Perovskite-type LaFeO₃: Photoelectrochemical Properties and Photocatalytic Degradation of Organic Pollutants Under Visible Light Irradiation

Round 1

Reviewer 1 Report

The paper "Perovskite-type LaFeO3: Photoelectrochemical properties and photocatalytic degradation of organic pollutants under visible light irradiation" shows the synthesis and a comprehensive characterization of perovskite-type LaFeO3. This photocatalyst was characterized 

as a function of calcination temperature. Several chemical-physical features as morphology, chemical composition, crystalline structure, textural properties and optical properties were taken into account and related to the photocatalytic performance of the LaFeO3. 

Authors state that the optima calcination temperature was 700°C because this experimental conditions allows to obtain an hight surface area and therfore improved photocatalytic activity. The work is well presented, complete and the conclusions are consistent with the experimental results.  In my opinion, it can be published on Catalyst in the present form.  

Author Response

Thanks for the positive comments

Reviewer 2 Report

The paper is well written, is innovative and interesting for the scientific community, the bibliography is correctly quoted. 

To better highlight the performance of these catalysts it is suggested to report the photodegradation experiment with the Tijare catalyst (reference 23 of the work) in the experimental conditions of the article. This experiment, in accordance with the "Best Practices for Reporting on Heterogeneous Photocatalysis" guidelines (dx.doi.org/10.1021/am504389z | ACS Appl. Mater Interfaces 2014, 6, 11815-11816) will highlight the properties of the new catalysts.

Author Response

Comment:

To better highlight the performance of these catalysts it is suggested to report the photodegradation experiment with the Tijare catalyst (reference 23 of the work) in the experimental conditions of the article. This experiment, in accordance with the "Best Practices for Reporting on Heterogeneous Photocatalysis" guidelines (dx.doi.org/10.1021/am504389z | ACS Appl. Mater Interfaces 2014, 6, 11815-11816) will highlight the properties of the new catalysts.

Reply:

Thank you for the comment. We have added reference 23 in the experimental section (Line 303). The changes in the preparation in comparison to Tijare et al. have been mentioned at the end of the Introduction in lines 67 and 68.

Reviewer 3 Report

Journal: Catalysts (ISSN 2073-4344)

Manuscript ID: catalysts-456336

Type: Article

Number of Pages: 23

Title: Perovskite-type LaFeO3: Photoelectrochemical properties and photocatalytic degradation of organic pollutants under visible light irradiation

Authors: Mohammed Ismael , Michael Wark *

 

This paper deals with the synthesis and characterisation of a perovskite-type material for possible applications in pollutants photodegration under visible light.

 

The authors report present with enough accuracy in a rather concise but clear manner all the aspects connected to synthesis and subsequent characterization.

 

The manuscript is well written, figure and SI material are well edited; references are numerous and well chosen.

 

In my opinion there a few points that need to be corrected and or implemented, as suggested in the following:

1.     The authors refer to SEM characterisation, and quote such an instrument, but in the manuscript they report TEM images (without quoting the relevant equipment): this point has to be clarified (end of pg. 4);

2.     Moreover, they report the EDS analysis obtained by SEM (beginning of pg. 5), indicating that C is ubiquitous and coming from the latex of the SEM sample holder: this is again to be clarified;

3.     In the FT-IR section, the authors refer to a very small peak located at ca. 1600 cm-1 as to be ascribable to “the H-O bending mode of absorbed water and some surface hydroxyl groups” (mid pg. 5). The spectral component they refer to is really almost not visible, but the stranger aspect is the parallel total absence in the 3600-3750 cm-1 range of the H-O vibration mode which should be the spectroscopic counterpart of the above H-O bending mode. Anyway the negligible (or absent) intensity of these modes could be ascribed to the very low specific surface areas (SSA) of the samples. This aspect should be commented;

4.     The “small peak at 2905 cm-1” is reported to be due to CO2 adsorbed during the processing of the FTIR sample: to which mode of CO2 do the authors think it should be due? As there are no IR active mode due to CO2 in that spectral region.

