Substantial Lifetime Enhancement of Si-Based Photoanodes Enabled by Amorphous TiO2 Coating with Improved Stoichiometry

Abstract

A post-ALD in-situ water treatment procedure is used to remove the ligand residues in amorphous TiO.sub.2 films coated on photoanode material to improve the film stoichiometry without introducing any additional crystallization. The processed amorphous TiO.sub.2 film showed drastically improved chemical stability, and thereby substantially elongated the lifetime of silicon-based photoanodes in alkaline electrolyte.

Claims

1. A method of removing precursor ligands and byproducts in a photoelectrode, the method comprising: (a) performing an atomic layer deposition of an oxide onto a photoanode material including the steps of: pulsing oxygen precursors onto a photoanode material surface; purging the photoanode material surface with an inert gas; pulsing metal precursors onto the photoanode material surface; and purging the photoanode material surface with the inert gas to deposit a thin film of oxide onto the photoanode material surface to produce a thin film matrix; (b) performing a water treatment of the thin film matrix including the steps of: pulsing the oxygen precursors onto the thin film matrix; and purging the thin film matrix with the inert gas; to reduce a ratio of precursor ligand to oxide from the thin film matrix.

2. The method of claim 1 wherein the water treatment reduces the ratio of precursor ligand to oxide by at least 20%.

3. The method of claim 2 wherein the water treatment reduces the ratio of precursor ligand to oxide by at least 25%.

4. The method of claim 1 wherein the photoanode material is silicon.

5. The method of claim 1 wherein the thin film of oxide is TiO.sub.2.

6. The method of claim 5 wherein the metal precursors are TiCl.sub.4.

7. The method of claim 6 wherein the precursor ligands are Cl ligands.

8. The method of claim 7 wherein the inert gas is N.sub.2 or Ar.

9. The method of claim 1 further comprising sputtering nickel (Ni) onto the thin film matrix.

10. The method of claim 1 where the step of water pulsing has a duration of at least 2 hours.

11. The method of claim 10 wherein the step of water pulsing onto the thin film matrix is pulsing a water pulse that is at least 0.5 second and the water pulse is repeated.

12. The method of claim 1 wherein the step of inert gas purging has a duration of at least 5 hours.

13. The method of claim 12 where the step of inert gas purging onto the thin film matrix is purging with an inert gas at least 0.5 second and the insert gas purging is repeated.

14. The method of claim 1 wherein the temperature of the atomic layer deposition is at less than or equal to 80 degrees Celsius.

15. The method of claim 1 wherein the temperature of the water treatment is at less than or equal to 80 degrees Celsius.

16. The method of claim 1 wherein the thin film of oxide has a thickness that is less than 15 nm.

17. The method of claim 1 wherein the thin film matrix has a thickness that is less than 40 nm.

18. The method of claim 1 wherein the atomic layer deposition and water treatment are performed in a vacuum.

19. A photoelectrode comprising: a thin film of oxide deposited onto a photoanode material via atomic layer deposition using a metal precursor reacted with an oxygen precursor resulting in a precursor ligand; wherein the precursor ligand to oxide ratio is less than 3%.

20. The photoelectrode of claim 19 wherein the photoanode material is silicon and the thin film of oxide is TiO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 is a schematic representation of a Si/TiO.sub.2/Ni photoanode of the present invention for PEC water oxidation having an n-type Si photoabsorber wafer coated with an ultrathin film of amorphous TiO.sub.2 via ALD and further coated with an outer layer of Ni film;

[0040] FIG. 2 is a schematic representation of pinhole formation in Cl-containing amorphous TiO.sub.2 films during PEC water oxidation;

[0041] FIG. 3 is a schematic representation of water treatment of the present invention showing the unreacted Cl ligands in amorphous TiO.sub.2 being removed by reacting with H.sub.2O molecules diffused through the film surface and forming a more continuous, interconnected TiOTi network;

[0042] FIG. 4 are X-ray photoelectron spectroscopy (XPS) core spectra of (a) Cl 2p where shaded areas are integrated Cl 2p.sub.1/2 and 2p.sub.3/2 peaks where dash lines are baselines for peak area integration and (b) Ti 2p shows an intensity comparison at different binding energies between pristine and water treated TiO.sub.2 thin films;

[0043] FIG. 5 are energy dispersive X-ray spectroscopy (EDS) spectra of pristine and water treated TiO.sub.2 collected from (a) the top and (b) the bottom regions from the cross sections (peaks were normalized by the Ti K peaks) and insets are enlarged Cl peaks showing the intensity change after water treatment where dash lines are baselines for peak area integration;

[0044] FIG. 6 are (a) electrochemical impedance spectroscopy (EIS) measurements of photoanodes protected by pristine and water-treated TiO.sub.2 films under 1 sun illumination and inset is the equivalent circuit for curve fitting; (b) Jph-V curves of Si/TiO2/Ni photoanodes protected by pristine and water treated amorphous TiO.sub.2 films; (c) chronoamperometry of the Si/TiO.sub.2/Ni electrode (with water-treatment) measured in 1.0 M KOH aqueous solution under 1 sun illumination at an external bias of 1.8 V vs. RHE. Dotted line marks 90% of the original Jph value; (d) Jph-V curves of Si/TiO2/Ni photoanode (with water treatment) obtained at a series of time points from the chronoamperometry test; and

[0045] FIG. 7 is a flow chart showing the steps of ALD followed by post-ALD water treatment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0046] Referring to FIG. 1, a typical configuration of a Si-based electrode or photoanode 10 is illustrated. The Si-based electrode 10 is composed of a light absorber or photoabsorber 12 of Si photoanode material, coated with an intermediate protective layer 14 of a conformal and pinhole-free inert oxide coating, and further coated with an outer catalyst layer 16 of a metal electrocatalyst coating for efficient oxygen evolution reaction as further described below.

