The Emerging Career of Strontium Titanates in Photocatalytic Applications: A Review

Round 1

Reviewer 1 Report

This review paper summarizes the synthesis and photocatalytic applications of strontium titanates. I reject this paper on the following basis.

 

1.         The submitted version is not appropriate as a review paper. The study is very limited.

 

2.         As per the title “A review on strontium titanates in photocatalytic applications”, the review must describe the photocatalytic applications of strontium titanates in detail. Whereas only one small paragraph is written on the application in hydrogen production and one short paragraph on photocatalytic degradation of organic pollutants. A full review paper should describe all applications in detail describing the effect of various reagents and reaction conditions on the efficiency of strontium titanates. The mechanism of reaction should also be mentioned describing how strontium titanates participate in the reaction. What is the role of strontium titanate in a particular reaction? Figures, charts, and flow diagrams should be added for various details.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript by Nikita Sharma et al. entitled " A review on strontium titanates in photocatalytic applications" is based on the strontium titanates (SrTiO3) that play an important role and have shown potential especially in the field of hydrogen production. This review summarizes the significance of (SrTiO3) in photocatalytic water splitting to produce hydrogen and photocatalytic degradation of the pollutants from the waste water. Different synthesis methods used for preparing SrTiO3 are also discussed. The work is original, well-organized, novel and present sufficient novelty for being accepted in Catalysts. However, there are few minor comments, which need to be properly addressed to improve the manuscript quality.

* The manuscript title can be further improved to make it more comprehensive and attractive reflecting the key content of this study.

* The last paragraph of introduction part needs improvement.
* The keywords can be written in Alphabetical order for better representation in the manuscript.

* Please readjust the font sizes in all figures to make them readable.
* There are several superscript and subscript errors in the manuscript. Please correct.

* Make sure all abbreviations are written out in full the first time used. This is particularly important in the abstract and the conclusions but work through the entire ms carefully from this perspective.
* Some related latest literature should be updated, such as Journal of Industrial and Engineering Chemistry 98, 283-288, 2021., etc.

* The conclusion part needs improvement.
* Manuscripts published in Catalysts must explain the significant advances provided in approaches and understanding compared to previous literature, and/or demonstrate convincingly potential in new applications. The Conclusions of your paper are especially important for this. Therefore, please try to sharpen this further. The optimal Conclusion should include:
- A summary of your key findings.
- A highlight of your hypothesis, new concepts, and innovations.
- A summary of key improvements compared to findings in the literature [provide a couple of references to indicate key improvements].
- Your vision for future work.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Referee report on the manuscript “A review on strontium titanates in photocatalytic applications”  by Nikita Sharma and Klara Hernadi.

This review looks good and can be recommended for publication, but only after a few significant improvements.

1. The introduction does not contain any supporting references and this is completely unacceptable. This manuscript is a review !!! Literature review .... At the same time, the first almost two pages do not contain links – this is nonsense !!!

2. “2. Properties of SrTiO3”. This part requires more factual data (lattice constants, band gap, properties of major impurities). See, for example, Table 1 in Eglitis, R. I., & Popov, A. I. (2018). Systematic trends in (0 0 1) surface ab initio calculations of ABO3 perovskites. Journal of Saudi Chemical Society, 22(4), 459-468.

https://doi.org/10.1016/j.jscs.2017.05.011

3. To interest more readers, a brief paragraph on recent theoretical work is also needed, especially since surface termination, point defects, band gap modulation are important.  See, for example:

Rusevich, L. L., et al (2020). Ab initio calculations of structural, electronic and vibrational properties of BaTiO3 and SrTiO3 perovskite crystals with oxygen vacancies. Low Temperature Physics, 46(12), 1185-1195.

Eglitis, Roberts, et al. "Systematic trends in YAlO3, SrTiO3, BaTiO3, BaZrO3 (001) and (111) surface ab initio calculations." International Journal of Modern Physics B 33 (2019): 1950390.

4. Table 1 should be supplemented with one or two columns, where it is necessary to demonstrate how the properties of nanocrystals differ from the method (one additional column is the size of nanoparticles, and the second is the band gap).

5. The same for Tables 2 and 3.

In principle, this is a rather interesting topic, which, of course, needs to be developed and promoted, the results obtained are interesting and can be recommended for publication after a detailed consideration and disclosure of the above-mentioned ambiguities.

 

 

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 4 Report

By careful reading the article, it can be found that the author has a clearer understanding of strontium titanates, but what is lacking in beauty is that the author has not systematically described the surface properties of strontium titanates and the essential reasons for the improvement of photocatalytic hydrogen production and degradation performance, which are some defects of this review. On the whole, it can meet the requirements of publication.

1. When describing the properties of SrTiO₃, the author could describe the special surface properties, because the special structure of strontium titanates conducive to improve the photocatalytic performance.

