Co3O4 Nanopetals on Si as Photoanodes for the Oxidation of Organics
, Niccolò Michieli 2, Boris Kalinic 2, Chiara Maurizio 2, Gian Andrea Rizzi 1,* and Giovanni Mattei 2Round 1
Reviewer 1 Report
Comments :
Overall Evaluation This manuscript describe an interesting approach to prepare a Si protective coating with thickness precisely controlled based on cobalt oxide. In particular different annealing process have been compared in order to obtain the best results. Nevertheless in order to make this perspective article publishable major revision are required.
Results and discussion 1) Page 5, line 108 the authors say that the “Electrochemical measurements were carried out either in 0.1M NaOH or in Na2SO4”. Later they report mainly the results obtained in 0.1 M Na2SO4 (Fig. 2 and Fig.3). Can they justify this choice? Can they added the LSV curves for all the samples in NaOH for comparison?. 2) Page 5, Fig. 2a: can the authors explain better in the text the experimental condition used during these measurements (i.e.. Dark and illuminant conditions)? 3) Page 5, Fig. 2b: instead of photocurrent vs concentration, to better understand the validity of the system I suggest to use a graphic derived by plotting the net charge (Q (C )) against theoretical COD values. For examples see: Electrochimica Acta 192 (2016) 319–327 , Zhang and alt. water research 43 (2009) 1986–1992, Chen and alt. Anal. Methods, (2012), 4, 1790. 4) Pag 7, line 156 and Fig. S3, SI: I suggest to stress in the text the choice of NaOH as electrolyte, instead of Na2SO4, for the mechanical analysis, in analogy with the reaction sequence proposed. 5) Pag.7, line 168: please cite Figure 4 in the text when start the discussion of EIS experiments; fig. 4 is never cited. Moreover in which electrolyte are obtained the impedance spectra?
References 8) check typos in ref 1, 2, 9, 18, 21,22, 30, 36 and 40.
Figure captions and Figures 9) Caption of Figure 4. Insert the electrolyte used for these measurements. 10) Figure 2, please insert the graphic J vs E of the different electrodes in NaOH electrolyte. |
Comments for author File: 
Comments.doc
Author Response
Comments :
Overall Evaluation This manuscript describe an interesting approach to prepare a Si protective coating with thickness precisely controlled based on cobalt oxide. In particular different annealing process have been compared in order to obtain the best results. Nevertheless in order to make this perspective article publishable major revision are required.
Results and discussion 1) Page 5, line 108 the authors say that the “Electrochemical measurements were carried out either in 0.1M NaOH or in Na2SO4”. Later they report mainly the results obtained in 0.1 M Na2SO4 (Fig. 2 and Fig.3). Can they justify this choice? Can they added the LSV curves for all the samples in NaOH for comparison?.
Answer: The COD and sensing measurements were performed in Na2SO4 because the difference in photocurrent/charge after addition of small volumes of analytes in NaOH is overshadowed by the already high photocurrent value, due to water oxidation. We have added a comment about this on lines 111-113 of the revised manuscript. A similar comment was already present on lines 125-127 in the un-revised version.
2) Page 5, Fig. 2a: can the authors explain better in the text the experimental condition used during these measurements (i.e.. Dark and Illuminant conditions)?
Answer: Figure 2a has been changed adding the measurements obtained in NaOH electrolyte and redrawing the curves indicating the dark currents in a clearer way.
3) Page 5, Fig. 2b: instead of photocurrent vs concentration, to better understand the validity of the system I suggest to use a graphic derived by plotting the net charge (Q (C )) against theoretical COD values. For examples see: Electrochimica Acta 192 (2016) 319–327, Zhang and alt. water research 43 (2009) 1986–1992, Chen and alt. Anal. Methods, (2012), 4, 1790.
Answer: We have changed Figure 2b according to the referee’s suggestions after acquisition of chronoamperometry to reach complete mineralization for different analytes.
4) Pag 7, line 156 and Fig. S3, SI: I suggest to describe in the text the choice of NaOH as electrolyte, instead of Na2SO4, for the mechanical analysis, in analogy with the reaction sequence proposed.
Answer: We have added a phrase, on line 157 of the revised manuscript, specifying that the cyclic voltammetries were acquired in NaOH in analogy with the paper of reference [38].
