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Crystal facet engineering of single-crystalline anatase TiO2 nanosheets with exposing dominate (0 1 0)-facet
Journal Pre-proofs Crystal facet engineering of single-crystalline anatase TiO2 nanosheets with exposing dominate (010)-facet S. Nithiananth, S. Harish, J. Archana, M. Navaneethan, M. Shimomura PII: DOI: Reference:
S0167-577X(20)30059-8 https://doi.org/10.1016/j.matlet.2020.127354 MLBLUE 127354
To appear in:
Materials Letters
Received Date: Accepted Date:
12 November 2019 12 January 2020
Please cite this article as: S. Nithiananth, S. Harish, J. Archana, M. Navaneethan, M. Shimomura, Crystal facet engineering of single-crystalline anatase TiO2 nanosheets with exposing dominate (010)-facet, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet.2020.127354
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Crystal facet engineering of single-crystalline anatase TiO2 nanosheets with exposing dominate (010)-facet S. Nithianantha,b, S. Harisha,b, J. Archanab*,, M. Navaneethanb, M. Shimomuraa* a
Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan b
Functional Materials and Energy Devices Laboratory, Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur-603 203, India Abstract While most anatase TiO2 crystals reported were in the form of nanoparticle, herein the tuneable and controlled (010)-faceted anatase phase were grown directly onto a conductive substrate. F- ions provided by addition of ammonium hexafluorotitanate (AHFT) play a synergistic effect in the crystals phase changed from rutile to anatase during crystal growth with low-index facets. The morphological transformation from vertically-aligned rutile-nanorods to highly crystalline anatase-nanosheets bounded with the most significant percentage of (001) facets with the effect of F- ions. At higher concentration of AHFT dominate the (010)-faceted anatase TiO2 with suppression of (001) facets of nanosheets (side length of 1.4 µm), which can suppress the charge recombination for an electron transport layer in solar cell applications, and thus, efficient charge transportation can be achieved compared to nanoparticles. Keywords: Rutile, Anatase, nanorods, nanosheets, facet 1.Introduction Among various metal oxide semiconductors (MOS), titanium dioxide (TiO2) has been widely used for many applications. TiO2 is the most commonly used for the electron transport layer (ETL) in perovskite solar cells, which is compared to other MOS, due to its chemical stability, abundance, low-cost, long-durability, quick electron transport, and long electron lifetime [1,2]. Generally, (101) facets are thermodynamically stable for anatase TiO2 than other facets due to low surface energy (0.44 Jm−2). Although (001) facets have a higher surface energy (0.90 Jm−2), they usually diminish rapidly during the crystal growth process. Therefore, single crystals anatase TiO2 with exposed (001) facets were much needed for improving the perovskite absorber on the ETL surface. The (001) facet growth percentage has been increased by exploiting various reaction systems and capping agents [3]. The introduction of F− ions as a capping agent stabilized the (001) facets of anatase TiO2, which increased 47% of (001) facets exposure [4]. Furthermore, similar methods were used to synthesize anatase TiO 2 nanosheets with 89% exposure of (001) facets, and F- and H+ ions have a significant role in changing the crystal structure of TiO2 [4,5]. The introduction of F- ion can be an effective strategy to control the growth to (001) facets of anatase TiO2 nanocrystals [6]. 1
However, these facet-controlled syntheses are resulted in the form of nanoparticles and could be unfavourable for the fabrication of the photovoltaic devices. The nanoparticles with large grain boundaries hinder the charge transportation because of the high charge-recombination possibility which depreciate the device performance [7]. To overcome, we should develop interparticle boundaries with a high reactive facet that is highly beneficial. Many reports suggest that nanostructures have been directly grown on conducting subtracts. Among that rutile TiO 2 nanorods grown on FTO substrate shows higher performance compared to other nanostructures [8]. However, in the 1D structures the attachment of dye molecules is less, and light trapping in the photoanode