Recent Breakthrough in Layered Double Hydroxides and Their Applications in Petroleum, Green Energy, and Environmental Remediation

Round 1

Reviewer 1 Report

The review proposed by Mostafa et al. is very well structured and documented, including the most representative and latest works for LDH applications in photocatalytic processes. The content of this review will be of great interest for the scientific community developing LDH materials and novel materials that can harvest more efficiently the solar spectrum. In general, I find this review of good quality to be published in Catalysts, and my recommendation is to be accepted after the following minor corrections:

 

1.       The authors discuss the application of LDHs as catalysts for petroleum, sulfurization, and water splitting processes. Though they discuss several results, it is recommended to include the main properties of the material that play a determinant role on the efficiency of the catalytic processes. E.g., surface area, pore size, crystallinity, active sites, cocatalysts, etc.

2.       Highlight the more prominent methods and the new methods developed in literature to obtain crystalline and high surface area LDHs, or other specific property, according to the application.

3.       Please discuss the stability and recyclability of the LDHs employed in the photocatalytic processes, and if they require a recovery process, e.g. drying, separation. Do the LDHs keep their crystallinity and surface area after their use in aqueous solution?

4.       In reviewing and comparing the efficiency of materials for photocatalytic water splitting process for hydrogen evolution, it is very important to discuss the quantum efficiency, or the solar to hydrogen efficiency, specially if the materials are presented as alternative for TiO2 that only absorb UV light. Please include data of quantum efficiency reports and the wavelength of analysis.

5.       For the photocatalytic application, please include the typical band gap energies found in LDHs and how the band gap can be tunned through modifications on the material.

6.       Other important property that needs to be discussed are the potentials of the valence band and conduction band in LDHs. Please include this information from literature, and discuss which processes are thermodynamically plausible e.g. water oxidation, water reduction, hydroxyl or superoxide radical generation, according to these potentials.

7.       Include references on the use of LDHs in photoelectrochemical systems, which are very important.

8.       When describing the water splitting application, please include the word photocatalytic water splitting. Since the process described by Fujishima and Honda in 1972 is photocatalytic water splitting. Water splitting alone can be misunderstood for other processes such as thermal water splitting, or electrolysis.

Author Response

Detailed Response for the reviewer comments

 

Reviewer 1:

1. The review proposed by Mostafa et al. is very well structured and documented, including teh most representative and latest works for LDH applications in photocatalytic processes. Teh content of this review will be of great interest for the scientific community developing LDH materials and novel materials that can harvest more efficiently the solar spectrum. In general, me find this review of good quality to be published in Catalysts, and my recommendation is to be accepted after the following minor corrections:
2. We acknowledge the efforts of the reviewer and his valuable comments that improved our review manuscript. I would like to emphasize that all comments have been considered and notified in our chapter manuscript. Please see our response and the accordance change in the entire manuscript (blue color).

 

 

Q1. The authors discuss the application of LDHs as catalysts for petroleum, sulfurization, and water splitting processes. Though they discuss several results, it is recommended to include the main properties of the material that play a determinant role on the efficiency of the catalytic processes. E.g., surface area, pore size, crystallinity, active sites, cocatalysts, etc.

A1. We really appreciate this intensive comment. In the revised manuscript, we attributed the activity of the used material to the corresponding LDH-property as surface area, pore size, crystallinity, and fine structure. These factors play an important role on the efficiency of the catalytic processes.

 

Q2. Highlight the more prominent methods and the new methods developed in literature to obtain crystalline and high surface area LDHs, or other specific property, according to the application.

A2. Thanks a lot for this valuable comment. In the revised manuscript, we highlighted the hydrothermal process at high temperature as the prominent method to get LDHs with high surface areas. Please see the revised version, page 8, section 5.2.5.2, lines 6-9 in blue.

Additionally, we highlighted our modified precipitation method from the vapor phase of TiCl₄ as the recent and effective method (section 5.2.5.4) to get highly active photocatalysts with lowered bandgap energies. Please see the revised version, page 8, section 5.2.5.4 in blue

5.2.5.4. Vapor diffusion method to prepare exceptional Ti-containing LDHs

The recent modified method to prepare highly crystalline and ultra-thin Ti-containing LDHs was introduced by us at 2020 [70]. Common Ti-based LDHs exhibit low crystallinities due to the segregation of Ti during the preparation by conventional precipitation methods [71-73]. For the first time, we applied the diffusion of TiCl₄ vapor method to avoid such this segregation and could synthesize novel CoTi-LDH nanorods of exceptional photocatalytic properties. The used [Cl^-]-precursors let to 1D-assembly by longitudinal propagation of CoCl₂ and TiCl₄ precursors and formation of [TiCl₆]^2- intermediate during the nucleation stages of the LDH-structure. This in-turn greatly enhanced the chemical activity by the ultrathin assembly of CoTi-LDH by presence of most of atoms on the surface of the nanorods [74] in contrast of the conventional 2D-structure of common LDHs. As a result, the prepared 1D CoTi-LDH exhibits lowered bandgap energy and great IR-responsivity (1.4 eV), high oxidation states of both Co and Ti and excellent catalyst distribution in the water splitting media. Scheme 1 shows the 1D formation mechanism of CoTi-LDH.  

 

 

Q3. Please discuss the stability and recyclability of the LDHs employed in the photocatalytic processes, and if they require a recovery process, e.g. drying, separation. Do the LDHs keep their crystallinity and surface area after their use in aqueous solution?

A3. We do value your concern. According to your comment, we discussed the stability and recyclability of the LDHs employed in the photocatalytic processes in the revised manuscript (Revised manuscript, page 22, section 9 in blue).

9. Durability of LDHs as potential photocatalytic materials

According our investigations [1, 6, 70, 121, 122], all the synthesized and applied LDH-photocatalysts exhibited stable structures in the photocatalytic water splitting under different light irradiation energies. Generally, LDHs are highly stable materials due to the perfect asymmetry and strong bonding of the interlayer anion (CO₃^2-, -CN, …) inside the layers, this makes it so difficult to substitute the interlayer anion except applying special treatment. To make anion exchange to the interlayer anion, it requires immersing the LDH in a solution with high concentration of interfering anion at high temperature and stirring for long time to achieve anion exchange. For example, the recent discovered CoTi, CoBi and CoBiTi LDHs photocatalysts showed great structural stabilities in respect to the primary hydrotalcite structure (XRD) as well as the existence of the interlayer CO₃^2- and CN anions (by FTIR) after many cycles of photocatalytic water splitting [1,101,103]. LDHs usually comprise heavy metal in their structures, the recovery of LDHs needs no complex separation methods, but just settling, filtration and drying at room temperature overnight or at 120 °C for 2-6 h. So that, LDHs usually possess their unique crystallinities and main properties even after long use in aqueous solutions.

