Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable silicon-based photocathode

The trade-offs between photoelectrode efficiency and stability significantly hinder the practical application of silicon-based photoelectrochemical devices. Here, we report a facile approach to decouple the trade-offs of silicon-based photocathodes by employing crystalline TiO2 with graded oxygen defects as protection layer. The crystalline protection layer provides high-density structure and enhances stability, and at the same time oxygen defects allow the carrier transport with low resistance as required for high efficiency. The silicon-based photocathode with black TiO2 shows a limiting current density of ~35.3 mA cm−2 and durability of over 100 h at 10 mA cm−2 in 1.0 M NaOH electrolyte, while none of photoelectrochemical behavior is observed in crystalline TiO2 protection layer. These findings have significant suggestions for further development of silicon-based, III–V compounds and other photoelectrodes and offer the possibility for achieving highly efficient and durable photoelectrochemical devices.

The main achievement of the manuscript is a promising stability/durability of the protection layer, however, the characterization, analysis and evaluation is to some extend made more difficult and obscured by the choice of black silicon (b-Si) as photo-absorber due the surface roughness. The case for the protection layer could probably have been more clearly made with a planar silicon photoabsorber, where corrosion or stability could be convincingly demonstrated for a Pd/b2-TiO2/Si sample. Then b-Si samples could have been added if they contributed to the case. In its present form the manuscript is quite confusing and not very clearly written, and the PEC performance is disappointing, even though you write: " In short, the crystalline TiO2 layer with graded oxygen defects supports the Si-based photocathode to achieve an outstanding PEC performance and a high stability synchronously." Which is hardly correct; the PEC performance is certainly not outstanding since hardly any energy is harvested, while the stability is very promising.
I cannot support publication in the prestigious Nature Communications.
A minor detail regarding the reply to my Question #1 about the silicon resistivity: Even though you do not supply details about how the four-point measurement was done, I suspect that the calculated resistivity is off by a factor of approximately 5 (i.e., exactly π/ln2) off; please consult literature on four-point measurements.
Reviewer #3 (Remarks to the Author): Reviewers Comment on the revised version submitted to Nat comm Crystalline TiO2 protective layer with graded oxygen defects for efficient and stable Si-based photocathode The manuscript titled "Crystalline TiO2 …... Si-based photocathode" deals with the black-Si photocathode for PEC water-splitting and black-TiO2 surface protection. The resultant photocathode shows excellent stability under harsh pH conditions. The authors have addressed two major revisions with more characterizations and measurements. Overall the manuscript has improved a lot and other than some inherent characteristics of p-Si without p-n junction. Overall, I would recommend the manuscript for publication in prestigious Nature communication with below modifications.