is also weaker which results in poor light harvesting [9]. Nevertheless, growing anatase TiO2 nanosheets directly on conducting substrate with facet controlled would be a benefit to the enhancement of interfacial materials improving the ETL/perovskite layers by the electron extraction [8]. In this work, a single-step hydrothermal method was used to grow on vertically aligned TiO2 nanostructures with tuneable directly and controlled (101), (001), and (010) facets on FTO substrate with uniform morphology and size. The sample with (010) and (001) facets dominated to the surface of particles can be used for the electron transport layer in solar cell applications. 2.Experimental details: 30 mL of hydrochloric acid and 30 mL of deionized water (DI-water) were stirred for 5 min. To the above solution, 1 mL of titanium tetrachloride was added and stirred for 10 h. The mixture solution was transferred into Teflon-lined autoclave, and FTO was placed inside and maintained at 160°C for 10 h. After the cooling process, the FTO substrate was taken out and rinsed with DI-water and dried at 80°C for 30 min. For the synthesis of anatase TiO2 same procedure was repeated with the addition of AHFT with various concentrations (0.00, 0.008, 0.016, 0.024 and 0.032 M). These samples are named as S1, S2, S3, S4, and S5. 3.Results and Discussion: Fig. 1a shows the XRD patterns of as-synthesized samples. The XRD pattern of FTO with the diffraction peaks corresponds to the tetragonal phase of SnO2 (JCPDS 41-1445). For pure TiO2 (Fig. 1a (S1)) peaks at 36.1º and 62.8º were well-matched with rutile phase (JCPDS 211276). With the addition of AHFT (0.008 M) peaks at 39.1º and 68.7º, corresponds to (200) and (116) planes of rutile and anatase phase were observed (Fig. 1a (S2)). In further, the increase in concentration from 0.008 to 0.016 M, the peak at 39.1º decreased with an increased peak intensity of 68.7º, which corresponds to anatase TiO2. For 0.024 and 0.032 M, new peaks observed at 25.2º, 48.1º, and 54.9º, which corresponds to (101), (200), and (211) planes of anatase (JCPDS 21-1272) without any secondary phase formation (Fig. 1a (S5)). Fig. 1b shows the Raman spectra of rutile and anatase TiO2. The peaks observed at 233, 443, and 609 cm-1 corresponds to Eg, Eg, and A1g modes of the rutile TiO2 (S1). The peak at 233 cm-1 is observed in the multi-phonon peak of the rutile phase, as shown in Fig. 1b (S1) [10,11]. After the addition of AHFT, phase transformation was observed. At lower 2
concentrations, the peaks at 233, 440, 520, and 613 cm-1 are assigned to Eg, Eg, A1g, and A1g, modes respectively, confirms the mixture of rutile and anatase phase formation [10-12]. In further increase in the concentration, pure anatase phase at 148 (E g), 402 (B1g), 513 (A1g), and 634 cm-1 (Eg) were observed without any secondary phase formation (Fig. 1b (S5)) [13, 14]. FESEM and HRTEM were performed to investigate the surface morphology of TiO2. Fig. 2(a) for the S1 sample without AHFT shows the FESEM image of vertically aligned rutile nanorods with an average size of about 166 nm. The insert image of Fig. 2(a) shows the precise formation of square-like vertically aligned nanorods. The cross-sectional image indicates the nanorods are vertically aligned on the FTO substrate with a thickness of 6.2 µm (Fig. 2(a1)). TEM and HRTEM confirm the formation of the bundle of nanofibers to form rod-like formation with good crystalline nature (Fig. 2(a2, a3)). The size of the nanofiber is around 3-4 nm. After the addition of AHFT (0.008 M) the formation of nanoparticles was observed (Fig. 2(b)). The nanoparticles were clearly observed in the HRTEM with size ranging from 5-10 nm (Fig. 2(b2, b3)). In further increase in the concentration to 0.1g, the mixture of nanoparticles and nanosheets was observed (Fig. 2c). The cross-sectional image shows the clear formation of nanoparticles (yellow dotted lines) and nanosheets (white dotted lines). For the S4 sample, large singlecrystalline nanosheets were observed with a size of about 1.2 x 1.2 µm (Fig. 2d). In further increase in the concentration (S5), densely well-aligned nanosheets with size and thickness of 1.4 x 1.4 µm and 168 nm were observed (Fig. 2e). Fig. 2(e1) shows the cross-sectional image of well-aligned nanosheets with thickness of 1.0 µm. Fig. 2(e2, e3) displays the HRTEM image of TiO2 