 

 

Q4. In reviewing and comparing the efficiency of materials for photocatalytic water splitting process for hydrogen evolution, it is very important to discuss the quantum efficiency, or the solar to hydrogen efficiency, especially if the materials are presented as alternative for TiO₂ that only absorb UV light. Please include data of quantum efficiency reports and the wavelength of analysis.

A4. We thank the reviewer so much for this nice comment. According to your comment, a new table (Table 2) was added to show the quantum yield efficiencies (QYE) of the materials used in the solar to hydrogen was added to the revised manuscript. There are few data discusses the QYE of LDHs compared to the other materials especially in the solar to hydrogen process. This section was also supplied by additional References [124-132]. Please see the revised manuscript, page 21, Table 2 as well as the references in page 28 in blue color.

Table 2: Quantum yield efficiency (QYE) of common photocatalysts in the solar to hydrogen process.

LDH-photocatalyst	Conditions	HER (µmolg-1h-1)	QYE	Ref.
CBT-LDH	Xe-200 W, <800 nm	272.8	-	[122]
CBT-LDH/CBTO	Xe-200 W, <800 nm	1255	-	[122]
CdS/ZnCr-LDH	Xe-340 W, <420 nm	374	42.6(420 nm)	[124]
NiO/NaTaO3 :La	Hg-400 W, full light	19800	56(420 nm)	[125]
Pt-Cd0.5Zn0.5S:Bi	Hg-400 W, >420 nm	55.9	9.7(420 nm)	[126]
Ni-CdS	Xe-300 W, >420 nm	25.8	26.8(420 nm)	[127]
Pt-La5Ti2AgS5O 7	Xe-300 W, >420 nm	225	1.2 (420 nm)	[128]
Pt-LaInS2O	Xe-300 W, >420 nm	9	0.2(420 nm)	[129]
P3HT/g-C3N4	Xe-300 W, >420 nm	3045	77.4(420 nm)	[130]
g-C3N4/N-rich-CNF	Xe-300 W, >420 nm	169	14.3(420 nm)	[131]
CdS-Au-HCNS Pt	Xe-300 W, >455 nm	277	8.7 (420 nm)	[132]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

124. Zhang, G.; Lin, B.; Yang, W.; Jiang, S.; Yao, Q.; Chen, Y.; Gao, B. Highly efficient photocatalytic hydrogen generation by incorporating CdS into ZnCr-layered double hydroxide interlayer. RSC Adv. 2015, 5, 5823-5829.
125. Kato, H.; Asakura, K.; Kudo, A. Highly efficient water splitting into H₂ and O₂ over Lanthanum-doped NaTaO₃ photocatalysts with high crystallinity and surface nanostructure. J. Am. Chem. Soc. 2003, 125 (10), 3082-3089.
126. Peng, S.; An, R.; Li, Y.; Lu, G.; Li, S. Remarkable enhancement of photocatalytic hydrogen evolution over Cd0.5Zn0.5S by bismuth-doping, J. Hydrogen Energy 2012, 37 (2), 1366-1374.
127. Wang, H.; Chen, W.; Zhang, J.; Huang, C.; Mao, L. Nickel nanoparticles modified CdS – A potential photocatalyst for hydrogen production through water splitting under visible light irradiation, J. Hydrogen Energy 2015, 40 (1), 340-345.
128. Suzuki et al., A titanium-based oxysulfide photocatalyst: La₅Ti₂MS₅O₇ (M = Ag, Cu) for water reduction and oxidation. Phys. Chem. Chem. Phys. 2012, 14 (44), 15475-15481.
129. Kiyonori, O.; Akio, I.; Kentaro, T.; Kenji, T.; Michikazu, H.; Kazunari, D. Lanthanum–indium oxysulfide as a visible light driven photocatalyst for water splitting. Lett. 2007, 36 (7), 854-855.
130. Zhang, L.; Han, Z.; Wang, W.; Li, X.; Su, Y.; Jiang, D.; Lei, X.; Sun, S. Solar-light-driven pure water splitting with ultrathin BiOCl nanosheets. Chemistry A Eur. J. 2015, 21 (50), 18089-18094.
131. Han, Q.; Han, Q.; Wang, B.; Gao, J.; Cheng, Z.; Zhao, Y.; Zhang, Z.; Qu, L. Atomically thin mesoporous nanomesh of graphitic C₃N₄ for high-efficiency photocatalytic hydrogen evolution. ACS Nano 2016, 10 (2), 2745-2751.
132. Zheng, D.; Pang, C.; Wang, X. Teh function-led design of Z-scheme photocatalytic systems based on hollow carbon nitride semiconductors, Commun. 2015, 51 (98), 17467-17470.

 

 

Q5. For teh photocatalytic application, please include the typical band gap energies found in LDHs and how the band gap can be tuned through modifications on the material.

A5. Thanks so much for this wonderful comment. In the revised version, the bandgap energies of many applied LDHs were mentioned through discussing the photocatalytic applications of LDHs. Also, the tuning of these bandgaps was highlighted from the previous literature. Please find these modifications in the revised manuscript in blue color through the whole text (such as page12 line 14-19 and page14, section 6.5, line 12-13 in blue).  

 

 

Q6. Other important property that needs to be discussed are the potentials of the valence band and conduction band in LDHs. Please include this information from literature, and discuss which processes are thermodynamically plausible e.g. water oxidation, water reduction, hydroxyl or superoxide radical generation, according to these potentials.

A6. We are grateful for this nice comment. In the revised manuscript, the positions of the valence band (VB) and conduction band (CB) edges of the common LDHs were highlighted in Figure 12. Also bandgap edges of some common oxide-photocatalysts were illustrated to show how the LDH-structure can lower the bandgap energies and alter their CB/VB potentials for enhancing the photocatalytic water splitting. This section was also supplied by additional References [133-137]. Please see the revised manuscript, pages 21 and 22, Figure 12 (a and b) as well as the references in page 28 in blue color.

Figure 12. The CB/VB potentials of common LDHs compared to (a) those of conventional metal oxide photocatalysts (b); it is obvious how the LDH-structure can greatly improve the photocatalytic properties and lower the bandgap energies.

 

References:

133. Nayak, S.; Parida, K. Recent progress in LDH@graphene and analogous heterostructures for highly active and stable photocatalytic and photoelectrochemical water splitting. Chem Asian J. 2021, 16, 2211- 2248.
134. Yang, Z.Z.; Zhang, C.; Zeng, G.M.; Tan, X.F.; Huang, D.L.; Zhou, J.W.; Fang, Q.Z.; Yang, K.H.; Wang, H.; Wei, J.; Nie, K. State-of-the-art progress in the rational design of layered double hydroxide based photocatalysts for photocatalytic and photoelectrochemical H2/O2 production, Coordinat. Chem. Rev. 2021, 446, 214103.
135. Zhang, J.; Zhang, W.; Yuan, F.; Yang, Z.; Lin, J.; Huang, Y.; Ding, M. Effect of Bi₅O₇I/calcined ZnAlBi-LDHs composites on Cr(VI) removal via adsorption and photocatalytic reduction. Appl. Surf. Sci. 2021, 562, 150129.
136. Gao, R.; Yan, D. Recent development of Ni/Fe‐based micro/nanostructures toward photo/electrochemical water oxidation. Adv. Energy Mater. 2020, 10, 1900954.
137. Zhao, D.; Jiang, K.; Pi, Y.; Huang, X. Superior electrochemical oxygen evolution enabled by three-dimensional layered double hydroxide nanosheet superstructures. ChemCatChem 2017, 9, 84-88.

