Strategies of Anode Materials Design towards Improved Photoelectrochemical Water Splitting Efficiency
Round 1
Reviewer 1 Report
The review proposed by Hu et al. concerns anode materials dedicated to photoelectrochemical water splitting. The paper is overall well written, well structured and illustrated and quite easy to read. This may also derive from the fact that most concepts are rather superficially considered. For newcomers to the field this may however be quite well suited and interesting.
1. A major concern I would like to raise is that the paper appears largely as a review of the author’s works rather than a real general review of the field. This should be stated clearly in the introduction or the author’s should give proper credit to a number of previous works, which they obviously choose to not cite. Alternatively, revisiting the references is also a possibility.
2. A second concern is that the charge separation chapter is incomplete. In recent years it has been shown, especially for PV cells, that the incorporation of perovskites is very favorable to favor charge separation due to the internal electrical potential. The authors may consider discussing this effect in the framework of water splitting.
3. Finally, the last section devoted to the upcoming challenges is very small. The authors should really consider improving this part.
There are a number of minor points that the authors should also consider.
4. Line 33: please add “reactions “ before R1 to R4
5. Lines 64-69: Should be summarized in one sentence.
6. Line 105: What is meant with “to accelerate” the hybridization? Do the authors mean “improve” ? Please rewrite.
7. Line 219: “found improve”, do the authors mean “improved” ?
8. Legend figure 19. In my version there are no blue arrows and is the same direction correct? Please clarify.
In summary the paper is of interest for readers who would like to discover the field and as such deserve publication after the authors have considered the reviewer’s comments. With some improvements it may become a nice reference for young researchers stating in the field.
Author Response
Reviewer 1
The review proposed by Hu et al. concerns anode materials dedicated to photoelectrochemical water splitting. The paper is overall well written, well structured and illustrated and quite easy to read. This may also derive from the fact that most concepts are rather superficially considered. For newcomers to the field this may however be quite well suited and interesting.
1. A major concern I would like to raise is that the paper appears largely as a review of the author’s works rather than a real general review of the field. This should be stated clearly in the introduction or the author’s should give proper credit to a number of previous works, which they obviously choose to not cite. Alternatively, revisiting the references is also a possibility.
A sentence has been added to state that this review is based mainly on our research work. It is noted that we have now added more references by other groups.
“In this review, we summarize recent progress in the development of anode materials for photoelectrochemical water splitting, which are mainly based on our research work.”
2. A second concern is that the charge separation chapter is incomplete. In recent years it has been shown, especially for PV cells, that the incorporation of perovskites is very favorable to favor charge separation due to the internal electrical potential. The authors may consider discussing this effect in the framework of water splitting.
The effect of internal electrical potential is added as 3.4 (as shown below):
”3.4 Internal electric field to improve the charge separation
Recent studies indicate that the internal electric field in photocatalyst can improve the charge separation [58]. The internal electric field provides a driving force for the separation of photoinduced charge carriers. Ferroelectric materials are a big family for having internal electric field due to spontaneous electric polarization property among various photocatalytic materials. The spontaneous polarization in ferroelectric materials are due to the non-centrosymmetric property and the polarization occurs because the positive and negative charges have different centers of symmetry [59]. Perovskite oxides (ABO3) are among the most extensively studied ferroelectrics, such as BaTiO3 [60], BiFeO3 [61]. Rohrer et al. found that BaxSr1-xTiO3 solid solution by replacing Ba with Sr can enlarge the space-charge region and improve the activity [62]. Zhang et al. also found that incorporation of carbon into the Bi3O4Cl lattice can increase internal electric field to boost bulk charge separation [63]. Hao et al. found that double perovskite ferroelectric Bi2FeMoxNi1-xO6 thin films also possess strong ferroelectric self-polarization, which has played a crucial part in improving the photovoltaic effect [64].”
3. Finally, the last section devoted to the upcoming challenges is very small. The authors should really consider improving this part.
We have improved the last part by discussing more challenges:
“A big challenge is to combine all the advantages and improve the overall efficiency for water splitting. Usually it is inevitable to introduce some other problems when improving one factor. For example, nanostructuring is mostly employed to enhance the charge separation and light absorption, however, more surface states will be formed which sometimes are unfavorable to the interfacial charge injection. Thus, co-catalysts loading is required to passivate the surface states. Then the challenge becomes to find a suitable co-catalyst to passivate and to control its thickness through a proper deposition technology. Recently, researchers are focusing on the formation of ultra-thin layer of co-catalyst on the surface, which will provide very efficient charge injection while avoiding the recombination in the surface layer and at the interface. But it is very difficult to precisely control the formation of a uniform cover layer of the co-catalyst on the surface with reasonable cost of production. New low-cost processing technology and materials are to be developed. Another related issue is the stability and durability from the point of view of practical application. Newly developed materials should be coated with good adhesion while having good chemical and photochemical stability.”