[0047] In one embodiment of the present invention, the photoabsorber 12 is an n-type Si wafer. The outer surface 18 of the photoabsorber 12 experiences rapid corrosion or dissolution in alkaline electrolytes 20, for example, NaOH or KOH aqueous solution or the like, which leads to instability and high overpotential of the Si-based electrode 10. To overcome the corrosion or dissolution of the photoabsorber 12, the photoabsorber 12 is coated with a thin and dense protective layer using high vacuum-based techniques such as atomic layer deposition (ALD).

[0048] The photoabsorber 12 may be coated via ALD with the intermediate protective layer 14 which may be an ultrathin film (e.g., about 24 nm thick or less than 25 nm thick) of amorphous TiO.sub.2. The conformal amorphous TiO.sub.2 protective layer 14 prohibits OH.sup. group 26 diffusion and thus protects the Si wafer outer surface 18 from chemical corrosion. The amorphous TiO.sub.2 protective layer 14 can also passivate Si wafer surface defect states to suppress charge recombination at the SiTiO.sub.2 interface. Meanwhile, the amorphous TiO.sub.2 protective layer 14 still permits adequate hole transport 22 through defect band conduction, allowing for unimpaired PEC efficiency.

[0049] Owing to the excellent conformality of ALD, the amorphous TiO.sub.2 protective layer 14 shows an extremely clear and smooth outer surface 24 without any observable particle features. The high homogeneity of the amorphous TiO.sub.2 protective layer 14 without any observable crystalline phases is a desired coating feature that is expected to bring high stability and long lifetime to the Si-based PEC system when submerged in alkaline electrolyte 20.

[0050] Although protection performance has been shown to be the highest with amorphous TiO.sub.2 and this amorphous TiO.sub.2 protective layer 14 is a preferred embodiment of the present invention, other amorphous ALD oxide films, such as Al.sub.2O.sub.3, HfO.sub.2, and ZrO.sub.2 may be used as the intermediate protective layer 14 in a similar manner to coat and protect the photoabsorber 12.

[0051] Finally, the amorphous TiO.sub.2 protective layer 14 is coated with an outer catalyst layer 16 of nickel (Ni) metal acting as the oxygen evolution reaction (OER) catalyst. The outer catalyst layer 16 is needed because the oxygen evolution kinetics of Si and TiO.sub.2 are inherently slow. The outer catalyst layer 16 for the OER may be an ultrathin film (e.g., about 12 nm thick or less than 15 nm thick) of Ni metal.

[0052] The Si-based electrode 10 as shown may be denoted as Si/TiO.sub.2/Ni. The Si-based electrode 10 may be the light facing photoanode, however, it is understood that the photoanode and the photocathode may be constructed in a similar manner. It is also understood that the photoabsorber 12, intermediate protective layer 14, and outer catalyst layer 16 may be interchanged with other suitable materials. For example, although Si is the preferred photoabsorber 12, it is understood that other single-crystalline semiconductors may be used such as III-V materials, for example, GaAs, InP, GaN and GaP, or II-VI materials, for example, CuS, CuSb, SnS, in a similar manner. Other outer catalyst layers 16 that can be used include platinum (Pt), ruthenium (Ru) and iridium (Ir) oxides, cobalt phosphate (CoPi), Fe, Ni, and Co-based oxides, Fe, Ni, and Fe-based (oxy)hydroxides, and the like.

[0053] To improve the lifetime of Si-based electrodes 10 and the surface protection of the intermediate protective layer 14, i.e., amorphous ALD oxide films, the present inventors have identified the essential structure-property relationships that dictate the protection lifetime control of the intermediate protective layer 14.

[0054] First, structural inhomogeneity in the intermediate protective layer 14 (e.g., imbedded intermediate phases) may induce highly localized current through the intermediate protective layer 14 and facilitate pinhole formation 28 contributing to intermediate protective layer 14 failure. Therefore, a low-temperature, crystalline-free intermediate protective layer 14 is preferable.

[0055] Second, unreacted precursor ligands and byproducts are another inevitable issue associated with the low temperature ALD processes with depositing the intermediate protective layer 14. These impurities may substantially change the properties of the intermediate protective layer 14, such as electronic and ionic transport properties, mechanical stability, and chemical reactivity. Therefore, removing the ligand residues, without introducing any additional crystallization, is preferable.

Example 1: Identifying the Role of Intermediates in the Amorphous Film

Methods

[0056] Atomic layer deposition of TiO.sub.2 on Si substrate: The 380 m-thick, 3 inch-diameter, single-side polished, <100> oriented, phosphorus doped, n-type Si wafers with resistivity of 1-10 .Math.cm are used for all TiO.sub.2 film deposition and photoelectrochemical (PEC) devices. Wafers are cleaned sequentially in the ultrasonic bath of acetone, isopropanol and deionized (DI) water for 20 min. Prior to TiO.sub.2 depositions, Si wafers are immersed in 5 wt % HF to remove the native oxide. TiO.sub.2 coating is conducted in a homemade atomic layer deposition (ALD) system.