5) Pag.7, line 168: please cite Figure 4 in the text when start the discussion of EIS experiments; fig. 4 is never cited. Moreover in which electrolyte are obtained the impedance spectra?
Answer: We have added a citation to Figure 4 (line 167 of the revised manuscript) and specified in the caption of Figure 4 that the measurements were acquired in Na2SO4.
References 8) check typos in ref 1, 2, 9, 18, 21,22, 30, 36 and 40.
Answer: We have fixed all the typos in ref. 1,2 , …40.
Figure captions and Figures 9) Caption of Figure 4. Insert the electrolyte used for these measurements.
Answer: We have added in the caption of Figure 4 the indication about the electrolyte used for the EIS measurements
10) Figure 2, please insert the graphic J vs E of the different electrodes in NaOH electrolyte.
Answer: We have added a graph with LVS (Figure 2(a)) obtained in NaOH as requested by the reviewer.
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
The submitted manuscript reports on the characterization of a thermally synthesized layer of Co3O4 and its performance towards photoelectrooxidation reactions of organic molecules. The authors have done significant changes in the content of the manuscript, but several aspects require to be modified. A more detailed review on the critical points of this manuscript is provided below.
1. In page 4, the formulas that are reported and obtained from reference 30 are not present in the actual reference. The values that seems a constant (+0.1 and -0.1) for the calculation of the Ecb are not clear where were they taken from and therefore seem arbitrary. A clarification to give meaning of these values should be mentioned. Moreover, the actual values for the Vfb for the interfaces are not mentioned. What was the criteria to select the frequency in the Mott-Schotky plots from which this value can be calculated (Figure s1 and s2 show many frequencies)?
2. In line 116, the authors make reference to a process of mineralization of organic compounds by describing an equation. The authors should state if they are proposing such equation or cite the proper reference if this is obtained from another published work.
3. From Figures 2c and S2, it can be observed a clear p-type behavior which is mentioned in the text, in page 6, line 139. Here, the authors state that the photoelectrochemical behavior that dominates is that from the Co3O4, which should be a good photocathode as this material is a p-type. In Figures 2(a-d) and 3a all show a more dominating positive photocurrent, which is often related to n-type semiconductors and at the same time they are good photoanodes. The photooxidation process of the glucose in Figure 2c and phtalate in Fig3a should then be favoured by a photoanodic material (n-type) which should not be the Co3O4.. Although a brief comment in line 159 is mentioned, this behavior contradicts the authors statement observed in the Mott-Schottky plots, for which an explanation to this situation should be provided in order to clarify how the target organic molecules are oxidized. Since the chopped photocurrents in Figure 2c are more spikes than constant plateaus, this resemble to photocorrosion of the system
4. In section 3.2 Mechanistic analysis, the authors have increased the description length of the EIS spectra and corrected the presentation of the plots. However, the attempt to describe the system is still vague, as a systematic analysis of the interfaces (Si/SiOx then Co3O4/SiOx/Si) would have helped to illustrate better the reactions taking place in the interface. Besides, the supporting electrolyte and concentration of glucose is not clearly stated. For example, for the sample PETALS in Figure 4a there is a clear Warburg-like impedance bound at lower frequencies which could be related to the diffusion-limited reaction, especially under illumination. This could be linked to a photocathodic reaction which is the contribution of the cobalt oxide, as the system is favored from a condition closer to the open-circuit potential when compared to the more anodically-polarized condition (1.0 V vs Ag|AgCl) where this reaction does not take place (Figure 4d). In this figure, the irradiated condition prevents the evolution of the features in the spectra (semicircles), where the proposed equivalent circuit practically does not apply. Contrasting the EIS with the chopped light LSV from Figure 2c, there is a high possibility that the layer of Co3O4 is prone to photocorrosion, which is characterized by the overshoots when light is shined on the interface. Also, having determined the values for the electrical elements (charge transfer resistances and CPE) would also contribute to a better comparison of the different interfaces to better elaborate the argument of the authors. The latter is clearly evident from the smaller impedance magnitudes under illumination conditions. Therefore, it is highly suggested to add a table with the calculated values.
