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## Creator

[Chenning Zhang](https://orcid.org/0000-0002-3372-3649), [Tetsuo Uchikoshi](https://orcid.org/0000-0003-3847-4781), Takaya Akashi

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[Visible-Photocatalytic performance of TiO2 particles simply achieved by surface nitridation and coatings fabricated by electrophoretic deposition (EPD) process](https://mdr.nims.go.jp/datasets/0fa32579-5b93-43c6-bb99-063550b28cdc)

## Fulltext

Materials Letters Visible-Photocatalytic Performance of TiO2 Particles Simply Achieved by SurfaceNitridation and Application for Self-Cleaning Glass Fabricated by ElectrophoreticDeposition (EPD) Process--Manuscript Draft-- Manuscript Number: MLBLUE-D-24-01217Article Type: Short CommunicationKeywords: Photocatalytic property;  Powder technology;  Coating;  Electrophoretic depositionprocessCorresponding Author: chenning ZHANG, Ph.DNational Institute for Materials ScienceTsukuba, Ibaraki JAPANFirst Author: chenning ZHANG, Ph.DOrder of Authors: chenning ZHANG, Ph.DTetsuo UchikoshiTakaya AkashiAbstract: Originally, surface-nitridated TiO2 particles were prepared by urea decompositionunder hydrothermal treatment. After the surface nitridation, the absorption onset wasextended to the visible region due to narrowed band gap of ~3.10 eV comparable tothat of ~3.28 eV for pristine TiO2. Due to narrowed band gap, the surface-nitridatedTiO2 distinguishably had the visible photocatalytic ability under 440 nm. The visible-photocatalytic nitridated TiO2 particles was successfully fabricated as dense coatingswith controllable transparency by electrophoretic deposition (EPD) process.Suggested Reviewers: chika Takai-YamashitaAssociate Professor, Gifu Universityc_takai@gifu-u.ac.jpKento IshiiAssistant Professor, Nagoya Institute of Technologyishii.kento@nitech.ac.jpYuji MasubuchiAssociate Professor, Hokkaido Universityyuji-mas@eng.hokudai.ac.jpPowered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation                  NATIONAL INSTITUTE FOR MATERIALS SCIENCE                                  Materials Processing Unit                     1 -2 -1 ,  Sengen ,  T SU KU B A ,  IB A RA KI ,  305 - 004 7 ,  J APA N       Dear Editor, We would like to submit the enclosed manuscript entitled “Visible-Photocatalytic Performance of TiO2 Particles Simply Achieved by Surface Nitridation and Application for Self-Cleaning Glass Fabricated by Electrophoretic Deposition (EPD) Process” to Materials Letters. This manuscript includes 4 figures and about 1945 words. In this work, surface-nitridated TiO2 particles were prepared by urea decomposition under hydrothermal treatment. After the surface nitridation, the absorption onset of nitridated TiO2 was extended to the visible region due to narrowed band gap of ~3.10 eV comparable to that of ~3.28 eV for pristine TiO2. The surface-nitridated TiO2 distinguishably had the visible photocatalytic ability under 440 nm, by comparing with that of untreated TiO2. The visible-photocatalytic nitridated TiO2 particles was successfully used to fabricate dense coatings with controllable transparency by electrophoretic deposition (EPD) process, which is expected to be potentially used as self-cleaning glass with enhanced photocatalytic efficiency in UV-visible wavelength region.   On behalf of my co-authors, I certify that this work is an original research that has neither been published previously nor is under any considerations for publication in another journal at the time of submission by any of the authors, in whole or in part. We deeply appreciate your consideration for our manuscript. We look forward to receiving the comments from reviewers. Best regards. Yours sincerely.         Dr. Chenning ZHANG Research Center for Electronic and Optical Materials National Institute for Materials Science (NIMS), Japan E-mail: zhang.chenning@nims.go.jp Cover letterhttp://www.nims.go.jp/units/u_materials-processing/index_e.htmlGraphical AbstractHighlights  Surface-nitridated TiO2 particles were originally prepared by hydrothermal treatment  After nitridation, band gap of TiO2 became narrowed responsible for visible light  Surface-nitridated TiO2 had a visible photocatalytic ability under 440 nm  Nitridated TiO2 particles is expected to be used as self-cleaning glass by EPD  Highlights1  Visible-Photocatalytic Performance of TiO2 Particles Simply Achieved by Surface Nitridation and Application for Self-Cleaning Glass Fabricated by Electrophoretic Deposition (EPD) Process  Chenning Zhang,a, b, * Tetsuo Uchikoshi,a, * Takaya Akashi,b   a Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan  b Department of Chemical Science and Technology, Hosei University, Koganei, Tokyo 184-8584, Japan                       * Corresponding author:  Dr. Chenning ZHANG E-mail: zhang.chenning@nims.go.jp Prof. Dr. Tetsuo UCHIKOSHI E-mail: UCHIKOSHI.Tetsuo@nims.go.jp   Manuscript Click here to view linked References 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 mailto:UCHIKOSHI.Tetsuo@nims.go.jphttps://www2.cloud.editorialmanager.com/mlblue/viewRCResults.aspx?pdf=1&docID=177552&rev=0&fileID=3055420&msid=77c52d0d-246c-42b4-bf0c-fbaec3955778https://www2.cloud.editorialmanager.com/mlblue/viewRCResults.aspx?pdf=1&docID=177552&rev=0&fileID=3055420&msid=77c52d0d-246c-42b4-bf0c-fbaec39557782  Abstract Originally, surface-nitridated TiO2 particles were prepared by urea decomposition under hydrothermal treatment. After the surface nitridation, the absorption onset was extended to the visible region due to narrowed band gap of ~3.10 eV comparable to that of ~3.28 eV for pristine TiO2. Due to narrowed band gap, the surface-nitridated TiO2 distinguishably had the visible photocatalytic ability under 440 nm. The visible-photocatalytic nitridated TiO2 particles was successfully fabricated as dense coatings with controllable transparency by electrophoretic deposition (EPD) process.                                 1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 3  1. Introduction Nano-sized particle of P25 TiO2 (commercial AEROXIDE® P25 TiO2 powder, anatase=~80 wt.% and rutile=~20 wt.%) is one of the promising photocatalysts because of its relatively high levels of activity in many photocatalytic reaction systems [1]. Despite this, P25 TiO2 has poor efficiency in the visible region of the solar spectrum due to its wide band gap of ~3.2 eV, which makes it typically requires exposure of ultraviolet (UV) light for photocatalytic reactions, therefore seriously limiting the photocatalytic application of P25 TiO2.  It has been reported that the band gap of the synthesized TiO2 was successfully narrowed by doping N into the lattice of TiO2, which effectively increased its photocatalytic activity induced by visible light [2]. However, the doping N into the lattice of TiO2 needs high synthesis temperature, special equipment, and complicated process, therefore, it is necessary to conceive a facial strategy for narrowing the band gap of TiO2. It has been known that urea starts to decompose and produce ammonium as nitrogen by hydrolysis reaction when temperature is more than ~130 oC [3], and hydrothermal treatment provides a condition of high pressure for reaction [4]. In this work, it is the first time to successfully modify particles of commercial P25 TiO2 by surface nitridation with using urea decomposition under hydrothermal treatment. The mechanism of nitridation was systemically investigated and the effect of nitridation on visible-photocatalytic performance was clearly elucidated in detail. The visible-photocatalytic coatings were effectively fabricated by EPD process as an application of the surface-nitridated P25 TiO2.  2. Experimental Commercial AEROXIDE® P25 TiO2 powder (Nippon Aerosil, Ltd., Tokyo, Japan) was dispersed in 50 mL of distilled water with dissolving urea (Co(NH2)2, Kanto Chemical Co., Inc. Tokyo, Japan) under continuous ultrasonic dispersion. The prepared P25 TiO2 suspension was put into a Teflon-lined autoclave under hydrothermal treatment at 180 oC for 24 h for achieving particle surface nitridation. The decomposition of urea occurred above ~130oC temperature and the decomposed product of NH4+ ions in the water as nitrogen source were adsorbed around the surface of the P25 TiO2 particles, then making the surface nitridation under the condition of high pressure during the process of hydrothermal treatment. After the hydrothermal treatment, the product was washed by distill water for four times and dried at 60 oC for 24h, finally followed by powder collection.  Photocatalytic performance was evaluated by bleaching 20 μM of methyl orange (MO) (C14H14N3NaO3S, reagent grade, Wako Pure Chemical Industries, Ltd, Osaka, Japan) solution mixing with the pristine and surface-nitridated P25 TiO2 powders inside a temperature-controlled chamber (SH-222, Espec Corp., Osaka, Japan) used as a dark box at a constant temperature of 25oC. The blue light was generated by a 300 W Xe light source (MAX-303, Asahi Spectra Co., Ltd., Tokyo, Japan) equipped with optical filters.  