[0057] Specifically, the target substrate is loaded on a quartz tube and placed at a position 5 cm away from the precursor inlet nozzle. N.sub.2 gas with a flow rate of 40 sccm is introduced into the chamber to serve as the carrier gas. The system's base pressure is kept at 3.8 Torr. The chamber temperature is maintained at 160 C. The deposition temperature is adjusted to 120 C. TiCl.sub.4 (Sigma-Aldrich, 99.9%)(i.e., metal precursor) and DI H.sub.2O vapors (i.e., oxygen precursor; can be H.sub.2O, H.sub.2O.sub.2 or O.sub.3) are pulsed into the deposition chamber separately with a pulsing time of 0.5 s and separated by 60 s N.sub.2 purging. Therefore, referring briefly to FIG. 7, one deposition cycle involves 0.5 s of H.sub.2O pulse+60 s of N.sub.2 purging+0.5 s of TiCl.sub.4 pulse+60 s of N.sub.2 purging with a TiCl.sub.4 pressure change of 120 millitorr (chamber pressure difference before and after ALD valve open), as seen in steps 50, 52, 54, and 56, respectively. The chamber is cooled down naturally under N.sub.2 flow after growth. The 24 nm and 2.5 nm-thick TiO.sub.2 coatings are received after 400 and 40 ALD cycles, respectively, corresponding to a growth rate of 0.06 nm per cycle.

[0058] TiCl.sub.4 is chosen as the metal precursor, rather than organometallic Ti precursors, since TiCl.sub.4 can avoid the involvement of big organic molecules, which may dim the structure and property comparison between intermediates and amorphous/crystalline counterparts. Being free of organic byproducts may improve the accuracy of the observation and analysis of the metastable intermediates. Moreover, metal halide precursors may promote the nucleation of ALD TiO.sub.2 and thereafter enrich the pool of intermediates.

[0059] Sputtering of Ni on TiO2 coated Si: The 12 nm Ni films are deposited on TiO.sub.2 coated Si through sputtering using a CVC 601 DC sputtering system. During the deposition the substrate is rotated at a speed of 5 rpm. The flow rate of argon gas is 25 sccm. The deposition time, pressure, power, voltage, current are: 120 s, 10 militorr, 200 W, 300 V, 0.65 A, respectively.

[0060] PEC Electrode fabrication: The back sides of Si wafer are first scratched with a diamond scribe (to remove the native oxides), and then are coated with Ga/In eutectic mixture and connected to a metal lead, forming an Ohmic back contact. Silver paint is then used to affix the lead. After drying in the fume hood, the entire back side and partial front side of the Si electrodes are encapsulated in Epoxy (Loctite, 9460), establishing an exposed active area of 0.1 cm.sup.2. Calibrated digital images and ImageJ are used to determine the geometrical area of the exposed electrode surface defined by epoxy.

[0061] Electrochemical measurements: The PEC tests are carried out in a typical three-electrode electrochemical setup with Si/TiO2/Ni as the working electrode, a Pt wire as the counter electrode and a Hg/HgO electrode as the reference electrode. The electrolyte is 1 M NaOH aqueous solution. During the cyclic voltammetry (CV) and chronoamperometry scanning, working electrodes are illuminated by a 150 W Xenon lamp coupled with an AM 1.5 global filter with a light intensity of 100 mW.Math.cm.sup.2 (one sun). Chronoamperometry curves are measured at a constant external bias of 1.8 V vs. RHE. Hg/HgO is converted to RHE using the following relationship: E(RHE)=E(Hg/HgO)+0.098 V+0.059pH. All electrochemical curves are recorded using an Autolab PGSTAT302N station.

[0062] SEM characterizations: Scanning electron microscopy (SEM) images are acquired on a Zeiss LEO 1530 field-emission microscope with a gun voltage of 5 kV and a working distance of 3 mm. SEM energy dispersive X-ray spectroscopy (EDS) is performed at a voltage of 10 kV and a working distance of 8 mm. The cross-sectional SEM images are obtained by physically breaking the PEC device and then choosing the piece of wafer with sharp fresh edge. To improve picture quality, the front side of the sample is electrically connected to the SEM stage through a conductive carbon tape.

[0063] AFM measurements: Atomic Force Microscopy (AFM) characterizations are conducted using a XE-70 Park System. For the c-AFM, the 24 and 2.5 nm-thick TiO.sub.2 films are grown on 380 m-thick, boron heavily doped, single-side polished, <100> oriented, p type wafer with a resistivity of 0.001-0.005 .Math.cm. The Si wafer are then electrically glued onto a steel disc using Ga/In eutectic and silver paste, creating an Ohmic contact between Si and steel disc, similar to the PEC electrode fabrication case. The AFM is operated in contact mode with platinum cantilevers and a complete circuit is formed by AFM tip-TiO.sub.2/Si-AFM stage. The current mappings are recorded under a constant bias of 3 V while the tomography images are probed simultaneously. The individual current-voltage curves are collected by swiping the bias between 3 V to 3 V.

[0064] STEM sample preparation: Cross sectional scanning transmission electron microscopy (STEM) samples are prepared by in-situ lift out using a Zeiss Auriga focused ion beam (FIB). The final FIB milling voltage is reduced to 2 kV to minimize damage from implanted Ga. The final milling is performed on a Fischione 1050 Nano mill equipment with accelerating voltage of 0.5 kV and an incident angle of 10.

[0065] STEM and EELS observations: STEM and electron energy loss microscopy (EELS) experiments are performed on a FEI Titan microscope with a CEOS probe aberration-corrector operated at 200 keV. The probe semi-angle is 24.5 mrad and the probe current is 25 pA. The estimated probe size is less than 1 . Annular bright field (ABF) STEM image is collected by Gatan 805 BF/DF detector spanning 5.7 to 12.6 mrad. EEL spectrum image is recorded with GIF 865 spectrometer, with energy dispersion of 0.2 eV/pixel, which allowed the simultaneous visualization of the Ti-L and OK EELS edges. The energy resolution is 0.85 eV measured from the full width at half maximum of zero-loss peak. Quantifications are calculated using the Digital Micrograph implementation of the standard quantification method.

