Development of Photocatalytic Coatings for Building Materials with Bi₂O₃-ZnO Nanoparticles

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors developed versatile coatings to protect the stone surfaces of Architectural Heritage. This work is interesting and this manuscript is well organized. It is recommended to accept this paper for publishing in this journal after minor revision based on the following comments:

Please consider the effects of coatings on the Architectural Heritage itself. In other words, whether do the coatings destroy the Architectural Heritage?

Author Response

Please consider the effects of coatings on the Architectural Heritage itself. In other words, whether do the coatings destroy the Architectural Heritage?

RESPONSE

We would like to reply that we see no risk of the coatings affecting the Architectural Heritage. In fact, they are prepared with the opposite intention, to achieve the preservation of the materials used in the Architectural Heritage. This is achieved by reducing the access of water due to the hydrophobic character of the coatings, the access of oily stains due to their oleophobic character and having self-cleaning capacity. In durability studies, no harmful effect on the materials has been observed, nor any acceleration of degradation, quite the contrary.

To clarify this fact, we have added some comments and pertinent references in the Introduction and Conclusions (current lines 88-91 and 686-688).

Reviewer 2 Report

Comments and Suggestions for Authors

1. Figure 2 is missing the label on the y-axis. Please improve the quality of the figure. What is the reason for the higher percentage of nanoparticles in the presence of visible light?

2. Fig. 9 and 10 are blurry, and not in good quality. Please improve the quality of the figure.

3. What's new in the article?

4. Are the results reproducible?

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Comments and Suggestions for Authors

1. Figure 2 is missing the label on the y-axis. Please improve the quality of the figure. What is the reason for the higher percentage of nanoparticles in the presence of visible light?

RESPONSE

The Y-axis has been indicated in Fig. 2: it is just the percentage of NO or NOx removal, as denoted by the legend in the same Figure. Its quality has been improved (see current line 130). The statement regarding the percentage of nanoparticles in the visible spectrum is not fully understandable for us. The mentioned figure does not depict the nanoparticles percentage; rather, it illustrates the continuous removal efficiency of nitrogen oxides by the photocatalytic system. The findings indicate that in the UV spectrum, the degradation capacity is marginally superior to that in the visible spectrum, as elucidated in the text.

2. Fig. 9 and 10 are blurry, and not in good quality. Please improve the quality of the figure.

RESPONSE

We have improved the quality of these Figures (current lines 390 and 406). However, please note that these are AFM images, in particular graphs b) and d) of these figures, of an area eroded by climatic aging, and therefore appears as diffuse and blurred by this fact.

3. What's new in the article?

RESPONSE

The work introduced in the objectives section encompasses several innovative aspects. One notable innovation is the incorporation of bismuth oxide nanoparticles alongside zinc oxide in architectural heritage building materials. Additionally, the creation of multifunctional coatings featuring two inorganic matrices—superhydrophobic and oleo-hydrophobic—is a unique advancement. This is particularly noteworthy as the latter matrix has not been previously documented in the literature, whereas many coatings typically rely on an organic base. Furthermore, the utilization of various non-ionic dispersants to enhance photocatalytic activity through nanoparticle dispersion represents another novel contribution.

All these aspects have been remarked in the Introduction (current lines 81-87; 97-102).

4. Are the results reproducible?

RESPONSE

Sections 3.2.1 and 3.2.2 (current lines 539-542 and 565-568) detail the number of replicates for each test to ensure repeatability of the results. The reproducibility of the results is supported by a detailed description of the experimental procedures for coating preparation and characterization. The different figures contain, when appropriate, error bars to indicate the standard deviation.

 

Comments on the Quality of English Language

Minor editing of English language required

RESPONSE

English have been checked and revised throughout the document.

Reviewer 3 Report

Comments and Suggestions for Authors

 

1. Many papers report the investigation of (photo)catalytic reactions to remove hazardous materials. However, the hazard of the materials used in the respective reactions and also the resulting residues are neglected in many cases. It would be valuable if the authors could provide the respective information for their synthesized compounds before and after the photo-catalytic reaction.

2. Some experimental details for measurements of UV-Vis spectra are missing.

3. Please provide the measured FTIR spectra in the supporting information.

4. Are the authors able to compare their results, from using these Bi2O3-ZnO nanoparticles, to the results of others in the literature?

5. Some work on the photo-catalytic reaction could be cited, such as Molecules 2023, 28, 6848; Colloid. Surface A., 656(2023)130475.

 

 

Author Response

Comments and Suggestions for Authors

1. Many papers report the investigation of (photo)catalytic reactions to remove hazardous materials. However, the hazard of the materials used in the respective reactions and also the resulting residues are neglected in many cases. It would be valuable if the authors could provide the respective information for their synthesized compounds before and after the photo-catalytic reaction.