5.     As for the very low SSA (begininng of pg. 6), the figures the authors obtain are referred to N2 ads/des isotherms: but when the SSA are so small, it would be more useful to use a different adsorbate, such as krypton, to investigate the SSA. Could the authors comment on this?

 

For the above reasons I propose to accept to be published in Catalysts this manuscript after minor revisions.

Author Response

Reviewer: 3

Comments:

The authors refer to SEM characterization, and quote such an instrument, but in the manuscript they report TEM images (without quoting the relevant equipment): this point has to be clarified (end of pg. 4).

Reply:

Sorry for the confusion. We have used SEM for the morphology determination on both samples as now been corrected in lines 97 and 98 and the caption of Figure 2.

Moreover, they report the EDS analysis obtained by SEM (beginning of pg. 5), indicating that C is ubiquitous and coming from the latex of the SEM sample holder: this is again to be clarified.

Reply:

Thank you for the comment. The carbon signals appearing in the EDX spectra are attributed to an incomplete coverage of the black carbon SEM holder with the samples (lines 107,108).

In the FT-IR section, the authors refer to a very small peak located at ca. 1600 cm^-1 as to be ascribable to “the H-O bending mode of absorbed water and some surface hydroxyl groups” (mid pg. 5). The spectral component they refer to is really almost not visible, but the stranger aspect is the parallel total absence in the 3600-3750 cm^-1 range of the H-O vibration mode which should be the spectroscopic counterpart of the above H-O bending mode. Anyway the negligible (or absent) intensity of these modes could be ascribed to the very low specific surface areas (SSA) of the samples. This aspect should be commented.

Reply:

The doubts of the reviewer are correct. It is unlikely that the small peak at 1600 cm^-1 results from O-H vibrations. We checked the assignment again and came to the conclusion that it is more likely, that small amounts of carboxyl groups from tiny residues of citric acid might be responsible for the vibration at 1600 cm^-1. This is now explained in lines 121 to 123.

The “small peak at 2905 cm-1” is reported to be due to CO₂ adsorbed during the processing of the FTIR sample: to which mode of CO₂ do the authors think it should be due? As there are no IR active mode due to CO₂ in that spectral region.

Reply:

Thanks to the reviewer for pointing this out. It is correct that the main vibrations (symmetric stretching) of CO₂ appear at around 2350 cm^-1. The observed very small vibrations at around 2905 cm^-1 might more likely result from C-H vibrations again in the tiny residues of citric acid. This is now stated on lines 121-123.

 As for the very low SSA (begininng of pg. 6), the figures the authors obtain are referred to N₂ ads/des isotherms: but when the SSA are so small, it would be more useful to use a different adsorbate, such as krypton, to investigate the SSA. Could the authors comment on this?

Reply:

The reviewer is correct; Kr in principle is a possible adsorptive for porous materials with small surface area. However, the problem with Kr is that the isotherms should be measured at 87 K in order to avoid the formation of a solid phase, which may lead to a mistake in the calculations. This means that liquid nitrogen cannot longer be used as a cooling agent. However, many researchers neglect this difficulty and measure at liquid N₂ temperature anyway. But we decided to stay at N₂ as adsorptive, since we believe that the SSAs obtained are reasonable (for sure with respect to the trends of the SSA among the different samples).

 

Reviewer 4 Report

1)The paper is not novel. There are several papers about the synthesis and photocatalytic properties of LaFeO3 prepared with the same method presented in this paper.