Results

[0066] The structural inhomogeneity in the amorphous TiO.sub.2 thin film (e.g., imbedded intermediate phases) induces highly localized current through the amorphous film and facilitates pinhole formation. When the amorphous TiO.sub.2 thin film is employed for PEC electrode protection, the intermediates raise local concentration of hydroxyls and promote the formation of pinholes through the amorphous matrix.

[0067] One effective kinetic principle to suppress the formation of intermediates is to confine the material to an extremely small volume, which may largely slow down the nucleation rate of a new phase as a result of reduced bulk free energy and limited space for atom rearrangement. This principle is thus implemented in the amorphous TiO.sub.2 thin films to restrict the formation of intermediates by reducing the film thickness to 2.5 nm, while other ALD growth conditions remained identical. For example, reducing the film thickness to 2.5 nm yielded an amorphous thin film with excellent homogeneity. The 2.5 nm protection film accomplished over 500 h electrode longevity at a photocurrent density of 30 mA.Math.cm.sup.2, largely exceeding the stability obtained from the 24 nm TiO.sub.2 protection.

[0068] In addition to film thickness, the deposition temperature is also decreased to 120 C., while other ALD growth conditions remained identical. By decreasing the deposition temperature, the PEC stability increased and had much fewer crystalline particles compared to the control. The presence of highly conductive intermediates also decreased.

[0069] These film thickness and temperature influences further confirmed that kinetics routes of suppressing the formation of intermediates are directly correlated to the protection performance. In addition to the control of film thickness and growth temperature, manipulating other deposition parameters (e.g., Ti precursors, pulse and purge times, and doping) may potentially further improve the protection performance by impeding the formation of intermediates.

[0070] Therefore, a low-temperature crystalline-free amorphous film is preferable for achieving a longer lifetime. This is accomplished by decreasing the film thickness and decreasing the deposition temperature.

Example 2: Identifying the Role of Unreacted Precursor Ligands and Byproducts in the Amorphous Film

Methods

[0071] ALD synthesis of TiO.sub.2 thin films: The n-type Si wafers in the experiments are 380 m-thick, 3 inch-diameter, single-side polished, <100> oriented, and have a resistivity of 1-10 .Math.cm. Prior to ALD, Si wafers are washed by acetone, isopropanol and deionized (DI) water in an ultrasonic bath for 20 min sequentially, followed by immersing in 5 wt % HF solution to remove the native oxide.

[0072] TiO.sub.2 is deposited in a homemade ALD system. Specifically, N.sub.2 gas with a flow rate of 40 sccm is introduced into the chamber to serve as the carrier gas. The system base pressure is kept at 780 mTorr. The chamber temperature is maintained at 100 C. for depositions. Precursors used for TiO.sub.2 deposition are TiCl.sub.4 (Sigma-Aldrich, 99.9%) (i.e., metal precursor) and DI H.sub.2O (i.e., oxygen precursor; can be H.sub.2O, H.sub.2O.sub.2 or O.sub.3). TiCl.sub.4 (Sigma-Aldrich, 99.9%). Both precursor vapors are pulsed into the deposition chamber separately with a pulsing time of 0.5 s each and separated by 60 s N.sub.2 purging. Therefore, referring briefly to FIG. 7, one deposition cycle involves 0.5 s of H.sub.2O pulse+60 s of N.sub.2 purging+0.5 s of TiCl.sub.4 pulse+60 s of N.sub.2 purging, as seen in steps 50, 52, 54, and 56, respectively. Through this procedure, 15 nm TiO.sub.2 film is obtained after 200 cycles. For TDMAT-TiO.sub.2 film, the film is deposited under recommended temperature of 250 C. in Fiji G2 ALD with TDMAT precursor (Sigma-Aldrich, 99.99%) with 300 cycles for comparison.

[0073] PEC electrode preparation: Ni films are deposited on TiO.sub.2-coated Si by sputtering using a CVC 601 DC sputtering system. The substrate is rotated at a speed of 5 rpm with argon flow at 25 sccm. The deposition is performed under 10 mTorr with 120 s deposition time. Then, the back side of Si wafer is scratched by a diamond scribe and covered by Ga/In eutectic mixture. Silver paste is applied to fix the metal lead to the Ga/In eutectic mixture to achieve good Ohmic contact. After drying in a fume hood, the entire back side and partial front side of the Si/TiO.sub.2/Ni electrodes are encapsulated by Epoxy (Loctite, 9460) with an exposed active area of 0.05 cm.sup.2. ImageJ is used to determine exposed electrode area.

[0074] Electrochemical characterizations: The PEC tests are carried out in a typical three-electrode electrochemical setup with Si/TiO.sub.2/Ni as working electrode, a Pt wire as counter electrode, and a Hg/HgO electrode as reference electrode. The electrolyte is 1 M KOH aqueous solution. For the cyclic voltammetry (CV) and chronoamperometry measurement, working electrodes are illuminated by a 150 W Xenon lamp coupled with an AM 1.5 global filter with a light intensity of 100 mW.Math.cm2 (one sun). Chronoamperometry curves are measured at a constant bias of 1.8 V vs. RHE. Electrochemical impedance spectroscopy (EIS) is conducted under open circuit voltage from 100 kHz to 0.1 Hz. All electrochemical curves are recorded using an Autolab PGSTAT101 station.