Author Response
1. In page 4, the formulas that are reported and obtained from reference 30 are not present in the actual reference. The values that seems a constant (+0.1 and -0.1) for the calculation of the Ecbare not clear where were they taken from and therefore seem arbitrary. A clarification to give meaning of these values should be mentioned. Moreover, the actual values for the Vfb for the interfaces are not mentioned. What was the criteria to select the frequency in the Mott-Schotky plots from which this value can be calculated (Figure s1 and s2 show many frequencies)?
Answer: The value for the Conduction Band edge for Si was obtained from the intercept of the Mott-Schottky plot at 1000 Hz. This frequency is commonly used in the literature to obtain the flat-band potential (Vfb) and the position of the CB edge for n-type semiconductors. We checked the reference about the position of CB edge for n-type semiconductors and it is correct. We agree with the referee that there is no clear indication, in the literature, about the position of the VB edge with respect to the Vfbin the case of p-type semiconductors and therefore we assume that the position of the VB edge is only roughly estimated. We have changed the text specifying that the position of the VB edge in the case of Co3O4is only indicative and added the values of Vfbfor Si and Co3O4as deduced from MS plots. We have added a proper reference in the case of p-type semiconductors (ref 31). Interestingly, the change in the shape of the MS plots, upon illumination, is well described in the recent publication by by Kirchartz et al. (Phys. Rev. Applied 7, 2017, 034018) and, as those authors write, is probably due to the obvious inhomogeneities of the electrical field (SiO2barrier layer) across the layer responsible for the photocurrent response that we see. In our case we think that most probably the formation of surface states, activated by light, is as well responsible for the change in the Mott-Schottky plots shape under illumination.
2. In line 116, the authors make reference to a process of mineralization of organic compounds by describing an equation. The authors should state if they are proposing such equation or cite the proper reference if this is obtained from another published work.
Answer: we have added the reference from which we have obtained the reported equation.
3. From Figures 2c and S2, it can be observed a clear p-type behavior which is mentioned in the text, in page 6, line 139. Here, the authors state that the photoelectrochemical behavior that dominates is that from the Co3O4, which should be a good photocathode as this material is a p-type. In Figures 2(a-d) and 3a all show a more dominating positive photocurrent, which is often related to n-type semiconductors and at the same time they are good photoanodes. The photooxidation process of the glucose in Figure 2c and phtalate in Fig3a should then be favoured by a photoanodic material (n-type) which should not be the Co3O4.. Although a brief comment in line 159 is mentioned, this behavior contradicts the authors statement observed in the Mott-Schottky plots, for which an explanation to this situation should be provided in order to clarify how the target organic molecules are oxidized. Since the chopped photocurrents in Figure 2c are more spikes than constant plateaus, this resemble to photocorrosion of the system.
Answer: In our opinion, as added in the text on line 99, in the caption of figure 1e and, of course, represented in figure 1e, this system is characterized by the formation of a tunnellingp-n junction between Co3O4and Si(100). Holes and electrons have to migrate through the thin SiO2barrier. These “tunnelling” junction are well known in the field of semiconductors and are reported for instance in ref (Journal of Applied Physics, 115, 2014, 033709 and in Chem. Phys. Lett., 388, 2004, 446). The behaviour shown in the LVS either in Na2SO4or in NaOH electrolytes is clearly that of a photoanode. We attribute the cathodic spikes of figure 2c to a high degree of electrons and holes recombination on the surface. This is a known behaviour described, for instance in (Phys. Chem. Chem. Phys., 16, 2014, 24610). A careful analysis of figure 2 c shows, in fact, that as soon as a hole scavenger like glucose is added the cathodic spikes are strongly attenuated. The formation of surface states (trap-states) might as well be the cause of such cathodic current overshoots. The presence of the thin barrier layer is interestingly beneficial in terms of photocurrent density since the treatment of the Si(100) wafer with HF immediately before the Co3O4deposition leads to a completely different interface and a very low photocurrent value. We attribute this behaviour to the probable formation of Co silicides after the thermal treatment. In conclusion, the presence of Co3O4improves the performances of this system in terms of photocurrent density due to the formation of a “tunnelling” p-n junction. This behaviour (enhancement of the photocurrent due to the presence of the Co3O4layer) is well represented in figure 2a. We attribute the change in the shape and slope of the Mott-Schottky plots, reported in the supporting materials, to the presence of the SiO2barrier layer that is clearly responsible for the non-homogeneous electric field and probably different mobility of electrons and holes along the layer where the photocurrent is build up. This behaviour is well described in a recent paper by Kirchartz et al. (Phys. Rev. Applied 7, 2017, 034018). A detailed analysis of the MS plots is beyond the scope of this paper, but, in any case, we have added the proper reference and comment in the text.