 

 

Q7. Include references on teh use of LDHs in photoelectrochemical systems, which are very important.

A7. Thanks a lot for this intensive comment. We supplied the revised version with cited references on the important LDHs used in photoelectrochemical systems, according to your comment. The added references are essential (especially [133] and [134]) to introduce much information about the photoelectrochemical process. We also highlighted the tangential role of LDHs in other fields of energy production as an important photoelectrochemical process as mentioned in references [133-137]. Please see the revised version in blue and the references [133-137] were mentioned in page 28.

 

 

Q8. When describing the water splitting application, please include the word photocatalytic water splitting. Since the process described by Fujishima and Honda in 1972 is photocatalytic water splitting. Water splitting alone can be misunderstood for other processes such as thermal water splitting, or electrolysis.

A8. We appreciate this wonderful comment. In the revised manuscript we replaced the term "water splitting" with the term "photocatalytic water splitting" in the whole manuscript to describe the water splitting application.

 

We again appreciate this thoughtful comment and we hope that the reviewer accepts the revised review manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The paper provides an overview of synthesis and applications of layered double hydroxides in chemical industry and nanotechnology. The topic received a large attention recently and a comprehensive review is highly desirable. However I have several comments both on the structure and the content of the manuscript that should be considered before its publication.

1. It is not very clear why semiconductor catalysts are reviewed in section 3.1? Also the choice of those catalysts is somewhat arbitrary as several other photocatalytic systems (for example MnO2, TiNx and hybrid materials TiO2/graphite, TiO2/hydroxyapatite, and TiO2/SiO2) are not mentioned while they are widely used for degradation of organic pollutants. The authors should justify the selection criteria in the corresponding section.

2.   Some sections are incomplete and have no references to literature data. For example section 5.2.2 has only 3 lines and should be combined with other sections. Also some characteristic values of surface area of LDHs should be listed in a Table and supported by references.

3. Section 5.2.5. Add references to the corresponding synthesis methods. They have been developed for a long time and should be properly cited. Any recent modifications of the original methods should be reviewed and discussed.

4. Application of LDH in photo- electro-chemistry and especially in catalysis for oxygen evolution reaction should be added and reviewed as a strategy for electrification of chemical industry. Various strategies including size modulation, heteroatom doping, and defect engineering, have been developed to further improve the electrocatalytic performance of LDHs towards OER. See for example: Gao, R., Yan, D., 2020. Recent Development of Ni/Fe‐Based Micro/Nanostructures toward Photo/Electrochemical Water Oxidation. Adv. Energy Mater. 10, 1900954.

5. Composite materials with LDH should be reviewed as well. For example, a  strategy to create ultrathin NiFe-LDHs nanosheets grown on carbon nanotubes (CNTs) was proposed by Zhao et al. Unlike previously reported NiFe-LDH nanomaterials, the newly formed materials demonstrated different characteristics, including ultrathin building blocks.

Zhao, D., Jiang, K., Pi, Y., Huang, X., 2017. Superior Electrochemical Oxygen Evolution Enabled by Three-Dimensional Layered Double Hydroxide Nanosheet Superstructures. ChemCatChem 9, 84–88.

6. The summary section should review novel strategies for control of properties of LDH and future research directions. For example, developing an effective strategy to exfoliate bulk NiFe-LDHs into stable single-layer NiFe-LDHs nanosheets with more exposed active sites remains challenging. The exfoliation of LDHs structure into a few layers and single sheets can expose more surface sites because of the large interlayer spacing between individual layers in the LDH structure. Yet the synthesis of exfoliated NiFe-LDHs nanosheets requires complex multistep process (i.e., hydrothermal process, anion exchange, and exfoliation) which takes several days to get the final product.  Some suggestions could be made how to improve some of those synthetic steps.

Author Response

Detailed Response for the reviewer comments

 

Reviewer 2:

1. The paper provides an overview of synthesis and applications of layered double hydroxides in chemical industry and nanotechnology. The topic received a large attention recently and a comprehensive review is highly desirable. However me has several comments both on the structure and the content of the manuscript that should be considered before its publication.
2. Thanks a lot for your instructive comments. We revised the entire manuscript carefully per of your comments. All amendments have been highlighted in the entire manuscript in blue color.

 

 

Q1. It is not very clear why semiconductor catalysts are reviewed in section 3.1? Also the choice of those catalysts is somewhat arbitrary as several other photocatalytic systems (for example MnO₂, TiNx and hybrid materials TiO₂/graphite, TiO₂/hydroxyapatite, and TiO₂/SiO₂) are not mentioned while they are widely used for degradation of organic pollutants. The authors should justify the selection criteria in the corresponding section.

A1. Thanks so much for the reviewer's valuable comment. In the revised version, we mentioned some common semiconductors which mainly concerns the LDH to illustrate how the LDH-structure can alter and enhance the photoelectrochemical properties of materials although LDHs are made of these metals or semiconductors. This review reflects simply prepared LDHs from Co, Cd, Ti, Cu, etc. which exhibit elevated activities compared to common semiconductors or metal oxides. The metal oxides as TiO₂-SiO₂, TiN_x, … are very important semiconductors. Please see the revised review, page 3, section 3.1 in blue as follow:

 

3.1.1. TiO₂

TiO₂ is the most common and first applied semiconductor in photocatalysis [12] as it is naturally abundant, environmentally friendly, chemically stable and it can be synthesized by versatile preparation routes in various morphologies [1,5]. Although, photocatalysis by pure TiO₂ is rare, which restricts its work in the UV-region counts only 5 % of sunlight radiation [1,5] due to its wide bandgap (3.2 eV) and the fast recombination of the electrons/hole pairs [1,5]. To overcome these drawbacks, many efforts has been done to prepare modified TiO₂-structures such as Pt-modified TiO₂ [13], Au-modified TiO₂ [14], Ag-modified TiO₂ [15], perovskite (CaTi, SrTi, BaTi)-modified TiO₂ [16], Carbon-modified TiO₂ [17], metal (Co, Ni, Zn)-modified TiO₂ [18], TiN-composites [19], graphine/TiO₂ nanoparticles [20], titania/hydroxyapatite (TiO₂/HAp) composites [21] and silica modified titania (TiO₂-SiO₂) [22] etc.