nanosheets with good crystalline nature. The morphology, size, thickness of coating with effect of AHFT concentration are shown in Table1. The elemental mapping confirms the uniform distribution of Ti, O, and F elements, in S5 (Fig. 2f). The substantial evidence indicates that the F- important ions role in phase change and reconstruction of a structure during the crystal growth. A reaction mechanism for the conversion of vertically aligned TiO2 nanorods to nanosheets is summarized from the above results (Fig. 3A). The AHFT, which acts like an Fsource plays a vital role, and the effect of F - ions was studied. Herein, F- ions promote the phase change from rutile to anatase and control the morphology with the gradual emergence of (010) facet and disappearance of (101) facet. When the amount of Ti4+ and F- ions was increased to 0.016 M there was a co-existence of (101), (010), and (001) facets, which resulted as shown in Fig. 3(A). For the S4 samples, the side-view image clearly shows that the cutting of (010) facet on (101) facet in the particles. The increment in concentration (S5 sample), faded (101) facet and dominancy of (010) and (001) facets was found. The elemental composition of rutile and anatase TiO2 was observed by XPS analysis, as shown in Fig. 3B. Fig. 3B(a) showed the survey spectra of S1 and S5. Ti 2p core-level spectrum of S1 (Fig. 3B(b)) revealed the peaks at 458.4 and 464.1 eV, corresponds to 2p3/2 and 2p1/2, respectively with Ti(IV) state. Fig. 3B(c) shows the O 1s spectrum with the peaks located at 529.6, 531.1, and 532.1 eV, which is assigned to the bulk oxygen, Ti-OH or oxygen vacancies, and surface adsorbed water molecules (Oad) [15]. Fig. 3B(d) shows the Ti 2p core-level spectrum (S5) obtained the peaks at 458.8 and 464.6 eV, correspond to 2p3/2 and 2p1/2, respectively. Fig. 3
3B(e) shows the O 1s spectrum observed the peaks are almost same as shown in Fig. 3B(c). However, the S5 samples of Ti 2p and O 1s peaks were shifted toward the higher binding energy side due to the influence of F- species at the surface compared to S1. Fig. 3b(f) shows the F 1s spectrum peaks at 684.5 eV, which is almost identical binding energy to the Ti-F species on the surface of TiO2 [16]. Thus, fluorine atoms are attached on the surface titanium atoms. 4.Conclusion: The controlled and tunable growth of single-crystalline anatase TiO2 with the exposed facet on the FTO substrate. F- ions from AHFT transforms of phase as well as morphology during the crystal growth to expose the low-index facet. The XPS analysis confirmed Ti-F species on the surface of TiO2. The elemental analysis confirms the existence of F - ions in the composition. At 0.032 M AHFT, (010) and (001) facets dominate the surface of particles with large surface nanosheets (side length of 1.4 µm), which can be a suitable material for the electron transport layer in solar cells applications. References 1. H.Yao, J.Ma, Y.Mu, Y.Chen, S.Su, P.Lv, X.Zhang, D.Ding, W.Fu, H.Yang, RSC Adv., 5 (2015) 6436. 2. U.Thakur, R.Kisslinger, K.Shankar, Nanomaterials, 7 (2017) 95. 3. T.Butburee, P.Kotchasarn, P.Hirunsit, Z.Sun, Q.Tang, P.Khemthong, W.Sangkhun, W.Thongsuwan, P.Kumnorkaew, H.Wang, K.Faungnawakij, J. Mater. Chem. A., 7 (2019) 8166. 4. K.Ha, Q.Wen, C.Wang, S.Yu, C.Hao, K.Chen, Soft Matter, 12 (2013) 7889. 5. J.Yang, Q.Wu, S.He, J.Yan, J.Shi, J.Chen, M.Wu, X.Yang, Nanoscale, 7 (2015) 13897. 6. S.Jiao, X.Fu, G.Lian, Z.Xu, Q.Wang, D.Cui, RSC Adv., 7 (2017) 20845. 7. I.Cho, Z.Chen, A.Forman, D.Kim, P.Rao, T.Jaramillo, X.Zheng, Nano. Lett., 11 (2011) 4984. 8. M.Noh, C.The, R.Daik, E.Lim, C.Yap, M.Ibrahim, N.Ludin, A.Yusoff, J.Jang, M.Teridi, J. Mater. Chem. C, 6 (2018) 712. 9. J.Liang, G.Zhang, J.Yang, W.Sun, M.Shi, AIP Advances, 5 (2015) 017141. 10. Y.Li, J.Wang, H.Sun, B.Wei, ACS Appl. Mater. Interfaces., 10 (2018) 11586. 11. B.Cai, D.Zhong, Z.Yang, B.Huang, S.Miao, W.Zhang, J.Qiu, C.Li, J. Mater. Chem., 3 (2015) 733. 12. Y.Du, Y.Deng, M.S.Zhang, Journal of Physics and Chemistry of Solids, 67 (2006) 2408. 13. L.K.Preethi, R.Antony, T.Mathews, L.Walczak, C.Gopinath, Scientific Reports, 7 (2017) 14314. 14. J.Diaz-Real, G.Dubed-Bandomo, J.Galindo-de-la-Rosa, l.Arriaga, J.Ledesma-Garcia, N.Alonso-vante, Beilstein J. nanotechnol., 9 (2018) 2643. 15. J.Zou, D.Wu, J.Luo, Q.Ximg, X.Luo, W.Dong, S.Luo, H.Du, S.Suib, ACS Catal., 6 (2016) 6867.
