Inkjet Printing of Sc-Doped TiO₂ with Enhanced Photoactivity

Round 1

Reviewer 1 Report

The author show a method for synthesizing Sc doped TiO2 to be processed by inkjet printing. Both the synthesis as well as processing of such materials is an important task for many applications. to the reviewers view the following aspects are missing:

Which are possible applications of such layers. TiO2 is broadly uses for many applications. The paper gives not hint what the possible applications of such materials could be, in particular what the effect of the Sc-doping is (except the shift of the optical characteristics). Where is the practical use?

A characterization of the inkjet printed layers is missing. There is only the SEM image and the droplet dimension. What were the jetting parameters for obtaining homogeneous and closed layers? Which layer thickness can be achieved. The images indicate a coffee stain effect. How can it be prevented? What is the height difference of edge and center?

Author Response

The author show a method for synthesizing Sc doped TiO2 to be processed by inkjet printing. Both the synthesis as well as processing of such materials is an important task for many applications. to the reviewers view the following aspects are missing:

 

QUESTION

Which are possible applications of such layers. TiO2 is broadly uses for many applications. The paper gives not hint what the possible applications of such materials could be, in particular what the effect of the Sc-doping is (except the shift of the optical characteristics). Where is the practical use?

 

ANSWER

Fabricated Sc-doped titania based layers are meant to be used in multijunction solar cell devices. This type of solar cells features the ultra-high efficiency achievable due to the multilayered deposition of semiconductors. The sequence of alternated multiple p-n junctions of different semiconductors generates the electric current in response to different wavelengths of light. This multilayered architecture brings the efficient photon conversion to electrons and their transport to electrodes. Due to the simplicity of the thin-films fabrication and straightforward influence of Sc on titania band-gap, Sc-doped TiO2 nanofilms can be implemented as one of the layers absorbing the specific region of the spectrum.  

Page 2 now states:

Solar cells play an important role as they allow to convert the abundant solar energy to electricity directly. During past decades, numerous studies have been devoted to improving the solar cells performance and lowering their price.[15]One of the widely explored approaches involves the use of the thin film multijunction solar cell. Such a composite architecture allows combining the efficient photon conversion to electrons and their transport from the multiple layers each absorbing different wavelengths of light. It has been shown that semi-conductor absorption capacity is a critical controllable parameter influencing the overall efficiency of the solar cell.[16,17]

One of the most widely used and extensively explored semiconductors is TiO2. Various approaches have been explored so far to improve the photochemical characteristics of TiO2 [18–20]among which the structural doping is one of the most widely used methods to shift the light absorption to the visible region and achieve an overall performance enhancement of the TiO2-based composites. Previously doping of the TiO2 structure with various metals (Fe, Mn, Sc, V, Cr, Mn), non-metals (N, C, F, S)[21–23]has been considered as an approach to enhance the photosensitivity of the tita- nia at lower wavelengths and, particularly, in the visible spectral range. 

 

 

QUESTION

A characterization of the inkjet printed layers is missing. There is only the SEM image and the droplet dimension. What were the jetting parameters for obtaining homogeneous and closed layers? Which layer thickness can be achieved. The images indicate a coffee stain effect. How can it be prevented? What is the height difference of edge and center?

 

ANSWER

We would like to emphasize that initial SEM images of separate drops were provided to illustrate the printing stability. To fulfill the reviewer’s request on the lack of the layers characterization, we included optical microscope images of inkjet printed Sc-TiO2 nanofilms and corresponding printing conditions to the manuscript. The thickness of the films varies from 100 nm to several micrometers. For example, we have printed a solid robust film with a thickness of 0.8 microns with the roughness of about 0.1 microns  (parameters were included in experimental section). The coffee-ring effect could be generally eliminated by reducing surface free energy. In our case, oxygen plasma treatment is the approach. Although the surface was plasma treated, the coffee-ring nevertheless is hard to dealwith and thereby it is seen on the Figure 6e with a height difference of 1 μm between center and edge of films.