4. In section 3.2 Mechanistic analysis, the authors have increased the description length of the EIS spectra and corrected the presentation of the plots. However, the attempt to describe the system is still vague, as a systematic analysis of the interfaces (Si/SiOx then Co3O4/SiOx/Si) would have helped to illustrate better the reactions taking place in the interface. Besides, the supporting electrolyte and concentration of glucose is not clearly stated. For example, for the sample PETALS in Figure 4a there is a clear Warburg-like impedance bound at lower frequencies which could be related to the diffusion-limited reaction, especially under illumination. This could be linked to a photocathodic reaction which is the contribution of the cobalt oxide, as the system is favored from a condition closer to the open-circuit potential when compared to the more anodically-polarized condition (1.0 V vs Ag|AgCl) where this reaction does not take place (Figure 4d). In this figure, the irradiated condition prevents the evolution of the features in the spectra (semicircles), where the proposed equivalent circuit practically does not apply. Contrasting the EIS with the chopped light LSV from Figure 2c, there is a high possibility that the layer of Co3O4 is prone to photocorrosion, which is characterized by the overshoots when light is shined on the interface. Also, having determined the values for the electrical elements (charge transfer resistances and CPE) would also contribute to a better comparison of the different interfaces to better elaborate the argument of the authors. The latter is clearly evident from the smaller impedance magnitudes under illumination conditions. Therefore, it is highly suggested to add a table with the calculated values.
Answer: The EIS experiments were performed in pure Na2SO4electrolyte and in Na2SO4containing a glucose solution. We reported only the EIS data acquired in Na2SO4electrolyte, without glucose. The data acquired from the glucose solution are very similar with the only difference that the charge transfer resistance under illumination is lower (glucose acts as a hole scavenger). We have changed Figure 4 adding an inset that show an enlargement of the fitting of EIS data under illumination for sample PETAL (Figure 4 d). The inset clearly show that the model used is reproducing the experimental data also under illumination. We also agree with the referee that a more detailed analysis of the Co3O4/SiO2/Si interface would be useful, but the determination of the SiO2barrier height requires the measurement of a V-I plot at low temperature and this is a measurement that we cannot do in our lab. An indication of the barrier height can be found in (Chem. Phys. Lett., 388, 2004, 446). We don’t think that there is any photo-corrosion of the Co3O4layer as written by the reviewer. We have used this electrode for many different measurements and the photocurrent density in either pure Na2SO4or NaOH is always very reproducible even after hours of photoelectrochemical work with bias applied and under illumination. We have acquired SEM images after hours of electrochemical work and the differences in morphology are barely visible. Moreover, the XPS spectra do not indicate any sign of photo-corrosion or variation of the Co oxidation state. Below we report a SEM image acquired after 24 hours of continuous EC work.
Author Response File: Author Response.pdf
Reviewer 3 Report
the changes are fine. No more modifications needed.
Author Response
The manuscript has been revised following suggestions of review 1 and 2
Round 2
Reviewer 1 Report
Comments:
All the questions have beeen answered.The results are presented exhaustively and a good comparison have been done with different systems.
I suggest to accept it in the present form.
Author Response
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Reviewer 2 Report
1. The authors have replied to the criteria used in the Mott-Schottky (MS) to evaluate the flat-band potential of their materials. Although nowadays is common to observe in different papers that the conditions used to evaluate the Efb is usually a frequency or range of frequencies this criterion is vague as the interfaces evaluated will differ from those where the reference frequency was taken from. The determination of such frequency(ies) is and has to be obtained from the Bode-phase representation of the impedance spectra for the evaluated system. Such interval of frequencies is taken where the real impedance becomes constant, and log (-Z'') vs. log (f) has a slope of -1. A courtesy example is provided in the following image, where the red band presented in the spectra represents the region where the criteria aforementioned is satisfied.