There are a number of minor points that the authors should also consider.
4. Line 33: please add “reactions “ before R1 to R4.
It was changed based on the suggestion, now R1 to R4 changed into R3 to R6.
5. Lines 64-69: Should be summarized in one sentence.
It has been revised as:
“Generally, light absorption of a photoelectrode depends on its bandgap. For a known photoelectrode with a fixed bandgap, the film thickness and structure would affect its light absorption efficiency. Some reported strategies used to enhance the light absorption are summarized below. “
6. Line 105: What is meant with “to accelerate” the hybridization? Do the authors mean “improve” ? Please rewrite.
“accelerate” is changed into “improve” based on the suggestion.
7. Line 219: “found improve”, do the authors mean “improved” ?
“improve” is changed into “improved” based on the suggestion.
8. Legend figure 19. In my version there are no blue arrows and is the same direction correct? Please clarify.
The blue arrows are added in Fig. 23 now.
In summary the paper is of interest for readers who would like to discover the field and as such deserve publication after the authors have considered the reviewer’s comments. With some improvements it may become a nice reference for young researchers stating in the field.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The present manuscript reviews some recent results concerning photoanode materials for photoelectrochemical water splitting (PEC-WS). After briefly introducing the basic concepts on PEC-WS, the authors discuss three main key aspects for the improvements of photoanodes: light absorption, charge separation, and surface charge injection. The authors finally conclude giving some insights about possible perspectives related to those strategies.
Although the overall purpose of this review is clear and well-motivated, the manuscript is affected by a number of flaws both regarding the organization and the contents. As such, it does not deliver an adequate amount of information to the reader, nor it results clear. For this reason, extensive corrections and editing need to be performed before the manuscript can be considered for publication. The points which should be addressed are reported below.
1. The first two sections, i.e. “1. Introduction” and “2. Fundamentals of PEC for enhancing the performance”, do not provide a sufficiently clear understanding to the reader about the basics about PEC-WS. In particular:
a. One single introductory section may be sufficient by combining those two ones.
b. The authors mention the OER and HER half-reactions (lines 28–29) without writing them. On the contrary, they report a detailed mechanism for the OER (lines 33–34), as described in Ref. 1, which in my view is unnecessary. By the way, Ref. 2 does not describe the OER process, so it should be removed. Also, the authors do not report the thermodynamic threshold for the WS reaction (1.23 eV) but the value of 4.92 eV (line 32), why?
c. The authors do not clearly distinguish photoanode materials (i.e. TiO2, Fe2O3) and catalysts, which may be applied on their surface (i.e. NixOy, MnOx), as in lines 39-40, but also throughout the overall manuscript. Indeed, Ref. 4 reviews about catalyst materials, not photoanodes (a suggested review is Nat. Rev. Mater. 2016, 1, 15010).
d. The authors mention the solar-to-hydrogen efficiency in lines 52–55. However, they do not report the formula to calculate it, nor the appropriate conditions for its reliable determination and other possible efficiency metrics commonly used by the community (see for instance J. Mater. Res. 2010, 25, 3). Having not explained the energy threshold for the WS reaction (see above), the authors do not clearly explain the importance of the bandgap in terms of maximum photocurrent/efficiency (e.g. theoretical photocurrent vs. bandgap plot). The same confusion appears at the beginning of Section 3 (lines 64–69).
e. Figure 1 (line 62) only reports the WS process at the photoanode surface, the authors may draw also the cathode part in order to clearly show the overall mechanism.
2. Section “3. Strategies to enhance the light absorption” includes nanostructure formation, band engineering (actually doping), and combination between two absorber materials. Also this section is affected by some issues:
a. Two important approaches have been neglected: the first one is the concept of reduction/hydrogenation of TiO2 (or other materials) to increase its light absorption and, consequently, its efficiency. This is also known as black titania, as discovered in 2011 by Chen and Mao (Science 2011, 331, 746). Although only the increased light absorption is not sufficient to explain the exceptional properties of the reduced/hydrogenated forms of TiO2, such strategies may be included in Section 3. A recent review about the latest findings on this topic is for instance ACS Catal. 2019, 9, 345.
