A Remarkable Photocatalyst Filter for Indoor Air Treatment

Round 1

Reviewer 1 Report

This manuscript reports a multi-purpose photocatalyst filter by loading nitrogen-doped titanium dioxide onto nonwoven polymer fibers (PNT/NWPF).This work improves the absorption of visible light by nitrogen doping of titanium dioxide, and the combination with NWPF gives it good stability, not only to trap PM2.5 to PM10 dust, but also to remove p-xylene and inhibit the growth of germs. The manuscript is well organized and can be accepted to publish after following some revisions.

 1. In abstract, the abbreviations PNT and NWPF appear for the first time and should be placed here to explain exactly what they stand for.

 2. In lines 68 to 71,this sentence “Because the composition of pollutant material in the air is so complexed and unregulated, it is a challenge to eliminate not only particulate pollutants such as PM2.5, gaseous pollutants,containing toxic chemicals and bacteria, effectively.”The word “complexed” is wrong,complex has the adjectival case,and this sentence has a mix of sentences and needs to be revised.

 3. In Figure 1,lacking X-ray diffraction patterns of titanium dioxide (P25) and PNT, the diffraction peaks of P25 and PNT should be compared first to show the structural differences between them, and then to compare the variations of the diffraction peaks of PNT and PNT/NWPF.

 4. In lines 108 to 110,the PNT coating thickness is obtained by Scherrer's equation, no specific calculation basis is given and needs to be added．

 5. In Figure 2c,the peak of nitrogen in the spectrum is too weak, which may have the effect of contamination, and it is difficult to indicate the successful doping. It is necessary to further show high-resolution XPS spectra of N1s, which shows the existence form of nitrogen and proves the doping into titanium dioxide.And in Figure 2d,the surface morphology of PNT is blurred, it is recommended to shoot at high magnification will be better.

 6. In Figure 5b,the inlet image of Figure 5b shows captured PM (size 43µm),the emphasis of the article is on capturing PM2.5 to PM10, and the size of the particles captured by the PNT/NWPF in the figure is too large.

 7. In line 261,“a total airflow rate of 0.05L”,the unit of flow rate in this sentence is wrong.

 8. In line 286,“The amount of photocatalyst coated on this filter is 1.4 to 4 mg/cm².”This sentence reads abruptly, with no mention of the corresponding load ratio in the preceding text, and no explanation of this point later.

 9. In line 295,“Photocatalytic degradation cannot be improved by excessive or insufficient moisture.”The meaning of this sentence is unclear.

 

10. In Figure 6,these four figures look blurry and almost impossible to see clearly.

11. Some old references should be updated, especially for air pollution, such as Catalysts 2022, 12(4), 400; Chinese Chemical Letters, 2022, 10, 4679.

Author Response

This manuscript reports a multi-purpose photocatalyst filter by loading nitrogen-doped titanium dioxide onto nonwoven polymer fibers (PNT/NWPF). This work improves the absorption of visible light by nitrogen doping of titanium dioxide, and the combination with NWPF gives it good stability, not only to trap PM_2.5 to PM₁₀ dust, but also to remove p-xylene and inhibit the growth of germs. The manuscript is well organized and can be accepted to publish after following some revisions.

Specific comments:

Comment 1. In abstract, the abbreviations PNT and NWPF appear for the first time and should be placed here to explain exactly what they stand for.

Response: The sentence has been modified with the given explanation for the abbreviations used for the first time in the abstract.

A multi-purpose photocatalyst filter, photocatalytic nitrogen-doped titanium dioxide (PNT) incorporated in non-woven polymer fibers (NWPF) with wide applications are focused on detail in this work.

Comment 2. In lines 68 to 71, this sentence “Because the composition of pollutant material in the air is so complexed and unregulated, it is a challenge to eliminate not only particulate pollutants such as PM_2.5, gaseous pollutants, containing toxic chemicals and bacteria, effectively.” The word “complexed” is wrong, complex has the adjectival case, and this sentence has a mix of sentences and needs to be revised.

Response: The sentence has been revised for clear understanding.

The contaminants in the air includes fine particulate (PM_2.5), toxic gaseous pollutants and harmful pathogens. So it is challenge to eliminate all sources of pollution in a most effective way.

Comment 3.   In Figure 1,lacking X-ray diffraction patterns of titanium dioxide (P25) and PNT, the diffraction peaks of P25 and PNT should be compared first to show the structural differences between them, and then to compare the variations of the diffraction peaks of PNT and PNT/NWPF.