2. In the description at 229-231, "its perovskite structure provides an advantage to adapt the dopants and introduce some defects which enhance the interaction with the advanced molecules." What is the specific perovskite structure and how to introduce defects?

3. In view of the problems of strontium titanates in photocatalytic hydrogen production and photodegradation of pollutants, can the author propose his own scheme?

 

Author Response

Many thanks for your positive feedback.

Q1. When describing the properties of SrTiO₃, the author could describe the special surface properties, because the special structure of strontium titanates conducive to improve the photocatalytic performance.

Ans. Thank you for your suggestion. We agree completely since the surface properties of SrTiO₃ are very important for catalysis. Therefore, we have added following paragraph into the manuscript highlighted in yellow.

The surface properties of SrTiO₃ is very important from the catalysis perspective. There are many works reported in the literature concerning SrTiO₃ (100) surface properties. The (100) surface of SrTiO₃ is known to be more stable and much easier to prepare than (110) and (111) surfaces. For the photocatalytic degradation of organic pollutants, the {100} surface works best while for hydrogen evolution, the {110} surface facets is the most active phase. The optical and photocatalytic properties are facet-dependent. Hseih et al. recently studied the necessity of incorporating tunable degrees of surface-dependent band bending. The {110} facets are much more efficient for hydrogen production reaction since it facilitates the migration of photoexcited electrons to this surface which promotes the H⁺ reduction to produce H₂ gas [1]. The study by Qiang et al. showed that the TiO₂-terminated surface behaves very differently from the SrO-terminated one and that the surface structure is a sensitive function of oxygen partial pressure and temperature [2]. These studies show that the careful designing of the facets of the photocatalyst may lead to an efficient photocatalyst for photocatalytic applications.

Q2. In the description at 229-231, "its perovskite structure provides an advantage to adapt the dopants and introduce some defects which enhance the interaction with the advanced molecules." What is the specific perovskite structure and how to introduce defects?

Ans. Thank you for your question. A perovskite is a material with the same crystal structure as the mineral calcium titanium oxide which was the first discovered perovskite crystal. The easiest way to describe perovskite structure is a cubic unit cell with titanium atoms at the corners, oxygen atoms at the midpoints of the edge and a calcium atom in the center. Any material that has the generic form ABX₃ and same crystallographic structure as perovskite mineral is considered to have perovskite structure. The ions A and B are the cations with a total charge of +6. The cations A are generally in low valence such as Ca, Pb, Sr, and La and occupies 12-fold oxygen coordinated holes. The cations B such as Co, Cr., Fe, Ni, Ti and Zr, reside in six-fold (octahedral) coordination holes. The oxygen vacancies are kind of defects which can be easily created in perovskite structures as mentioned above. These vacancies can be created by the substitution of A or B cations, fully or partially, with lower-valence cations such that the total charge of these cations together is less than +6.  The ions A or B can be selectively substituted by introducing aliovalent or isovalent cations. Therefore, they have flexible structure and can accommodate considerable amount of vacancies. The advantage of perovskite structure is the high sensitivity to compositional and oxygen stoichiometry which optimizes their catalytic and electrical properties which is an-added advantage.

It is known that by introducing defects photocatalytic efficiency of SrTiO₃ could be enhanced. There are different types of lattice defects but oxygen vacancy is the majority defect that appears in metal oxide photocatalysts. These defects can be introduced in many ways or can be created accidentally during the synthesis. A small amount of lattice oxygen is released to the gas phase due to the dissociation of Ti-O bonds, which leads to the creation of oxygen vacancies along with the release of free electrons in the lattice. The free electron reduces Ti⁴⁺ to Ti³⁺. Therefore, relevant defect species are Ti³⁺ and oxygen vacancy. The defect caused by oxygen vacancy is called surface defect. Ion doping is another very common method that create some defects in the form of ion vacancies. In this way, the photocatalytic property of SrTiO₃ has been reported to improve [3–6]. Doping with metal or non-metal is known to induce defects in the lattice structure of SrTiO₃. The defect engineering is an emerging area of study and is an efficient way of narrowing band gap instead of introducing impurity phase. This prevents the inhomogeneity and instability of the catalyst [7]. Strontium vacancies have also been reported and have shown promising results, for instance, inducing ferroelectricity in thin films [8].

Q3. In view of the problems of strontium titanates in photocatalytic hydrogen production and photodegradation of pollutants, can the author propose his own scheme?

Ans. Thank you for your question. This is an interesting point. Since this is a review article, we haven’t shared our results so far. We plan to submit a separate manuscript dealing with our own results and proposing our own scheme in near future. We hope we could answer your queries.