b. The second relevant approach consists in the combination between the classical photoanodes for PEC-WS (especially TiO2) and plasmonic nanoparticles (NPs), which, through different mechanisms, can allow a sort of visible-light sensitization of wide-bandgap materials, or anyway increase their performance in PEC-WS experiments. A suggested review about this topic is Adv. Mater. 2019, doi: 10.1002/adma.201805513.
c. Sections “3.1 Nanostructure formation” and “3.2 Band engineering” somehow appear also in Section 4, “4.1 Doping” and “4.2 Nanostructure to shorten the diffusion length”. This is indeed due to the fact that both doping and nanostructuring can induce more effects in the material. However, such repetition can easily create confusion among the readers. It is better to explain doping in Section 3, because the main effect is related to the increased light absorption. Conversely, the discussion about nanostructuring is more suitable to Section 4, as its main effect is to decouple the short hole diffusion length with the long electron diffusion length (as typical in 1D or quasi-1D nanostructures, see also Chem. Soc. Rev. 2017, 46, 3716). In this way, the authors can better focus their explanation.
d. In Section 3.2, line 103, the authors mention that anion doping is relatively rare. This is not true: an extensive amount of work has been carried out about anion-doped TiO2, among others (see for instance Chem. Rev. 2014, 114, 9824; Energy Environ. Sci. 2012, 5, 9603). Also, the authors could include a scheme about the difference between cation- and anion-doping (similar to Figs. 6 and 10 in Chem. Rev. 2010, 110, 6503) for a clearer understanding.
3. Section “4. Strategies to improve the charge separation”, should be improved in these specific points.
a. Re-organize the sections 4.1 and 4.2 based on Comment 3c.
b. The numbering of the sub-sections is not correct (all of them are 4.1).
c. Figure 9 should be changed by including a schematics about the concept of nanostructuring to address the problem of short diffusion length (see for instance Fig. 7 in J. Mater. Chem. 2008, 18, 2311); the acronym “ZFO” in the caption must be explained; at least one example of change in photocurrent before/after the nanostructuring should be included.
d. The section “Heterojunction” should also mention those formed between TiO2 and hematite (Fe2O3), see again Chem. Soc. Rev. 2017, 46, 3716.
4. Regarding Section “5. Strategies to enhance surface charge injection”:
a. The title of 5.1, “co-catalyst loading”, is conceptually wrong. The term “co-catalyst” indeed refers to the photocatalytic case, in which typically semiconductor nanoparticles drive the overall WS reaction, so that they are functionalized with a oxygen-evolution catalyst and a hydrogen-evolution catalyst (co-catalyst), see Chem. Soc. Rev. 2008, 38, 253. The authors should just refer to “catalysts” for the OER reaction.
b. In “5.2 Surface treatment and surface passivation”, the authors mention that point defects at the surface of photoelectrodes act as recombination centers (lines 284–285). This is correct in some cases, but in other cases such defects are in fact deliberately introduce to increase the efficiency, as in the case of black TiO2 (see Comment 2a). The authors should better clarify this point (see also J. Phys. Chem. Lett. 2013, 4, 1624). Indeed, later the authors mention that oxygen vacancy sites on BiVO4 are active sites for water oxidation (line 375).
c. In the same section, Figure 17 is not really a good choice as it is almost completely covered by a text. A clearer/better figure is suggested.
d. In the same section, Figure 18 could be placed at the beginning of the section since it helps illustrating the concept of a passivation layer to “heal” surface trap states.
e. Section “5.2 Active sites” presents a list of equations describing the free energy of the intermediate steps in the OER reaction. This discussion should either be simplified/clarified or removed, because it does not deliver a clear message.
f. In the same section, lines 356–362, the authors discuss about Ni(1–x)FexOOH and doped graphene frameworks for the OER. However, such materials are employed as electrocatalysts, they are not photoanodes.
g. In the same section, line 375, the authors mention that vacancies at the BiVO4 surface are active sites for water oxidation. Several works have been also performed on TiO2, see for instance Science 2008, 320, 1755 (and Comment 4b).
To conclude, the authors are encouraged to carefully edit and improve their manuscript. I am available to review a revised version of the manuscript.
Author Response
Reviewer 2
The present manuscript reviews some recent results concerning photoanode materials for photoelectrochemical water splitting (PEC-WS). After briefly introducing the basic concepts on PEC-WS, the authors discuss three main key aspects for the improvements of photoanodes: light absorption, charge separation, and surface charge injection. The authors finally conclude giving some insights about possible perspectives related to those strategies.