[0075] Materials characterizations: Scanning electron microscopy (SEM) images are acquired on a Zeiss LEO 1530 field-emission microscope with a gun voltage of 5 kV and a working distance of 3.5 mm. X-ray photoelectron spectroscopy is acquired by Thermo Scientific K-alpha XPS instrument. Atomic Force Microscopy (AFM) topography is obtained using an XE-70 Park System. Device corrosion area percentages are statistically analyzed by ImageJ. Four dimensional scanning transmission electron microscopy (4D-STEM) is performed using Thermofisher Scientific Themis Z STEM operated at 300 kV and equipped with an Electron Microscopy Pixel Array Detector (EMPAD) to acquire nano-diffraction patterns from different sampling areas within the films. Intensity variance of acquired nano-diffractions are calculated. EDS is performed using FEI Themis Z microscope at 300 kV equipped with 4 Super-X detectors, and the chemical composition of amorphous films is obtained by analyzing EDS spectra using FEI Vlox software and K energies for Ti, Cl, and O. The presence of crystalline phases within the amorphous matrix is investigated by observation of the film using low angle annular dark field (LAADF) STEM imaging, including diffraction contrast.

Results

[0076] Chronoamperometry test at a bias of 1.8 V versus reversible hydrogen electrode (RHE) revealed a quick photocurrent density (J.sub.ph) decay, where the original value dropped by 10% within just 30 hours. At the point of failure (defined as the time point when J.sub.ph reached <90% of its original value), a large number of interconnected pores appeared on the electrode surface, suggesting low stability of this TiO.sub.2 coating. These large pores evolved from small pinholes as early as a few hours of operation, while J.sub.ph is still >95% of its original value. The large pores would isolate the Si photoabsorber from Ni catalyst layer and facilitate the formation of insulating SiO.sub.x that limits the hole transport from Si to Ni catalyst. As a result, the Si/TiO.sub.2/Ni photoanode PEC performance is impaired.

[0077] The J.sub.ph vs. potential (V) curves are recorded at a few time points through the chronoamperometry test. In the first hour, the water oxidation onset potential is 1.08 V versus RHE and J.sub.ph reached a saturated value of 31.5 mA/cm.sup.2 at 1.8 V versus RHE. The saturated J.sub.ph and onset potential are on par with reported benchmark n-Si-based photoanodes, indicating the high quality of the Si/TiO.sub.2/Ni system. This onset potential is kept steady for the first 15 hours, but quickly shifted positively to 1.11 V and 1.24 V versus RHE at the 30-hour and 35-hour operation time points, respectively. The higher onset potential implied the increase of charge transfer resistance in the PEC system, which is typically induced by the formation of insulating SiO.sub.2 layer due to Si corrosion. Accordingly, saturated J.sub.ph dropped continuously to 30.2, 29.8, 28.8, and 21.9 mA/cm.sup.2 at the PEC operation time of 5, 15, 30 h, and 35 h respectively, consistent with the decay trend in the stability test. During PEC operation, because no extra redox peaks other than the Ni(OH).sub.2/NiOOH couple are observed from the J.sub.ph-V curves, the corrosion of TiO.sub.2 layer is a result of chemical dissolution without valence change.

[0078] The TiO.sub.2-coated Si wafer is immersed in a 1M KOH aqueous solution to evaluate the chemical stability of the amorphous TiO.sub.2 film. Without an external bias, the TiO.sub.2-coated Si wafer still exhibited obvious corrosion but with square-like pores, a typical morphology of chemically corroded Si wafer in alkaline solution. This type of corrosion is also started from pinholes as a result of TiO.sub.2 dissolution confirmed by XPS analyses, suggesting the chemical reactivity of as-deposited TiO.sub.2 would primarily be responsible for the failure.

[0079] Referring to FIG. 2, the existence of Cl elements may be attributed to the unreacted Cl ligands 30 from the TiCl.sub.4 precursor at a relatively low deposition temperature (100 C.). Considering the presence of an appreciable amount of precursor ligand residues (Cl from TiCl.sub.4 precursor) in the film, the unreacted Cl ligands 30 act as the terminating point in the TiOTi network 32, and thus introduce a more permeable amorphous lattice allowing fast reaction 34 between OH.sup. groups 26 and TiO.

[0080] In addition, DFT calculation revealed that residual Cl ligands may also induce in-gap defect states at the middle of the band gap and shifted Fermi level from 2.71 eV to 4.14 eV closer to conduction band, which can enhance the hole conductivity of the amorphous TiO.sub.2 film. This localized conductivity increase can induce local OH.sup. accumulation at the electrode surface, resulting in pinhole formation 28, which facilitates the reaction between OH.sup. and TiO.sub.2. Together, in the PEC system, Cl residues may enable a faster dissolution of TiO.sub.2 and a rapid diffusion of OH.sup. to reach the vulnerable Si surface.

[0081] Due to the destructive residual Cl ligands, a longer protection lifetime may be achieved by reducing the residual Cl ligands in amorphous TiO.sub.2 film. Furthermore, no crystallization should be induced, as crystalline phases can introduce structural inhomogeneity and jeopardize the film's lifetime.

Post-ALD In-Situ Water Treatment

[0082] Referring to FIG. 3, in amorphous TiO.sub.2 films deposited at low temperature, residual ligands are inevitable. Although the present inventors do not wish to be bound by any particular theory, it is believed that their presence may significantly increase the film reactivity in alkaline electrolytes, resulting in pinhole formation and quick dissolution. Removing the residual Cl ligands by completing the reaction with extra water exposure leads to closer-to-ideal stoichiometry of TiO.sub.2 film with improved TiOTi network continuousness.

[0083] Referring also to FIG. 7, a post-ALD water treatment procedure has been developed to reduce the residual Cl ligands and maintain amorphous film homogeneity simultaneously. After the regular TiO.sub.2 ALD cycles are completed, the amorphous TiO.sub.2 matrix 40 sample is kept in the growth chamber and subjected to alternating water pulses 42 and inert gas purges 43 under the same low temperature, for example, at less than or equal to 100 degrees Celsius and at less than or equal to 80 degrees Celsius, and in a vacuum chamber. Therefore, one post-ALD water treatment cycle may involve 0.5 s of H.sub.2O pulse+10 s of N.sub.2 purging, as seen in steps 42 and 43, respectively. Water treatment steps may be completed after 1 cycle to thousands of cycles depending on the length of each water pulse and length of each inert gas purge.