RESPONSE

The reviewer suggests a very interesting idea to which, we agree, little attention is paid. The present work focused on non-toxic or very slightly toxic compounds such as zinc oxide or bismuth oxide. It was stated in the introduction why bismuth oxide, probably one of the lowest toxicity heavy metals, was chosen. It has been safely tested for use in dental cements meeting the criteria for radiopacity, without previous studies demonstrating cellular inflammation or irrefutable manifestations of toxicity (Y.C. Hwang, et al. Chemical composition, radiopacity, and biocompatibility of Portland cement with bismuth oxide, Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod., 107 (2009), pp. e96-e102). With this background, we think that the choice of bismuth oxide nanoparticles (at 8%) and mainly zinc oxide do not represent a problem of toxicity or hazardous materials. In the present study, after photocatalytic degradation, only the modification of the composition of the surrounding atmosphere has been assessed, studying the possible emission of NO₂, as discussed in the work, more toxic than NO, and focusing the discussion on the selectivity of the photocatalysts. No study has been carried out on possible compositional modifications in the photocatalyst after the PCO process, which, on the other hand, are considered unlikely in any case.

We have added this reference in the introduction mentioning the low toxicity of bismuth compounds (current lines 72-73).

2. Some experimental details for measurements of UV-Vis spectra are missing.

RESPONSE

Section 3.2.1 details the laminar flow reactor used (according to ISO standard), the lamp used, its spectrum, its power at the irradiation distance (the lamp's nominal irradiance at a distance of 0.5 m after 1 h was measured to be 41.4 Wm^-2 (780-400 nm), 13.6 Wm^-2 (400-315 nm), and 3.0 Wm^-2 (315-280 nm)), as well as the filter and references. The study procedure has also been conveniently detailed (lines 524-542).

3. Please provide the measured FTIR spectra in the supporting information.

RESPONSE

We have not performed FTIR measurements as the reviewer seems to suggest, so we cannot provide such a spectrum. Unfortunately, nor do we know exactly what spectrum the reviewer is referring to or of which components.

4. Are the authors able to compare their results, from using these Bi2O3-ZnO nanoparticles, to the results of others in the literature?

RESPONSE

The first version included a comparison of the photocatalytic activity of the coatings in terms of NOx degradation with respect to the UNE 127197-1:2013 standard.

The comparison is difficult with other bibliographic references, because the measurement conditions change (irradiation power, type of illumination -generally UV-, quantity of the photocatalyst, exposed surface, concentration of inlet gases, flow, contact time, etc.). If one also wants to consider the nanoparticle included in the coating, it is even more complicated, by changing, besides the aforementioned parameters, thicknesses, substrates and concentration of the photocatalyst. For example, Ballari et al. observed in coatings with pure TiO₂ NO degradations between 43 and 83% but with strict UV illumination, while Dylla et al. measured degradations between 20 and 60%, or Cros et al. observed average degradations of 43% (Ballari, M.M., et al. 2011. Experimental study of the NO and NO2 degradation by photocatalytically active concrete. Catal. Today 1:175-180. Dylla, H., M.M. Hassan, L.N. Mohammad, T. Rupnow, and E. Wright. 2010. Evaluation of environmental effectiveness of titanium dioxide photocatalyst coating for concrete pavement. Trans. Res. Rec. 2164:46-51. Clement J. Cros, Alexandra L. Terpeluk, Neil E. Crain, Maria C.G. Juenger & Richard L. Corsi (2015) Influence of environmental factors on removal of oxides of nitrogen by a photocatalytic coating, Journal of the Air & Waste Management Association, 65:8, 937-947).

Regarding powdered nanoparticles, the results for Bi₂O₃-ZnO heterostructures showed better NO and NOx degradation performances under both sunlight and visible light compared to TiO₂-ZnO (50:50) and (10:90) heterostructures [Speziale et al. Development of Multifunctional Coatings for Protecting Stones and Lime Mortars of the Architectural Heritage. Int. J. Archit. Herit. 2020 14:1008–29]. The data were very similar with respect to heterostructures of the same composition, TiO₂-ZnO, but in ratio (25:75), in particular with strictly visible illumination (percentage differences of only 3%, while with solar these differences were 12.7%, always referring to NO degradation).

We have included some comments and new references in the text regarding these comparisons (current lines 127-129 and 266-268).

5. Some work on the photo-catalytic reaction could be cited, such as Molecules 2023, 28, 6848; Colloid. Surface A., 656(2023)130475.

RESPONSE

The two references have been cited, as requested (current references 32 and 33).