 

19. Xie, Z.; Liu, X.; Zhan, P.; Wang, W.; Zhang, Z. Tuning the optical bandgap of TiO₂ -TiN composite films as photocatalyst in the visible light. AIP Adv. 2013, 3, 062129.
20. Guidetti, G.; Pogna, E.A.A.; Lombardi, L.; Tomarchio, F.; Polishchuk, I.; Joosten, R.R.M.; Ianiro, A.; Soavi, G.; Sommerdijk, N.A.J.M.; Friedrich, H.; Pokroy, B.; Ott, A.K.; Goisis, M.; Zerbetto, F.; Falini, G.; Calvaresi, M.; Ferrari, A.C.; Cerullo, G.; Montalt, M. Photocatalytic activity of exfoliated graphite -TiO₂ nanoparticle composites. Nanoscale, 2019,11, 19301-19314.
21. Yao, J.; Zhang, Y.; Wang, Y.; Chen, M.; Huang, Y.; Cao, J.; Ho, W.; Lee, S.C. Enhanced photocatalytic removal of NO over titania/hydroxyapatite (TiO₂/HAp) composites with improved adsorption and charge mobility ability. RSC Adv. 2017, 7, 24683.
22. Urkasame, K.; Yoshida, S.; Takanohashi, T.; Iwamura, S.; Ogino, I.; Mukai, S.R. Development of TiO₂- SiO₂ photocatalysts having a microhoneycomb structure by the ice templating method. ACS Omega 2018, 3, 14274−14279.

 

Also, we added a new section (3.1.6.) to discuss the MnO₂ photocatalyst (Revised manuscript, page 4 in blue).

 

3.1.6. MnO₂

Manganese element has variable oxidation states and provides various oxide forms. The alpha-form (α-MnO₂) is the common form in photocatalysis, it has the advantage over many catalysts due to its capability to work effectively even at very low temperatures. Due to the efficient lattice vacancies and oxygen defects both act as charge carriers, MnO₂ finds wide applications in photocatalysis and especially in photodegradation of organic pollutants [26].

26. Warsi, M.F.; Bilal, M.; Zulfiqar, S.; Khalid, M.U.; Agboola, P.O.; Shakir, I. Enhanced visible light driven photocatalytic activity of MnO₂ nanomaterials and their hybrid structure with carbon nanotubes. Mater. Res. Express 2020, 7, 105015.

 

 

Q2. Some sections are incomplete and has no references to literature data. For example, section 5.2.2 has only 3 lines and should be combined with other sections. Also some characteristic values of surface area of LDHs should be listed in a Table and supported by references.

A2. We thank the reviewer so much for this nice comment. In the revised manuscript, we combined the HRTEM section with the SEM section (Please see the revised version, page 9, section 5.2.6.2. Surface features of LDHs) such as;

5.2.6.2. Surface features of LDHs; Scanning electron microscopy (SEM), High resolution transmission electron microscope (HRTEM) and Surface area

SEM is the second analytical tool that reflects the hydrotalcite structure of LDHs. The SEM images of LDHs usually reveal plate-like morphologies with well-ordered hexagonal crystallites [6] of highly porosity as shown in Figure 4. In contrast to SEM and XRD which is a characteristic analytical tool for LDH and provides a fingerprint image of the LDH-structure, HRTEM imaging of LDHs is variable where sometimes it clearly reflects the 2D-assembly of the hexagonal plate crystallites of LDHs (as SEM) [63] but in other times it provides unclear or confusing images due to the finite LDH-structure and the new electro/electromagnetic properties of elements in the LDH-configuration [64]. Figure 5 (A and B) shows variable HRTE-images of common LDHs.

Surface area is a common analysis of LDHs especially in the applications of adsorption where it plays a predominant role in sorption of different ions. Table 1 shows the surface areas of various LDHs; it is very variable and dramatically alters with the used preparation method of the LDH, M^II/M^III ratio and the metal type.

 

Table 1. Correlation between the chemical composition and synthesis route of LDHs and surface area.

LDH	MII/MIII ratio	Synthesis route	SE	Ref.
CoAlCl-	4:1	Hydrothermal/acid salt template	26.5	[75]
CoAlCl-	4:1	Coprecipitation	30.9	[75]
CoAlCl-	4:1	Hydrothermal	19.7	[75]
NiAlCO32-	1:1	Coprecipitation	133	[76]
NiCoNO3-	1:1	Hydrothermal	37	[77]
NiFeCO3-	2:1	Coprecipitation	427.00	[78]
NiAlCl-	4:1	Coprecipitation	95.44	[79]
NiAlCO3-	4:1	Hydrothermal	13	[79]
MgFeCl-	2:1	Solvothermal with SDS	70.19	[80]
ZnAlCl-	2:1	Urea	253	[81]
ZnMgAlCO32-	2:1	Coprecipitation	28.12	[82]
NiFeNO3-	4:1	Coprecipitation	17.84	[83]

 

 

 

 

 

 

 

 

 

 

 

75. Chen, Y.; Jing, C.; Zhang, X.; Jiang, D.; Liu, X.; Dong, B.; Feng, L.; Li, S.; Zhang, Y. Acid- salt treated CoAl layered double hydroxide nanosheets with enhanced adsorption capacity of Methyl Orange dye, J. Colloid Interface Sci. 2019, 548, 100 –109.
76. El Hassani, K.; Beakou, B.H.; Kalnina, D.; Oukani, E.; Anouar, A. Effect of morphological properties of layered double hydroxides on adsorption of azo dye Methyl Orange: a comparative study, Clay Sci. 2017, 140, 124 –131.
77. Guan, T.; Fang, L.; Lu, Y.; Wu, F.; Ling, F.; Gao, J.; Hu, B.; Meng, F.; Jin, X.; A facile approach to synthesize 3D flower-like hierarchical NiCo layered double hydroxide microspheres and their enhanced adsorption capability, Colloids Surf. A Physicochem. Eng. Asp. 2017, 529, 907 –915.
78. Jiang, Z.; Yan, L.; Wu, J.; Liu, X.; Zhang, J.; Zheng, Y.; Pei, Y. Low-temperature synthesis of carbonate-intercalated NixFe-layered double hydroxides for enhanced adsorption properties, Surf. Sci. 2020, 531, 147281.
79. Jing, C.; Chen, Y.; Zhang, X.; Guo, X.; Liu, X.; Dong, B.; Dong, F.; Zhang, X.; Liu, Y.; Li, S.; Zhang, Y. Low carbonate contaminative and ultrasmall NiAl LDH prepared by acid salt treatment with high adsorption capacity of Methyl Orange, Ind. Eng. Chem. Res. 2019, 58, 11985 –11998.
80. Shabbir, R.; Gu, A.; Chen, J.; Khan, M.M.; Wang, P.; Jiao, Y.; Zhang, Z.; Liu, Y.; Yang, Y. Highly efficient removal of congo red and Methyl Orange by using petal-like Fe-Mg layered double hydroxide, Int. J. Environ. Anal. Chem. 2022, 102 (5), 1-18.
81. -F. Chao, J.-J. Lee, S.-L. Wang, Preferential adsorption of 2,4-dichlorophenoxyacetate from associated binary-solute aqueous systems by Mg/Al-NO₃ layered double hydroxides with different nitrate orientations, J. Hazard. Mater. 2009, 165, 846-852.
82. Zheng, Y.M.; Li, N.; Zhang, W.D. Preparation of nanostructured microspheres of Zn-Mg-Al layered double hydroxides with high adsorption property, Colloids Surf. A Physicochem. Eng. Asp. 2012, 415, 195-201.
83. Lu, Y.; Jiang, B.; Fang, L.; Ling, F.; Gao, J.; Wu, F.; Zhang, X. High performance NiFe layered double hydroxide for Methyl Orange dye and Cr(VI) adsorption, Chemosphere 2016, 152, 415 –422.

