Heteroepitaxial Growth of GaP Photocathode by Hydride Vapor Phase Epitaxy for Water Splitting and CO2 Reduction
, Yanqi Yuan 2,†, Feng Li 2, Balaji Manavaimaran 1,3, Sebastian Lourdudoss 1, Peng Zhang 2,* and Yanting Sun 1,*Round 1
Reviewer 1 Report
The paper reports Zn-doped p-GaP grown on GaAs and Si substrates for solar-driven PEC H2 generation by water splitting and CO2 reduction.Each sample has been very carefully characterized, and the conclusions are supported by a large body of data. Therefore, I consider this paper worthy of publication as is.
Author Response
Thank you for the recommendation
Reviewer 2 Report
In this work, the authors have demonstrated heteroepitaxial Zn doped p-GaP growth on different (001) GaAs, (001) Si and (111) Si substrates via hydride vapor phase epitaxy. The heteroepitaxial p-GaP photoelectrode performance is quite impressive for solar driven photoelectrochemical applications for water splitting and CO2 reduction, which is comparable to homoepitaxial p-GaP/GaP. The results are interesting and supported by solid experimental evidences. However, following issues should be addressed before the publication on Catalysts.
1. Why choose GaP as photocathode in this work? Have authors considered other III-V semiconductors on Si for PEC applications?
2. WP nanoparticles were not used on photocathode for CO2 reduction. Can authors explain the reason? Will other catalysts be applied in future that may improve the efficiency of CO2 reduction?
Author Response
In this work, the authors have demonstrated heteroepitaxial Zn doped p-GaP growth on different (001) GaAs, (001) Si and (111) Si substrates via hydride vapor phase epitaxy. The heteroepitaxial p-GaP photoelectrode performance is quite impressive for solar driven photoelectrochemical applications for water splitting and CO2 reduction, which is comparable to homoepitaxial p-GaP/GaP. The results are interesting and supported by solid experimental evidences. However, following issues should be addressed before the publication on Catalysts.
- Why choose GaP as photocathode in this work? Have authors considered other III-V semiconductors on Si for PEC applications?
Our response: Thank you for the comment. The manuscript has been revised as follows: “In addition to its well-suited band parameters, GaP also has a lower lattice mismatch to Si compared to most other III-V compounds (0.37%), which makes it a good candidate for III-V/Si integration strategies.” (Page 2, lines 49-51)
- WP nanoparticles were not used on photocathode for CO2 reduction. Can authors explain the reason? Will other catalysts be applied in future that may improve the efficiency of CO2 reduction?
Our response: Thank you for the comment. More details about the catalysts used have been added to the manuscript as follows: “WP particles were omitted from the cathodes used for CO2 reduction as this would enhance the competing hydrogen evolution reaction. WP has been proved as a great catalyst in HER, the presence of WP can not only increase the concentration of active sites but also provide a suitable hydrogen adsorption free energy for HER. In the field of CO2 reduction, Cu-based catalysts have attracted much attention which is the only heterogeneous catalyst catalyzing the formation of various products of hydrocarbons and oxygenates. The Cu-based catalysts will be applied in the further for CO2 reduction.” (page 13, line 424 – page 14, line 431)
Reviewer 3 Report
Strömberg et al reported Heteroepitaxial growth of GaP photocathode by hydride vapor phase epitaxy for water splitting and CO2 reduction. However, the author needs to improve the manuscript to be published in this journal. Comments are given below.
1- Please explain more about the experimental for CO2 reduction.
2- Faradic efficiency is an important parameter with stability results. Please provide it.
3- Mechanism of CO2 reduction with band diagram should be provided.
4- English and grammatical mistakes should be addressed while revising the manuscript.
Author Response
Reviewer 3’s Comments and Suggestions for Authors
Strömberg et al reported Heteroepitaxial growth of GaP photocathode by hydride vapor phase epitaxy for water splitting and CO2 reduction. However, the author needs to improve the manuscript to be published in this journal. Comments are given below.
1- Please explain more about the experimental for CO2 reduction.
Our response: Thank you for the feedback. Additional experimental details have been provided as: “Photoelectrochemical CO2 reduction reaction measurements were conducted un-der AM 1.5G irradiance in a three-electrode cell configuration with TiO2/p-GaP as the WE, Ag/AgCl as the RE and Pt as the CE (Fig. 2(b)). An electrolyte solution with final concentration of 0.5 M NaCl and 10 mM pyridine was used. In order to ensure a CO2 - response, the linear sweep voltammetry (LSV) curves under Ar-saturated and CO2-saturated conditions were measured separately. Initially, argon gas was injected into the electrolyte for half an hour to ensure it was saturated. After the electrochemical test, the gas was replaced with CO2 and ventilated for half an hour before measuring the LSV curve again.” (Page 15, lines 491-498)
2- Faradic efficiency is an important parameter with stability results. Please provide it.
Our response: We agree with the reviewer about the importance of the Faradic efficiency. Since the Faradic efficiency and stability of CO2RR are both very important and comprehensive points to explore and they will be tested and discussed in the follow-up research.
3- Mechanism of CO2 reduction with band diagram should be provided.
Our response: Thank you for the comment. A detailed description of the CO2RR mechanisms for TiO2/GaP photocathodes including a reference has been provided as: “The conduction bands of GaP and TiO2 are located at −0.7 and −0.2 VNHE, respectively. To reduce CO2, photoelectrons need to overcome a 1.2 V energy barrier. In theory, according to the calculation of the flat band potential, the internal pn-junction can produce nearly 0.7 V. This leads to an −0.5 V externally applied overpotential being required to drive the CO2 reduction reaction. However, the actual initial potential re-quired is less than -0.5 V due to the reduced recombination of photogenerated carriers and the additional pn-junction photovoltage derived from the TiO2 layer. A more detailed description of the CO2RR mechanisms for TiO2/GaP photocathodes can be found in ref [17].” (page 10, lines 322-329)
Figure R1. Energy band alignment of GaP and TiO2 together with the
relevant redox potentials of CO2. (Data from Ref.17)
4- English and grammatical mistakes should be addressed while revising the manuscript.
Our response: Thank you for pointing that out. The manuscript has been proof-read and corrected.
Author Response File: 
Author Response.docx
Round 2
Reviewer 3 Report
yes now we can accept this manuscript for publication.