Author Response File: Author Response.docx

Reviewer 2 Report

Authors report on the inkjet printing of Sc-doped TiO2 ink with enhanced photoactivity. The manuscript show novelties but the part dealing with inkjet printing is not enough detailed and important missing explanations and experimental details must be added to the revised manuscript. However, the reviewer recommends the acceptance of the manuscript for publication in coating with the following issues addressed:

 

_ The abstract section has to be revised to clearly highlight the main results.

Authors claim in the introduction section: “ Being one of the most accurate techniques for thin films production, inkjet printing technology is suitable for laboratory as well as for industry.” This assertion is not true and must be revised. Inkjet printing cannot be considered as one of the most accurate methods to fabricate thin films! It exists other techniques such as thermal evaporation, Chemical Vapor Deposition… used in industry that can be considered as accurate techniques but not inkjet printing!

 

_ In the introduction section, authors claimed: “the high printing accuracy” The inkjet printing technology cannot be considered as an accurate techniques to fabricate patterns in comparison with photolithography. Authors have to remove all these kind of assertions in the revised manuscript.

 

_ In the experimental sections, details are missing: For instance, concerning the pulse waveforme, the firing voltage, and the printing frequency is mentioned but not the duration of the firing pulse (the duration when the firing voltage is applied to the nozzle). The experimental section has to be more accurately described to allow the reader to reproduce experiment.

_ The main weakness of the document is experiments and observation dealing with inkjet printing. For instance authors claims: “Figure 6a,b show SEM images of the printed 10 wt.% Sc-TiO2 drops evidencing their highly uniform deposition at equal distance of 120 μm (center-to-center) with no traces of satellite droplets.”

This assertion is not true. The deposition is not uniform! The images highlight coffee ring effect that is one drawback of inkjet printing technology. Authors have to discuss about this problem. The following work dealing with problems due to non-uniformity of a printed film should be cited in the revised manuscript:

“Tao, Z., Le Borgne, B., Mohammed-Brahim, T., Jacques, E., & Harnois, M. (2018). Spreading and drying impact on printed pattern accuracy due to phase separation of a colloidal ink. Colloid and Polymer Science, 296(11), 1749-1758.”

_ The stability of the ink over the time is not mentioned. Additional experiments should be added to the revised manuscript (e.g, no sedimentation).

_ Film of the jetting behavior should be added to the manuscript in supplementary information to show the robustness of the ink jetting (e.g., no satellites droplets)

 

 

 

Comments for author File: Comments.docx

Author Response

Authors report on the inkjet printing of Sc-doped TiO2 ink with enhanced photoactivity. The manuscript show novelties but the part dealing with inkjet printing is not enough detailed and important missing explanations and experimental details must be added to the revised manuscript. However, the reviewer recommends the acceptance of the manuscript for publication in coating with the following issues addressed:

 

We are thankful to the reviewer for the high evaluation of our work and going to address the following issues in accordance to the reviewer’s request. 

 

QUESTION

The abstract section has to be revised to clearly highlight the main results.

 

ANSWER

As referee requested, the abstract section was thoroughly revised.

Now the Page 1 states:

Abstract:Here we report the methodology for the nanocomposites fabrication based on the inkjet printing technique. The doped TiO₂nanoparticles with Sc contents up to 10 wt. % were synthesized and adapted towards a facile fabrication of microscale structures and thin films printing. Implementation of the state-of-the-art low-temperature synthesis allowed to successfully incorporate high concentrations of Sc³⁺ions into TiO₂lattice and improve the light absorption characteristics of the resulting materials. Without affecting the anatase structure substantially, Sc doping gave rise to the intensified absorbance capacity and provided means for efficient fabrication of Sc-TiO2 microarchitectures via inkjet printing technique. The changes in the spectral and structural characteristics of the Sc-TiO₂composites were observed by the EDX, XRD, UV-vis methods. The rheological parameters of the colloidal suspension based on the synthesized Sc-TiO₂nanoparticles were adapted for inkjet printing in terms of optimal viscosity, morphology and surface tension. The developed individual inks characteristics allowed to produce the close coherence between the enhanced optical properties of the sol-gel prepared Sc-TiO₂and the respective inkjet-printed films. The introduced methodology features the possibility to jet doped and pure TiO₂robust films for potential large-scale fabrication.