2) The authors claim that "...LaFeO3 is not able to form hydrogen via water splitting under light irradiation, confirming our negative results in respective experiments employing photodeposited platinum nanoparticles as co-catalyst and methanol as sacrificial agent, which of course stand in contrast to H2 formation reported earlier...." and that ".based on our present results, some doubts on the H2 production results come up".  I believe that the authors should be cautious in their statements. In fact, from the experimental section it is reported that they used Pt/LaFeO3 catalyst prepared by photodeposition but no physical-chemical characterization of this catalyst is reported. It is mandatory to analyze the distribution and size of Pt particles and it is necessary also to understand if the Pt species are present as metallized particles or as PtOx. In fact, as it is well known, for Pt content higher than certain values, Pt particles become the center for recombining photoinduced electron and hole pairs and so the photoactivity of photocatalysts is negligible  (I suggest to strongly study the literature about this aspect). In addition, the authors used methanol as sacrificial agent and a light source with a very high power. Due to the high volatility of the methanol, the temperature of photoreactor must be mantained at very low value avoiding the heating phenomena induced by the lamp. Did the authors used a cooling system for the suspension under irradiation?

3) The authors used Rhodamine B dye as model pollutant. This pollutant may sensitize the photocatalyst surface inducing activity in presence of visible light. Indeed, the used a colourless pollutant (4CP), but they analyzed the concentration of this pollutant by spectrophotometer. Are they sure that reaction intermediates did not influence the correct analysis of 4CP concentration? Additionally, they also should perform TOC analysis in order to analyze the mineralization of the targer pollutant. They also should compare their results with the literature about the degradation of 4CP.

Author Response

Reviewer: 4

Comments:

1)      The paper is not novel. There are several papers about the synthesis and photocatalytic properties of LaFeO₃ prepared with the same method presented in this paper.

 

Reply:

Thank you for the comment; it is correct that the photocatalytic properties of LaFeO₃ have also been reported by others. We tried to cite these papers thoroughly. We have added reference 23 in the experimental section (Line 303). The changes in the preparation in comparison to Tijare et al. have been mentioned at the end of the Introduction in lines 67 and 68.
A focus in our paper is put on the photoelectrochemical characterization of the LaFeO₃ (Fig. 8, lines 172-222). This kind of characterization is often neglected by others, however, it provides valuable insight on band positions, being important to see what photocatalytic reactions are possible, and charge carrier dynamics, i.e. charge carrier separation.  

 

2)      The authors claim that "...LaFeO₃ is not able to form hydrogen via water splitting under light irradiation, confirming our negative results in respective experiments employing photodeposited platinum nanoparticles as co-catalyst and methanol as sacrificial agent, which of course stand in contrast to H₂ formation reported earlier...." and that ".based on our present results, some doubts on the H₂ production results come up".  I believe that the authors should be cautious in their statements. In fact, from the experimental section it is reported that they used Pt/LaFeO₃ catalyst prepared by photodeposition but no physical-chemical characterization of this catalyst is reported. It is mandatory to analyze the distribution and size of Pt particles and it is necessary also to understand if the Pt species are present as metallized particles or as PtOx. In fact, as it is well known, for Pt content higher than certain values, Pt particles become the center for recombining photoinduced electron and hole pairs and so the photoactivity of photocatalysts is negligible (I suggest to strongly study the literature about this aspect). In addition, the authors used methanol as sacrificial agent and a light source with a very high power. Due to the high volatility of the methanol, the temperature of photoreactor must be maintained at very low value avoiding the heating phenomena induced by the lamp. Did the authors use a cooling system for the suspension under irradiation?

 

Reply:

Thank you for stressing these critical points.
LaFeO₃ does not produce hydrogen due to the positive conduction band potential measured by the Mott Schottky plot. The position of the conduction band is often not measured (refs. 37 and 38). We have added another reference reporting onH₂ formation from LaFeO₃/g-C₃N₄ composites to make clear that in that case the g-C₃N₄ with a conduction band at strongly negative potential (-0.85 V vs NHE) is the photocatalyst at which the hydrogen is formed (ref. 39). Of course, other ferrites (CuFe₂O₄ and NiFe₂O₄, new refs. 40 and 41) have activity for hydrogen production due to their more negative conduction band positions. (Line 198-203).

The Pt particles photodeposited are very small (1-2 nm) being hardly visible even by transmission electron microscopy. The amount deposited is much below 1 wt.-%. We now added this information in lines 194 and 373.