A reference that is helpful to understand the classical conditions required for the recording of MS plots is J. Phys. D: Appl. Phys., Vol. 11, 1978. However, nanosized geometries can lead to distortion of the MS and their interpretation, as presented elegantly by Muñoz in his work (Electrochimica Acta 52 (2007) 4167–4176). This is evidenced in the image above where a donor distribution is observed. The authors are invited to revise their data to check the validity of the assumptions.
2. In their reply, the authors mention that when a hole scavenger is added to the electrolyte, the cathodic spikes are strongly attenuated, while in fact the spikes that should be attenuated are the anodic ones. When the hole scavenger (glucose) is added to the system, the organic compound is prone to oxidation via the hole left in the semiconductor. Then the electron gained from the glucose will increase the current flow to the electrode giving as a result a positive current which is associated to an oxidation process. When the light is shined on the surface of the junction, an anodic photocurrent is developed and thus confirms the n-type behavior. Interestingly, after the light is interrupted, a very high cathodic photocurrent is developed in the form of a spike of almost the same magnitude as the anodic one but with a longer decay time. This is a very interesting process that is not discussed in the paper and cannot be disregarded by the authors. Although the authors suggest that the spikes are a consequence of the fast recombination of electron-hole pairs, this is only valid immediately after the light is turned off. An explanation for the longer decay time to zero-current (which is not obtained in the Figure 2c) is required as this is not a typical observation. A reference that might be used to understand such overshoots and that can be used in the future is (Berger, T.; Monllor-Satoca, D.; Jankulovska, M.; Lana-Villarreal, T.; Gómez, R., The Electrochemistry of Nanostructured Titanium Dioxide Electrodes. ChemPhysChem 2012, 13 (12), 2824-2875. An image taken from that article which depicts the mechanistic development of the photocurrent can be found below.
A useful tool for the explanation of the cathodic spikes could be the Pourbaix diagrams (E.M. Garcia et al. / Journal of Power Sources 185 (2008) 549–553), where Co3O4 does not seems to be a very stable oxide to the polarization and the development of the overshoots in the current could be interpreted in these terms.
3. In the section 3.2 entitled as “Mechanistic analysis”, the authors present an examination of their EIS plots. The discussion of the results is limited from the conceptual point of view, for which the contribution to the knowledge is restricted to a comparative description rather than to an explanation of the involved phenomena. While comparing the magnitude of the features (sizes and shapes) in the spectra can lead to some theorization of the differences based on the morphology of the samples, a more insightful explanation can be derived from the spectra. This information is directly obtained from the fitting of the spectra and the numerical values for the electrical elements can be compared precisely instead of the sizes and shapes of the semicircles only. In the previous review round, it was suggested to include the table with this information and was not added. In this regard, there are a few questions that should be clarified:
a. Why was the OCP condition not evaluated or preferred over 0.1V? Was the stability of the electrochemical system (net current density = 0) obtained before doing the experiments?
b. For the sample PETALS, the proposed equivalent circuit includes 3 parallel circuit in series which differ from the other two samples. The theory suggests that each parallel circuit (R-CPE) represents an electrode (interface), for which the use of the 3 series-parallel circuit should be explained. To what interfaces are ascribed each of the nodes in parallel? And in which of the semicircles is occurring the reaction of interest? This should be included to provide a solid argument seeking to give mechanistic support of this section.
c. The same sample (PETALS) has only two features in illuminated-condition spectra, for which the same model used under darkness should not apply. What are the values for the electrical elements under illumination and darkness?
d. What is the justification, other than obtaining a good numerical fit, for using the two different electrical circuits? This justification should be included in the text.
e. Is it correct that units in all the experiments are in kOhm? It is a little too high for the first semicircle. What is the solution resistance?
f. In line 191, the authors associate the processes above 10kHz to a “residual contribution” related to charge transfer at the working electrode. If we consider the well-accepted idea that each semicircle corresponds to an electrode, then this is not a residual contribution but another electrodic process (this partly responds to the question “b”).