Comments from Referee#1:
Firstly, we are of great gratitude to your kindness for providing us with so much profound knowledge and significant information in all the review processes. Your comments offer us many brand-new viewpoints including the resistivity calculated by four-point measurement, the relationship between the resistivity of Si and doping density, and ICP analysis of the electrolyte post-electrolysis.
Q1) The main achievement of the manuscript is a promising stability/durability of the protection layer, however, the characterization, analysis and evaluation is to some extend made more difficult and obscured by the choice of black silicon (b-Si) as photo-absorber due the surface roughness. The case for the protection layer could probably have been more clearly made with a planar silicon photo-absorber, where corrosion or stability could be convincingly demonstrated for a Pd/b2-TiO 2 /Si sample. Then b-Si samples could have been added if they contributed to the case. Our revision and explanation: Thank you very much for your instructive suggestion. Based on your suggestion, the characterization and PEC performance of Pd nanoparticles/black TiO 2 /planar Si (Pd/b2-TiO 2 /planar Si) have been analyzed and measured to further confirm the characterization and effect of the protection layer. As shown in the revised manuscript, the black TiO 2 layer showed graded oxygen defects. Even though Pd/b2-TiO 2 /planar Si photocathode had a lower limiting current than Pd/b2-TiO 2 /b-Si photocathode, the black TiO 2 layer still provided a promising stability in 1.0 M NaOH. Thus, the Si with or without nanostructure hardly affects the characterization, analysis and evaluation of the protection layer.   Q2) In its present from the manuscript is quite confusing and not very clearly written, and the PEC performance is disappointing, even though you write:"In short, the crystalline TiO 2 layer with graded oxygen defects supports the Si-based photocathode to achieve an outstanding PEC performance and a high stability synchronously." Which is hardly correct; the PEC performance is certainly not outstanding since hardly any energy is harvested, while the stability is very promising. I cannot support publication in the prestigious Nature Communications. Our revision and explanation: Firstly, we are sorry for the poor expressions. We have revised the WHOLE manuscript carefully and tried to avoid any grammar errors or confusing expressions and asked several native English writers to check the paper. We believe that the language is now acceptable for publication. The changes have been highlighted in the updated manuscript. Even though the photovoltage generated by p-Si is definitely lower than that obtained from n + p-Si, p-Si is often used as the photocathode materials due to low cost and good thermal stability. In addition, high quality black n + p-Si can be difficult to be formed by metal-catalyzed electroless etching method. Therefore, p-Si that natively had a low photovoltage for HER was chosen as the photocathode in this work. Furthermore, the aim of the manuscript is focusing on decoupling the trade-offs between stability and efficiency of p-Si photocathode by the crystalline TiO 2 with graded oxygen defects. To demonstrate the PEC performance of Pd/b2-TiO 2 /b-Si photocathode concisely, Table R1 summarizes the reported PEC performance of Si-based photocathodes without buried p-n junction in recent years. On the basis of the data from Table R1, Pd/b2-TiO 2 /b-Si photocathode shows a comparable PEC performance to those p-Si-based photocathodes regardless of the durability and the electrolyte. However, the inaccurate sentence "…… the crystalline TiO 2 layer with graded oxygen defects supports the Si-based photocathode to achieve an outstanding PEC performance ……" has been corrected as below. As mentioned in the manuscript, the use of crystalline TiO 2 layer with oxygen defects can effectively decouple the trade-offs between stability and efficiency of p-Si photocathode. The proposed method in this manuscript can make an important contribution to promoting the solar-to-hydrogen conversion and relieving the global warming, which is fit for the scope of Nature Communications.
{Characterization and photoelectrochemical profile of cocatalyst/protective layer/b-Si: "…… the crystalline TiO 2 layer with graded oxygen defects supports the p-Si photocathode to achieve a comparable PEC performance ……" Table R1. Comparison of selected representative state-of-the-art p-Si-based photocathodes for HER. V OS is the potential measured at a water reduction current density of 1 mA·cm -2 ; is the current density at 0 V vs RHE; J lim is the limiting current of the photocathode under illumination. Configutation Ref.

Our revision and explanation:
Thanks a lot for pointing out our mistake and providing a useful advice. Firstly, we used a wrong equation to calculate the silicon (Si) resistivity in the previous response. According to your recommendation, we carefully consult the literatures on four-point measurements and recalculate the Si resistivity as follows. As shown in Supplementary Fig. 33a, the thickness of Si wafer (d), the tip-tip distance (S) and the distance between tip and sample edge (L) are 0.5, 1 and 2 mm, respectively. The resistivity (ρ) of Si wafer is calculated as follows: where I is the applied current, V the corresponding voltage, S the tip-tip distance and B 0 the correction factor (3.104) determined by S/d and L/S. Based on the resistance (R) ranged from 10 to 20 Ω, the real resistivity of Si is around 2-4 Ω·cm. In addition, the Mott-Schottky plot of the planar Si has been added in the updated manuscript. Supplementary Information:   Figure 34. Mott-Schottky plot of the planar Si from capacitance measurement as a function of potential vs RHE under dark conditions. The Mott-Schottky plot was acquired at a frequency of 1 KHz in 0.5 M H 2 SO 4 solution by a CHI 660 potentiostat. The Mott-Schottky equation is shown below: where C is capacitance, q the charge of an electron (1.60 × 10 -19 C), ε 0 the vacuum permittivity (8.85 × 10 -14 F·cm -1 ), ε s the permittivity of silicon (1.05 × 10 -14 F·cm -1 ), A the area of the sample, N D the donor density, V the appied bias, V fb the flat band voltage, k Boltzmann's constant (1.38 × 10 -23 J·K -1 ), and T the temperature (25 °C). The x-intercept of the Mott-Schottky plot was reached at the bias that needs to be applied to cause the bands to become flat. Also, the slope of the plot can be used to calculate the donor density of the electrode. The x-intercept plus kT/q (~0.025 V) equals the flat band voltage. The N D can be calculated using the equation: are the dielectric constant of silicon (11.68) and the slope of the sharp increase from 0-0.12 V region. Thus, N D for the planar Si can be calculated to be 3.21 × 10 15 cm -3 , corresponding to the resistivity of Si wafer (2-4 Ω·cm) basically.