We carefully checked the influence of co-catalysts (Pt or Rh) in other publications before, e.g. in O. Merka, O. Raisch, F. Steinbach, D.W. Bahnemann, M. Wark, J. Am. Ceram. Soc., 96 (2013) 634 – 642 (DOI: 10.1111/jace.12013) or J. Soldat, R. Marschall, M. Wark, Chem. Sci. 5 (2014), 3746-3752 (DOI: 10.1039/c4sc01127a), however, we never found indication that the Pt or Rh co-catalysts act as recombination centers.

The temperature of the methanol-containing aqueous solution was kept at slightly below room temperature (at about 10 °C) by use of a flow of cooling water through the double-walled quartz glass mantle of the reactor during the photocatalytic reaction (Lines 366 and 373).

3)      The authors used Rhodamine B dye as model pollutant. This pollutant may sensitize the photocatalyst surface inducing activity in presence of visible light. Indeed, the used a colorless pollutant (4CP), but they analyzed the concentration of this pollutant by spectrophotometer. Are they sure that reaction intermediates did not influence the correct analysis of 4CP concentration? Additionally, they also should perform TOC analysis in order to analyze the mineralization of the target pollutant. They also should compare their results with the literature about the degradation of 4CP.

Reply:

Thank you for your comment. We are aware on the problem of possible self-sensitization of the Rhodamine B. Thus, of course, we checked that RhB is not self-degrading. As seen in Fig. 9b the degree of self-degrading is very low and is negligible compared to the degree of decomposition with LaFeO₃.
We also performed a degradation test of RhB with SiO₂ as a photocatalytic inactive material. This was negative as well. This information is now added in line 235. However, we did not include this result in Fig. 9b, since the experimental conditions (oxide to RhB ratio) were slightly different.

For 4-CP in a HPLC analysis performed with the reaction mixture after 5 hours of illumination, no significant amounts of decomposition products were found. TOC experiments were performed as well; confirming the decomposition (55 % loss of TOC). With that our LaFeO₃ photocatalyst shows higher activity for 4-CP degradation compared to that reported by Pirzada et al. [59] and Hu et al. [60]. These information and references were now added (Line 289-294).

Round 2

Reviewer 4 Report

2.1. Structural and optical characterization of LaFeO3

Line 193-194 “…confirming our negative results in 193 respective experiments employing photodeposited platinum nanoparticles as co-catalyst (< 1 wt.-% (0.5 wt.-%), particle size < 2 nm)…” Precise better the Pt content. Is it less than 1 wt.% or  is 0.5wt.%?

 

Line 197 “Thus, based on our present results, some doubts on the H2 production results come up.” The previous phrase should be removed, since the authors tested their LaFeO3 in the presence of cocatalyst under visible light irradiation and was inactive since the slightly positive CB potential.

 

3.2. Photocatalytic properties

 “Also with SiO2, a photocatalytically inactive material, no other loss of RhB from solution than by adsorption (in the dark) occurs. “

It must be precised that sensitization is the phenomenon for which, using the visible-light absorption of dye molecules, photo-excited electrons are transferred to inorganic semiconductors, and these electrons are able to initiate redox reactions. The dye molecules are used as the light absorber and the inorganic semiconductors need to be UV-responsive materials (>3.0 eV). 

In this case the authors added a test on a degradation  of RhB with SiO2 a phoactive inactive material that of course gives a negative response.

The authors should test a semiconductor in the visible light sensitization by the dye.

 

. 3.4. Photocatalytic degradation activity measurement.

“As co-catalyst 0.5 wt.% Pt (particle size < 2 380 nm) were deposited on the LaFeO3 powder via reductive photodeposition from H2PtCl6^.6H2O” In this part the conditions of the photoreduction should be added, since they do a parallel with other photocatalysts active in hydrogen production

 

Author Response

Comments and Suggestions for Authors

2.1. Structural and optical characterization of LaFeO3

Line 193-194 “…confirming our negative results in 193 respective experiments employing photodeposited platinum nanoparticles as co-catalyst (< 1 wt.-% (0.5 wt.-%), particle size < 2 nm)…” Precise better the Pt content. Is it less than 1 wt.% or  is 0.5wt.%?