Comments for author File: Comments.docx
Author Response
Journal: Surfaces (ISSN 2571-9637)
Manuscript ID: surfaces-397757
Type: Article
Number of Pages: 13
Title: Co3O4 nanopetals on Si as photoanodes for the oxidation of organics
Round: 3
1. The authors have replied to the criteria used in the Mott-Schottky (MS) to evaluate the flat-band potential of their materials. Although nowadays is common to observe in different papers that the conditions used to evaluate the Efb is usually a frequency or range of frequencies this criterion is vague as the interfaces evaluated will differ from those where the reference frequency was taken from. The determination of such frequency(ies) is and has to be obtained from the Bode-phase representation of the impedance spectra for the evaluated system. Such interval of frequencies is taken where the real impedance becomes constant, and log (-Z'') vs. log (f) has a slope of -1. A courtesy example is provided in the following image, where the red band presented in the spectra represents the region where the criteria aforementioned is satisfied.
A reference that is helpful to understand the classical conditions required for the recording of MS plots is J. Phys. D: Appl. Phys., Vol. 11, 1978. However, nanosized geometries can lead to distortion of the MS and their interpretation, as presented elegantly by Muñoz in his work (Electrochimica Acta 52 (2007) 4167–4176). This is evidenced in the image above where a donor distribution is observed. The authors are invited to revise their data to check the validity of the assumptions.
Answer: We thank the reviewer for his suggestion. We have applied the procedure to choose correctly the frequency and added in the text the references he suggested. The frequency range we obtained is between 1000 and 3000 Hz. We have updated the value in the text together with the new Vfb.
2. In their reply, the authors mention that when a hole scavenger is added to the electrolyte, the cathodic spikes are strongly attenuated, while in fact the spikes that should be attenuated are the anodic ones. When the hole scavenger (glucose) is added to the system, the organic compound is prone to oxidation via the hole left in the semiconductor. Then the electron gained from the glucose will increase the current flow to the electrode giving as a result a positive current which is associated to an oxidation process. When the light is shined on the surface of the junction, an anodic photocurrent is developed and thus confirms the n-type behavior. Interestingly, after the light is interrupted, a very high cathodic photocurrent is developed in the form of a spike of almost the same magnitude as the anodic one but with a longer decay time. This is a very interesting process that is not discussed in the paper and cannot be disregarded by the authors. Although the authors suggest that the spikes are a consequence of the fast recombination of electron-hole pairs, this is only valid immediately after the light is turned off. An explanation for the longer decay time to zero-current (which is not obtained in the Figure 2c) is required as this is not a typical observation. A reference that might be used to understand such overshoots and that can be used in the future is (Berger, T.; Monllor-Satoca, D.; Jankulovska, M.; Lana-Villarreal, T.; Gómez, R., The Electrochemistry of Nanostructured Titanium Dioxide Electrodes. ChemPhysChem 2012, 13 (12), 2824-2875. An image taken from that article which depicts the mechanistic development of the photocurrent can be found below.
A useful tool for the explanation of the cathodic spikes could be the Pourbaix diagrams (E.M. Garcia et al. / Journal of Power Sources 185 (2008) 549–553), where Co3O4 does not seems to be a very stable oxide to the polarization and the development of the overshoots in the current could be interpreted in these terms.
Answer: We thank again the reviewer for the suggestions. A detailed analysis of the kinetics of holes and electrons recombination is out of the scope of this paper and will be the subject of a future article. To this purpose we have prepared Co3O4 samples grown on SiO2/Si substrates where the oxide thickness has been accurately measured by ellipsometry. We have, to this purpose, acquired curves representing the photocurrent decay after applying light pulses. In any case, we have added the suggested reference and a comment about the LVS under chopped light (lines 122-132 of the revised manuscript). “The electrode response to chopped illumination is characterized by ''spike and overshoot'' photocurrent transients. We attribute the photocurrent spikes of figure 2(c) (blue and orange curves) to a high degree of electrons and holes recombination on the surface and to the formation of surface states (trap-states). This is a known behavior described, for instance, in [34, 35]. When the light is turned on, holes generated in the space charge region are swept rapidly towards the semiconductor electrolyte junction. Due to the slow kinetics of the 4-hole oxidation of water to molecular oxygen, the concentration of holes increases considerably at the interface until the rate of arrival of holes is balanced in the steady state by the rates of charge transfer and recombination. Since surface recombination leads to a flux of electrons towards the surface, the resulting photocurrent transient is the sum of the hole and electron contributions. A careful analysis of figure 2(c) shows, also, that as soon as a hole scavenger like glucose is added the cathodic spikes are strongly attenuated.”