Although the overall purpose of this review is clear and well-motivated, the manuscript is affected by a number of flaws both regarding the organization and the contents. As such, it does not deliver an adequate amount of information to the reader, nor it results clear. For this reason, extensive corrections and editing need to be performed before the manuscript can be considered for publication. The points which should be addressed are reported below.
1. The first two sections, i.e. “1. Introduction” and “2. Fundamentals of PEC for enhancing the performance”, do not provide a sufficiently clear understanding to the reader about the basics about PEC-WS. In particular:
a. One single introductory section may be sufficient by combining those two ones.
The revised version has merged the two into one.
b. The authors mention the OER and HER half-reactions (lines 28–29) without writing them. On the contrary, they report a detailed mechanism for the OER (lines 33–34), as described in Ref. 1, which in my view is unnecessary. By the way, Ref. 2 does not describe the OER process, so it should be removed. Also, the authors do not report the thermodynamic threshold for the WS reaction (1.23 eV) but the value of 4.92 eV (line 32), why?
The OER and HER half-reactions are written in line 30 to 31 now.
Ref.2 is deleted.
4.92 eV means 1.23*4=4.92, In order to clearly express the energy, “with the energy 4.92 eV” are change into “with 1.23 eV for thermodynamic threshold energy”
c. The authors do not clearly distinguish photoanode materials (i.e. TiO2, Fe2O3) and catalysts, which may be applied on their surface (i.e. NixOy, MnOx), as in lines 39-40, but also throughout the overall manuscript. Indeed, Ref. 4 reviews about catalyst materials, not photoanodes (a suggested review is Nat. Rev. Mater. 2016, 1, 15010).
The catalysts, which may be applied on their surface, are deleted now, and the reference has been replaced by the suggested one (Nat. Rev. Mater. 2016, 1, 15010).
d. The authors mention the solar-to-hydrogen efficiency in lines 52–55. However, they do not report the formula to calculate it, nor the appropriate conditions for its reliable determination and other possible efficiency metrics commonly used by the community (see for instance J. Mater. Res. 2010, 25, 3). Having not explained the energy threshold for the WS reaction (see above), the authors do not clearly explain the importance of the bandgap in terms of maximum photocurrent/efficiency (e.g. theoretical photocurrent vs. bandgap plot). The same confusion appears at the beginning of Section 3 (lines 64–69).
Some formula and figure have been added to explain the STH and the relationship of theoretical STH and band gap:
“The solar-to-hydrogen (STH) efficiency is an important metric for benchmarking and performance evaluation. The STH for a PEC system constructed by photoelectrodes can be expressed by [6]:
(7)
where is the photocurrent observed from the experiment, and is the power intensity of the sunlight.
For a photoelectrode, is determined by the photogeneration absorption efficiency (ηabs), the product of the injection efficiency (ηsep), and the injection efficiency (ηinj) of the photogenerated carrier to the reactant [7], which can be described by:
(8)
where is the theoretical photocurrent when 100% of the photons in the solar spectrum with energies exceeding the bandgap are absorbed and converted. Theoretical STH and solar photocurrent of a photoelectrode under AM 1.5 G irradiation (100 mW cm-2) are displayed in Fig. 1 [6]. Figure 1. Dependence of theoretical STH and solar photocurrent density of photoelectrodes on their bandgap absorption edges. Reproduced from Ref. 5。 with permission from The Royal Society of Chemistry.”
e. Figure 1 (line 62) only reports the WS process at the photoanode surface, the authors may draw also the cathode part in order to clearly show the overall mechanism.
The cathode part has now been added in Figure 2.
2. Section “3. Strategies to enhance the light absorption” includes nanostructure formation, band engineering (actually doping), and combination between two absorber materials. Also this section is affected by some issues:
a. Two important approaches have been neglected: the first one is the concept of reduction/hydrogenation of TiO2 (or other materials) to increase its light absorption and, consequently, its efficiency. This is also known as black titania, as discovered in 2011 by Chen and Mao (Science 2011, 331, 746). Although only the increased light absorption is not sufficient to explain the exceptional properties of the reduced/hydrogenated forms of TiO2, such strategies may be included in Section 3. A recent review about the latest findings on this topic is for instance ACS Catal. 2019, 9, 345.
Some discussion has been added in part 3.2.
“Another strategy for enlarging the light absorption is by intrinsic doping, such as introducing oxygen vacancy in oxides. The typical example is black TiO2, in which lots of Ti3+ species are generated accompanied with the oxygen vacancy, forming self-doping [20,21]. The disorder caused by vacancies and self-doping introduces mid-gap states which forms the band tail states and narrows the band gap. The light absorption of TiO2 was enlarged to around 1000 nm from the 400 nm of white TiO2. However, it is noted that the enlarged light absorption has little contribution to the enhanced performance, because the there is little visible light response for the black TiO2 (Fig. 6) [22]. The enhanced performance is possibly from the better conductivity.