References:

[1]      P.L. Hsieh, G. Naresh, Y.S. Huang, C.W. Tsao, Y.J. Hsu, L.J. Chen, M.H. Huang, Shape-Tunable SrTiO3 Crystals Revealing Facet-Dependent Optical and Photocatalytic Properties, J. Phys. Chem. C. 123 (2019) 13664–13671. https://doi.org/10.1021/acs.jpcc.9b02081.

[2]      Z.Q. Li, J.L. Zhu, Relaxations of- and SrO-terminated (001) surfaces, Phys. Rev. B - Condens. Matter Mater. Phys. 58 (1998) 8075–8078. https://doi.org/10.1103/PhysRevB.58.8075.

[3]      Y. Wu, T. He, Ag loading induced visible light photocatalytic activity for pervoskite SrTiO3 nanofibers, Spectrochim. Acta - Part A Mol. Biomol. Spectrosc. 199 (2018) 283–289. https://doi.org/10.1016/j.saa.2018.03.078.

[4]      E. Grabowska, M. Marchelek, T. Klimczuk, W. Lisowski, A. Zaleska-Medynska, TiO2/SrTiO3 and SrTiO3 microspheres decorated with Rh, Ru or Pt nanoparticles: Highly UV–vis responsible photoactivity and mechanism, J. Catal. 350 (2017) 159–173. https://doi.org/10.1016/j.jcat.2017.04.005.

[5]      X. Li, Z. Ge, F. Xue, H. Liu, B. Lyu, M. Liu, Lattice-oriented contact in Pd/SrTiO3 heterojunction for rapid electron transfer during photocatalytic H2 production, Mater. Res. Bull. 123 (2020) 110722. https://doi.org/10.1016/j.materresbull.2019.110722.

[6]      T.H. Chiang, H. Lyu, T. Hisatomi, Y. Goto, T. Takata, M. Katayama, T. Minegishi, K. Domen, Efficient Photocatalytic Water Splitting Using Al-Doped SrTiO3 Coloaded with Molybdenum Oxide and Rhodium-Chromium Oxide, ACS Catal. 8 (2018) 2782–2788. https://doi.org/10.1021/acscatal.7b04264.

[7]      L. Gu, H. Wei, Z. Peng, H. Wu, Defects enhanced photocatalytic performances in SrTiO3 using laser-melting treatment, J. Mater. Res. 32 (2017) 748–756. https://doi.org/10.1557/jmr.2016.461.

[8]      Y.S. Kim, J. Kim, S.J. Moon, W.S. Choi, Y.J. Chang, J.G. Yoon, J. Yu, J.S. Chung, T.W. Noh, Localized electronic states induced by defects and possible origin of ferroelectricity in strontium titanate thin films, Appl. Phys. Lett. 94 (2009) 88–91. https://doi.org/10.1063/1.3139767.

 

Round 2

Reviewer 1 Report

The authors do not seem to be welcoming to reviewers' comments. Reviewers give comments to improve the quality of work and the authors should take these positively and the disagreement should be shown in a decent way.

This review paper is still showing limited study. A review paper should describe various studies already done on the same or related materials. It is not copying and pasting. Review means to give a detailed understanding of all the aspects to the reader in light of recent developments. The authors are suggested to improve the review by adding descriptions on following

1. Various factors or reaction conditions affecting the efficiency of the SrTiO3 should be discussed for both applications.

2. How SrTiO3 as a support material helps to improve the efficiency of various metals loaded on it.

3. The authors should describe various advantages, drawbacks/challenges, and future aspects for improvement of the efficiency of SrTiO3 for each application. The authors should also describe the recent advantages in the field.

4. The old references should be replaced by the recent ones within the past five years to make the review more up-to-date and authentic for recent advantages.

Author Response

Many thanks for your feedback and further comments which are highly appreciated to improve the quality of the manuscript.

Q1. Various factors or reaction conditions affecting the efficiency of the SrTiO₃ should be discussed for both applications.

Ans. Thank you for your suggestion. We agree with your suggestion, indeed, there are important factors that can have an effect on the overall efficiency of SrTiO₃. Below we have discussed some factors. Different synthesis procedure and calcination temperature can affect significantly the performance of strontium titanates.

Effect of calcination time and temperature:

Calcination temperature is a very important factor that can remarkably affect the photocatalytic performance of SrTiO₃. This is because a number of properties like surface area, porosity, adsorption rate or crystallization can be affected with changes in temperature conditions. Aizhong et al. studied the removal of malachite green dye by Ni,La-SrTiO₃ and reported the effect of calcination temperature on the performance of SrTiO₃ [9]. The low calcination temperature resulted in higher photocatalytic degradation due to the combination of properties obtained like large surface area and pore volume, high visible light absorbing ability and high adsorption rates. The study of Hong et al. discloses their effect in addition to sulphur content on the decomposition of methylene blue by SrTiO₃. Other than this, the heating rate during the crystallization process is also known to affect the overall efficiency of SrTiO₃. For instance, in the study of Luis et al. the lowest heating rate were found to result in the best photocatalytic activity of SrTiO₃ nanoparicles for methylene blue removal [10].

