tunneling behavior. A shadow mask is used to define the circular top metal contacts of 100 m diameter. The thickness of Ir top metal contact is 50 nm. The substrate bias was scanned from 0 to 1 V while monitoring the current density, as shown in Figure S5. Transmission electron microscopy of Ir/TiO 2 /p + -Si anode after stability test. Cross-sectional TEM was conducted after the 3 hr CP stability test (constant current at 5 ma/cm 2 ) for the Ir/TiO 2 /p + -Si sample (Figure S6). Comparing Figure S6 with Figure 1B, it is evident that cross-sectional TEM images detect no apparent structural change in the TiO 2 -protected anodes after the life test. X-ray photoelectron spectroscopy (XPS). The XPS measurements were performed using a PHI VersaProbe system with a 100 W Al-Kα X-ray source on a spot NATURE MATERIALS www.nature.com/naturematerials 5
size of 100 µm at a 45 incident angle. The binding energy scan range was 0-1200 ev in 1 ev steps, and the pass energy was 117.4 ev, which provides the optimal balance between scan resolution and counting statistics in our system. A dual beam neutralizer (7 V Ar + and 30 V electron beam) was used to neutralize sample charging. XPS depth profiling was performed using an Ar + ion beam at 2 kv and 1 A with an area of 2x2 cm. Spectra were collected at intervals of 3 s during sputtering. Atomic Force Microscopy (AFM). The AFM images for these samples were acquired using a Park XE-70 instrument set on a non-contact tapping mode with a scan rate of 1 Hz. A representative AFM image of the nanocomposite anode after synthesis is shown in Figure S9. Figure S1. Electrochemical cell setup without illumination. Crosssectional schematic of experimental setup utilized for dark electrochemical experiments on Ir/TiO 2 /p + -Si anodes. 6 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION Figure S2. Electrochemical cell setup with illumination. Crosssectional schematic of experimental setup utilized for stability tests with n-si/tio 2 /Ir under solar illumination. This geometry allows for unobstructed illumination of the sample. NATURE MATERIALS www.nature.com/naturematerials 7
8 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION Figure S3. Impedance spectroscopy of aqueous solutions. Nyquist plots from series resistance measurements from (A) acidic, (B) neutral and (C) basic solutions measured from 300 khz to 0.1 Hz. Figure S4. Chronoamperometry in neutral solution. Constant potential measurement at 1.5 V vs. Ag/AgCl/sat. KCl in ph 7 buffered solution without illumination for 18 hours for Ir/p + -Si ( ) and Ir/TiO 2 /p + -Si (-). NATURE MATERIALS www.nature.com/naturematerials 9
Figure S5. Temperature dependent tunneling current measurement for the Ir/TiO 2 /p + -Si electrode. The ALD-grown TiO 2 thicknesses were (A) 2 nm, (B) 4 nm, and (C) 10 nm. Measurements were taken at (-) 23 o C, (-) 50 o C, and (-) 75 o C. The line (-) indicates the compliance of the meter. 10 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION Figure S6. Cross-sectional TEM image of Ir/TiO 2 /p + -Si anode. Taken after 3 hr constant current (5 ma on a sample area of 0.196 cm 2 ) stability test. NATURE MATERIALS www.nature.com/naturematerials 11
Figure S7. Equilibrium band diagrams. (A) Ir/TiO 2 /p + -Si and (B) Ir/TiO 2 /n-si anode in contact with ferrocyanide solution. It is assumed that the redox level of the solution equilibrates with the Ir metal catalyst. 12 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION Figure S8. Light saturated current density. Cyclic voltammogram of Ir/TiO 2 /n-si in 1M H 2 SO 4 under simulated solar illumination at 1 sun. Scan rate is 0.1 V/s NATURE MATERIALS www.nature.com/naturematerials 13
Figure S9. AFM image of Ir covered ALD-TiO 2 sample. Root-mean-square roughness ~0.2 nm 14 NATURE MATERIALS www.nature.com/naturematerials
SUPPLEMENTARY INFORMATION Table S1. Summary of peak-to-peak potential separation for anodes measured in ferri/ferrocyanide solution Substrate E anodic to E cathodic (mv) TiO 2 /Si No observable peaks Ir/TiO 2 /Si 130 Indium Tin Oxide a 220 a Delta Technologies, Product # CB-40IN-S211, 4-8 Ω/ with a small area, front-side electrical contact outside the Teflon cone perhaps inducing a series resistance that accounts for the larger peak-to-peak separation. NATURE MATERIALS www.nature.com/naturematerials 15