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 4  Phase identification was performed by X-ray diffraction (XRD) on X-ray diffractometer (model RINT 2200, Rigaku Corp., Tokyo, Japan). Observations for particle morphology and coating microstructure were performed by field-emission scanning electron microscopy (FE-SEM) (model S-4800, Hitachi, Ltd., Tokyo, Japan). Element mapping was detected by energy dispersive X-ray (EDX) spectrometer (model EDAX Apollo XL, EDAX Inc., Mahwah, NJ, USA). Ultraviolet-visible (UV-vis) spectra was made on Jasco V-570 spectrophotometer (Jasco Corp., Hachioji, Tokyo, Japan) after baseline calibration. The thickness of the coating fabricated by the EPD process was measured by constant pressure thickness gauge (PG-20J, Teclock Co., Ltd. Nagano, Japan). 3. Results and discussion                                  Fig. 1 XRD patterns of (a) P25 TiO2 powders before and after surface nitridation, (b) local region in 2Theta range of 20–30o, and micrographs of morphologies of (c) pristine and (d)  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 5  surface-nitridated P25 TiO2 particles, respectively. The insets in (b) are interstitial N doping in the lattices of anatase-and rutile-TiO2, respectively.  The influence of the surface nitridation on phase composition was investigated in the XRD patterns of the TiO2 P25 powders before and after surface nitridation, as exhibited in Fig. 1a. By identification, the phase composition of P25 TiO2 is mixture of anatase and rutile, in which the anatase fraction was estimated as ~82.5 wt.% from the integrated intensity of the anatase (101) diffraction peak, according to the equation [5]. After the surface nitridation, no any obvious change was found on the anatase fraction of ~82.7 wt.%, indicating that the phase composition of P25 TiO2 was almost invariable after the surface nitridation at 180oC in this work. Fig. 1b shows the XRD patterns of pristine and surface-nitridated P25 TiO2 powders in a local region of 20‒30o. By comparing with the XRD patterns of the P25 TiO2 powders before and after surface nitridation, the diffraction peak of anatase (101) and rutile (110) was found to be slightly shifted towards high angle by 0.1o and 0.14o, respectively. Moreover, after the surface nitridation, the axes of a and c, and unit cell volume of v all became small (a=3.78982→3.78125 Å, c=9.51346→9.49348 Å, and v=136.64→135.74 Å3), therefore reasonably deducing that N ions did not substitute for O ions in the lattice of TiO2 after the surface nitridation, instead, came into the interstitial site of lattice, as a result, causing oxygen defects in the lattice, as below: (1) where TiTi×  denotes the Ti ion at Ti site, 𝑉𝑂‥is oxygen defect, and Ni′′′ is N at interstitial site, and symbols of •, Χ, and ′are the Kröger-Vink notations for net charge +1, the zero net charge, and net charge -1, respectively [6], as shown in the insets of interstitial N doping in the lattices of anatase-and rutile-TiO2 in Fig. 1b. Similar reports concerning lattice shrinkage caused by the formation of oxygen defects are also found in other literatures [7]. Figures 1c and 1d demonstrate the morphology micrographs of the pristine and surface-nitridated P25 TiO2 particles, respectively. It is comparably distinguished that no any obvious changes in particle morphology (particle size=~25 nm) after hydrothermal treatment. Thus, it is more accurate to confirm the contribution from the surface nitridation on photocatalytic property. UV-vis spectra of the P25 TiO2 powders before and after the surface nitridation and calculated band-gap energies of these samples are shown in Figs. 2a and 2b, respectively. Comparably, the absorption onset of the P25 TiO2 powder extended to visible region after the surface nitridation. The band-gap energies (Eg) of the pristine and surface nitridated P25 TiO2 powders were calculated following the equations [8]. Before the surface nitridation, the calculated band-gap energy of P25 TiO2 is ~3.28 eV, however, after the nitridation, that became narrowed to ~3.14 eV. The narrowed Eg provides an evidence that the nitridation was effectively performed after the hydrothermal treatment in this work. TiO2+NH4+→TiTi× +2𝑉𝑂‧‧+Ni′′′ +2H2O  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 6   Fig. 2 (a) UV-vis spectra of the pristine and surface-nitridated P25 TiO2 powders and (b) their calculated band-gap energies.    