This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article Cite This: ACS Omega 2019, 4, 5944−5949
http://pubs.acs.org/journal/acsodf
In Situ Probing of Photoinduced Hydrophilicity on Titania Surface Using Dye Molecules Hiromasa Nishikiori,*,†,‡ Kotaro Tagami,† Shingo Matsunaga,† and Katsuya Teshima†,‡ †
Department of Environmental Science and Technology, Faculty of Engineering and ‡Center for Energy and Environmental Science, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan
Downloaded via 178.159.97.38 on March 28, 2019 at 16:59:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: The titania film surface exhibits superhydrophilicity after UV irradiation due to its photocatalytic function. This is caused by a change in the density of the surface hydroxyl groups, which affects the surface charge density. Titania films were prepared to observe the change in the surface charge during UV irradiation. The amounts of some xanthene dyes adsorbed and deposited on the titania films after UV irradiation were evaluated as a function of the irradiation time. The increase in the adsorption and deposition amounts of the more negative dye versus the UV irradiation time was greater. These results indicated that the titania surface charge became more positive by the UV irradiation. It was reported that basic hydroxyl groups formed on the titania particle surface during the UV irradiation. The surface dissociates hydroxyl ions into the water phase and has a positive charge, which was supported by the present study.
■
surface.11 There are several xanthene dyes possessing different charge or pKa values, which are available to probe the titania surface.12−15 In this study, titania films were prepared by a simple sol−gel method to observe changes in the titania surface charge during UV irradiation. The dye molecules were “adsorbed” and “deposited” on the titania films from their aqueous solutions. The amounts of several xanthene dyes adsorbed and deposited on the titania films after UV irradiation were evaluated as a function of the irradiation time. The UV−vis absorption spectra were obtained to estimate the dye amounts and titania surface acidity. The amounts of the adsorbed dyes were estimated by subtracting those of the dyes remaining in the solutions from the initial amounts. The amounts of the deposited dyes were those of the dyes loaded on the substrates after withdrawing them from the solutions. The xanthene dyes, erythrosin B (anionic, pKa 2.35, 3.79), eosin Y (anionic, pKa 2.02, 3.80), fluorescein (anionic, pKa 2.08, 4.31, 6.43), rhodamine 6G (cationic), and rhodamine B (cationic or neutral) were used as probe molecules. Their molecular forms are shown in Scheme 1.
INTRODUCTION Titania is one of the active photocatalysts that is well known and widely studied.1,2 The titania surface is reported to exhibit superhydrophilicity after UV irradiation due to its photocatalytic function. The hydrophilicity increased during UV irradiation. This is caused by a change in the density of the surface hydroxyl groups, which affects the surface charge density. The reported mechanism is as follows.3,4 First, photoproduced holes are diffused on the titania surface and trapped by lattice oxygens during the photocatalytic process. Second, the distance of the Ti−O bond increases and then the Ti−OH interaction weakens. Third, a water molecule is coordinated to Ti and new surface OH groups are formed. The density of the OH groups on the titania surface increases. In addition to this, degradation of organic molecules on the surface also increases its hydrophilicity. However, the status of the titania surface is not easy to evaluate because the surface is actually covered with many water molecules and some impurities. It is important that the surface should be probed by various approaches and insights. Some dye molecules are used for analyzing a solid surface and its inside because their structures are sensitive to the surrounding properties, such as acidity and polarity.5−11 Their UV−vis absorption spectra are convenient for probing their structures. Xanthene dyes used for laser media are also used as the in situ probe molecules for spectroscopic measurements.5,6,11 In our previous study, a small amount of silica deposited on the titania surface was detected by the adsorption of a fluorescein dye such that its absorption spectrum indicated a higher acidity than that when adsorbed on the titania © 2019 American Chemical Society
■
RESULTS AND DISCUSSION The measurement of the contact angle of a water droplet on a solid surface is a general and simple method to evaluate the surface hydrophilicity or hydrophobicity. Figure S1 shows the Received: January 16, 2019 Accepted: March 19, 2019 Published: March 28, 2019 5944
DOI: 10.1021/acsomega.9b00151 ACS Omega 2019, 4, 5944−5949
ACS Omega
Article
Scheme 1. Molecular Forms of Xanthene Dyes Used in This Study