 

QUESTION

Authors claim in the introduction section: “Being one of the most accurate techniques for thin films production, inkjet printing technology is suitable for laboratory as well as for industry.” This assertion is not true and must be revised. Inkjet printing cannot be considered as one of the most accurate methods to fabricate thin films! It exists other techniques such as thermal evaporation, Chemical Vapor Deposition… used in industry that can be considered as accurate techniques but not inkjet printing!

 

ANSWER

We agree with the referee and apologize for a misleading selection of words. Indeed, such methods as CVD or photolithography allow for more precise modification of surfaces and generation of well-defined films. Here we implied that inkjet combines sufficient precision at the micro- and mesoscale with low costs and easy scale-up. The text has been revised accordingly. Although the inkjet printing technique might exhibit high precision under thoroughly controlled parameters, it is indeed less accurate than conventional fabrication methods. It is true that inkjet printing could hardly suppress CVD and PVD methods in some aspects, nevertheless, the inkjet printing technique has its strengths including flexibility, diversity of printed materials and its possible implementations as thin films or other microstructures. We thank the reviewer for the valuable remark and the mentioned sentence is rewritten accordingly. 

 

QUESTION

In the introduction section, authors claimed: “the high printing accuracy” The inkjet printing technology cannot be considered as an accurate techniques to fabricate patterns in comparison with photolithography. Authors have to remove all these kind of assertions in the revised manuscript.

 

ANSWER

We thank the reviewer for the valuable point. The sentence regarding inkjet printing accuracy was considerably refined.

 

QUESTION

In the experimental sections, details are missing: For instance, concerning the pulse waveform, the firing voltage, and the printing frequency is mentioned but not the duration of the firing pulse (the duration when the firing voltage is applied to the nozzle). The experimental section has to be more accurately described to allow the reader to reproduce experiment.

 

ANSWER

According to the referee’s request, we have added missing experimental details to experimental section. 

 

Now Page 3 states:

Figure S9 shows waveform used in the experiment and detailed information on duration of the process and level of the signal. 

A film on a glass substrate was printed with drop spacing of 10 μm in 4 layers and calcinated at 370 ºC for 30 minutes. Drop array was printed on the silicon wafer with drop spacing of 50 μm and drop velocity of 8 m/s at table the temperature of 35 ^oC.

 

The data concerning the waveform’s parameters were included in the manuscript’s supplementary information (Figure S9).

 

 

QUESTION

The main weakness of the document is experiments and observation dealing with inkjet printing. For instance authors claims: “Figure 6a,b show SEM images of the printed 10 wt.% Sc-TiO2 drops evidencing their highly uniform deposition at equal distance of 120 μm (center-to-center) with no traces of satellite droplets.”

This assertion is not true. The deposition is not uniform! The images highlight coffee ring effect that is one drawback of inkjet printing technology. 

Authors have to discuss about this problem. The following work dealing with problems due to non-uniformity of a printed film should be cited in the revised manuscript:

“Tao, Z., Le Borgne, B., Mohammed-Brahim, T., Jacques, E., & Harnois, M. (2018). Spreading and drying impact on printed pattern accuracy due to phase separation of a colloidal ink. Colloid and Polymer Science, 296(11), 1749-1758.”

 

 

ANSWER

We thank the referee for the valuable remark. We agree that the coffee-ring effect is observed in the printed drops apparently due to the high contact angle of  the silicon substrate. To clarify the nature of the fabricated films, we included the optical microscope images of printed film on the preliminary plasma treated glass substrate. Now the Figure 6 a-e shows the drop array indicating the highly stable printing conditions and final nanofilms with relatively low roughness. Although the surface was plasma treated, the coffee-ring nevertheless is hard to copy with and thereby it is seen on the Figure 6e with  a height difference of 1 μm between center and edge of films.

According to the referee’s request, we briefly highlighted the possible limitations of inkjet printing.

 

Now Page 2 states:

The potential limitations of multicomponent systems inkjet process are phase separation and coffee-ring formation.[29]These problems were explicitly studied in [30,31]. Based on previous experience we tried to prevent those issues by finding optimal printing parameters and drying conditions for uniform film deposition.

 

QUESTION

The stability of the ink over the time is not mentioned. Additional experiments should be added to the revised manuscript (e.g, no sedimentation).

