Seebeck-voltage-triggered self-biased photoelectrochemical water splitting using HfOx/SiOx bi-layer protected Si photocathodes

The use of a photoelectrochemical device is an efficient method of converting solar energy into hydrogen fuel via water splitting reactions. One of the best photoelectrode materials is Si, which absorbs a broad wavelength range of incident light and produces a high photocurrent level (~44 mA·cm−2). However, the maximum photovoltage that can be generated in single-junction Si devices (~0.75 V) is much lower than the voltage required for a water splitting reaction (>1.6 V). In addition, the Si surface is electrochemically oxidized or reduced when it comes into direct contact with the aqueous electrolyte. Here, we propose the hybridization of the photoelectrochemical device with a thermoelectric device, where the Seebeck voltage generated by the thermal energy triggers the self-biased water splitting reaction without compromising the photocurrent level at 42 mA cm−2. In this hybrid device p-Si, where the surface is protected by HfOx/SiOx bilayers, is used as a photocathode. The HfOx exhibits high corrosion resistance and protection ability, thereby ensuring stability. On applying the Seebeck voltage, the tunneling barrier of HfOx is placed at a negligible energy level in the electron transfer from Si to the electrolyte, showing charge transfer kinetics independent of the HfOx thickness. These findings serve as a proof-of-concept of the stable and high-efficiency production of hydrogen fuel by the photoelectrochemical-thermoelectric hybrid devices.

hybrid system, the overall current is determined by the high photocurrent level of the PEC cell (i.e., the TE is only used as a voltage booster). This feature of the PEC-TE hybrid system can decouple the tradeoff relation between the voltage and current, thus maximizing the H 2 power generation (P Max ).
Various light-absorbing semiconductors including narrow bandgap (Si, InP, and GaAs) and wide bandgap (TiO x , CuO x , FeO x , and BiVO x ) materials have been used as photoelectrodes of PEC cell 30 . In the PEC-TE hybrid systems, because the output Seebeck voltage of the TE can further applied to drive the water splitting reaction, the narrow bandgap semiconductors with a higher photocurrent relative to the wide bandgap semiconductors are effective in maximizing the H 2 power generation. Thus, Si with comparatively narrow bandgap (1.1 eV) is capable of generating a high photocurrent of ~44 mA·cm −2 via harnessing a large portion of the solar spectrum and is a suitable material as a light-absorbing semiconductor photoelectrode 31 . However, the poor corrosion-resistance of Si is an inherent drawback in terms of the long-term stable operation of the PEC reaction 32 . One suitable strategy for offsetting this drawback is to use a high-atomic-density insulating material as a protective layer that would enable the complete blocking of the electrolyte permeation [33][34][35][36][37] . In general, a high-atomic-density insulator of significant thickness provides excellent protection ability but results in an increase in the transfer resistance of the light-induced charge carriers owing to the tunneling barrier of the insulator 38 . Therefore, for Si PEC devices, it is of paramount importance to design a protection layer that guarantees both long-term stability and facile charge-transfer kinetics.
In this study, we present a PEC-TE hybrid system with a HfO x /SiO x bilayer protected Si photocathode to ensure its long-term stability without the deterioration of the charge-transfer kinetics. The PEC-TE hybrid device based on an antireflective Si nanostructure photocathode enables the self-biased PEC water-splitting reaction at a high photocurrent level of ~42 mA·cm −2 , which has a maximum H 2 power generation of 55 mW·cm −2 . The HfO x , which has strong corrosion resistance in an acidic electrolyte and a relatively higher atomic density (9.68 g·cm −3 ) than other dielectric materials such as SiO x (2.65), AlO x (3.95), and TiO x (4.23), physically prevents the permeation of the electrolyte into the Si surface, thereby resulting in long-term stable PEC operation for 200 h. Moreover, on applying V TE , the electrical tunneling barrier of HfO x becomes irrelevant to the charge transport because the potential drop caused by the application of V TE mainly occurs in the SiO x layer, which induces the tunneling barrier of HfO x to be located at a level lower than the conduction band of Si. Therefore, V TE is not only applicable for driving a water-splitting reaction but also for reducing the kinetic overpotential required for charge transfer.