 

 

Q3. Section 5.2.5. Add references to the corresponding synthesis methods. They have been developed for a long time and should be properly cited. Any recent modifications of the original methods should be reviewed and discussed.

A3. We do value your concern. According to your comment, we cited many references to the corresponding synthesis methods in the revised manuscript. Also we discussed the recent modified preparation methods in separate sections (Revised version, pages 7 and 8 in blue color), as follow;

 

5.2.5.1. Co-precipitation method 

It is the most common method applied for the preparation of LDHs [63-65] and nanomaterials generally in which the metal cations are mixed in a dilute solution under vigorous stirring at 50-90 °C. On the other hand, the precipitation of the metal cations is performed through a suitable base titration in presence of a carbonate source. The common bases applied for precipitation are NaOH and NH₄OH, the carbonate species are applied in general by Na₂CO₃. Carbonate is the most stable and common interlayer anion so it is extremely difficult to prepare LDH material free of carbonate without severs precautions. The factors leading to the formation of LDH rather than amorphous hydroxide or gel formation are aging time, rate of titration, and pH. Long aging time (12-36 h) and slow rate of titration improve the crystallinity and the general morphology of the produced LDH, while pH must be adjusted between 9 and12.

 

5.2.5.2. Hydrothermal method

It is similar to the co-precipitation method but the process is completed under the vapor pressure produced from the entrapped solution of the metal cations and precipitating agents in a Teflon-lined autoclave at elevated temperature (120-180 °C) for a long time. The hydrothermal method can be performed by firstly precipitating the starting cations as in the precipitation method and then transporting the precipitate into the autoclave or it can be completely performed in the autoclave [66,67]. Due to the high temperature and long-time of synthesis, the hydrothermal method usually yields LDHs with sharp crystalline, high surface area, porous and in-turn highly active adsorbents/catalysts.

 

5.2.5.3. Sol-gel method

In which the precursors of the metals to be precipitated are in form of alkoxides such as aluminum tri-isoperoxide and titanium tetra- isoperoxide rather than the conventional inorganic anions as chlorides or nitrates. The precipitation procedure can be completed by the self-hydrolysis of the used alkoxides by heating to 70 °C for a definite time or by applying a suitable hydroxide to control the final pH [68,69].

 

5.2.5.4. Vapor diffusion method to prepare exceptional Ti-containing LDHs

The recent modified method to prepare highly crystalline and ultra-thin Ti-containing LDHs was introduced by us at 2020 [70]. Common Ti-based LDHs exhibit low crystallinities due to the segregation of Ti during the preparation by conventional precipitation methods [71-73]. For the first time, we applied the diffusion of TiCl₄ vapor method to avoid such this segregation and could synthesize novel CoTi-LDH nanorods of exceptional photocatalytic properties. The used [Cl^-]-precursors let to 1D-assembly by longitudinal propagation of CoCl₂ and TiCl₄ precursors and formation of [TiCl₆]^2- intermediate during the nucleation stages of the LDH-structure. This in-turn greatly enhanced the chemical activity by the ultrathin assembly of CoTi-LDH by presence of most of atoms on the surface of the nanorods [74] in contrast of the conventional 2D-structure of common LDHs. As a result, the prepared 1D CoTi-LDH exhibits lowered bandgap energy and great IR-responsivity (1.4 eV), high oxidation states of both Co and Ti and excellent catalyst distribution in the water splitting media. Scheme 1 shows the 1D formation mechanism of CoTi-LDH. 

63. Chen, J.; Wang, C.; Zhang, Y.; Guo, Z.; Luo, Y.; Mao, C.J. Engineering ultrafine NiS cocatalysts as active sites to boost photocatalytic hydrogen production of MgAl layered double hydroxide. Surf. Sci. 2020, 506, 144999.
64. Gonçalves, J.M.; Martins, P.R.; Angnes, L.; Araki, K. Recent advances in ternary layered double hydroxide electrocatalysts for the oxygen evolution reaction. New J. Chem. 2020, 44, 998-9997.
65. Claydon, R.; Wood, J. A mechanistic study of layered-double hydroxide (LDH)-derived nickel-enriched mixed oxide (Ni-MMO) in ultradispersed catalytic pyrolysis of heavy oil and related petroleum coke formation. Energy Fuels 2019, 33, 10820-10832.
66. Li, G.; Zhang, J.; Li, L.; Yuan, C.; Weng, T.C. Boosting the electrocatalytic activity of nickel-iron layered double hydroxide for the oxygen evolution reaction by terephthalic acid. Catalysts 2022, 12(3), 258.
67. Nayak, S.; Parida, K. MgCr-LDH nanoplatelets as effective oxidation catalysts for visible light-triggered Rhodamine B degradation. Catalysts 2021, 11(9), 1072.
68. Agostino, L.C.D.; Gonçalves, R.G.L.; Santilli, C.V.; Pulcinelli, S.H.; Effect of solvent in the sol-gel synthesis of layered double hydroxides as catalysts for the ethanol steam reforming reaction. Proceedings of the 18. Brazil MRS Meeting 2019.
69. Zhao, Y.; He, S.; Wei, M.; Evans D.G.; Duan, X. Hierarchical films of layered double hydroxides by using a sol–gel process and their high adaptability in water treatment. Commun. 2010, 46, 3031-3033.
70. Mostafa, M.S.; Lan, C.; Betiha, M.A.; Zhang, R.; Gaoa, Y.; Ge, G. Enhanced infrared-induced water oxidation by one-pot synthesized CoTi-Nanorods as highly infrared responsive photocatalyst. Power Sources 2020, 464, 228176.
71. Chowdhury P.R.; Bhattacharyya, K.G. Synthesis and characterization of Co/Ti layered double hydroxide and its application as a photocatalyst for degradation of aqueous Congo Red. RSC Adv. 5 (112), 2015, 92189-92206.
72. Chowdhury, P.R.; Bhattacharyya, K.G. Ni/Ti layered double hydroxide: synthesis, characterization and application as a photocatalyst for visible light degradation of aqueous methylene blue. Dalton Trans. 2015, 44,6809 –6824.
73. Chen, G.; Qian, S.; Tu, X.; Wei, X.; Zou, J.; Leng, L.; Luo, S. Enhancement photocatalytic degradation of rhodamine B on nano Pt intercalated Zn–Ti layered double hydroxides. Surf. Sci. 2014, 293, 345– 351.
74. Ji-yong, X.; Mo-tang, T.; Cui, C.; Sheng-ming, J.; Yong-ming, C. Preparation of α-Bi₂O₃ from bismuth powders through low-temperature oxidation. Nonferrous Metals Soc. China 2012, 22, 2289-2294.