4
16. H.Yang, G.Liu, S.Qiao, C.Sun, Y.Jin, S.Smith, J.Zou, H.Cheng, G.Lu, J. AM, CHEM, SOC,
131 (2009) 4083.
Table 1: Fabricated TiO2 nanostructures with the effect of AHFT concentration.
5
Figure 1: (a) XRD, (b) Raman spectra of S1, S2, S3, S4, and S5 samples.
6
Figure 2: FESEM and TEM images for (a, a1, a2, a3) S1, (b, b1, b2, b3) S2, (c, c1, c2, c3) S3, (d, d1, d2, d3) S4, (e, e1, e2, e3) S5, (f, f1, f2, f3) elemental mappings of sample S5.
7
Figure 3: (A) Schematic diagram of rutile to anatase phase transformation, (B) XPS analysis of sample S1 and S5. 8
Highlights •Tunable and controlled (010)-faceted anatase single crystals were grown by hydrothermal method
•F- ions play a synergistic effect in the crystals phase from rutile to anatase with low-index facets
•At higher concentration of F- ions dominate the (010)-faceted anatase TiO2
•Over 1.4-m-size anatase TiO2 single crystal sheets were obtained
9
S0167-577X(20)30059-8 https://doi.org/10.1016/j.matlet.2020.127354 MLBLUE 127354
To appear in:
Materials Letters
Received Date: Accepted Date:
12 November 2019 12 January 2020
Please cite this article as: S. Nithiananth, S. Harish, J. Archana, M. Navaneethan, M. Shimomura, Crystal facet engineering of single-crystalline anatase TiO2 nanosheets with exposing dominate (010)-facet, Materials Letters (2020), doi: https://doi.org/10.1016/j.matlet.2020.127354
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Crystal facet engineering of single-crystalline anatase TiO2 nanosheets with exposing dominate (010)-facet S. Nithianantha,b, S. Harisha,b, J. Archanab*,, M. Navaneethanb, M. Shimomuraa* a
Graduate School of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan b
Functional Materials and Energy Devices Laboratory, Department of Physics and Nanotechnology, SRM Institute of Science and Technology, Kattankulathur-603 203, India Abstract While most anatase TiO2 crystals reported were in the form of nanoparticle, herein the tuneable and controlled (010)-faceted anatase phase were grown directly onto a conductive substrate. F- ions provided by addition of ammonium hexafluorotitanate (AHFT) play a synergistic effect in the crystals phase changed from rutile to anatase during crystal growth with low-index facets. The morphological transformation from vertically-aligned rutile-nanorods to highly crystalline anatase-nanosheets bounded with the most significant percentage of (001) facets with the effect of F- ions. At higher concentration of AHFT dominate the (010)-faceted anatase TiO2 with suppression of (001) facets of nanosheets (side length of 1.4 µm), which can suppress the charge recombination for an electron transport layer in solar cell applications, and thus, efficient charge transportation can be achieved compared to nanoparticles. Keywords: Rutile, Anatase, nanorods, nanosheets, facet 1.Introduction Among various metal oxide semiconductors (MOS), titanium dioxide (TiO2) has been widely used for many applications. TiO2 is the most commonly used for the electron transport layer (ETL) in perovskite solar cells, which is compared to other MOS, due to its chemical stability, abundance, low-cost, long-durability, quick electron transport, and long electron lifetime [1,2]. Generally, (101) facets are thermodynamically stable for anatase TiO2 than other facets due to low surface energy (0.44 Jm−2). Although (001) facets have a higher surface energy (0.90 Jm−2), they usually diminish rapidly during the crystal growth process. Therefore, single crystals anatase TiO2 with exposed (001) facets were much needed for improving the perovskite absorber on the ETL surface. The (001) facet growth percentage has been increased by exploiting various reaction systems and capping agents [3]. The introduction of F− ions as a capping agent stabilized the (001) facets of anatase TiO2, which increased 47% of (001) facets exposure [4]. Furthermore, similar methods were used to synthesize anatase TiO 2 nanosheets with 89% exposure of (001) facets, and F- and H+ ions have a significant role in changing the crystal structure of TiO2 [4,5]. The introduction of F- ion can be an effective strategy to control the growth to (001) facets of anatase TiO2 nanocrystals [6]. 1