Figure 6. Applied bias photon-to-current efficiency (ABPE) of the TiO2 nanotubes (TNTs) and black titania nanotube (B-TNTs) as a function of applied potential. (b) Incident-Photon-to-Current-conversion Efficiency (IPCE) spectra in the region of 300–700 nm at 0.23 V vs. Ag/AgCl. Reproduced from Ref. 24 with permission from The Royal Society of Chemistry. “
b. The second relevant approach consists in the combination between the classical photoanodes for PEC-WS (especially TiO2) and plasmonic nanoparticles (NPs), which, through different mechanisms, can allow a sort of visible-light sensitization of wide-bandgap materials, or anyway increase their performance in PEC-WS experiments. A suggested review about this topic is Adv. Mater. 2019, doi: 10.1002/adma.201805513.
Discussion on the plasmonic effect has been added to consider this issue.
“Noble metal nanoparticles can also enhance the absorption light scattering/trapping, such as Au nanoparticles coated BiVO4 and hematite [29,30]. However, the plasmonic effect also involves direct electron transfer mechanism (DET) and plasmon induced resonance electron transfer (PIRET) to enhance the performance except the light absorption. Currently the full understanding of the involved physical mechanisms remains elusive, which needs more work to clarify it [31].”
c. Sections “3.1 Nanostructure formation” and “3.2 Band engineering” somehow appear also in Section 4, “4.1 Doping” and “4.2 Nanostructure to shorten the diffusion length”. This is indeed due to the fact that both doping and nanostructuring can induce more effects in the material. However, such repetition can easily create confusion among the readers. It is better to explain doping in Section 3, because the main effect is related to the increased light absorption. Conversely, the discussion about nanostructuring is more suitable to Section 4, as its main effect is to decouple the short hole diffusion length with the long electron diffusion length (as typical in 1D or quasi-1D nanostructures, see also Chem. Soc. Rev. 2017, 46, 3716). In this way, the authors can better focus their explanation.
After carefully thinking about the valuable comments, we still feel it’s better to discuss them in the two separate parts, as our discussion is centered around the key contributing steps of a PEC water splitting process rather than the materials techniques used. We appreciate your suggestion, however, if we group the discussion by the nanostructuring and doing, it will disrupt our current paper structure and flow of discussion
d. In Section 3.2, line 103, the authors mention that anion doping is relatively rare. This is not true: an extensive amount of work has been carried out about anion-doped TiO2, among others (see for instance Chem. Rev. 2014, 114, 9824; Energy Environ. Sci. 2012, 5, 9603). Also, the authors could include a scheme about the difference between cation- and anion-doping (similar to Figs. 6 and 10 in Chem. Rev. 2010, 110, 6503) for a clearer understanding.
Thank you This part has been revised as follows.
“Anion doping was also employed extensively to improve the activity of catalytic activity due to significant adjustments in electronic structure. A lot of attention has been paid on the nonmetal doping into TiO2 to tune the light absorption, such as N doping as well as C, F, S, B doping [15]. For example, N doping greatly expands the light absorption to 700 nm [16].”
3. Section “4. Strategies to improve the charge separation”, should be improved in these specific points.
a. Re-organize the sections 4.1 and 4.2 based on Comment 3c.
We have re-organized this part according to comment 3c, and the gradient doping has been brought forward to doping as one example of doping.
b. The numbering of the sub-sections is not correct (all of them are 4.1).
It has been corrected.
c. Figure 9 should be changed by including a schematics about the concept of nanostructuring to address the problem of short diffusion length (see for instance Fig. 7 in J. Mater. Chem. 2008, 18, 2311); the acronym “ZFO” in the caption must be explained; at least one example of change in photocurrent before/after the nanostructuring should be included.
This part has been revised according to the suggestion. Schematics has been included and also we provide one more figure to illustrate the performance enhancement before and after nanostructure.
the acronym “ZFO” is “ZnFe2O4”, this is added in Figure 16.
d. The section “Heterojunction” should also mention those formed between TiO2 and hematite (Fe2O3), see again Chem. Soc. Rev. 2017, 46, 3716.
Relevant part has been added.
