Figures of Merit for Photocatalysis: Comparison of NiO/La-NaTaO₃ and Synechocystis sp. PCC 6803 as a Semiconductor and a Bio-Photocatalyst for Water Splitting

Round 1

Reviewer 1 Report

The manuscript "Figures of Merit for Photocatalysis: Comparison of NiO/La-NaTaO3 and Synechocystis sp. PCC 6803 as a Semiconductor and a Bio-Photocatalyst for Water Splitting" is interesting and well written with an adequate introduction regarding the importance of the FOM. Nevertheless, some key points must be reviewed and discussed as follows:

1. The described and considered FOM are those usually reported. However, transport phenomena have an important impact on catalysis. Mixing, for example, could improve the catalyst's global performance, and this is not considered, in deep. 
2. The dry cell base to "normalize" the catalyst's performance does not consider the liquid contribution to the catalytic process. Please discuss in detail this.
3. The radiation intensity has a key role in catalytic performance. How do the authors corroborate the equivalence between 10 and 200 mW cm-2 to construct Figure 1?
4. The relation to catalyst mass is useful for scaling porpoises and comparing solid heterogeneous catalysts, which is not exactly the case in this study. Active sites can be useful for comparing results in such catalysts. The authors could provide the threshold for each section, in terms of the surface area of bacteria or catalyst, because the mentioned limits in this section are very wide established.
5. The bandgap is reported in the supplementary information; however, it is not related to the effect of the intensity of light on the initial reaction rate. Please enrich the discussion considering the bandgap.
6. Particle size in the case of photocatalyst is another key parameter for performance. This is not discussed so far. Please provide discussion and related references on this subject.
7. The initial rate usually is determined during the initial minutes of reaction, not during 3 hours. Please explain what are the considerations for using such time, especially when the experimental data show an important lag of H2 or O2 production. 
8. The statement in lines 529-533 is partially true since not all the photocatalysts work in the UV range. Indeed, many researchers are looking for activation range toward the visible light range, and some others have been achieved this with very competitive and promissory results.
9. Please provide the JCPDS card to establish the planes of the photocatalyst. 

Author Response

In Response to Reviewer 1

1. Comment: “The described and considered FOM are those usually reported. However, transport phenomena have an important impact on catalysis. Mixing, for example, could improve the catalyst's global performance, and this is not considered, in deep”

Response and action taken: We thank the reviewer for her/his helpful comment. It is indeed true that transport phenomena may have a major impact on photocatalytic reactions. However, a limitation by mass transfer effects is likely not the case in the present study as severe limitations of light absorption due to limited light intensity were observed. Therefore, the following paragraph was added to the introduction: “Additionally, other authors [52,53] pointed out that mass transfer has so far not been investigated in detail for photocatalytic reactions, and even less for the specific reaction of photocatalytic water splitting. In this context, Ballari et al. [54] investigated mass transfer limitations in photocatalytic slurry reactions proving their existence. Though they stated that photocatalytic reactions using semiconductors in general can be considered not to be mass transfer limited at low catalyst loadings (< 1 g dm^-3), low irradiation rates (< 1·10^-6 mol cm^-2 s^-1), appropriate mixing conditions or for slow photocatalytic reactions. As the conditions for light intensity are matched in this study (cf. section 2.3.2), thorough mixing of gas and liquid phase is ensured by the experimental setup (cf. section 3.3) and the investigated reaction takes place in a timespan of hours mass-transfer limitations are not considered to be relevant for the discussion of the presented results.”. Further the paragraph “However, it is likely that the observed dependence will change at even higher light intensities towards a response equal to the square root of the light intensity, as shown for example by Tabata et al. [72] for photocatalytic water splitting over K₄Nb₆O₁₇. Further, according to Bloh [73] this implies that the reaction over NiO/La-NTO is purely governed by the photon flux and effects like mass-transfer limitations can be neglected. This is likely to be caused by the wide band gap of NiO/La-NTO, i.e., 4.08 eV based on the UV/Vis diffuse reflectance determination (see ESI Figure S20), which restricts the utilizable spectral range.” was added in the discussion (section 2.3.2) to clarify this issue. In this context, the following references were added: [52] Energy Environ. Sci., 2021, 11, 60; [53] ACS Catal., 2017, 7, 8006; [54] Chem. Eng. Sci., 2010, 65, 4931-4942; [72] Catal. Lett., 1994, 28, 417-422; [73] Front. Chem., 2019, 7, 128.