Results
H-terminated and SiO x -grown p-Si (100) wafers were prepared through the treatment of diluted hydrofluoric acid and hot-water-oxidation, respectively, and these wafers were deposited with an amorphous HfO x thin film using the atomic layer deposition (ALD) process [39][40][41] . The chemical composition of HfO x was confirmed using energy-dispersive X-ray spectroscope (EDXS) mapping and X-ray photoelectron spectroscopy (XPS) spectra (See Supplementary Figs 1 and 2). Figure 1a,b show the high-resolution transmission electron microscope (HRTEM) images for the ALD HfO x thin film deposited on the H-terminated and SiO x -grown Si wafers. The HfO x deposited on H-terminated Si was observed to have a 3-nm-thick HfSiO x interfacial layer, which is caused by a diffusion of the Si atoms during the ALD (see Fig. 1a) 42 . In contrast, the interfacial layer was not found in the SiO x -grown Si. Figure 1b shows the TEM image for the atomically thin SiO x interlayer inserted between the HfO x and Si without the formation of a HfSiO x layer. This difference is likely due to the bond strength of Si-O (≈190 kcal·mol −1 ) 43 being higher than that of Si-H (≈80 kcal·mol −1 ) 44 . The stronger Si-O binding formed by SiO x growth prevents the Si diffusion on the Si surface, thereby creating the HfO x /SiO x bilayer without formation of HfSiO x . The chemical quality of the interface between HfO x and Si is clearly confirmed by the angle-resolved X-ray photoelectron spectroscopy (ARXPS) results. For the Si2p XPS spectra at the low incident angle that represents the interface information, the H-terminated Si wafer has one dominant state of Si 3+ originating from the HfSiO x interfacial layer ( Fig. 1c), whereas the SiO x -grown Si wafer has various sub-oxide states of Si 1+ , Si 2+ , Si 3+ , and Si 4+ (corresponding to Si 2 O, SiO, Si 2 O 3 , and SiO 2 , respectively; see Fig. 1d) 45,46 .
Linear sweep voltammetry (LSV) responses for the Si photocathodes protected with HfO x , SiO x , and HfO x / SiO x were obtained in 0.5-M H 2 SO 4 to compare the charge transfer kinetics for the PEC water-splitting reaction (Fig. 2a). Two electrode systems consisting of a Si photocathode and Pt anode were used to determine the potential requirement for the self-biased water-splitting reaction. With a thermodynamic potential barrier of 1.23 eV, additional kinetic overpotential is required for the water-splitting reaction. Owing to the sluggish kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), the application of a minimum potential of ~2.8 V vs. Pt is required for realizing a high photocurrent level of 30 mA·cm −2 . The deposition of a 5-nm-thick HfO x results in an additional overpotential of 1.1 V at 30 mA·cm −2 owing to the tunneling barrier of HfO x , and the increase in the thickness to ~9 nm further increases the overpotential by 1.2 V (indicated by blue curves in Fig. 2a). Interestingly, on inserting a thin SiO x interlayer between the HfO x and Si (creating a HfO x / SiO x bilayer), the additional overpotential is completely reduced regardless of the HfO x thickness (indicated by the pink curves in Fig. 2a). The same behavior was observed in the dark LSV characteristics for the degenerated doped n-type (n + -Si) wafer, which is only attributed to the charge transfer kinetics ( Supplementary Fig. 3). This result indicates that the insertion of SiO x makes the charge transport independent of the tunneling barrier of the HfO x , which is also verified by the electrochemical impedance spectra ( Supplementary Fig. 4).
The negligible tunneling barrier can be explained by the effective engineering of an energy band after the insertion of SiO x under the application of a voltage for driving the PEC water-splitting reaction. When applying potential to the electrolyte/insulator/semiconductor configuration, the electric field can be built up across the insulator, thereby inducing a potential drop at the insulator layers 47 . When the potential is applied across the HfO x /SiO x bilayer, the potential drop can be expressed as www.nature.com/scientificreports www.nature.com/scientificreports/ where E and t are the electric field and thickness, respectively. E SiOx and E HfOx are related as The electric field is dominantly applied in the SiO x layer rather than in HfO x owing to a dielectric constant of SiO x (3.9) being much lower than that of HfO x (11) 47 . This allows a predominant potential drop across the SiO x layer. The potential drop across the SiO x layer under the application of potential results in a lying conduction band of HfO x that is relatively lower than that of Si, as described in Fig. 2b, which shows the band diagram of the HfO x /SiO x bilayer. As a result, a tunneling barrier of SiO x only affects the transport of the charged carrier in the HfO x /SiO x bilayer, and consequently, the effective tunneling distance becomes equal to the thickness of the SiO x monolayer. In contrast, the potential drop for the HfO x monolayer occurred entirely in the HfO x monolayer, thus creating a triangular barrier as described in Fig. 2c 48 . Because of the triangular barrier, the tunneling resistance is differentiated with the current density. In the case of a low current level (<~10 mA·cm −2 ), the charge carriers are mostly transferred through the upper region of the triangular barrier in order to minimize the tunneling distance and resistance. Consequently, in the low-current level, the overpotential induced by the tunneling resistance is independent of the thickness. However, in the case of a high photocurrent region (>~10 mA·cm −2 ), charge carriers transfer through all the regions of the barrier and suffer from a large tunneling distance and resistance. Therefore, the overpotential in the high-current level is greatly increased with the thickness.