“Another case of heterojunction is α-Fe2O3–TiO2 heterojunction. Some research works found that the Fe2O3–TiO2 heterojunction showed enhancement in the photocurrent [55, 56]. However, somehow it is not consistent with our general understanding on the heterojunction, because hematite has a more positive conduction band potential than TiO2, which prohibits the electron transfer from hematite to TiO2. Further study shows that this kind of heterojunction is through iron titanates (e.g., Fe2TiO5, Fe3TiO4, and FeTiO3) formed at the interface [57].”
4. Regarding Section “5. Strategies to enhance surface charge injection”:
a. The title of 5.1, “co-catalyst loading”, is conceptually wrong. The term “co-catalyst” indeed refers to the photocatalytic case, in which typically semiconductor nanoparticles drive the overall WS reaction, so that they are functionalized with a oxygen-evolution catalyst and a hydrogen-evolution catalyst (co-catalyst), see Chem. Soc. Rev. 2008, 38, 253. The authors should just refer to “catalysts” for the OER reaction.
It has been revised into catalysts.
b. In “5.2 Surface treatment and surface passivation”, the authors mention that point defects at the surface of photoelectrodes act as recombination centers (lines 284–285). This is correct in some cases, but in other cases such defects are in fact deliberately introduce to increase the efficiency, as in the case of black TiO2(see Comment 2a). The authors should better clarify this point (see also J. Phys. Chem. Lett. 2013, 4, 1624). Indeed, later the authors mention that oxygen vacancy sites on BiVO4 are active sites for water oxidation (line 375).
It has been revised to clarify this issue.
“Point defects are usually found on the surface of photoelectrode. In some cases, defects play a positive role in the surface catalysis. However, in some other cases defects serve as recombination centers. Thus, it is important to suppress the recombination on these surface states acting as recombination centers. “
c. In the same section, Figure 17 is not really a good choice as it is almost completely covered by a text. A clearer/better figure is suggested.
Figure 22 (in the revised manuscript) is changed and the text is removed.
d. In the same section, Figure 18 could be placed at the beginning of the section since it helps illustrating the concept of a passivation layer to “heal” surface trap states.
Figure 21 (in the revised manuscript) is now placed at the beginning of “5.2. Surface treatment and surface passivation” .
e. Section “5.2 Active sites” presents a list of equations describing the free energy of the intermediate steps in the OER reaction. This discussion should either be simplified/clarified or removed, because it does not deliver a clear message.
This details OER reaction are now deleted following your suggestion.
f. In the same section, lines 356–362, the authors discuss about Ni(1–x)FexOOH and doped graphene frameworks for the OER. However, such materials are employed as electrocatalysts, they are not photoanodes.
This analysis about “Ni(1–x)FexOOH” is now deleted.
g. In the same section, line 375, the authors mention that vacancies at the BiVO4 surface are active sites for water oxidation. Several works have been also performed on TiO2, see for instance Science 2008, 320, 1755 (and Comment 4b).
To conclude, the authors are encouraged to carefully edit and improve their manuscript. I am available to review a revised version of the manuscript.
It has been included in the revised manuscript.
“Similarly, Ti3d defect state in the band gap of titania also plays an important role in the surface catalysis of defect contained TiO2 [87].”
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
The manuscript is a revised version of a review concerning photoanode materials for photoelectrochemical water splitting (PEC-WS). With respect to the previous version, several corrections have been made, thus increasing the overall quality of the manuscript. Nevertheless, few flaws are still present, which should be amended before the final publication. These points are reported below.
1. The authors have corrected the Introduction section, but some inaccuracies are still present. The stoichiometric coefficients of reaction R2 (the HER) should be multiplied by 2 to balance it with the reaction R1 (OER). In addition, the description provided in the paragraph in lines 33–35 appears misleading. The authors should mention that the two half-reactions together constitute the overall WS process (2H2O → 2H2 + O2), with the thermodynamic threshold energy E0=1.23 eV. Ref. 1 should be substituted with a more appropriate and general source (for instance the book Photoelectrochemical Hydrogen Production by R. van de Krol and M. Grätzel, Springer 2012); Ref. 3 should be deleted (it has not been removed, contrary to the Authors’ response letter); the symbol * in R3–R6 should be defined. In Equation 7 (line 52), the value of the threshold energy should be explicitly indicated (either 1.229 or the approximated value 1.23 should be chosen). Finally, the authors should also explicitly mention that this value limits the bandgap energy of any possible photoanode, i.e. Eg > 1.23 eV.
2. Figure 2 has been updated based on my previous comment 1e. However, in the updated figure the authors have included both a photoanode and a photocathode, while the review focuses only on photoanode materials. So, a picture highlighting a complete cell with a photoanode and a metal cathode is better suited (see for instance Fig. 2c in Chem. Soc. Rev. 2014, 43, 7520). I apologize for the lack of clarity in my previous comment.