 

ANSWER

As referee requested, results of the measurements of inks zeta-potential were added to the Supplementary Information. 

 

Now the Page 8 states:

To study ink stability and impact of additives on the sedimentation process, inks zeta potential was measured. Figure S10 shows minor effect of additives presence on the stability of ink with corresponding zeta-potential being higher than 30 mV in all cases.

 

QUESTION

Film of the jetting behavior should be added to the manuscript in supplementary information to show the robustness of the ink jetting (e.g., no satellites droplets).

 

ANSWER

Requested video has been added to supplementary materials.

Author Response File: Author Response.docx

Reviewer 3 Report

the paper is focused on the optimization of inkjet printing route of titania-based nanocomposites thin films and on the characterisation of these nanocomposites.

The paper is well written and clear in each section. The obtained results are well discussed.

Author Response

Reviewer 3

The paper is focused on the optimization of inkjet printing route of titania-based nanocomposites thin films and on the characterization of these nanocomposites.

The paper is well written and clear in each section. The obtained results are well discussed.

 

We are grateful for the high evaluation of our work.

Author Response File: Author Response.docx

Reviewer 4 Report

The manuscript “Inkjet printing of Sc-doped TiO₂with enhanced photo-activity”explores the possibility to fabricate nanocomposites by the inkjet printing technique. The doped TiO₂with high Sc contents was synthesized and adapted towards an easy and scalable fabrication of microscale structures and thin films printing. 

The argument is not new, many are the previous works that include different particles like doping of the anatase but here the novelty consists in having applied a low temperature method to make the gel and then make a gel optimized for inkjet. The authors argue that the inclusion of SC in the anatase lattice provokes only small changes in the lattice parameters without substantial structural changes. The authors have performed many different techniques to characterize the material with and without the inclusion of Sc contents. In this regard, it would be useful for the authors to verify what was stated with a Raman spectroscopy analysis which turns out to be an elective technique for these investigations, verifying that no substantial structural changes occur in the anatase. Unfortunately, the article presents different typos and repetitions in the text and the English language should be improved.

Moreover, in order to be able to replicate the experiment, the authors should provide more details both for the synthesis process and for the preparation of the ink.

Author Response

Reviewer 4

The manuscript “Inkjet printing of Sc-doped TiO2 with enhanced photo-activity” explores the possibility to fabricate nanocomposites by the inkjet printing technique. The doped TiO₂with high Sc contents was synthesized and adapted towards an easy and scalable fabrication of microscale structures and thin films printing. 

 

Authors are grateful to referee for such valuable review. 

 

QUESTION

The argument is not new, many are the previous works that include different particles like doping of the anatase but here the novelty consists in having applied a low temperature method to make the gel and then make a gel optimized for inkjet. The authors argue that the inclusion of Sсin the anatase lattice provokes only small changes in the lattice parameters without substantial structural changes. The authors have performed many different techniques to characterize the material with and without the inclusion of Sc contents. In this regard, it would be useful for the authors to verify what was stated with a Raman spectroscopy analysis which turns out to be an elective technique for these investigations, verifying that no substantial structural changes occur in the anatase. 

 

ANSWER

Indeed, analysis with Raman spectroscopy allows unambiguously determine structural changes and probably will be very useful to complete the experimental part of present work. Unfortunately, we do not have access to this particular equipment and, besides, we believe that provided characterization is adequate enough for purposes of present study. Provided XRD analysis shows no traces of other phases but anatase. In addition to evidence of Sc presence obtained from the EDX measurements, Sc incorporation into titania lattice cell was confirmed by observing the peaks shift on XRD patterns of Sc-doped samples. Furthermore, obtained results are in a good correlation with previous studies reporting Sc-doping procedures.

 

 

 

QUESTION

Unfortunately, the article presents different typos and repetitions in the text and the English language should be improved.

 

ANSWER

The manuscript has been carefully proof-read, we have tried to correct all mistakes and typos.

 

 

QUESTION

Moreover, in order to be able to replicate the experiment, the authors should provide more details both for the synthesis process and for the preparation of the ink.

 

Thank you for this comment, we have added some additional experimental data, including the printing parameters for uniform robust films. 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

This work is worthy to be published in Coating

Reviewer 4 Report

Accept in present form