Peer review of: Direct and indirect Z-scheme heterostructure-coupled photosystem enabling cooperation of CO2 (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
In this manuscript, the authors fabricated Cu2O-Pt/SiC/IrOx composite as an effective photocatalyst 
to achieve CO2-to-HCOOH conversion and stoichiometric O2 evolution in a spatially separated setup. 
The photocatalyst can be prepared by photo-deposition and electron transfer proceeds effectively 
following a Z-scheme configuration. I think some key aspects of the photocatalytic process should be 
clarified before publication of this manuscript, and my concerns are as follows: 
1. During photocatalytic experiments, HCOOH is produced as the predominant CO2 reduction product 
with negligible CH4, HCHO and CO being generated. An explanation for this high selectivity is 
recommended. 
2. In this work, Fe2+ is added to the CO2RR counterpart of the setup and Fe3+ is added to the OER 
counterpart that is separated from the other with a Nafion membrane. Can Fe2+/Fe3+ permeate 
through the membrane? If not, after reaction for a certain period, Fe2+ should be depleted in CO2RR 
part while simultaneously, Fe3+ in the OER chamber is quantitatively converted into Fe2+. In this 
case, I think the authors should examine the color change of each counterpart (caused by Fe3+/Fe2+ 
conversion) and check whether the reaction slows down or ceases after complete Fe3+/Fe2+ inter-
conversion (for instance, the catalytic behavior beyond 8 hours without freshly added Fe3+/Fe2+). 
3. In a strong reducing environment, is Cu2O stable? Similarly, is Fe3+ or Fe2+ stable in a water-
oxidation system? Relative evidence should be given, such XPS and STEM-EDS. 
4. We noticed that the total number of e- in the typical CO2 reduction for 8h was about 0.7 mmol, and 
the number of e- provided by Fe2+ solution was about 0.6 mmol. We suggest that the reaction should 
be operated more than 16h to ensure the roles of Fe3+/Fe2+. 
5. If Nafion membrane can relay the redox couple Fe2+/Fe3+ freely, why in the beginning of the 
reaction the Fe2+ and Fe3+were separately located in the CO2-reduction compartment and the H2O-
oxidation compartment? What would it be when Fe2+/Fe3+ are evenly dispersed in both cell? 
6. The low performance of one-pot reaction may not from the coexistence of HCOOH and O2 but the 
competing reactions from the reduction of Fe3+ and oxidation of Fe2+ with high concentration of 
Fe3+ and Fe2+. 
7. It seemed that two different chambers were contacted with Fe3+/Fe2+, but the reaction rate was 
independent of concentrations of redox couple (Figure 6a). We wonder why the ratio of product was 
stoichiometric in two independence chamber. Does the Pt capacity of Pt/WO3 influence this process? 
Relative experiment data should be given. 
8. As demonstrated in Figure 9a, Pt acts as a mediator between SiC and Cu2O. In Figure 6c, control 
experiments with Cu2O/SiC/IrOx in which Pt is absent and Pt-Cu2O/SiC/IrOx where Pt and Cu2O are 
deposited in a reversed order are recommended, in order to reinforce the function of Pt in this work. 
9. Line 130, the authors said that in Figure 2c and S4, the distributions of Pt and Cu in the selected 
area are coincident and appear at almost the same position. While the corresponding EDS mapping 
profiles for Pt-L and Cu-K don't look exactly the same, especially in Figure S4. Can more direct 
evidence be provided to prove the existence of core-shell structure between them, rather than the 
deposition of Cu2O on the whole material surface？ 
10. The setup photographed in Figure S6, in many ways, resembles the H-cell in electrocatalysis. 
What is the applicability of the Cu2O-Pt/SiC/IrOx composite material for electrocatalysis or photo-
electrocatalysis? 
11. In this work, especially in Table S2, the catalytic performances of catalysts are evaluated by the 
rate of HCOOH generation. What about the solar-to-HCOOH conversion efficiency for Cu2O-
Pt/SiC/IrOx under optimum conditions? 
12. According to the SI, both the 13CO2 and D2O were employed as the reactants in the isotope 
labelling comparison reaction instead of CO2 and H2O, respectively. While the authors only provided 
the results of 13C in Line 206. What was the result of oxygen labeling? 
13. In the table shown in Figure 8, considering that all lifetimes were quite short, was any effort 
expended to ensure that the temporal response was independent of optical pulse energy? 
14. In Figure 6b, cycle experiment of HCOOH evolution in the spatially separated reaction system was 
measured. Since the system can operate without sacrificial reagents, why not measure the stability of 
the system for a long time instead of cycle experiment? Whether the generation of oxygen will affect 
the relay of Fe2+/Fe3+? 
15. Line 318, the authors said that the indirect Z-scheme electron transfers from the CB of SiC to the 
valent band (VB) of Cu2O would be accelerated by the Pt nanoparticles located between Cu2O and 
SiC, due to the excellent conductivity and high work function of Pt. So, may be the catalytic efficiency 
of Cu2O/SiC and Cu2O/Pt/SiC should be compared to illustrate the role of Pt. 
 