3. In Figure 3, line 96, all the letters for the sub-figures should be included.
4. At line 107, the phrase “a valence band above the O 2p valence band” appears confusing, it is better to mention that metal ions introduce donor or acceptor levels above the valence band.
5. The phrase “anion doping has also been extensively employed to improve the activity of catalytic activity…” at line 113 must be corrected: anion doping has been employed to shift the absorption edge from the UV to the visible region, indeed it is one of the strategies to enhance the light absorption.
6. At line 116, the authors refer to the case of “red anatase” (ref. 116), which was obtained by co-doping anatase TiO2 with both N and B, not only N.
7. Figures 11 and 12 may be merged in a single one as they both illustrate the case of the gradient-doped BiVO4 of ref. 39.
8. The references 55-57 inserted in the paragraph at lines 269–275 are not related to the case of heterojunction between α-Fe2O3 and TiO2, but rather to the use of ferroelectrics to improve charge separation (lines 283–295). Appropriate references can be found, for instance, in the suggested review Chem. Soc. Rev. 2017, 46, 3716.
9. In the section 4.1 “Catalysts loading”, the suggested correction from “co-catalyts” to “catalyst” has been performed only in the title. The same should be done throughout all the text, as well as in the Conclusion section. In addition, the acronym “OEC” (line 310) and the nomenclature “R-Fe2O3” (line 312) should be explained.
Finally, some phrases with an incorrect use of English or an unclear meaning should be corrected, for instance: lines 43–44; line 137; lines 194–195; lines 328–329; lines 342–345; lines 353–354; line 361; line 418 (why oxygen-deficient?).
Author Response
The manuscript is a revised version of a review concerning photoanode materials for photoelectrochemical water splitting (PEC-WS). With respect to the previous version, several corrections have been made, thus increasing the overall quality of the manuscript. Nevertheless, few flaws are still present, which should be amended before the final publication. These points are reported below.
1. The authors have corrected the Introduction section, but some inaccuracies are still present. The stoichiometric coefficients of reaction R2 (the HER) should be multiplied by 2 to balance it with the reaction R1 (OER). In addition, the description provided in the paragraph in lines 33–35 appears misleading. The authors should mention that the two half-reactions together constitute the overall WS process (2H2O → 2H2 + O2), with the thermodynamic threshold energy E0=1.23 eV. Ref. 1 should be substituted with a more appropriate and general source (for instance the book Photoelectrochemical Hydrogen Production by R. van de Krol and M. Grätzel, Springer 2012); Ref. 3 should be deleted (it has not been removed, contrary to the Authors’ response letter); the symbol * in R3–R6 should be defined. In Equation 7 (line 52), the value of the threshold energy should be explicitly indicated (either 1.229 or the approximated value 1.23 should be chosen). Finally, the authors should also explicitly mention that this value limits the bandgap energy of any possible photoanode, i.e. Eg > 1.23 eV.
R2 has been corrected.
R2: 4H+(aq)+4e-=2H2
lines 33–35 are corrected according to the comments.
The two half-reactions constitute the overall water splitting process (2H2O → 2H2 + O2), with the thermodynamic threshold energy E0=1.23 eV. Although both reactions are important for the overall water splitting efficiency, the entire process is mainly hindered by the hypokinetic four-electron water oxidation.
Ref. 1 has been replaced by Photoelectrochemical Hydrogen Production by R. van de Krol and M. Grätzel, Springer 2012.
Ref. 3 has been deleted in the revised manuscript.
Symbol * is now explained to indicate a surface site.
One sentence has been added to explain 1.23.
“The number 1.23 in the equation is the approximated value of the threshold energy for water splitting which requires the bandgap energy of any possible photoanode lager than 1.23 eV. ”
2. Figure 2 has been updated based on my previous comment 1e. However, in the updated figure the authors have included both a photoanode and a photocathode, while the review focuses only on photoanode materials. So, a picture highlighting a complete cell with a photoanode and a metal cathode is better suited (see for instance Fig. 2c in Chem. Soc. Rev. 2014, 43, 7520). I apologize for the lack of clarity in my previous comment.
The figure has been changed as suggested.
3. In Figure 3, line 96, all the letters for the sub-figures should be included.
Changed as suggested.
4. At line 107, the phrase “a valence band above the O 2p valence band” appears confusing, it is better to mention that metal ions introduce donor or acceptor levels above the valence band.