2. Comment: “The dry cell base to "normalize" the catalyst's performance does not consider the liquid contribution to the catalytic process. Please discuss in detail this.”

Response: The reviewer is right that the cell dry weight does not consider the contribution of liquid in the cells. In the manuscript page 10, lines 300-310 (former lines 280-290) the effect on the photocatalyst comparison and related figures of merit is discussed exemplarily for the comparison of 16.3 mg Synechocystis and 100 mg NiO/La-NTO.

3. Comment: The radiation intensity has a key role in catalytic performance. How do the authors corroborate the equivalence between 10 and 200 mW cm-2 to construct Figure 1?

Response and action taken: We thank the reviewer for her/his care and precision. Both, 10 and 200 mW cm^-2 (without light filter) were used for Synechocystis and NiO/La-NTO respectively, as these were the light intensities which lead to the highest observed photocatalytic activity during the investigation for the dependence of the photocatalytic activity on the light intensity displayed in Figure 3. We have thus added this information together with a reference to Figure 3 in the figure caption of Figure 1: “…at their respective light intensity for maximum photocatalytic activity (see Figure 3), i.e., using 10 mW cm^-2 for Synechocystis and 200 mW cm^-2 for NiO/La-NTO”.

4. Comment: “The relation to catalyst mass is useful for scaling porpoises and comparing solid heterogeneous catalysts, which is not exactly the case in this study. Active sites can be useful for comparing results in such catalysts. The authors could provide the threshold for each section, in terms of the surface area of bacteria or catalyst, because the mentioned limits in this section are very wide established.”

Response: We agree that a comparison based on the number of active sites available for each photocatalyst could provide meaningful insights. However, identification of the rate-limiting active site in the case of semiconductor photocatalysts is not trivial and sometimes even elusive. With respect to the surface area, we consider a comparison based on the surface (both external surface and if either the cell interior in the case of bio-photocatalysts or the pores in the case of semiconductor photocatalysts are included) does not provide meaningful insight and might even be misleading, since the difference between the systems for such a comparison is too general. To shortly illustrate this, the surface area of La-NTO was determined via N₂-sorption to be 2-3 m² g^-1. Though we did not determine the exact surface area of Synechocystis in our study, an approximate value for the surface area of the wild strain also used in this study can be found in the literature (J. Appl. Phycol., 2014, 26, 1, doi: 10.1007/s10811-013-0103-7), which is around 80.71 μm² cell^-1. While the OD₇₅₀ = 1 corresponds to a cell density of approximately 4·10⁸ cells mL^-1, resulting in 4·10¹⁰ cells for 100 mL of reaction solution used in a typical photocatalytic reaction in the study. This would give a total external surface area of 3200000 m² for the bio-photocatalyst, compared to 0.2 m² of accessible surface area for La-NTO. Thus, the resulting difference in surface area is seven orders of magnitude, while the difference of the initial reaction rate is one to two orders of magnitude. As can be seen from this calculation, the result of the comparison of NiO/La-NTO and Synechocystis could be very misleading. Therefore, the goal of this study was to discuss suitable Figures of merit to compare these rather different catalyst systems.