[0084] Each water pulse may be at least 0.5 seconds and at least 1 second and at least 1.5 seconds. The total time for the combined water pulses during the total water treatment may be at least 2 hours and at least 3 hours and 4 hours and at least 5 hours. The water pulse may be pulses of oxygen precursors such as H.sub.2O, H.sub.2O.sub.2 or O.sub.3. Although referred to herein as water pulses and water treatment it is understood that this may refer to pulsing an oxygen precursor including H.sub.2O.sub.2 or O.sub.3 and not just water.

[0085] Each inert gas purge may be at least 10 seconds and at least 15 seconds and at least 20 seconds. The total time for the combined inert gas purges during the total water treatment may be at least 5 hours and at least 6 hours and at least 7 hours and at least 8 hours. The insert gas may be N.sub.2 or Ar.

[0086] Therefore, post-ALD water treatment may involve repeated cycles of H.sub.2O pulse+inert gas purging+H.sub.2O pulse+inert gas purging . . . etc. The total water treatment duration (total time for water pulses plus total time for gas purging) may be at least 8 hours and at least 9 hours and at least 10 hours.

[0087] During this treatment, water molecules 44 diffuse into the TiOTi network 32 of the amorphous TiO.sub.2 matrix 40 and reacts with the dangling unreacted Cl ligands 30, which raises the film's stoichiometry and improves the TiOTi network 32 continuity.

[0088] The resulting amorphous TiO.sub.2 matrix 48 following the water treatment is less permeable thus limiting the fast reaction between OH.sup. and TiO. Desirably, the water treatment reduces the amount of unreacted Cl ligands and thus the Cl to Ti (Cl:Ti) ratio is reduced to below 3% and below 2.5% and the Cl to Ti (Cl:Ti) ratio is reduced by at least 1% and by at least 1.5% by the water treatment.

[0089] The amorphous film structure is well retained during this extended ALD process, possibly due to the limited mobility of TiO polyhedrons inside the amorphous matrix. When applied as a Si photoanode protection layer, this homogeneous amorphous TiO.sub.2 film exhibits an ultra-stable protection performance in alkaline solution, maintaining a very high saturated J.sub.ph at 30 mA/cm.sup.2 for 600 hours.

[0090] This discovery provides a promising solution to decouple the crystallization from raising the ALD reaction completeness. The amorphous ALD film with controlled stoichiometry may enable an essential manufacturing capability leading the PEC photoelectrodes to meet the industrial standard.

[0091] Therefore, post-ALD water treatment, as further described in the examples below, may at least partially remove the unreacted Cl ligands in ALD amorphous TiO.sub.2 films without introducing additional crystallization, and thereby largely improved the film's lifetime for Si-photoelectrode protection.

Example 3: Reducing Unreacted Precursor Ligands and Byproducts in the Amorphous Film Via Post-ALD Water Treatment

Methods

[0092] Post-ALD water treatment: Immediately after the normal ALD procedure being completed, an additional 2400 water pulses are introduced to the ALD chamber. Each water pulse is separated by 10 s N.sub.2 purging. The other chamber conditions remained the same. After this water treatment procedure is completed, the chamber is cooled down under N.sub.2 flow naturally before the sample is removed.

[0093] Free-standing TiO.sub.2 film preparation: To avoid the strong background signal from Si wafer in STEM nano diffraction, free-standing TiO.sub.2 films are prepared by ALD on a sacrificial PVP (Polyvinylpyrrolidone) layer. 2% PVP aqueous solution is prepared and spin-coated on Si wafer at 3000 rpm for 30 s. PVP-coated Si wafer is used in the ALD synthesis of amorphous TiO.sub.2 films under 100 C. for 200 cycles deposition. Each cycle includes 0.5 s of H.sub.2O pulse+60 s of N.sub.2 purging+0.5 s of TiCl.sub.4 pulse+60 s of N.sub.2 purging.

[0094] Accordingly, the additional 2400 cycles of water treatment are conducted after normal growth. After synthesis, samples are immersed in water at room temperature for 2 hours to release the TiO.sub.2 film. The free-standing TiO.sub.2 films are scooped by TEM grids for STEM characterizations.

[0095] Materials characterizations: Scanning electron microscopy (SEM) images are acquired on a Zeiss LEO 1530 field-emission microscope with a gun voltage of 5 kV and a working distance of 3.5 mm. X-ray photoelectron spectroscopy is acquired by Thermo Scientific K-alpha XPS instrument. Atomic Force Microscopy (AFM) topography is obtained using an XE-70 Park System. Device corrosion area percentages are statistically analyzed by ImageJ. Four dimensional scanning transmission electron microscopy (4D-STEM) is performed using Thermofisher Scientific Themis Z STEM operated at 300 kV and equipped with an Electron Microscopy Pixel Array Detector (EMPAD) to acquire nano-diffraction patterns from different sampling areas within the films. Intensity variance of acquired nano-diffractions are calculated. EDS is performed using FEI Themis Z microscope at 300 kV equipped with 4 Super-X detectors, and the chemical composition of amorphous films is obtained by analyzing EDS spectra using FEI Vlox software and K energies for Ti, Cl, and O. The presence of crystalline phases within the amorphous matrix is investigated by observation of film using low angle annular dark field (LAADF) STEM imaging, including diffraction contrast.