Comments from Referee#3:
The manuscript titled "Crystalline TiO 2 …… Si-based photocathode" deals with the black-Si photocathode for PEC water-splitting and black-TiO 2 surface protection. The resultant photocathode shows excellent stability under harsh pH conditions. The authors have addressed two major revisions with more characterizations and measurements. Overall the manuscript has improved a lot and other than some inherent characteristics of p-Si without p-n junction. Overall, I would recommend the manuscript for publication in prestigious Nature Communications with below modifications. We are of great gratitude to your positive appraisal and instructive comments. We have revised our manuscript and provided a point-by-point response to your comments as follows: Figure S22). For more details pl. refer recent article in Nature Energy, 3, 185-192 (2018), Nano Lett. 15, 2817-2824 and other papers cited therein. Our revision and explanation: Thank you very much for the articles.

Q1) As the MS deals only with photocathode the half-cell efficiencies (Solar to hydrogen conversion efficiencies (SHCE)) should be shown. (Not as in
According to the methods reported by the articles, the solar-to-hydrogen conversion efficiencies (SHCE) has been provided in the updated manuscript as below: {Characterization and photoelectrochemical profile of cocatalyst/protective layer/b-Si: "…… But regrettably, the solar-to-hydrogen (STH) efficiency and solar-to-hydrogen conversion efficiency (SHCE) of Pd/b2-TiO 2 /b-Si (Supplementary Figs. 21 and 22) were low because of the lack of p-n junctions. ……" Supplementary Information: Figure 22. Electrochemical characterization of Pd nanoparticles deposited on ITO in a 1.0 M NaOH electrolyte in dark. The inset is the characteristics of Pd/b2-TiO 2 /b-Si photocathode in a 1.0 M NaOH electrolyte under simulated AM 1.5 G illumination. V OC is the open-circuit voltage of the photocathode; V OS is the potential measured at a water reduction current density of 1 mA·cm -2 ; E 0 is the equilibrium water reduction potential in the 1.0 M NaOH electrolyte, which is 0 V vs RHE; is the current density at E 0 ; FF is the fill factor of the photocathode; and SHCE is the solar-to-hydrogen conversion efficiency of the photocathode. The preparation conditions of Pd nanoparticles on ITO are identical to those in b2-TiO 2 /b-Si. The V OC is calculated as follows: ca,10 10 , ph OC where V ph,10 is the potential of Pd/b2-TiO 2 /b-Si photocathode at a current density of 10 mA·cm -2 under illuminated, and V ca,10 the potential of Pd nanoparticles on ITO at a current density of 10 mA·cm -2 in dark. The SHCE is calculated as follows: where P incident is the illumination power density. where V ph,10 is the potential of Pd/b2-TiO 2 /b-Si photocathode at a current density of 10 mA·cm -2 under illuminated, and V ca,10 the potential of Pd nanoparticles on ITO at a current density of 10 mA·cm -2 in dark. The SHCE is calculated as follows: where P incident is the illumination power density.

Q3) Still, the presentation and language can be improved for better readability. Our revision and explanation:
We are sorry for our poor expressions. We have revised the WHOLE manuscript carefully and tried to avoid any grammar errors or confusing expressions and asked several native English writers to check the paper. We believe that the language is now acceptable for publication. The changes have been highlighted in the updated manuscript.