However, these facet-controlled syntheses are resulted in the form of nanoparticles and could be unfavourable for the fabrication of the photovoltaic devices. The nanoparticles with large grain boundaries hinder the charge transportation because of the high charge-recombination possibility which depreciate the device performance [7]. To overcome, we should develop interparticle boundaries with a high reactive facet that is highly beneficial. Many reports suggest that nanostructures have been directly grown on conducting subtracts. Among that rutile TiO 2 nanorods grown on FTO substrate shows higher performance compared to other nanostructures [8]. However, in the 1D structures the attachment of dye molecules is less, and light trapping in the photoanode is also weaker which results in poor light harvesting [9]. Nevertheless, growing anatase TiO2 nanosheets directly on conducting substrate with facet controlled would be a benefit to the enhancement of interfacial materials improving the ETL/perovskite layers by the electron extraction [8]. In this work, a single-step hydrothermal method was used to grow on vertically aligned TiO2 nanostructures with tuneable directly and controlled (101), (001), and (010) facets on FTO substrate with uniform morphology and size. The sample with (010) and (001) facets dominated to the surface of particles can be used for the electron transport layer in solar cell applications. 2.Experimental details: 30 mL of hydrochloric acid and 30 mL of deionized water (DI-water) were stirred for 5 min. To the above solution, 1 mL of titanium tetrachloride was added and stirred for 10 h. The mixture solution was transferred into Teflon-lined autoclave, and FTO was placed inside and maintained at 160°C for 10 h. After the cooling process, the FTO substrate was taken out and rinsed with DI-water and dried at 80°C for 30 min. For the synthesis of anatase TiO2 same procedure was repeated with the addition of AHFT with various concentrations (0.00, 0.008, 0.016, 0.024 and 0.032 M). These samples are named as S1, S2, S3, S4, and S5. 3.Results and Discussion: Fig. 1a shows the XRD patterns of as-synthesized samples. The XRD pattern of FTO with the diffraction peaks corresponds to the tetragonal phase of SnO2 (JCPDS 41-1445). For pure TiO2 (Fig. 1a (S1)) peaks at 36.1º and 62.8º were well-matched with rutile phase (JCPDS 211276). With the addition of AHFT (0.008 M) peaks at 39.1º and 68.7º, corresponds to (200) and (116) planes of rutile and anatase phase were observed (Fig. 1a (S2)). In further, the increase in concentration from 0.008 to 0.016 M, the peak at 39.1º decreased with an increased peak intensity of 68.7º, which corresponds to anatase TiO2. For 0.024 and 0.032 M, new peaks observed at 25.2º, 48.1º, and 54.9º, which corresponds to (101), (200), and (211) planes of anatase (JCPDS 21-1272) without any secondary phase formation (Fig. 1a (S5)). Fig. 1b shows the Raman spectra of rutile and anatase TiO2. The peaks observed at 233, 443, and 609 cm-1 corresponds to Eg, Eg, and A1g modes of the rutile TiO2 (S1). The peak at 233 cm-1 is observed in the multi-phonon peak of the rutile phase, as shown in Fig. 1b (S1) [10,11]. After the addition of AHFT, phase transformation was observed. At lower 2