 

Q4. Application of LDH in photo-electro-chemistry and especially in catalysis for oxygen evolution reaction should be added and reviewed as a strategy for electrification of chemical industry. Various strategies including size modulation, heteroatom doping, and defect engineering, has been developed to further improve teh electrocatalytic performance of LDHs towards OER. See for example:

Gao, R., Yan, D., 2020. Recent Development of Ni/Fe‐Based Micro/Nanostructures toward Photo/Electrochemical Water Oxidation. Adv. Energy Mater. 10, 1900954.

A4. We thank the reviewer so much for this comment. In the revised manuscript, we mentioned the applications of LDHs in the field of photo-electro-chemistry and cited the mentioned interesting reference to cover this important field. Please see the revised version, the end of section 8, page 21, The Recent Breakthrough in synthesis and applications of 1D LDHs and Reference [136]. Other references were added to support the revised version [132-137].

 

 

 

 

 

 

 

 

 

Q5. Composite materials with LDH should be reviewed as well. For example,

a strategy to create ultrathin NiFe-LDHs nanosheets grown on carbon nanotubes (CNTs) was proposed by Zhao et al. Unlike previously reported NiFe-LDH nanomaterials, the newly formed materials demonstrated different characteristics, including ultrathin building blocks. Zhao, D., Jiang, K., Pi, Y., Huang, X., 2017. Superior Electrochemical Oxygen Evolution Enabled by Three-Dimensional Layered Double Hydroxide Nanosheet Superstructures. ChemCatChem 9, 84–88.

A5. We are grateful for this wonderful comment. According to your comment, we reviewed many composite materials with LDH and supplied the revised version with the aforementioned references.

Table 2 shows the quantum yield efficiency (QYE) and the effectiveness of CBT and CBTO IR-responsives [124-132] in the solar to hydrogen process compared to conventional photocatalysts. Figure 12 exhibits the CB/VB potentials of common LDHs compared to ordinary oxide photocatalysts. We cannot forget or overcome the tangential role of LDHs in other fields of energy production as the important photoelectrochemical process [133-137]. We also highlighted the tangential role of LDHs in other fields of energy production as the important photoelectrochemical process [104,105].

133. Nayak, S.; Parida, K. Recent progress in LDH@graphene and analogous heterostructures for highly active and stable photocatalytic and photoelectrochemical water splitting. Chem Asian J. 2021, 16, 2211- 2248.
134. Yang, Z.Z.; Zhang, C.; Zeng, G.M.; Tan, X.F.; Huang, D.L.; Zhou, J.W.; Fang, Q.Z.; Yang, K.H.; Wang, H.; Wei, J.; Nie, K. State-of-the-art progress in the rational design of layered double hydroxide based photocatalysts for photocatalytic and photoelectrochemical H2/O2 production, Chem. Rev. 2021, 446, 214103.
135. Zhang, J.; Zhang, W.; Yuan, F.; Yang, Z.; Lin, J.; Huang, Y.; Ding, M. Effect of Bi₅O₇I/calcined ZnAlBi-LDHs composites on Cr(VI) removal via adsorption and photocatalytic reduction. Surf. Sci. 2021, 562, 150129.
136. Gao, R.; Yan, D. Recent development of Ni/Fe‐based micro/nanostructures toward photo/electrochemical water oxidation. Energy Mater. 2020, 10, 1900954.
137. Zhao, D.; Jiang, K.; Pi, Y.; Huang, X. Superior electrochemical oxygen evolution enabled by three-dimensional layered double hydroxide nanosheet superstructures. ChemCatChem 2017, 9, 84-88.

 

 

Q6. The summary section should review novel strategies for control of properties of LDH and future research directions. For example, developing an effective strategy to exfoliate bulk NiFe-LDHs into stable single-layer NiFe-LDHs nanosheets with more exposed active sites remains challenging. The exfoliation of LDHs structure into a few layers and single sheets can expose more surface sites coz of teh large interlayer spacing between individual layers in teh LDH structure. Yet the synthesis of exfoliated NiFe-LDHs nanosheets requires complex multistep process (me. hydrothermal process, anion exchange, and exfoliation) which takes several days to get teh final product.  Some suggestions could be made how to improve some of those synthetic steps.

A6. Thanks so much for this nice comment. The summary section was revised to review novel strategies for control of properties of LDH and future research directions. The above mentioned references were highlighted in the revised version (Revised manuscript, page 23, section 10, line 25-31 in blue).

 

Hope this amendment will take your consideration. We again appreciate this thoughtful comment.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The authors answered most of my questions. There are a few more suggestions on the revised version which can help to improve the paper:

 

It would be good to make the list of all notations and abbreviations used in the paper.

1) Figure 2. Add in the Figure caption: “Adopted from ... with permission from …”

2) Section 5.2.2 is still incomplete. Add typical values of specific surface area (could be in a form of Table)

3) Figure 6. Is that according to Ref 91? Then it needs to be stated in the Figure caption: “Adopted from ... with permission from …”

4) Figure 8. Explain abbreviations on the y-axis. In Figure 8b, units are missing on the x-axis.

5) Table 1. Column SE. I assume this is specific surface area (SSA)? Please write in full. Also provide the units as it should be m2/g. Why the data are presented with different accuracy?

6) Line 306-307 Section 5.2.5.1. The pH is adjusted on the range 9-12. COuydl you elaborate how the pH influences and the l morphology of the produced LDH? Why there are studies where pH is 9 while in others pH =12? The same comment is related to aging time. Why it is changed over such a wide range (12-36h)? Could you elaborate on structure-property relationship and support your discussion with several references where the effect of synthesis time is demonstrated.

7) Line 314-315. The same as above. “Due to the high temperature and long-time of synthesis”. Could you provide more information on synthesis time and how it influences the properties such as surface area, crystallinity and other?

8) Also synthesis method (5.2.5) could be done in flow. See for example: Li et al., Synthesis of nano-catalysts in flow conditions using millimixers. Advanced Nanomaterials for Catalysis and Energy. doi: 10.1016/B978-0-12-814807-5.00001-2 Chapter 1.