Response: The sentence has been revised as per the suggestion.

An XRD analysis is performed on the prepared materials to determine their crystal phase structure (Figure 3). PNT prepared from TiO2 (P25) has both the anatase phase and rutile phase (in accordance with JCPDS 21-1272 and JCPDS 21-1276 standard reference pattern) which is consistent with the biphasic structure of commercial P25 [37]. Also, there were no other characteristic peaks from impurities as indicated by the XRD patterns. Further, PNT is coated on non-woven polymer fabric which is composed of double layered polymer-polyethylene terephthalate (PET) fiber and polypropylene. Among the double layer, the photocatalyst PNT preferred selective coating on the polyethylene terephthalate layer only because of its superior hydrophilic nature than to polypropylene. According to the literature, the crystal faces of high crystal PET fibers have been assigned to the following numerous diffraction peaks positioned at 17.5⁰, 21.5⁰ and, 26.1⁰ values of 2θ corresponding to 010, 110, and 100 crystal planes, respectively [38, 39]. Meanwhile ,the lattice plane 110 appeared at 13.95⁰ represents for the polypropylene layer [40]. After the PNT coating on the non-woven polymer fabric, its XRD patterns indicated high intense crystalline peaks at 25.47⁰, 27.54⁰, 38.05⁰, 48.26⁰, 54.15⁰, 62.88⁰, 69.03⁰ in both 1% and 2% photocatalyst coated NWPF. There is not much difference in the intensities between 2% and 5% PNT coated NWPF. Using Scherrer's formula, the crystallite size of synthesized materials was calculated from the half-width of anatase peak (101). The mean crystallite size of the 1%, 2% and 5% PNT coated NWPF is 13, 15 and 15.2 nm respectively.

Comment 4.  In lines 108 to 110, the PNT coating thickness is obtained by Scherrer's equation, no specific calculation basis is given and needs to be added.

Response: Thank you for the comment. We have added the information about Scherrer equation under “Material Characterization” section.

The average crystallite size is calculated using Scherrer formula [D = kλ/Wcosθ] in which k is the numerical constant (k=0.93), λ is CuKα wavelength (λ= 1.5406 Å).

Comment 5. In Figure 2c, the peak of nitrogen in the spectrum is too weak, which may have the effect of contamination, and it is difficult to indicate the successful doping. It is necessary to further show high-resolution XPS spectra of N1s, which shows the existence form of nitrogen and proves the doping into titanium dioxide. And in Figure 2d, the surface morphology of PNT is blurred, it is recommended to shoot at high magnification will be better.

Response: We have done high-resolution XPS spectra of PNT and also curve fitting for the individual elements with brief explanation. Also, the morphology image of PNT (Figure 2d) is also changed with clear image.

The XPS spectra of PNT nanoparticle is shown in Figure 1c-e for N1s, O1s and Ti2p energy levels respectively. In Figure 1c, it is has been clearly demonstrated that the core level of Nitrogen 1s peak in PNT occurred at 398.2 eV since the anionic nitrogen present in O-Ti-N linkage [32]. It is expected that either simple chemisorbed nitrogen or TiN will appear at ≤ 397.5 eV, and NO or NO₂ species perhaps at 400 eV or even higher. On the contradict, N1s peak at 398.2 support the hypothesis that nitrogen is associated into the TiO₂ lattice as N-Ti-O linkage indicates substitutional doping [33]. Figure 1d represents the O1s spectra of PNT with a peak at 530.28 eV indicates a Ti-O bond [34]. The PNT sample however shows a broadening on the higher BE side at 531.5 eV. There might be possibilities that PNT has a divergent type of oxygen owing to its characteristic covalent nature. This is likely that both oxygen and nitrogen elements are accessible in TiO₂ from the same lattice units. Ti2p spectrum of the PNT sample displayed two distinct energy levels at 458.05 and 463.74 eV confirms the Ti2p_3/2 and Ti2p_1/2, which supports the presence of Ti⁴⁺ species as depicted in Figure 1e [35].Thereby, a small shift in the binding energy is observed for PNT compared with P25 [36] due to the interaction between nitrogen and titanium.

 Comment 6. In Figure 5b, the inlet image of Figure 5b shows captured PM (size 43µm), the emphasis of the article is on capturing PM2.5 to PM10, and the size of the particles captured by the PNT/NWPF in the figure is too large.

Response: We have incorporated the new inlet image in Figure 5b which shows the capture of PM which is around 8-11 µm.