This sentence changed to “incorporation of metal ions will introduce donor level by hybridizing the O 2p valence orbitals with a metal ion having high-lying valence orbitals such as Bi(III), Ag(I), Sn(II), Pb(II), or Cu(I).”
5. The phrase “anion doping has also been extensively employed to improve the activity of catalytic activity…” at line 113 must be corrected: anion doping has been employed to shift the absorption edge from the UV to the visible region, indeed it is one of the strategies to enhance the light absorption.
This sentence is changed to “Anion doping has been employed to shift the absorption edge from the UV to the visible region, indeed it is one of the strategies to enhance the light absorption.”
6. At line 116, the authors refer to the case of “red anatase” (ref. 116), which was obtained by co-doping anatase TiO2 with both N and B, not only N.
This sentence is changed to “N and B doping greatly expands the light absorption to 700 nm.”
7. Figures 11 and 12 may be merged in a single one as they both illustrate the case of the gradient-doped BiVO4 of ref. 39.
Figures 11 and 12 have been merged in Figures 11.
8. The references 55-57 inserted in the paragraph at lines 269–275 are not related to the case of heterojunction between α-Fe2O3 and TiO2, but rather to the use of ferroelectrics to improve charge separation (lines 283–295). Appropriate references can be found, for instance, in the suggested review Chem. Soc. Rev. 2017, 46, 3716.
references 55-57 have been moved into the 3.4 Internal electric field to improve the charge separation
“Another case of charge separation is α-Fe2O3–TiO2 heterojunction. Some research works found that the Fe2O3–TiO2 heterojunction showed enhancement in the photocurrent [63,64]. However, somehow it is not consistent with our general understanding on the heterojunction, because hematite has a more positive conduction band potential than TiO2, which prohibits the electron transfer from hematite to TiO2. Further study shows that this kind of heterojunction is through iron titanates (e.g., Fe2TiO5, Fe3TiO4, and FeTiO3) formed at the interface [55].”
and Chem. Soc. Rev. 2017, 46, 3716 have been added in 3.3. Heterojunction
“Another case of heterojunction is α-Fe2O3–TiO2 heterojunction. Holes generated in TiO2 will be transferred to α-Fe2O3 with electrons preferentially accumulating in TiO2 at the same time. Under specific conditions, hybrid composites such as Fe2TiO5, Fe3TiO4, and FeTiO3 form at the interface between hematite and titania that, despite some controversial results, exhibit remarkable performances as photoanode materials in PEC devices [55]”
9. In the section 4.1 “Catalysts loading”, the suggested correction from “co-catalyts” to “catalyst” has been performed only in the title. The same should be done throughout all the text, as well as in the Conclusion section. In addition, the acronym “OEC” (line 310) and the nomenclature “R-Fe2O3” (line 312) should be explained.
“co-catalyts” and “ cocatalyts” are changed to “catalysts loading”
“OEC” is changed to “oxygen evolution catalyst”
“R-Fe2O3 ” is changed to “Fe2O3”
10.Finally, some phrases with an incorrect use of English or an unclear meaning should be corrected, for instance: lines 43–44; line 137; lines 194–195; lines 328–329; lines 342–345; lines 353–354; line 361; line 418 (why oxygen-deficient?).
lines 43–44
This sentence is changed into
“However, there is still lack of cheap and stable photoanode materials with sufficient efficiency for water oxidation. Although significant progress has been made by developing new materials, nanostructures, and other new concepts over the past decades, efficiencies are still below our expectation.”
line 137
This sentence is changed into
“Doping and presence of lattice vacancies introduce mid-gap states which form the band tails and narrow the band gap.”
lines 194–195
This sentence is changed into
“Recently, the performance of a new promising material Zn2FeO4 for PEC water splitting was improved by Ti doping”
lines 328–329
This sentence is changed into
“Thus, it is important to suppress the recombination on these surface centers.”
This sentence is changed into
“This is mainly due to the pretreatment that can remove the surface recombination center MoOx which is segregated during the preparation of the photoelectrode.”
lines 353–354
This sentence is changed into
“Originally, passivation layers were applied to semiconductor photoelectrodes to improve their photochemical or chemical stability.”
line 361
This sentence is changed into
“Besides, IrO2 and CoOx thin layers on TaON photoanodes were found to improve the durability by reducing the self-oxidation of the electrode”
line 418
we have changed “oxygen-deficient” into “n-type” in the sentence
“We have summarized the strategies to enhance the light absorption, charge separation, and surface charge injection of this class of n-type semiconductor oxide materials.”
Author Response File: Author Response.pdf