It is necessary to describe flow chemistry synthesis next to batch synthesis. According to the characterization results, it is often observed that the hydrotalcite obtained from flow synthesis configurations shows much higher BET surface area than that produced with a classical coprecipitation method.

Author Response

Detailed Response for the reviewer comments

 

 

Reviewer#1:

1. Comments and Suggestions for Authors The authors answered most of my questions. There are a few more suggestions on the revised version which can help to improve the paper:
2. We acknowledge the efforts of the reviewer and his valuable comments that improved our review manuscript. I would like to emphasize that all comments have been considered and notified in our chapter manuscript. Please see our response and the accordance change in the entire manuscript (blue color).

 

Q1. It would be good to make the list of all notations and abbreviations used in the paper.

A1. We thank the reviewer so much for this valuable comment. In the revised manuscript, we added a list of all notations and used abbreviations according to your comment (Please see the revised version. Page 1, abbreviations).

 

Abbreviations:

LDH:   Layered Double Hydroxide; UV: Ultraviolet; IR: Infrared; 1D: One dimensional; D2: Two dimensional; D3: Three dimensional; SOx: Sulfur Oxides; NOx: Nitrogen Oxides; ppm: Part per million;  WMO: World Metrological Organization; COP26: Conference of the Parties no 26; CNT: Carbon; nanotubes;  CB: Conduction band; VB: Valence band; IUPAC: International Union of Pure and Applied Chemistry; LbL: Layer-by-layer; XRD: X-ray diffraction; JCPDS: Joint Committee on Powder Diffraction Standards; SEM: Scanning electron microscopy; HRTEM: High resolution transmission electron microscope; MOF: Metal organic framework; CBT-LDH: CoBiTi LDH: CBTO: CoBiTi oxide; eV: Electrovolt

 

Q2. Figure 2. Add in the Figure caption: “Adopted from ... with permission from …”

A2. We really appreciate this intensive comment. In the revised manuscript, we adapted the caption of figure 2:

Figure 2. Structures of (a) Mg (OH)₂ brucite (Adopted from Cavani et al. [59] with permission from Elsevier) and (b) LDH (Adopted from Mochane et al. [60] with permission from MDPI).

59. Cavani, F.; Trifiro, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and applications. Tod. 1991, 11, 173-302.
60. Mochane, M.J.; Magagula, S.I.; Sefadi, J.S.; Sadiku, E.R.; Mokhena, T.C. Morphology, thermal stability, and flammability properties of polymer-layered double hydroxide (LDH) nanocomposites: A review. Crystals2020, 10 (7), 612.

 

Q3. Section 5.2.2 is still incomplete. Add typical values of specific surface area (could be in a form of Table)

A3. We do value your concern. The typical values of specific surface area were listed in Table 1 in the revised manuscript, and we referred to this modification in section 5.2.2 (Please see the revised manuscript, page 7, section 5.2.2 and the modified Table 1 in page 11 in blue) as follow;

 

5.2.2. High surface area

LDHs have great surface areas due to their low dimension, infinite lamellar (layered) structure, and very small particle sizes. According to the IUPAC classification, LDHs usually exhibit IV-type isotherms with H3 hysteresis loop characteristics for the mesoporous materials [1,6]. Although LDHs exhibit usual IV-type isotherms, the values of their surface areas are variable. Table 1 shows the variation of the surface area of LDHs with their type and synthesis route.

Table 1. Correlation between the chemical composition and synthesis route of LDHs and surface area.

LDH	MII/MIII ratio	Synthesis route	Specific surface area (m2/g)	Ref.
CoAlCl-	4:1	Hydrothermal/acid salt template	26.5	[80]
CoAlCl-	4:1	Coprecipitation	30.9	[80]
CoAlCl-	4:1	Hydrothermal	19.7	[80]
NiAlCO32-	1:1	Coprecipitation	133	[81]
NiCoNO3-	1:1	Hydrothermal	37	[82]
NiFeCO3-	2:1	Coprecipitation	427.00	[83]
NiAlCl-	4:1	Coprecipitation	95.44	[84]
NiAlCO3-	4:1	Hydrothermal	13	[84]
MgFeCl-	2:1	Solvothermal with SDS	70.19	[85]
ZnAlCl-	2:1	Urea	253	[86]
ZnMgAlCO32-	2:1	Coprecipitation	28.12	[87]
NiFeNO3-	4:1	Coprecipitation	17.84	[88]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Q4. Figure 6. Is that according to Ref 91? Then it needs to be stated in the Figure caption: “Adopted from ... with permission from …”

A4. We thank the reviewer so much for his nice comment. Figure 6 caption was revised according to your comment. According to the modifications in the references, Ref. 91 was renumbered to be 96 in the revised manuscript. Please see the revised version, Page 3, Figure 6 caption in blue.

Figure 6. The photo-desulfurization mechanism over (NiCo₂-LDH/Fe₃O₄)-composite. (Adopted from Masoumi and Hosseini [96] with permission from Elsevier).

 

Q5. Figure 8. Explain abbreviations on the y-axis. In Figure 8b, units are missing on the x-axis.

A5. We are grateful for this intensive comment. We have checked and modified Figure 8 accurately as per of your comment. We explained the abbreviation on the y-axis. Also, we mentioned the unit on the x-axis in Figure 8b (Please see the revised version, Figure 8, page 18).

 

Q6. Table 1. Column SE. I assume this is specific surface area (SSA)? Please write in full. Also provide the units as it should be m²/g. Why the data are presented with different accuracy?

A6. Thanks a lot for this intensive comment. The SSA column in Table was modified to include the full name (Specific surface area) in the revised version and the unit (m²/g) was mentioned according to your comment.

The data are presented with different accuracy according to the cited literatures (such as [65]) based on many factors such as the type of used cations, preparation method, and the preparation conditions. From our point of view, the measurement conditions/parameters of the surface area by the BET instruments can greatly affect the accuracy of the giving data. Taking into account the fine structure of LDHs and the ultra-narrow d-spacing (interlayer distance = 0.76 nm for CO₃^2- and similar anions), the N₂ adsorption by the LDH surface and pores is very complex and may need special precaution and treatment of the LDH-sample rather than conventional materials. Even in sorption of ions rather than N₂, the role of surface area of LDHs is still unclear and sometimes let to opposite values [65].

 

65. Johnston, A.M.; Lester, E.; Williams, O.; Gomes, R.L. Understanding layered double hydroxide properties as sorbent materials for removing organic pollutants from environmental waters. J. Environ. Chem. Engin. 2021, 9 (4), 105197

 

 

Q7. Line 306-307 Section 5.2.5.1. The pH is adjusted on the range 9-12. Could you elaborate how the pH influences and the l morphology of the produced LDH? Why there are studies where pH is 9 while in others pH =12? The same comment is related to aging time. Why it is changed over such a wide range (12-36 h)? Could you elaborate on structure-property relationship and support your discussion with several references where the effect of synthesis time is demonstrated.