 
 
Reviewer #2: 
Remarks to the Author: 
This work described the direct and indirect Z-Scheme system composed by Cu2O-Pt/SiC/IrOx used in 
artificial photosynthesis to produce HCOOH and O2 simultaneously with high efficiency. Interestingly, 
the separated reaction chambers connected with Nafion membrane were adopted and the artificial 
photosynthesis was realized with high efficiency. As a result, under visible light irradiation, the 
production rate of HCOOH and O2 is up to 896.7 and 440.7 μmol g-1 h-1 respectively, which is much 
higher than that of the reported materials. To make this work more scientific and rigorous, major 
revision is needed before publishing in Nature Communications. My comments and suggestions are the 
following: 
 
1. As mentioned in this work, the core-shell like structure composed of Cu2O and Pt on the SiC 
surface was constructed. The author claimed that the Cu2O was deposited on Pt. Have the authors 
obverted the growth of Cu2O partials on the Pt surface over time? And is there any evidence to 
describe the interaction between Cu2O and Pt? According to Figure 2a, Cu2O was just surround with 
Pt and most of Cu2O was located on SiC surface. It was unavoidable that Cu2O was formed on the SiC 
with the reduction process because the excited electrons also can be existed on SiC surface. Thus, I 
am afraid that the illustration of the location of Cu2O in the target system in Figure 1 is not accurate. 
 
2. What is the special advantage of the indirect Z-Scheme system? Without the presence of Pt, the 
electrons still can transfer from SiC to Cu2O along with the energy diagram in Figure 9. To confirm the 
merit of the indirect Z-Scheme, the control experiment with two direct Z-Scheme (Cu2O/SiC/IrOx) 
should be supplemented. 
 
3. In this work, two reaction chambers were adopted and this separated half reaction system was 
effective in the production of the target product. From the results in Figure 6, this device was more 
useful than that of Gu2O-Pt/SiC/IrOx system in enhancement of the artificial photosynthesis 
efficiency. But why both the proton reduction in Pt/WO3 system and the oxidation of water in Gu2O-
Pt/SiC/IrOx system can be suppressed with the presence of redox couple Fe2+/Fe3+? What is the 
driving force of the diffusion of Fe2+/Fe3+ between in the two system? 
 
4. It is impossible all the Pt is in the form of metallic state, especially in the Cu2O-Pt/SiC and Cu2O-
Pt/SiC/IrOx. Because the XPS peaks are wide and the area ratio of Pt Peaks is not 4:3. Thus XPS 
peaks of Pt, Cu2O and Ir should be reanalyzed. 
 
5. Why the selectivity of the HCOOH is high? The possible HCOOH production process and the 
intermediate products should be given with the help of theoretical calculation or testing technology. 
6. Please give the AQY of this artificial photosynthesis. 
 
7. Please check the descriptions carefully. In page 11, the “Pt/TiO2” in line 279 and 284 should be 
corrected. Keep the identical form in figures in the whole manuscript, especially the sequence. 
 
 
 
Reviewer #3: 
Remarks to the Author: 
This manuscript described an interesting spatial separation photocatalytic system for CO2 reduction to 
HCOOH. Remarkable activity for COOH was achieved in this work, while stoichiometric O2 was 
produced. Still, it needs a minor revision before we accepted this manuscript: 
1. Author described that “no substantial change after the deposition of various cocatalysts (Figure 
S2)” when discussing the influence of surface modification to the BET specific surface area of SiC. 
However, according to Figure S2, it cannot be neglected that the BET specific surface of Cu2O-
Pt/SiC/IrOx (~14 m2 g-1) reduced about 22 % compared to that of bare SiC(~18 m2 g-1). More 
accurate description should be added. 
2. When discussing the distribution of IrOx on the surface of SiC, it was described that “it is separated 
from Cu2O-Pt on the SiC surface”. In Figure 2, Ir is evenly distributed on the surface of SiC, even in 
the blank background of EDS. Despite the overlap of Cu and Pt on the EDS, Cu is also evenly 
distributed on SiC. This makes the conclusion of spatial separation of Cu and Ir lack of support. The 
EDS profile for Ir may also lead to a conclusion that Ir is not firmly combined with SiC. More solid 
evidence to your conclusion should be given. 
3. Some references on CO2 photoreduction should be cited, such as Nature Communications 2018, 9, 
1252 etc.