Effect of the metal type and loading concentration of the metal:

The effect of metal loading was seen on the light harvesting ability of SrTiO₃, however, no effects were seen on the physical properties of the material. The electrochemical properties of the doped-metal was the deciding factor. The thorough study of Tarawipa et al. summarizes the effect of using different metals (Au, Ni, Pt, Ag, Ce and Fe) [11]. The highest hydrogen efficiency was obtained by using Au at a loading of 1 wt.% while Ce and Fe had a negative effect on hydrogen generation. Similarly, the addition of Na⁺ ion to SrTiO₃ by impregnation method led to high photocatalytic activity of SrTiO₃ [12]. This effect can be presumed to be coming out from the effect of adding Na⁺ ion which may be due to the substitution of Sr⁺ ions and therefore, inducing similar effects as in the case of Na⁺ ion doping at the preparation step.

And in some cases, the purity of the raw materials played an importance role, for example, in the study of Junzhe et al. [13]. The enhanced photocatalytic activity for water splitting and extended lifetime of the photogenerated charge carriers was attributed to the doping by Na⁺ ions. The performance of the photocatalyst was dependent on the purity of the raw material rather than the ionic state obtained during the preparation method.

Q2. How SrTiO₃ as a support material helps to improve the efficiency of various metals loaded on it.

Ans. Thank you for your question. The whole idea of doping or metal loading is to enhance the efficiency of SrTiO₃ photocatalyst. The metal alone does not undergo photocatalytic reaction. SrTiO₃ is a photocatalytic material that serves more than just a support material. It has been seen that in majority of cases by loading a metal catalyst or cocatalyst increases the charge transfer, thereby, reducing recombination rate. This loaded metal or co catalyst act as an active site or charge transferring site in the photocatalytic reaction. This way the efficiency of SrTiO₃ could be enhanced. For instance, in the study of Tarawipa et al. to produce hydrogen, the loading with different metals (Au, Ag, Pt and Ni) led to increase in the diffusivity of hydrogen from liquid to gas phase which enhanced the hydrogen production efficiency [11].

Q3. The authors should describe various advantages, drawbacks/challenges, and future aspects for improvement of the efficiency of SrTiO₃ for each application. The authors should also describe the recent advantages in the field.

Ans. Thank you for your suggestion. This is an important area of discussion, we agree. The part of future tendency has already been discussed briefly at the end of conclusion. Concerning advantage/drawback section, the authors have inserted some lines as can be seen below. The author would like to draw your attention to the fact that throughout the text, the authors have discussed recent progress with SrTiO₃ in a variety of sections.

Advantage and drawback of SrTiO₃ in H₂-production: Strontium titanates are considered to be catalytically active when it comes to the application of water splitting. Although the band gap of SrTiO₃ is similar to TiO₂ (3.2 eV) but the conduction band is more negative than TiO₂ that makes it a good candidate for hydrogen production application. However, the efficiency of pure SrTiO3 is still too low for hydrogen generation because of fast recombination rate of photogenerated electron/hole pairs. Hence, there is a need to modify SrTiO3 to obtain high efficiency.

Regarding future tendencies, it is very likely that strontium titanate is a worthy candidate for further similar research, sooner or later even for industrial applications. The dynamically evolving studies so far have made it clear (see Fig. 2) that both doping/composite formation and modification of morphology/specific surface area/stability or a combination of these parameters could provide a number of breakthrough results in the near future.

Q4. The old references should be replaced by the recent ones within the past five years to make the review more up-to-date and authentic for recent advantages.

Ans. Thank you very much for the suggestion. This is, indeed, true and we completely agree that recent work citations are good for the review papers. This is why our previous revised version already has quite a good number of recent references. Below the author has summarized the number of references w.r.t recent years in a tabular form to bring to the notice. Additionally, in order to give a thorough review and compiling the information from the past until present, the authors have included work from the whole period. This is why old references are also a part of this review. We regret that this doesn’t seem appropriate to the reviewer. We hope that we met with your expectations.

Year of publication	Reference number in the previous revised version	Total number of ref.
2022	21, 23, 30, 55, 62, 65	6
2021	6, 40, 45, 48	4
2020	2, 39, 42, 51, 54, 59	6
2019	19, 31, 33, 35, 37, 43, 46, 61, 63	9

 

 

Reviewer 3 Report

The author have added  new  paragraph "2. Properties of SrTiO3", which is actually very important, but forget to update the reference list

Author Response

Many thanks for your positive feedback. The reference list was updated.