[0096] Computational Method: The amorphous TiO.sub.2 structure is obtained through a melt-quenched process using the MA potential. The structure is firstly melted in a NVT ensemble at 5000 K for 50 ps with a timestep of 0.5 fs, and cooled down to 3000 K with a timestep of 0.5 fs and 200 ps simulation time. Next, the model is equilibrated at 3000 K for another 50 ps. Finally, the model is annealed from 3000 K down to 300 K at a cooling step of 1 K/ps, equilibrated at 300 K for 100 ps, and statically optimized to minimal energy to obtain the final quenched atomic structure. The a-TiO.sub.2 model consists of 87 atoms within a cubic box. The density of the structure model is consistent with the typical experimental density value of amorphous TiO.sub.2 at room temperature (3.84 g/cm.sup.3). The electronic property of amorphous TiO.sub.2 with and without Cl ligands is then calculated using density functional theory (DFT) implemented in the Vienna ab initio simulation (VASP) package. The generalized gradient approximation exchange correlation functional Perdew, Burke, and Ernzerhof (PBE) with the Hubbard U correction is applied for the structure optimization and the density of states calculation. The U value of 4.2 eV is applied on Ti atoms. The projector augmented wave method (PAW) is used for the effective potential for all atoms. The PAW potentials used in these calculations have valence electron configurations of 3p.sup.64s.sup.23d.sup.2 for Ti, 2s.sup.22p.sup.4 for O, and 3s.sup.23p.sup.5 for Cl. The plane wave cutoff energy of 520 eV is used in all the calculations. The convergence criteria are 10.sup.6 eV/cell for electronic self-consistent and 0.05 eV/ for ionic relaxation, respectively. Tetrahedron method with Blchl corrections is applied for the density of states calculation.

Results

[0097] The post-ALD water treatment preserves the surface flatness and conformality of the as-deposited TiO.sub.2 film. No additional nanoparticles are observed at the film surface. AFM topography scan revealed that the TiO.sub.2 surface kept the same extremely low roughness of 0.3 nm.

[0098] Referring to FIG. 4(a), XPS Cl 2p spectra showed an apparent intensity drop of both Cl 2p.sub.3/2 (198.16 eV) and Cl 2p.sub.1/2 (199.82 eV) peaks (corresponding to the ClTi bonding) after water treatment. Referring to FIG. 4(b), the fine Ti 2p spectra of both pristine and water-treated films showed an almost identical shape with the dominating Ti.sup.4+ chemical state located at 459 eV, implying the water treatment did not change the chemical state of Ti.sup.4+ cation in the network.

[0099] By integrating the Cl, O, and Ti peaks areas as a half-quantitative analysis, the Cl:Ti ratio is found reduced by 27% (from 0.062 to 0.045), confirming a substantial removal of residual Cl ligands from the amorphous TiO.sub.2 film.

[0100] Four-dimensional scanning electron microscopy (4D-STEM) based characterizations are further used to confirm the elemental and structural change from water treatment. The presence of two burry rings in the averaged nano diffraction patterns revealed that no additional crystallization is induced by the water treatment as compared to the pristine TiO.sub.2 film.

[0101] Diffraction also suggested the existence of the medium-range ordering (MRO) in both TiO.sub.2 amorphous films. MRO refers to the nanoscale volumes with relatively high structural ordering within amorphous materials. Here, the variance (V) of the nano diffraction intensity as a function of reciprocal lattice vector k, V(k) is used to measure the degree of MRO. The magnitude of the peak is related to the degree of structural fluctuation created by the distribution of MRO domains, and the peak position is related to the type of MRO. The broad peak at 3.0 nm.sup.1 indicates the MRO is substantially disordered, confirming the amorphous phase. After water treatment, the broad peak position is retained without shifting, demonstrating that the disordered MRO structure is well maintained. The slightly increased peak intensity may be related to Cl ligand removal.

[0102] Referring to FIG. 5, the change of Cl distribution is directly visualized using long-time energy dispersive spectroscopy (EDS) Cl mapping acquired from the cross-section of both TiO.sub.2 film samples. Compared to the uniform distribution of Cl signal in the pristine TiO.sub.2 film, the Cl signal in the water treated TiO.sub.2 film is clearly decreased with a concentration gradient, where the Cl signal is nearly undetectable from the top 5 nm region. EDS spectrums are then collected to quantify the location-dependent chemical composition change (all peak intensities are normalized by the Ti K peaks for comparison).

[0103] Comparing the water treated TiO.sub.2 film to the pristine TiO.sub.2 film, the Cl:Ti ratio in the bottom region is reduced from 3.86% to 2.41% and in the top region is reduced from 2.46% to 1.81%. These characterizations further confirmed that post-ALD water treatment is able to partially remove residual Cl ligands without introducing additional crystallization to the amorphous ALD TiO.sub.2 films.

[0104] The chemical stability of water treated TiO.sub.2 films is then evaluated by immersing the treated Si/TiO.sub.2 sample in a 1M KOH aqueous solution without applying any external bias. Compared to pristine TiO.sub.2-coated Si, the density of square-like corrosion spots is substantially reduced from 300 mm.sup.2 to 30 mm.sup.2, with smaller spot sizes. XPS elemental analysis revealed nearly identical Cl 2p, Ti 2p, and O 1s peak intensities after immersion, indicating that the TiO.sub.2 film after water treatment may be largely preserved when facing alkaline solution.

[0105] Electrochemical impedance spectroscopy (EIS) of Si/TiO.sub.2 electrodes is applied to study the possible TiO.sub.2 film conductivity change induced by Cl removal. The Nyquist plots are fitted based on the equivalent circuit model. The Si/TiO.sub.2 electrodes with and without water treatment showed similar charge transfer resistance at the high-frequency region, suggesting partially removing Cl did not impair the charge transport property of the TiO.sub.2 film.

[0106] The above characterizations confirmed that post-ALD water treatment is able to largely improve the chemical stability and preserve the good electrical conductivity.

