Operando deconvolution of photovoltaic and electrocatalytic performance in ALD TiO2 protected water splitting photocathodes

The dual-working-electrode technique enables the deconvolution of the intrinsic properties of the buried p–n junction and the electrocatalyst on the surface for water splitting photocathodes.

3 Atomic layer deposition of TiO 2 and Ga 2 O 3 . TiO 2 on Si wafer and Ga 2 O 3 -TiO 2 on Cu 2 O plate were deposited by atomic layer deposition (ALD) using a Picosun R200 tool. Before the deposition, the sample was rinsed with deionized water and dried under a stream of N 2 . The samples were then placed inside the ALD chamber, which was already heated to 120 °C. Tetrakis(dimethylamino)titanium (Sigma-Aldrich) and H 2 O were used as the precursor for Ti and O, respectively. The Ti precursor was heated to 85 °C and a 1.6 s pulse was used (with software boost function), followed by a 6.0 s N 2 purge. H 2 O was held at room temperature and a 0.1 s pulse was used, followed by a 6.0 s N 2 purge. To reach 100 nm of thickness for TiO 2  For the Ga 2 O 3 layer, bis(µ-dimethylamino)tetrakis-(dimethylamino)digallium (STREM, 98%) was used as Ga precursor. The ALD chamber temperature was kept at 160 °C during deposition. The Ga precursor was held at 150 °C and a 2.5 s pulse was used (with software boost function), followed by a 7.0 s N 2 purge. The H 2 O was held at room temperature, and a 0.1 s pulse was used, followed by a 4.0 s N 2 purge time. To deposit a 20 nm-thick Ga 2 O 3 thin film, 250 cycles were used. In order to avoid ALD growth on the back side of the sample (potentially leading to shunting problems), teflon tape was used to cover 4 the back side of the Si wafers and Cu 2 O plates during the ALD process. Supplementary Fig. S13 present the Cu 2 O/Ga 2 O 3 /TiO 2 multilayered structure.
Fabrication of Si-based DWE. After ALD TiO 2 deposition, the working electrode 1 (WE1) contact was made to the back side of the p-type silicon by scratching the wafer, applying Ga-In eutectic (Aldrich) and attaching copper foil (Aldrich). A layer of epoxy resin (Loctite Epoxide-resin EA 9461) was then used to cover and glue the electrode to a glass microscope slide, with a certain portion of the TiO 2 surface left uncovered for use as the electrochemical active area. For making the front contact, a 20 nm-thick Au layer was sputtered (Safematic CCU-010) onto the epoxy as well as a small part of the exposed TiO 2 . A copper wire was connected to the Au layer with Ag paint (Ted Pella, Inc.) on top of the epoxy, as a connection for the second working electrode (WE2). Finally, the front contact was protected from the electrolyte by masking it with a second epoxy layer. Supplementary Fig. S2 shows an optical photograph and the structure scheme of an as-fabricated pn + Si/TiO 2 DWE.