5. Comment: “The bandgap is reported in the supplementary information; however, it is not related to the effect of the intensity of light on the initial reaction rate. Please enrich the discussion considering the bandgap”

Response and action taken: We thank the reviewer for her/his suggestion. To clarify that the band gap is the crucial material property we rephrased lines 213-214 on page 7 (former lines 193-194) and added the value of the band gap along with an explanation how the band gap leads to a restricted light absorption and thus affects the dependency of the light intensity observed in the study: “Further, according to Bloh [72] this implies that the reaction over NiO/La-NTO is purely governed by the photon flux and effects like mass-transfer limitations can be neglected. We assume this to be caused by the wide band gap of NiO/La-NTO, i.e., 4.08 eV based on the UV/Vis diffuse reflectance determination (see ESI Figure S20), which restricts the utilizable spectral range.”

6. Comment: “Particle size in the case of photocatalyst is another key parameter for performance. This is not discussed so far. Please provide discussion and related references on this subject.”

Response and action taken: We thank the reviewer for her/his helpful comment and agree that in semiconductor based photocatalysis the particle size plays an important role. Although this was originally not considered due to the implications with cell growth and decay of bio-photocatalysts we added a paragraph in the introduction to discuss this matter: “A further aspect to be considered in semiconductor photocatalysis is the catalyst particle size. Many studies have investigated particle size effects for different photocatalysts and found that the interplay of factors such as specific surface area [55,56], charge-carrier dynamics [57,58] and light absorption [58,59] is crucial. Especially, if photocatalysts of similar composition are compared, the explanation of observed differences in catalytic activity needs to consider particle size effects, which are hardly covered by existing FOMs [60]. However, particle size effects are not considered in this study, because the cell size of the bio-photocatalyst cannot be controlled and is subject to changes due to growth and decay processes.” In this context the following references were added: [55] Nanomaterials, 2020, 10; [56] Catal. Sci. Technol., 2020, 10, 6274-6284; [57] J. Phys. Chem. C, 2013, 117, 22584-22590; [58] Adv. Funct. Mater, 2021, 31 2102468; [59] Appl. Catal. B, 2018, 221, 1-8; [60] Electrochim. Acta, 1995, 40 1277-1281.

7. Comment: “The initial rate usually is determined during the initial minutes of reaction, not during 3 hours. Please explain what are the considerations for using such time, especially when the experimental data show an important lag of H2 or O2 production.”

Response and action taken: We agree with the reviewer that typically the initial reaction rate is only determined right at the beginning of the reaction. However, considering that this is typically done to avoid deactivation phenomena, which we do not observe even at longer reaction times, and since in our case we need to form enough product to reliably detect the product in the gas phase via GC-TCD, we decided to increase the reaction time for the determination of the initial reaction rate to improve data quality. This information is now added in the experimental section “The initial reaction rate (r_ini) was determined based on the first 3 h of observed product evolution. This period was chosen to ensure that enough product was present in the gas phase to enable detection via GC-TCD,...”. In addition, Figure S18 was added to the ESI showing the H₂ evolution over NiO/La-NTO for 20 h of reaction, where no deactivation can be observed.

8. Comment: “The statement in lines 529-533 is partially true since not all the photocatalysts work in the UV range. Indeed, many researchers are looking for activation range toward the visible light range, and some others have been achieved this with very competitive and promissory results”

Response and action taken: We thank the reviewer for his precision and fully agree that many researchers successfully work on the extension of photocatalysis to the visible spectral range. Hence, we rephrased the statement to be clear that the semiconductor used in this study is referred to: “The semiconductor photocatalyst of this study, on the other hand, requires UV-light with a high light intensity.”

9. Comment: “Please provide the JCPDS card to establish the planes of the photocatalyst.”

Response and action taken: The reference diffraction patterns for NaTaO₃ (Pbnm space group) and NiO (Fm-3m space group) were added to the graph displaying the XRD patterns. In this respect, the references [80] (Dalton Trans., 2020, 49, 10994-11004) and [81] (J. Solid State Chem., 2016, 238, 109-112) were added.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript reports the comparison of NiO/La-Na- 2 TaO3 semiconductors and Synechocystis sp. PCC 6803 bio-Photocatalyst for photocatalytic Water Splitting. The concept of figures of merit (FOMs)  (i.e reaction rate, photocatalytic space time yield, a redefined apparent quantum yield, reactor geometry, reactor size, light source and illuminated area) for comparing catalytic performance is interesting for phtotcatalytic applications. The article is nicely presented and has a broad interest to the readers. Hence, this article could be accepted. However, I have the following queries.