concentrations, the peaks at 233, 440, 520, and 613 cm-1 are assigned to Eg, Eg, A1g, and A1g, modes respectively, confirms the mixture of rutile and anatase phase formation [10-12]. In further increase in the concentration, pure anatase phase at 148 (E g), 402 (B1g), 513 (A1g), and 634 cm-1 (Eg) were observed without any secondary phase formation (Fig. 1b (S5)) [13, 14]. FESEM and HRTEM were performed to investigate the surface morphology of TiO2. Fig. 2(a) for the S1 sample without AHFT shows the FESEM image of vertically aligned rutile nanorods with an average size of about 166 nm. The insert image of Fig. 2(a) shows the precise formation of square-like vertically aligned nanorods. The cross-sectional image indicates the nanorods are vertically aligned on the FTO substrate with a thickness of 6.2 µm (Fig. 2(a1)). TEM and HRTEM confirm the formation of the bundle of nanofibers to form rod-like formation with good crystalline nature (Fig. 2(a2, a3)). The size of the nanofiber is around 3-4 nm. After the addition of AHFT (0.008 M) the formation of nanoparticles was observed (Fig. 2(b)). The nanoparticles were clearly observed in the HRTEM with size ranging from 5-10 nm (Fig. 2(b2, b3)). In further increase in the concentration to 0.1g, the mixture of nanoparticles and nanosheets was observed (Fig. 2c). The cross-sectional image shows the clear formation of nanoparticles (yellow dotted lines) and nanosheets (white dotted lines). For the S4 sample, large singlecrystalline nanosheets were observed with a size of about 1.2 x 1.2 µm (Fig. 2d). In further increase in the concentration (S5), densely well-aligned nanosheets with size and thickness of 1.4 x 1.4 µm and 168 nm were observed (Fig. 2e). Fig. 2(e1) shows the cross-sectional image of well-aligned nanosheets with thickness of 1.0 µm. Fig. 2(e2, e3) displays the HRTEM image of TiO2 nanosheets with good crystalline nature. The morphology, size, thickness of coating with effect of AHFT concentration are shown in Table1. The elemental mapping confirms the uniform distribution of Ti, O, and F elements, in S5 (Fig. 2f). The substantial evidence indicates that the F- important ions role in phase change and reconstruction of a structure during the crystal growth. A reaction mechanism for the conversion of vertically aligned TiO2 nanorods to nanosheets is summarized from the above results (Fig. 3A). The AHFT, which acts like an Fsource plays a vital role, and the effect of F - ions was studied. Herein, F- ions promote the phase change from rutile to anatase and control the morphology with the gradual emergence of (010) facet and disappearance of (101) facet. When the amount of Ti4+ and F- ions was increased to 0.016 M there was a co-existence of (101), (010), and (001) facets, which resulted as shown in Fig. 3(A). For the S4 samples, the side-view image clearly shows that the cutting of (010) facet on (101) facet in the particles. The increment in concentration (S5 sample), faded (101) facet and dominancy of (010) and (001) facets was found. The elemental composition of rutile and anatase TiO2 was observed by XPS analysis, as shown in Fig. 3B. Fig. 3B(a) showed the survey spectra of S1 and S5. Ti 2p core-level spectrum of S1 (Fig. 3B(b)) revealed the peaks at 458.4 and 464.1 eV, corresponds to 2p3/2 and 2p1/2, respectively with Ti(IV) state. Fig. 3B(c) shows the O 1s spectrum with the peaks located at 529.6, 531.1, and 532.1 eV, which is assigned to the bulk oxygen, Ti-OH or oxygen vacancies, and surface adsorbed water molecules (Oad) [15]. Fig. 3B(d) shows the Ti 2p core-level spectrum (S5) obtained the peaks at 458.8 and 464.6 eV, correspond to 2p3/2 and 2p1/2, respectively. Fig. 3