A7. We thank the reviewer so much for these valuable notifications. In the revised manuscript we elaborated the roles/influences of pH, aging time and temperature on the LDH structure and morphology. Please find our modifications in blue color in page 8, section 5.2.5.1. as follow;

 

5.2.5.1. Co-precipitation method 

It is the most common method applied for the preparation of LDHs [63-65] and nanomaterials generally in which the metal cations are mixed in a dilute solution under vigorous stirring at 50-90 °C. On the other hand, the precipitation of the metal cations is performed through a suitable base titration in presence of a carbonate source. The common bases applied for precipitation are NaOH and NH₄OH, the carbonate species are applied in general by Na₂CO₃. Carbonate is the most stable and common interlayer anion so it is extremely difficult to prepare LDH material free of carbonate without severs precautions. The factors leading to the formation of LDH rather than amorphous hydroxide or gel formation are aging time, rate of titration, and pH. Long aging time (12-36 h) and slow rate of titration improve the crystallinity and the general morphology of the produced LDH, while pH must be adjusted between 9 and12. The range of pH is wide (≥ 8) where it must be above the neutralization point (pH = 7) to ensure full precipitation of the cations (since LDH is a true hydroxide). High pH values can affect the morphology and crystallinity of LDHs [66] as it increases the cationic content (M^II-M^III) in the final LDH [67].

 

The added references:

66. Wu, L.; Pan, F.; Liu, Y.; Zhang, G.; Tang, A.; Atrens. A. Influence of pH on the growth behaviour of Mg-Al LDH films. Engin. 2018, 34 (9), 674-681.
67. Zhao, Y.; Xiao, F.; Jiao, Q. Hydrothermal synthesis of Ni/Al layered double hydroxide nanorods. J. Nanotechnol. 2011, 11,

 

Q8. Line 314-315. The same as above. “Due to the high temperature and long-time of synthesis”. Could you provide more information on synthesis time and how it influences the properties such as surface area, crystallinity and other?

A8. We really appreciate this comment. This section was revised to discussed the influence of both time and temperature on LDH structure/morphology as mentioned in page 8, section 5.2.5.1 in the revised version, as follow:

Long aging time (≥12 h) increases the crystallinity as well as the cationic content of the formed LDH [67]. High temperatures accelerate the full precipitation of LDHs and increase their crystallinities [68].

 

67. Zhao, Y.; Xiao, F.; Jiao, Q. Hydrothermal synthesis of Ni/Al layered double hydroxide nanorods. J. Nanotechnol. 2011, 11, 646409.
68. Iqbal, M.A. Effect of synthesis conditions on the controlled growth of MgAl–LDH corrosion resistance film: Structure and corrosion resistance properties. Michele Fedel. Coat. 2019, 9, 30.

 

 

Q9. Also synthesis method (5.2.5) could be done in flow. See for example: Li et al., Synthesis of nano-catalysts in flow conditions using millimixers. Advanced Nanomaterials for Catalysis and Energy. doi: 10.1016/B978-0-12-814807-5.00001-2 Chapter 1.

It is necessary to describe flow chemistry synthesis next to batch synthesis. According to the characterization results, it is often observed that the hydrotalcite obtained from flow synthesis configurations shows much higher BET surface area than that produced with a classical precipitation method.

Q9. Thanks for this valuable comment. We revised section 5.2.5. to highlight the flow method and flow chemistry which is the recent breakthrough in preparation methods for LDHs. Also, we cited the abovementioned flow chemistry reference to support our manuscript (Ref. 78 and 79 in the revised version). Please find our modifications in blue color in section 5.2.5.4 and the new section 5.2.5.5 in the revised manuscript (pages 8 and 9) which separately describes the flow chemistry method as follow;

 

5.2.5.4. Vapor diffusion method to prepare exceptional Ti-containing LDHs

The recently modified method to prepare highly crystalline and ultra-thin Ti-containing LDHs was introduced in 2020 by Mostafa et al. [73]. Common Ti-based LDHs exhibit low crystallinities due to the segregation of Ti during the preparation by conventional precipitation methods [74-76]. For the first time, we applied the diffusion of TiCl₄ vapor method to avoid such segregation and could synthesize novel CoTi-LDH nanorods of exceptional photocatalytic properties. The used [Cl^-]-precursors let to 1D-assembly by longitudinal propagation of CoCl₂ and TiCl₄ precursors and formation of [TiCl₆]^2- intermediate during the nucleation stages of the LDH-structure. This in turn greatly enhanced the chemical activity by the ultrathin assembly of CoTi-LDH by the presence of most of the atoms on the surface of the nanorods [77] in contrast to the conventional 2D-structure of common LDHs. As a result, the prepared 1D CoTi-LDH exhibits lowered bandgap energy and great IR-responsivity (1.4 eV), high oxidation states of both Co and Ti, and excellent catalyst distribution in the water splitting media. This innovative vapor diffusion method can be industrially commercialized especially for massive production of Ti-containing LDHs for prospective water splitting, solar cells’ materials and other catalytic and photocatalytic applications. The nature liquid state of TiCl₄ facilitates the usage of vapor diffusion method by the self-supplied TiCl₄–vapor or by use of inert carrier as N₂ without the need of dosing pump as in continuous flow preparation methods.

 

5.2.5.5. Continuous flow preparation method

Because LDHs are highly active and promising materials in fields of energy and environment, development of innovative methods for scale up preparation of LDHs is necessary. The abovementioned preparation methods of LDHs are common, feasible and long-term trailed since the discovery of LDHs, but they still suffer some drawbacks which hinders the industrial production of LDHs. The recent continuous flow preparation methods achieve great advantages over conventional methods (batch precipitation) in the preparation of LDHs such as the feasible scaling up and better physical/chemical properties of the manufactured materials [78]. The synthesized LDHs by flow routes exhibit narrower particle sizes, much fine morphologies and high surface areas. By the flow chemistry routes, it is possible now to avoid the drawbacks of batch methods such as unstable pH, super saturation stage, agglomeration and time consumption especially in the conventional co-precipitation methods [79]. The continuous flow preparation methods have various reactor designs as well as diverse operation means and configurations for preparation of LDHs and other materials in specific morphologies and compositions [79]. The great benefits of flow chemistry technique make it the promising route for supplying the research and industrial communities with smart and tailored LDH and other valuable materials.

 

The added references

78. Chang dong Li, Maoshuai Li, Andre C. van Veen, Synthesis of Nano-Catalysts in Flow Conditions Using Millimixers. Nanomater. Catal. Energy 2019, 1-28.
79. Tichit, D.; Layrac, G.; Gérardin, C. Synthesis of layered double hydroxides through continuous flow processes: A review. Chem. Engin. J. 2019, 369, 302-332.

 

Hope this amendment will take your consideration. We again appreciate this thoughtful comment.

 

Author Response File: Author Response.pdf