Thanks for this comment.  We now clearly say that the Pt content was determined to 0.5 wt-%.

Line 197 “Thus, based on our present results, some doubts on the H2 production results come up.” The previous phrase should be removed, since the authors tested their LaFeO3 in the presence of cocatalyst under visible light irradiation and was inactive since the slightly positive CB potential.

We agree that there was some need for clarification. On lines 190-197 we now write: The slightly positive CB potential explains that the LaFeO₃ is not able to form hydrogen via water splitting under light irradiation. This is confirmed by respective experiments attempting photocatalytic H₂ formation with our LaFeO₃ samples on which platinum nanoparticles (0.5 wt.-%, particle size < 2 nm) were photodeposited as co-catalyst. Even by use of light of λ ≥ 320 nm and methanol as sacrificial agent no hydrogen was detected with all the Pt/LaFeO₃ samples. This result stands in contrast to H₂ formation reported earlier by Tijare et al. [23], Parida et al. [37] and Vaiano et al. [38], who, however, performed no analysis on conduction band positions. Thus, some doubts regarding the H₂ production reported in their papers exist.

 

3.2. Photocatalytic properties

 “Also with SiO2, a photocatalytically inactive material, no other loss of RhB from solution than by adsorption (in the dark) occurs. “

It must be precised that sensitization is the phenomenon for which, using the visible-light absorption of dye molecules, photo-excited electrons are transferred to inorganic semiconductors, and these electrons are able to initiate redox reactions. The dye molecules are used as the light absorber and the inorganic semiconductors need to be UV-responsive materials (>3.0 eV). 

In this case the authors added a test on a degradation  of RhB with SiO2 a phoactive inactive material that of course gives a negative response.

The authors should test a semiconductor in the visible light sensitization by the dye.

The reviewer is right; SiO₂ is not a good reference material to check for sensitization effects. Thus, we now performed a reference experiment with SnO₂ which we had in hand for another project. The obtained data for the photocatalytic degradation of RhB with SnO₂ under illumination with light of
l > 420 nm are now included in Fig. 9b.  It can be seen that the degradation is not stronger than for the blank experiment with RhB alone. Thus, since the semiconductor SnO₂ cannot be excited with 420 nm light, no photocatalytic degradation occurs.

We thus replaced the sentence concerning SiO₂ with the following text describing shortly the results with SnO₂ (lines: 234-236): Also in presence of SnO₂, a semiconductor with a band gap of 3.0 eV, which can, thus, not be excited by light of λ ≥ 420 nm only negligible degradation was found. Thus, sensitization effects can be ruled out as well.

 

 

. 3.4. Photocatalytic degradation activity measurement.

“As co-catalyst 0.5 wt.% Pt (particle size < 2 380 nm) were deposited on the LaFeO3 powder via reductive photodeposition from H2PtCl6^.6H2O” In this part the conditions of the photoreduction should be added, since they do a parallel with other photocatalysts active in hydrogen production

Thanks to the reviewer for this comment; we now describe the experimental conditions for the attempted photocatalytic hydrogen production in more detail in the Experimental part (chapter 3.4) in lines 382 till 397. We also renamed this subchapter to: “3.4. Photocatalytic degradation activity and hydrogen evolution measurements”.
In lines  393-397 we now describe the photodeposition of Pt: “Upon light irradiation, metallic Pt nanoparticles are photodeposited onto the photocatalyst surface sites preferentially accessible for electrons, while CO₂ is formed from methanol being employed as sacrificial reagent as qualitatively detected with our multichannel analyzer (Emerson). The standard redox potential for the reduction of Pt²⁺ ions to metallic Pt is at + 1.2 V vs. NHE, thus the electrons in the CB of LaFeO₃ are able to initiate this reduction.”