Fabrication of Cu 2 O-based DWE.
After the deposition of the Ga 2 O 3 -TiO 2 overlayer, a 100 nm-thick Au layer was then sputtered onto the back side of the Cu 2 O plate (the front side was protected with teflon tape), followed by connecting an Ag wire with Ag paint as WE1. Epoxy resin was then used to cover the whole back side of the electrode to provide protection and enhance the stiffness of the Cu 2 O plate. WE2 was connected to the ALD-TiO 2 surface using the same method as for the Si-based DWE, described above. and Pt(sp) on FTO slides were recorded with a Shimadzu UV-3600Plus UV/Vis/NIR spectrometer equipped with an integrating sphere.
Photoelectrochemical Measurements. Photoelectrochemical measurements were performed in a four-electrode configuration using a BioLogic SP-300 bipotentiostat.
The reference electrode was Ag/AgCl (0.197 V vs. NHE) and a Pt wire served as the counter electrode. All electrode potentials were converted into RHE scale: at room temperature, V RHE = V NHE + 0.059 × pH = V Ag/AgCl + 0.059 × pH + 0.197. Before the measurements, the electrolyte was sparged with N 2 for at least 10 min to remove dissolved oxygen. Simulated one sun illumination (100 mW cm -2 ) was provided by a 150 W Xe-lamp with AM 1.5 G filter from LOT Oriel, and the intensity was  with an Ag/AgCl reference electrode and a Pt wire counter electrode. The photocathodes were covered with epoxy to fix the active area to ~0.08 cm 2 . The measurement was performed in the same electrolyte as in the PEC measurements (described above). The electrolyte was stirred and constantly sparged with Ar gas at a rate of 20 ml/min. The gas outlet from the cell was connected to a 450-GC Gas Chromatograph Bruker Daltonics GmbH for gas analysis. One LSV scan was first performed for choosing a suitable V1 potential for the Faradaic efficiency tests. During the measurement, the exposed area of the photocathode was illuminated with a white-light LED. The intensity of the light was calibrated to reach a similar photocurrent density as obtained under simulated one sun illumination, as described above  When V1 is more positive than 0 V RHE , V2 is similar to V1, thus ΔV is 0, indicating that no voltage difference is generated under dark conditions in this potential region. When V1 is scanned to more negative potentials than 0 V RHE , V2 stays mainly unchanged with a value slightly more negative than 0 V RHE , contributing to the tiny dark current. This region of very negative V1 corresponds to a reverse bias across the p-n Si homojunction, as is evident from the ΔV-V1 curve.
Under illumination, ΔV maintains a constant value of 475 mV until V1 reaches ~0.47 V RHE . In this positive potential region, ΔV equals the output V oc as there is no current flow across the interface. The potential where ΔV begins to shrink indicates the real onset potential of hydrogen production, which appears in this case at ~0.47 V RHE . As the photocurrent increases while sweeping the potential negatively, ΔV decreases until a saturation photocurrent density is obtained, and then continues to shrink as the photocurrent remains saturated. Supplementary Table S1 J-V parameters extracted from the J-ΔV behavior of the water splitting photocathodes. We expect the pn + -Si junction samples from the same wafer to generate the same V oc . The small V oc variation among pn + Si/TiO 2 /Pt(ed), pn + Si/Ti/Pt(ed), pn + Si/TiO 2 /Pt(sp) is likely due to the different light intensity as a result of light absorbtion by the catalyst and overlayers, or perhaps passivation and recombination at the Si/overlayer interface (through pinholes). The power conversion efficiency (η) is given for completeness, and is defined as:

J-ΔV
where P in of one sun illumination is the incident illumination power density (100 mW cm -2 ). For the pn + Si/TiO 2 /Pt(sp) sample, an increased light intensity (P in = ~137 mW cm -2 ) was also used. Photoexcited electrons cannot be transferred into solution since the potential of the photoexcited electrons is still not high enough to drive proton reduction. In this region of V1 (before the onset of hydrogen generation), ΔV values remain constant and equal to V oc . The flat band potential of TiO 2 itself is located near E(H + /H 2 ). As soon as V2 is slightly more negative than 0 V RHE , hydrogen evolution is thermodynamically allowed, and electrons flow into the electrolyte by reducing protons into hydrogen gas. The V1 potential at this point is defined as the onset potential.
As V1 becomes more negative, the ΔV value shrinks as the photocurrent corresponding to hydrogen generation increases. When V1 catches up to V2, ΔV is 0 and the buried p-n junction is at the short-circuit condition. Finally, as soon as the photocurrent becomes saturated due to the photon flux and recombination, V2 reaches a steady value independent of V1. The reverse bias across the p-n junction becomes stronger as V1 level continues to move to more negative potential.
The V oc appears at a more negative potential than the onset of photocurrent under certain sweeping conditions, as shown in Supplementary Fig. S5. This means that V2 is more positive than 0 V RHE while current is flowing. In order to confirm that these small photocurrents 13 correspond to hydrogen evolution and not to e.g. proton intercalation, we carried out faradaic efficiency measurements (See Supplementary Fig. S6). Hydrogen was indeed observed at very small cathodic photocurrent densities, such as -0.08 mA cm -2 , at potentials more positive than we would expect from the J-ΔV analysis. This phenomenon likely arises from the fact that the slightly lower electron density than in the illuminated area and consequently an actually slightly reduced V oc is detected compared to the illuminated area (as shown in Fig. 1b). Thus, the onset potential appears earlier than the V oc in Supplementary Fig. S5. This V oc loss is highly dependent on the distance between the Au contact and the illumination area, represented by d in this scheme. For all the samples we measured, d is ~1 mm. When intentionally lengthening d, V oc further decreases, yielding ~75 mV loss at d = 3 mm. From our experience with more than fifty samples, the offset between the measured V oc and the apparent onset from the J-V1 curves is typically a few tens of mV with a platinum catalyst.