[0107] Referring to FIG. 6, to further evaluate its influences to the PEC performance, the same amount of Ni catalysts is deposited on Si/TiO.sub.2 for water oxidation reaction under the same conditions. The Ja-V curves of Si/TiO.sub.2/Ni photoanodes with and without water treatment exhibited similar onset potential and saturated J.sub.ph. The slightly increased slope of the J.sub.ph-V curve from the water-treated TiO.sub.2 sample suggests there might be less amount of charge recombination due to the improved charge transport property. With the improved PEC performance, the chronoamperometry test is conducted at an external bias of 1.8 V versus RHE. A stable J.sub.ph at 30 mA/cm.sup.2 is recorded for up to 600 hours, about one order of magnitude longer than the pristine TiO.sub.2-protected Si photoanodes. J.sub.ph-V curves are also recorded at a series of reaction time points to understand the PEC property change during this long operation period. The J.sub.ph-V curve maintained an identical shape for the first >100 hours, where the electrode surface is nearly intact showing an extremely high stability. The slope started to show a subtle decrease till 250 hours. By that time, only a few small pinholes evolved on the electrode surface. As the reaction time extended, the Ni(OH).sub.2/Ni(OOH) redox peaks and the saturated potential gradually shifted anodically, and the J.sub.ph-V curve slope slightly decreased. Both can be attributed to the increase of charge transfer resistance due to the formation of SiO.sub.x on the Si surface. The saturated J.sub.ph almost maintained at the same 30 mA/cm.sup.2 throughout the entire testing period, evidencing the long term high water oxidation performance.

[0108] Owing to the much higher chemical stability of the water treated TiO.sub.2 films, the evolution from several pinholes to large, interconnected pores is substantially suppressed. Therefore, Si photoabsorber and the TiO.sub.2/Ni catalyst layer may still maintain a tight connection and allow nearly impaired charge flow before the formation of large pores. When the photoanode approached its failure point at 600 hours, the J.sub.ph-V curve exhibited a drastic anodic shift and the saturated photocurrent density decreased apparently. At this point, large and interconnected pores would be observed on the electrode surface, indicating the water treated TiO.sub.2 film may still share the same failure mechanism as the pristine TiO.sub.2 films.

[0109] The post-ALD water treatment is fundamentally different from other regular approaches to improve the stoichiometry of ALD films. Reducing the residual ligands (i.e., raising the ALD reaction completeness to improve the stoichiometry) and suppressing the crystallization are coupled and anticorrelated in regular ALD processes. For example, extending the length of the ALD water pulse may improve the reaction completeness per cycle. After elongating water pulse time from 0.5 s to 6.5 s, Cl 2p core spectrum showed apparent peak intensity drop, validating the improved ALD reaction completeness.

[0110] However, extending the reaction time may also facilitate surface diffusion of as-deposited species that are loosely bonded to the surface. This may enable the rearrangement of surface atoms and form local nuclei on the surface, which may serve as seeds for TiO.sub.2 to grow into nanoparticles. For example, as-prepared TiO.sub.2 film exhibited a large number of nanoparticles on the surface and the corresponding structure heterogeneity yields an impaired protection performance.

[0111] Similarly, due to the lower energy barrier for nucleating, particularly the intermediate phases of TiO.sub.2 at the active growth surfaces, other regular approaches to achieving complete ALD reactions, such as raising the temperature or introducing plasma, are associated with undesirable crystallization, which also jeopardizes the protection lifetime as earlier research discovered. For example, TiO.sub.2 film deposited under 160 C. yielded a large number of crystalline nanoparticles among the amorphous matrix. The corresponding TiO.sub.2-protected Si photoanode only showed a 70 h lifetime for PEC operation.

[0112] The critical role of water vapor exposure in the post-ALD treatment is further demonstrated by annealing the pristine TiO.sub.2 films in an N.sub.2 atmosphere or under TiCl.sub.4 exposure instead. Annealing TiO.sub.2 films in N.sub.2 atmosphere showed almost the same peak intensity in XPS Cl 2p, Ti 2p core spectrum as compared to pristine TiO.sub.2 film, suggesting the inert gas atmosphere will not change the chemical composition of the ALD film. Thus, the PEC performance of N.sub.2-annealed TiO.sub.2 film had a similar lifetime performance as pristine TiO.sub.2 (10% decay within 29 hours).

[0113] Annealing Si/TiO.sub.2 in TiCl.sub.4 atmosphere brought extra Cl impurities to the film and exhibited acute chemical instability. A large amount of corrosion spots emerged after 1 day of immersion. The drastic drop of peak intensity in XPS Cl 2p, Ti 2p, and O 1s spectra demonstrated the quick dissolution of TiO.sub.2 from the film. Correspondingly, TiCl.sub.4-treated TiO.sub.2 film showed a very short PEC protection lifetime with 10% photocurrent decay in only 1 hour. These control experiments also proved the critical role of Cl impurities in determining the chemical stability and protection lifetime of ALD TiO.sub.2 films.

[0114] These results are described in the inventors' publication, Dong, Yutao, et al. Substantial lifetime enhancement for Si-based photoanodes enabled by amorphous TiO.sub.2 coating with improved stoichiometry. Nature Communications 14.1 (2023): 1865, and is hereby incorporated by reference.

[0115] Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as upper, lower, above, and below refer to directions in the drawings to which reference is made. Terms such as front, back, rear, bottom and side, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms first, second and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context.

[0116] When introducing elements or features of the present disclosure and the exemplary embodiments, the articles a, an, the and said are intended to mean that there are one or more of such elements or features. The terms comprising, including and having are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[0117] It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.

[0118] To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. 112(f) unless the words means for or step for are explicitly used in the particular claim.