1.Authors should give more clear details about the purpose of the work in the introduction section. The novelty or significance of this work is not clearly stated. The strategies of the FOMs selection must be fundamentally illustrated. Improve the introduction with more recent literature.

2.Materials section must be presented clearly. Most of the details of the instrument have not been described in the manuscript.

3.Every part of the figures in the article needs to be carefully checked and modified.

3.There are some errors in the paper, so the Authors are encouraged to review the form and the English of the manuscript.

Author Response

In Response to Reviewer 2

Comment 1: “Authors should give more clear details about the purpose of the work in the introduction section. The novelty or significance of this work is not clearly stated. The strategies of the FOMs selection must be fundamentally illustrated. Improve the introduction with more recent literature.”

Response and action taken: We appreciate the reviewer’s suggestion to improve the introductory section and emphasize the significance of the presented work more strongly. For this reason, we rephrased the final part of the introduction stating the goals of the study and contribution to the field to make the contribution of our work clearer: “This will also provide further insight into the functions and operation principles of different photocatalyst types allowing to connect especially the fields of heterogeneous, semiconductor photocatalysis and bio-photocatalysis.”

In view of the strategy and reasoning for the figure of merit selection, we would like to highlight chapter 2.1 (“Selection of Figures of Merit”), where the selection of the used figures of merit has been extensively explained and justified. Additionally, as we also stated later in the conclusions, figures of merit should be easily accessible to ensure widespread general use and thus provide a broad basis of data amongst which scientific publications  in the field can be compared.

With respect to the recommendation to extend the cited literature by more recent works, we added the following citations throughout the introductory section: [6] Sol. RLL, 2021, 5, 2100177; [11] Environ. Technol. Innov., 2021, 22, 101471; [12] Nat. Commun., 2021, 12, 2528; [13] J. Phys. Chem. C, 2021, 125, 9730; [14] J. Colloid Interface Sci., 2022, 605, 727-740; [15] Catalysts, 2020, 10, 1166; [16] J. Hazard. Mater., 2022, 421, 126719; [17] Chem. Eng. J., 2021, 426, 131217; [18] Chem. Eng. J., 2020, 374, 123258; [35] Adv. Energy Mater., 2021, 11, 2101566; [44] Chem. Eng. Res. Des., 2020, 154, 135-150; [52] Energy Environ. Sci., 2021, 11, 60; [53] ACS Catal., 2017, 7, 8006; [54] Chem. Eng. Sci., 2010, 65, 4931-4942; [55] Nanomaterials, 2020, 10; [56] Catal. Sci. Technol., 2020, 10, 6274-6284; [57] J. Phys. Chem. C, 2013, 117, 22584-22590; [58] Adv. Funct. Mater, 2021, 31 2102468; [59] Appl. Catal. B, 2018, 221, 1-8; [73] Front. Chem., 2019, 7, 128.

Comment 2: “Materials section must be presented clearly. Most of the details of the instrument have not been described in the manuscript.”

Response: We thank the reviewer for her/his comment and thoroughness. In this respect, we would like to direct the reviewer’s attention to the ESI, where details such as conditions and equipment used for the catalyst characterization (section 2 and 3) and photocatalytic reaction (section 4) are given.

Comment 3: “Every part of the figures in the article needs to be carefully checked and modified.”

Response and action taken: We thank the reviewer for her/his attention to detail. We checked all figures of the manuscript including the ESI again. Different details in the figures 1 and 3 in the manuscript, and figures S9b, S14 – S17 and S19 in the ESI have been corrected.