It is also worth noting that the formation of the HfSiO x interfacial layer in the HfO x monolayer induces a hysteresis phenomenon owing to a negative charge trapping/detrapping mechanism at the interfacial layer 49 ; the LSV curve is shifted into the anodic direction during the 10 cycles of the LSV scan (see Supplementary Fig. 5). This suggests that the HfSiO x interfacial layer with a high defect density acts as a recombination site for the charge carrier and also causes an increase in the charge transfer resistance. By contrast, the HfO x /SiO x bilayer prevented the formation of HfSiO x interfacial layer and the hysteresis. To observe the effect of the SiO x interlayer on the PEC performance of the bilayer protected Si photocathodes, the LSV responses were characterized for the samples with www.nature.com/scientificreports www.nature.com/scientificreports/ the wet-chemically-grown SiO x interlayer with oxidation times of 10 min (thin-SiO x ) and 30 min (thick-SiO x ) and with a thermally grown 1.8-nm-thick SiO 2 interlayer (Supplementary Fig. 6). A wet-chemically-grown thin-SiO x shows the lowest overpotential as compared to the thicker one and the thermally grown SiO 2 interlayer.
The engineering of the HfO x barrier height under the application of a voltage gives a further advantage in the PEC-TE hybrid system based on the HfO x /SiO x bilayer protected Si photocathode. As demonstrated in previous works, a hybrid system in which a PEC device is electrically combined in series with a TE device could promote a self-biased solar water-splitting reaction. The Seebeck voltage generated in the TE device owing to the thermal gradient (V TE ) causes the Fermi level of the counter electrodes to be lower than the oxygen evolution level, as depicted in Fig. 3a. This can effectively offset the potential required for driving the self-biased water-splitting reaction while the overall current is determined by the photocurrent of the PEC cell (i.e., the TE is only used as a voltage source) 26 . This was proven by the LSV responses with the driven V TE ; the LSV curve for the bare Si photocathode (with DHF treatments) was shifted in the anodic direction in proportion to the applied V TE (see  www.nature.com/scientificreports www.nature.com/scientificreports/ Supplementary Fig. 7). Interestingly, in the case of the HfO x /SiO x bilayer protected Si photocathode, an anodic shift degree is larger than the applied V TE , and the overpotential is further reduced by 350 mV at a 70 °C temperature gradient (see Fig. 3b). To clearly differentiate the shifts, the anodic shift degrees of the LSV curve (reducing overpotential) at a photocurrent of 30 mA·cm −2 were plotted as a function of ΔT along with the V TE (Fig. 3c). It should be noted that the slope of the linear plot for V TE is the Seebeck coefficient and is represented by the relation V TE = SΔT. It is apparent that the slop for the HfO x /SiO x bilayer protected Si photocathode (41 mV·K −1 ) is much greater than that for V TE (35 mV·K −1 ). For the purpose of comparison, we also characterized the slope for the Si photocathodes grown with monolayers of HfO x and SiO x (see Supplementary Fig. 8 for the LSV curves). The SiO x has a slope (36 mV·K −1 ) that is analogous with that of V TE . In contrast, the slope for the HfO x is much greater than that for V TE but smaller than that for the HfO x /SiO x bilayer. These results are obtained owing to a different tunneling barrier height and potential drop under the applied V TE (see Supplementary Fig. 9 for the band diagrams).