Fig. 3 UV-vis spectra of the MO solutions after photocatalytic reaction (a) with mixing the P25 TiO2 powders before and after the surface nitridation under 440 nm irradiation for 48 h and (b) with mixing the surface nitridated powders under 440 nm irradiation for 0‒48 h. The insets in (a) and (b) are the photographs of MO solutions after the photocatalytic reaction. The absorption spectra of the MO solution without any catalyst are used as reference herein.  As seen in Fig. 3a, after the irradiation, the intensity of absorption peak at ~460 nm became slightly decreased for the pristine P25 TiO2, but became seriously weaken for the surface-nitridated one, suggesting that the photocatalytic performance under 440 nm irradiation was significantly improved after the surface nitridation, due to the narrowed band gap from ~3.28 eV to ~3.14 eV responsible for extending the absorption region to 440 nm. The quite difference in the color of bleached MO solution is shown in the inset of Fig. 3a. The resaon for visible-photocatalytic property after the surface nitridation upon P25 TiO2 is explained as that when N  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 7  incorporated into the lattice of anatase-and rutile-TiO2 as intensitital doping after the surfac nitridation upon P25 TiO2, the N 2p band above O 2p valence band formed a new mid-gap energy state and eventually narrowed the band gap of TiO2 P25 and shifted the optical absorption to visible light region. For the MO solution with mixing the surface nitridated P25 TiO2 powders, the intensity of absorption peak at ~460 nm became weak with 440 nm irradiation time from 0 h to 48 h, as shown in Fig. 3b. In particularly, after 48 h irradiation, the absorption peak became almost flat, suggesting the almost MO molecules were decomposed by the photocatalytic reaction, which is clearly demonstrated by the color variation of MO solutions in the inset of Fig. 3b. According to the (L−H) model [9], when initial concentration is very diluted (20 μM in this experiment), reaction-rate constant was calculated as ~5.60×10-6 s-1 for the surface nitridated P25 TiO2 powders under 440 nm irradiation.     Fig. 4 (a) UV-vis spectra of transmittance for the coatings of surface nitridated P25 TiO2 powders deposited by the EPD process at various depositing time of 0s–60min, (b) thickness of the coating as a function of depositing time and their appearances, and (c) FE-SEM image of cross-section for the coating fabricated by the EPD process under 10 V at 3 min depositing time with its EDX element mapping.  Fig. 4 exhibites (a) the transmittance in UV-vis spectra for the coatings of surface nitridated  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 8  P25 TiO2 deposited onto the surface of ITO glass by the EPD process at various depositing time of 0s–60min, (b) the relationship between coating thickness and depositing time, and (c) cross-section of the deposited coating with its EDX element mapping. The variation in the transmittances of coatings fabricated by the EPD process as a function of depositing time is obvious and the fabricated coating (thickness as ~80 μm) is dense with component elements.  4. Conclusions Commercial P25 TiO2 particles were surface-nitridated by urea decomposition under hydrothermal treatment. After the surface nitridation, the absorption onset was extended to the visible region. The narrowed band gap played a role in photocatalytic performance under visible irradiation. Visible-photocatalytic coatings with controllable transparency were successfully fabricated by EPD process.  References 1. T. Ishigaki, Y. Nakada, N. Tarutani, T. Uchikoshi, Y. Tsujimoto, M. Isobe, et al, Roy. Soc. Open Sci. 7 (2020) 191539. 2. S. Ansari, M. Khan, M. Ansari, M. Cho, New J. Chem. 40 (2016) 3000-3009. 3. W. H. R. Shaw, J. J. Bordeaux, J. Am. Chem. Soc. 77 (1955) 4729‒4733. 4. K. Byrappa, M. Yoshimura, Handbook of hydrothermal technology, William Andrew, 2012. 5. R. A. Spurr, H. Myers, Anal. Chem. 29 (1957) 760‒762. 6. F.A. Kröger, H.J. Vink, Solid State Phys. 3 (1956) 307‒435. 7. B. Choudhury, A. Choudhury, Mater. Chem. Phys. 131 (2012) 666‒671. 8 A. Hagfeldt, M.Gratzel, Chem. Rev. 95 (1995) 49-68. 9. H. Alekabi, N. Serpone, J. Phys. Chem. 92 (1988) 5726‒5731.      1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 Declaration of interests  ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.  ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:        Declaration of Interest Statement