Comment 4: “There are some errors in the paper, so the Authors are encouraged to review the form and the English of the manuscript.”

Response and action taken: We thank the reviewer for her/his diligence. We thoroughly checked the manuscript and corrected spelling mistakes and grammatical errors where necessary.

Author Response File: Author Response.pdf

Reviewer 3 Report

This work entitled “Comparison of NiO/La-NaTaO3 and Synechocystis sp. PCC 6803 as a Semiconductor and 3 a Bio-Photocatalyst for Water Splitting” found that the combination of reactor and photocatalyst design both semiconductor and bio-photocatalyst is conducive maximizing the photocatalytic performance. Interesting work. The photocatalytic performance has been studied and compared. However, some issues should be solved before publication:

1. The standard XRD card of pristine NiO and Na-TaO₃ samples should be inserted in Figure S18.
2. Authors may request to discuss about the importunate of semiconductor-based photocatalysis in the introduction part. J. Colloid Interface Sci., 2022, 605, 727-740; Catalysts, 2020, 10, 1166; Solar RRL 2021, 5, 2100177; Dalton Transactions, 50, 13801.
3. In the TEM image (Figure S20), the NiO and La-NTO should be clearly indexed.
4. How about the stability?

Author Response

In Response to Reviewer 3

Comment 1: “The standard XRD card of pristine NiO and Na-TaO3 samples should be inserted in Figure S18.”

Response and action taken: The reference diffraction patterns for NaTaO₃ (Pbnm space group) and NiO (Fm-3m space group) were added to the graph displaying the XRD patterns (now Figure S19). In this respect, the references [80] (Dalton Trans., 2020, 49, 10994-11004) and [81] (J. Solid State Chem., 2016, 238, 109-112) were added.

Comment 2: “Authors may request to discuss about the importunate of semiconductor-based photocatalysis in the introduction part. J. Colloid Interface Sci., 2022, 605, 727-740; Catalysts, 2020, 10, 1166; Solar RRL 2021, 5, 2100177; Dalton Transactions, 50, 13801.”

Response and action taken: We thank the reviewer for referring to the literature references which were not cited in our manuscript. To include these important issues, we made the following addition to the introduction: “Besides, photocatalysts can be used in a wide range of applications to tackle problems important to society and environment, e.g., air [11,12] and water purification [13,14], organic pollutant degradation [15,16], or bacterial disinfection [17,18].” In this respect, the following references were added: [6] Sol. RLL, 2021, 5, 2100177; [11] Environ. Technol. Innov., 2021, 22, 101471; [12] Nat. Commun., 2021, 12, 2528; [13] J. Phys. Chem. C, 2021, 125, 9730; [14] J. Colloid Interface Sci., 2022, 605, 727-740; [15] Catalysts, 2020, 10, 1166; [16] J. Hazard. Mater., 2022, 421, 126719; [17] Chem. Eng. J., 2021, 426, 131217; [18] Chem. Eng. J., 2020, 374, 123258.

Comment 3: “In the TEM image (Figure S20), the NiO and La-NTO should be clearly indexed.”

Response: We thank the reviewer for his suggestion. Unfortunately, the resolution of the TEM image displayed in Figure S20 is not high enough to allow for a lattice plane determination. Further, the NiO content (0.16 wt.-% Ni) in combination with a high dispersion according to literature (reference [31] – J. Am. Chem. Soc., 2003, 125, 3082-3089) does not allow to observe NiO particles. The image was meant to give information on the semiconductor particle size.

Comment 4: “How about the stability?”

Response and action taken: Except for the special case of deactivation of the bio-photocatalyst discussed on page 9 (chapter 2.3.2) we did not observe deactivation of the studied photocatalysts during the time of the experiments. The photocatalyst stability was investigated for longer periods in case of NiO/La-NTO. Over 20 h of reaction, no deactivation was observed. To illustrate this, Figure S18 was added to the ESI.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Thank you for attending the reviewer's comments