As a result of applying V TE , a saturated photocurrent of 34 mA·cm −2 was obtained at a zero potential (i.e., external bias-free). In the PEC-TE hybrid device, an improvement in the photocurrent level is directly correlated with the solar-to-hydrogen conversion efficiency. Thus, we used antireflective Si nanostructures on Si photocathode, which completely absorbed the incident light 50,51 and greatly improved external quantum efficiency (EQE) for the light-induced charge carrier, maximizing the photocurrent of 42 mA·cm −2 (see Fig. 3d and Supplementary  Figs 10 and 11). While the PEC-TE hybrid system with planar Si provided the maximum H 2 power generation (P Max ) of 42 mW·cm −2 , the antireflective Si nanostructure photocathode achieved a P Max of 55 mW·cm −2 , which is four times greater than that of a photovoltaic-electrolysis combination (P Max of ~12 mW·cm −2 indicated by the grey line in Fig. 3d).
The long-term stabilities of the HfO x /SiO x bilayer protected Si photocathode were performed using chronoamperometry at a photocurrent level of 10 mA cm −2 and using chronopotentiometry at an applied potential of −3 V. Due to the HfO x thin film which has an (electro)chemical stability in the acidic electrolyte and transparency for all wavelength region of the incident light, the Si photocathode exhibited a stable water-splitting reaction for over 200 h at high photocurrent level of 33 mA/cm 2 (Fig. 4a). The stability achieved at high photocurrent by HfO x thin film is the best result compared to other metal oxide thin films (see Supplementary Table 1). TiO x thin films chemically stable in the acidic and alkaline electrolytes have been widely used as a protection layer and have demonstrated long-term stability over 960 hr 52 . However, it has been recently reported ultraviolet (UV) light absorbed in TiO x causes the electrochemical reduction of TiO x 53 . Because of this issue, the TiO x protected Si photocathode operates reliably only when there is no UV irradiation, resulting in low photocurrent. Under the stable operation, the H 2 evolution was characterized by measuring a volume of the H 2 gas collected under a simulated AM 1.5 G 1-Sun illumination while operating under a certain photocurrent level of 32 mA·cm −2 for 50 min (Supplementary Fig. 12). The measured H 2 volumes were corresponded well with the theoretically calculated values at a 100% Faraday efficiency based on the total charge passed. After the PEC operation for 200 h, interestingly, the potential was finally shifted in the anodic direction by 200 mV, which was identified by the www.nature.com/scientificreports www.nature.com/scientificreports/ anodic shift of the LSV curve (see Fig. 4b). This was found to be the result of the crystallization of the as-deposited a-HfO x during the PEC water-splitting reaction as observed in the TEM images and X-ray diffraction (XRD) results ( Supplementary Fig. 13) of the HfO x layer before and after the 200 h stability test (see Fig. 4c,d). Because of a bulk defect state in the a-HfO x that trapped the charge carrier, a substantial resistance was induced for the charge carrier across the a-HfO x , as depicted in Fig. 4e. In contrast, the crystallized HfO x significantly reduced the bulk defect density in the a-HfO x in order to improve the charge transfer kinetics. The crystallization during the solar water splitting reaction at the room temperature condition is an intriguing phenomenon, as it is known to crystallize a-HfO x in the monoclinic phase at temperature of 500 °C 54 . This can be attributed to an electroforming phenomenon wherein the transition metal oxide can be reformed under the electrical field 55 . This has been observed in transition-metal-oxide-based solid-state electronic devices. This finding presents a new method of improving the insulator quality used in PEC devices.

Discussion
We have presented the PEC-TE hybrid device using HfO x /SiO x bilayer protected the Si photocathode in which the HfO x thin film has a strong corrosion-resistance against the acidic electrolyte. The application of the Seebeck voltage generated in the TE device to the PEC device not only drove the self-biased solar water-splitting reaction but also promoted the ease of charge transfer through the HfO x tunneling barrier. On applying the Seeback voltage to the HfO x /SiO x bilayer/Si photocathode, the conduction band of HfO x reached an energy level lower than the conduction band of Si, thereby allowing the electron transfer to become independent of the HfO x tunneling barrier. Because of the kinetic benefit, the voltage required for the self-biased water-splitting reaction of the Si photocathode is lowered by 0.35 V. As a result, the planar-Si-photocathode-based PEC-TE hybrid realized the self-biased water-splitting reaction with the evolved H 2 power of 40 mW cm −2 at a temperature difference of 70 °C. By adopting antireflective Si nanostructures that improve the photocurrent level to 42 mA·cm −2 , the stored H 2 power reached ~52 mW·cm −2 . The HfO x thin-film protection ensured long-term stable PEC operation for over 200 h. During the stability test, we observed the phase transformation of HfO x from the amorphous structure into a crystalline structure, which reduces the bulk defect density and thus the kinetic overpotential for the charge transfer through the HfO x layer.