3B(e) shows the O 1s spectrum observed the peaks are almost same as shown in Fig. 3B(c). However, the S5 samples of Ti 2p and O 1s peaks were shifted toward the higher binding energy side due to the influence of F- species at the surface compared to S1. Fig. 3b(f) shows the F 1s spectrum peaks at 684.5 eV, which is almost identical binding energy to the Ti-F species on the surface of TiO2 [16]. Thus, fluorine atoms are attached on the surface titanium atoms. 4.Conclusion: The controlled and tunable growth of single-crystalline anatase TiO2 with the exposed facet on the FTO substrate. F- ions from AHFT transforms of phase as well as morphology during the crystal growth to expose the low-index facet. The XPS analysis confirmed Ti-F species on the surface of TiO2. The elemental analysis confirms the existence of F - ions in the composition. At 0.032 M AHFT, (010) and (001) facets dominate the surface of particles with large surface nanosheets (side length of 1.4 µm), which can be a suitable material for the electron transport layer in solar cells applications. References 1. H.Yao, J.Ma, Y.Mu, Y.Chen, S.Su, P.Lv, X.Zhang, D.Ding, W.Fu, H.Yang, RSC Adv., 5 (2015) 6436. 2. U.Thakur, R.Kisslinger, K.Shankar, Nanomaterials, 7 (2017) 95. 3. T.Butburee, P.Kotchasarn, P.Hirunsit, Z.Sun, Q.Tang, P.Khemthong, W.Sangkhun, W.Thongsuwan, P.Kumnorkaew, H.Wang, K.Faungnawakij, J. Mater. Chem. A., 7 (2019) 8166. 4. K.Ha, Q.Wen, C.Wang, S.Yu, C.Hao, K.Chen, Soft Matter, 12 (2013) 7889. 5. J.Yang, Q.Wu, S.He, J.Yan, J.Shi, J.Chen, M.Wu, X.Yang, Nanoscale, 7 (2015) 13897. 6. S.Jiao, X.Fu, G.Lian, Z.Xu, Q.Wang, D.Cui, RSC Adv., 7 (2017) 20845. 7. I.Cho, Z.Chen, A.Forman, D.Kim, P.Rao, T.Jaramillo, X.Zheng, Nano. Lett., 11 (2011) 4984. 8. M.Noh, C.The, R.Daik, E.Lim, C.Yap, M.Ibrahim, N.Ludin, A.Yusoff, J.Jang, M.Teridi, J. Mater. Chem. C, 6 (2018) 712. 9. J.Liang, G.Zhang, J.Yang, W.Sun, M.Shi, AIP Advances, 5 (2015) 017141. 10. Y.Li, J.Wang, H.Sun, B.Wei, ACS Appl. Mater. Interfaces., 10 (2018) 11586. 11. B.Cai, D.Zhong, Z.Yang, B.Huang, S.Miao, W.Zhang, J.Qiu, C.Li, J. Mater. Chem., 3 (2015) 733. 12. Y.Du, Y.Deng, M.S.Zhang, Journal of Physics and Chemistry of Solids, 67 (2006) 2408. 13. L.K.Preethi, R.Antony, T.Mathews, L.Walczak, C.Gopinath, Scientific Reports, 7 (2017) 14314. 14. J.Diaz-Real, G.Dubed-Bandomo, J.Galindo-de-la-Rosa, l.Arriaga, J.Ledesma-Garcia, N.Alonso-vante, Beilstein J. nanotechnol., 9 (2018) 2643. 15. J.Zou, D.Wu, J.Luo, Q.Ximg, X.Luo, W.Dong, S.Luo, H.Du, S.Suib, ACS Catal., 6 (2016) 6867.
4
16. H.Yang, G.Liu, S.Qiao, C.Sun, Y.Jin, S.Smith, J.Zou, H.Cheng, G.Lu, J. AM, CHEM, SOC,
131 (2009) 4083.
Table 1: Fabricated TiO2 nanostructures with the effect of AHFT concentration.
5
Figure 1: (a) XRD, (b) Raman spectra of S1, S2, S3, S4, and S5 samples.
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Figure 2: FESEM and TEM images for (a, a1, a2, a3) S1, (b, b1, b2, b3) S2, (c, c1, c2, c3) S3, (d, d1, d2, d3) S4, (e, e1, e2, e3) S5, (f, f1, f2, f3) elemental mappings of sample S5.
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Figure 3: (A) Schematic diagram of rutile to anatase phase transformation, (B) XPS analysis of sample S1 and S5. 8
Highlights •Tunable and controlled (010)-faceted anatase single crystals were grown by hydrothermal method
•F- ions play a synergistic effect in the crystals phase from rutile to anatase with low-index facets
•At higher concentration of F- ions dominate the (010)-faceted anatase TiO2
•Over 1.4-m-size anatase TiO2 single crystal sheets were obtained
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