Comment 7. In line 261,“a total airflow rate of 0.05L”,the unit of flow rate in this sentence is wrong.

Response: We have changed the unit for “total airflow rate as 50 mL/min”.

Comment 8. In line 286,“The amount of photocatalyst coated on this filter is 1.4 to 4 mg/cm².”This sentence reads abruptly, with no mention of the corresponding load ratio in the preceding text, and no explanation of this point later.

Response: This has been modified and rearranged in the manuscript with clear explanation (page and line). Also, the representation of amount of catalyst coated is consistently maintained in g/m² throughout the manuscript. [1% PNT/NWPF = 45.8 g/m² 2% PNT/NWPF = 64 g/m² 5% PNT/NWPF = 90.1 g/m²]

The amount of photocatalyst loaded on NWPF is in the range of 45.8 to 64 g/m² for different concentrations. When higher amount of PNT is loaded on NWPF, the efficiency of photocatalyst (5% PNT/NWPF) is greatly increased to 94.2% under normal visible daylight.

Comment 9. In line 295,“Photocatalytic degradation cannot be improved by excessive or insufficient moisture.” The meaning of this sentence is unclear.

Response: The sentence has been revised for clear understanding.

Photocatalytic degradation can also be influenced by moisture content, but, an optimal RH level is required for maximum photocatalytic degradation.

  Comment 10. In Figure 6,these four figures look blurry and almost impossible to see clearly.

Response: We have inserted the new image with high resolution range.

 Comment 11. Some old references should be updated, especially for air pollution, such as Catalysts 2022, 12(4), 400; Chinese Chemical Letters, 2022, 10, 4679.

Response: Thank you for the recent citation. We have added the mentioned references in the introduction part of the manuscript [Ref 18, 19].

Photocatalysts are commonly utilized to remove toxic chemical compounds effectively [17-19]

Author Response File: Author Response.pdf

Reviewer 2 Report

My comments are listed in the following: 

-  In results and discussion part, it needs some better interpretation with detail discussion for all of data with published paper citation.

-  The authors should recheck this sentence “The main characteristic crystal faces 93 of polyethylene terephthalate polymer positioned at 17.29⁰, 22.48⁰, and 25.37⁰ values of 2θ 94 corresponding to 010, 110, and 100 crystal planes, respectively” How polymer material has a crystallity? 

- “After the PNT coating on the 96 nonwoven polymer fabric, the XRD patterns show high intense crystalline peaks at 25.47⁰, 97 27.54⁰, 38.05⁰, 48.26⁰, 54.15⁰, 62.88⁰, 69.03⁰ in both 1% and 2% photocatalyst coated NWPF.” I cannot understand this sentence. 1 or 2 % sample is not easily response for X-Ray.

-  SEM images are not clear, I recommend TEM analysis.

-  This part needs Bandgap energy value evaluation and position with Mott-Schottky data. From these data, it needs Bandgap energy location. Is it p-type or n-type ?

-  And, it requires some more detail data such PL and EIS data.

- From, Figure 3, authors suggest 4 kinds of samples (a) Pristine NWPF. (b) Pretreated NWPF. (c) SiO2 coated NWPF. (d) PNT 147 coated NWPF. All of samples require same analysis such as XRD, SEM, N2 adsorption-desorption isotherm, DRS, and Photocataytic reactions.

Author Response

In results and discussion part, it needs some better interpretation with detail discussion for all of data with published paper citation.

Comment 1. The authors should recheck this sentence “The main characteristic crystal faces 93 of polyethylene terephthalate polymer positioned at 17.29⁰, 22.48⁰, and 25.37⁰ values of 2θ 94 corresponding to 010, 110, and 100 crystal planes, respectively” How polymer material has a crystallinity? 

Response: The explanation for the crystal faces of polyethylene terephthalate (PET) polymer has been supported and explained with the reported literature works [38, 39].

PET prepared in the fiber production process undergoes annealing which bears 25% crystallinity has broad X-ray diffraction peaks with poor intensities at 17.29⁰, 22.48⁰, and 25.37⁰ respectively.

Comment 2.  “After the PNT coating on the 96 nonwoven polymer fabric, the XRD patterns show high intense crystalline peaks at 25.47⁰, 97 27.54⁰, 38.05⁰, 48.26⁰, 54.15⁰, 62.88⁰, 69.03⁰ in both 1% and 2% photocatalyst coated NWPF.” I cannot understand this sentence. 1 or 2 % sample is not easily response for X-Ray.