Method
Preparation of HfO x thin-film protected Si photocathodes. A single-crystalline Si wafer with a resistivity of 1-10 Ω cm and degenerately doped n + -type Si (100) wafer with a resistivity of 0.001-0.002 Ω cm were used to characterize the PEC response and charge transfer kinetics, respectively. Prior to the deposition of HfO x , the Si wafer was dipped into diluted hydrofluoric acid to form an H-terminated surface while removing the native oxide. For growing the SiO x thin film on the surface of the Si wafer, the H-terminated Si wafer is oxidized through treatment with hot water at 90 °C for 10-30 min. ALD of the HfO x thin film was performed for both the H-terminated and SiO x -grown Si wafers to obtain the HfO x monolayer and HfO x /SiO x bilayer coated Si wafers, respectively. An HfO x thin film was deposited at 280 °C in a 4-inch traveling-wave type ALD reactor (CN-1 Co.) using Tetrakis (ethylmethylamino) hafnium (TEMAHf) and H 2 O precursors with a carrier gas of high purity N 2 (99.999%). The TEMAHf, N 2 , and H 2 O were sequentially injected for 2.5, 30, and 1.5 s, respectively.
Characterization of HfO x thin films. A HRTEM (JEOL, JEM-2100F, Japan) equipped with an EDXS operated at an accelerating voltage of 200 kV was used to observe the morphology and elemental composition of the HfO x -thin-film-coated Si wafers. The thicknesses of the HfO x and SiO x thin films were measured using spectroscopic ellipsometry (Sopra GES 5E, fitted to a Tauc-Lorentz function using the Cauchey model). The chemical states of the HfO x and SiO x thin films were investigated using ARXPS equipped with a monochromatic Al Kα (1.486 eV) source.
Setup of PEC-TE hybrid devices. The PEC device consists of the Si photocathode and Pt mesh anode for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. The back of the Si photocathode was treated with DHF soaking and subsequent scribing using an In-Ga eutectic alloy (Sigma-Aldrich) to fabricate back contacts with a Cu electrode. PEC-TE hybrid systems were used by electrically connecting the PEC device with a TE device in series; the Si photocathode is connected to an anode of the TE device, and a cathode of the TE device is connected to the Pt anode. TE devices with a Seebeck coefficient of 35 mV·K −1 and internal resistances of 2.1 Ω, which are commercially available from Kryotherm (Saint-Petersburg, Russia), were employed.
Characterizations of PEC performances. The PEC properties of the Si photocathodes were measured using a potentiostat (Iviumstat, Eindhoven, Netherlands) with a two-electrode configuration (Si photocathode and Pt mesh anode). The PEC responses were characterized using various methods of LSV, chronopotentiometry, and chronoamperometry in 0.5-M sulfuric acid under a 100-mW·cm −2 illumination (Xenon lamp equipped with AM 1.5 G filter) and calibrated by a Si photodiode standard cell (PV Measurements, Inc.). The LSV responses were measured under dark and illuminated conditions at a scan rate of 50 mV·s −1 . The stability and H 2 evolution measurements were performed using chronopotentiometry and chronoamperometry. Home-made heating and cooling systems were fabricated to control the temperature and were used for applying a constant temperature difference between the front and back of the TE device. The temperature difference was monitored using K-type thermocouples (Center 306 Data Logger, New Taipei City Taiwan). The quantity of hydrogen evolved at the Si photocathode under illumination and the Seebeck voltage application was measured using the volume displacement method wherein the volume of a water-filled flask changes with the evolution of hydrogen gas. A Pt counter electrode covered with a quartz cylinder and proton exchange membrane was used in order to separately collect hydrogen and oxygen gas. All the PEC-TE hybrid system is shown in Supplementary Fig. 14 as a digital image. www.nature.com/scientificreports www.nature.com/scientificreports/

Data Availability
The data supporting the findings of this study are available within the article and its Supplementary Information files. All other relevant source data are available from the corresponding author upon reasonable request.