Inorganic chemistry texts report Co3O4 as the thermodynamically stable phase for Co oxide. CoO naturally transforms into Co3O4 if left at the atmosphere. In any case, many different Pourbaix diagrams containing Co3O4 can be found in the literature. In the Pourbaix diagram included below (Atlas of Eh-pH Diagrams – Intercomparison of thermodynamic databases - Geological Survey of Japan Open File Report No. 419 – 2005 - Research center for Deep Geological Environments), the stability range of Co3O4 phase is rather large, although we agree with the reviewer that at pH values lower than 6 this material is clearly unstable. In any case, this Pourbaix diagram clearly shows that Co3O4 is an unstable phase only if the electrode is polarized below 0.2 V vs SHE.
3. In the section 3.2 entitled as “Mechanistic analysis”, the authors present an examination of their EIS plots. The discussion of the results is limited from the conceptual point of view, for which the contribution to the knowledge is restricted to a comparative description rather than to an explanation of the involved phenomena. While comparing the magnitude of the features (sizes and shapes) in the spectra can lead to some theorization of the differences based on the morphology of the samples, a more insightful explanation can be derived from the spectra. This information is directly obtained from the fitting of the spectra and the numerical values for the electrical elements can be compared precisely instead of the sizes and shapes of the semicircles only. In the previous review round, it was suggested to include the table with this information and was not added. In this regard, there are a few questions that should be clarified:
a. Why was the OCP condition not evaluated or preferred over 0.1V? Was the stability of the electrochemical system (net current density = 0) obtained before doing the experiments?
Answer: Yes, we have checked the stability of the system at OCP. The system has always shown very good stability. We have acquired EIS measurements with different polarizations and decided to present the data where the photocurrent was absent or, instead, was fully developed.
b. For the sample PETALS, the proposed equivalent circuit includes 3 parallel circuit in series which differ from the other two samples. The theory suggests that each parallel circuit (R-CPE) represents an electrode (interface), for which the use of the 3 series-parallel circuit should be explained. To what interfaces are ascribed each of the nodes in parallel? And in which of the semicircles is occurring the reaction of interest? This should be included to provide a solid argument seeking to give mechanistic support of this section.
Answer: We assign the R-CPE elements in the following sequence: the first one to the double-layer, the second one to the Co3O4 layer with presence of surface states and the last one to the Co3O4/SiOx/Si structure. In the case of samples Hybrid and No-Petals the absence of surface states requires only the presence of 2 R-CPE elements (double layer and Co3O4/SiOx/Si structure), although, as correctly pointed out by the reviewer, in the case of sample Hybrid a slightly more satisfactory fitting could be obtained also with 3 R-CPE elements in dark conditions. The assignment of the R-CPE elements is specified in the text from lines 229-236 of the revised manuscript.
c. The same sample (PETALS) has only two features in illuminated-condition spectra, for which the same model used under darkness should not apply. What are the values for the electrical elements under illumination and darkness?
Answer: we have added a table in the SI reporting the fitted values for capacity and resistances for the 3 electrodes in the dark and under illumination at 0.1 V and 1.0 V. The table contains a row where the assignment of resistances and capacitances values to each interface is specified. A satisfactory fitting for sample PETALS can only be obtained with 3 R-CPE elements.
d. What is the justification, other than obtaining a good numerical fit, for using the two different electrical circuits? This justification should be included in the text.
Answer: We have already written that the presence o surface states, (trap states), requires a model with 3 R-CPE elements (lines 229-236) of the revised manuscript. We have added a table in the SI reporting the fitted values for capacity and resistances for the 3 electrodes in the dark and under illumination at 0.1 V and 1.0 V. The table contains a row where the assignment of resistances and capacitances values to each interface is specified.
e. Is it correct that units in all the experiments are in kOhm? It is a little too high for the first semicircle. What is the solution resistance?
Answer: we have checked the units in the reported EIS plots and are all correct. The solution resistance was about 85 Ohms in all three cases.
f. In line 191, the authors associate the processes above 10kHz to a “residual contribution” related to charge transfer at the working electrode. If we consider the well-accepted idea that each semicircle correspond to an electrode, then this is not a residual contribution but another electrodic process (this partly responds to the question “b”).
Answer: the reviewer is right. We have deleted the phrase.
Author Response File: Author Response.docx