Response: The comment has been addressed in materials and methods section. Different weight percentage of PNT solution has been prepared and applied for coating. The performance of different coating density has been evaluated for its coating stability and also tested its performance for xylene removal.

Comment 3.  SEM images are not clear, I recommend TEM analysis.

Response: We have analysed samples again for SEM and TEM and inserted in the revised manuscript

The surface morphology of the PNT nanoparticles are photographed by scanning electron microscopy (SEM) in Figure 2a; the spherical shaped nanoparticles are clearly seen. Figure 2b depicts the HR-TEM image of the synthesized PNT nanoparticles. A lattice fringe of 0.350 nm and 0.240 nm was observed because of the anatase lattice plane (101) of TiO₂ nanoparticles and 0.320 nm because of the rutile lattice plane (110) of pure TiO₂ (Figure 2c-d)

Comment 4.   This part needs Bandgap energy value evaluation and position with Mott-Schottky data. From these data, it needs Bandgap energy location. Is it p-type or n-type ?

Comment 5.  And, it requires some more detail data such PL and EIS data.

Response: Since our research lab is currently in developing state, we lack some experimental facilities which are not available at this time. We apologize for missing these mentioned experiments (Bandgap position by Mott-Schottky data and EIS). PL experiment has been carried out and the data has been added in the revised manuscript.  Hopefully, we have tried to satisfy the discussion part with reference citations and also compared our material with other progressive works (Table-3) as suggested. However, we have also added some more supporting experiments for material characterization such as XPS (Figure 1c-e), TEM (Figure 2b-d) and FT-IR spectroscopy (Figure 1f). Kindly, consider it.

 

The surface morphology of the PNT nanoparticles are photographed by scanning electron microscopy (SEM) in Figure 2a; the spherical shaped nanoparticles are clearly seen. Figure 2b depicts the HR-TEM image of the synthesized PNT nanoparticles. A lattice fringe of 0.350 nm and 0.240 nm was observed because of the anatase lattice plane (101) of TiO₂ nanoparticles and 0.320 nm because of the rutile lattice plane (110) of pure TiO₂ (Figure 2c-d). Photoluminescence (PL) spectra is an excellent tool to study how well charge carriers trap, migrate and transfer because the PL emissions results from the recombination of free carriers [37]. TiO₂ absorbs incident photons with energy equal to or higher than the band gap energy, producing photoinduced charge carriers (e^- h⁺⁾. Further, PL emission spectra are produced by the recombination of electrons and holes. Thus, a low PL intensity specifies less charge recombination. The PL emission spectra for TiO₂ and PNT are shown in Figure 2e. The faster recombination of photoinduced charge carriers in TiO₂ resulted in a higher PL intensity. When nitrogen (N) is introduced to TiO₂, the intensity is lowered.  Hence, the PL results confirm that N states serve as a hole reservoir for inducing excitations and improving electron-hole separation [38].

Comment 6.  From, Figure 3, authors suggest 4 kinds of samples (a) Pristine NWPF. (b) Pretreated NWPF. (c) SiO2 coated NWPF. (d) PNT 147 coated NWPF. All of samples require same analysis such as XRD, SEM, N2 adsorption-desorption isotherm, DRS, and Photocataytic reactions.

Response: Since non-woven fabric (NWPF) is used as substrate for the material coating (dip coating process), we have showed SEM data for the detailed observation. But, the pretreated NWPF substrate and role of binder SiO₂ does not have significant photocatalytic properties. Therefore, we have not analyzed the above characterizations before.

Additional information

As per the suggestion, we have compared this study with other published research works mentioned in Table-3 

Table 3 Photocatalytic activity of PNT/NWPF of current work and other reported literature data for xylene pollutant decomposition

Photocatalyst	Pollutant	Light source	Removal efficiency (%)	Reference
Blackened TiO2	xylene	UV+ Visible light (365-375nm + 470-618nm)	35.64%	[54]
TiO2, Mn-doped TiO2 on Ceramic Honeycomb substrate	xylene	UV light UV254+185 nm	33.5% 94%	[55]
5% (wt) Degussa-TiO2 coated on high efficiency particulate air filter (HEPA)	xylene	36 W UV lamp	49%	[51]
Degussa-TiO2 0.1% Mn-TiO2	xylene	300W UV light 300W UV, 500W Halogen light	58% 58%  22%	[56]
1% (wt) PNT/NWPF 2% (wt) PNT/NWPF 5% (wt) PNT/NWPF	xylene	24W Fluorescent lamp	62.1% 86.5% 94.2%	Current work

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The revision can be accepted now.