change in the contact angle of a water droplet (0.50 mm3) on the titania film UV-irradiated as a function of time. The angle decreased with an increase in the irradiation time from 40° to less than 10°, indicating that the surface became more hydrophilic by further irradiation. This is a well-known property of the titania exhibiting superhydrophilicity. The determination of the amount of the adsorbed water is also a method to evaluate the surface hydrophilicity. Figure S2 shows the Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS) spectra of the adsorbed water on the titania films UV-irradiated as a function of time and the relative amount of the adsorbed water estimated from them. The amount was normalized by that on the titania film before UV irradiation (0 h). The peaks at around 3500−3000 and at 1640 cm−1 can be assigned to the O−H stretching and vending vibrations of the surface OH groups and adsorbed water molecules, respectively.16−19 The intensity of these bands clearly increased with an increase in the irradiation time. This behavior also indicated an increase in the amount of the adsorbed water due to an enhancement of the hydrophilicity of the titania surface. Figure S3 shows the X-ray photoelectron spectroscopy (XPS) spectra for the O 1s electrons of the titania film before and after the UV irradiation for 5 h. Both spectra consisted of three bands assigned to the O−Ti bond of the titania, OH group, and the adsorbed water.19−22 The peak observed at 529.8 eV was assigned to the lattice oxygen of the titania crystal. The bands at around 532−533 eV can be assigned to the OH group and adsorbed water. Table S1 shows the relative amounts of the elements (Ti, O, and C) and the components for the O 1s bands. The percentages of the O−Ti bond and OH group increased by the 5 h irradiation. The ratios of the OH group to the Ti were 0.73 and 0.78 before and after the irradiation, respectively. The carbon atoms due to some impurities also decreased. It is important that the amount of the OH group clearly increased by the irradiation. Figure 1 shows the UV−vis absorption spectra of the dyes in their basic solutions and those after reaching their adsorption equilibria on the titania plates unirradiated and UV-irradiated for 5 h. The absorbance of all of the dyes more significantly decreased due to the adsorption onto the irradiated titania than the unirradiated one. Figure 2 shows the UV−vis absorption spectra of the dyes deposited from their basic solutions on the titania films UV-irradiated as a function of
time. The absorbance of all of the dyes generally increased with an increase in the irradiation time in consideration of some experimental deviations. The absorbance of erythrosin B slightly changed after the 1 h irradiation. The absorbance of rhodamine B slightly changed during the 5 h irradiation. These results indicated that the amounts of the dye molecules adsorbed and deposited on the titania films increased due to an increase in the hydrophilicity. Table 1 shows the absolute adsorption and deposition amounts of the dye molecules before and after the 5 h UV irradiation. Figure 3 shows the changes (increments) in the adsorption and deposition amounts of the dye molecules after the 5 h UV irradiation. The amounts of the deposited dyes were significantly higher than those of the adsorbed dyes. The relative adsorption and deposition amounts were also calculated by normalizing the amount of each dye adsorbed and deposited before the irradiation (0 h). The pKa value for the anionic dye of the stronger acid, i.e., erythrosin B, eosin Y, and fluorescein, is lower, indicating that the dye possesses a more negative charge.12−15 The pKa values for the proton dissociation between the anion and dianion for erythrosin B, eosin Y, and fluorescein are 3.79, 3.80, and 6.43, respectively.13,14 The adsorption or deposition of the neutral or cationic dye (rhodamine B) and cationic dye (rhodamine 6G) did not significantly increase. The relative increases in both the adsorption and deposition amounts of the more negative dye more significantly increased with the UV irradiation time. Figure 4 shows the dependence of the relative deposition amounts of the dyes onto the titania films on the UV irradiation time compared to that of the relative adsorption amount of water and the surface acidity estimated from the dianion/anion ratio of fluorescein. The relative adsorption and deposition amounts were calculated by normalizing the amounts of water adsorbed and each dye deposited before the irradiation (0 h). The acid−base equilibria of fluorescein are shown in Scheme 2. The rate of the increase in the deposition amounts of the more negative dyes was greater than that of the water adsorption. The dye molecules are deposited on the titania film surface during dip-coating, i.e., the competition process between running down of the solution and water evaporation, which increases and decreases the dye deposition amount, respectively. Therefore, the deposition amount depends on not only the hydrophilicity of the surface but also the electrostatic interaction between the surface and 5945
DOI: 10.1021/acsomega.9b00151 ACS Omega 2019, 4, 5944−5949
ACS Omega
Article
Figure 2. UV−vis absorption spectra of (a) erythrosin B, (b) eosin Y, (c) fluorescein, and (d) rhodamine B deposited from their basic solutions on the titania films UV-irradiated as a function of time.
Figure 1. UV−vis absorption spectra of (a) erythrosin B, (b) eosin Y, (c) fluorescein, and (d) rhodamine B in their basic solutions and those after reaching their adsorption equilibria on the titania plates unirradiated and UV-irradiated for 5 h.
amounts did not significantly increase by the UV irradiation as expected. The acidity of the titania particle surface is equivalent to around pH 7 in water. Fluorescein exhibits the pKa of 6.43 for the proton dissociation between the anion and dianion. The change in the acidity of the titania particle surface can be effectively probed using fluorescein. Previously, the spectra of the fluorescein adsorbed on the silica and titania indicated the acidity corresponding to pH values about 5.8 and 6.1 in water, respectively, estimated from the ratio of the dianion species to the anion species.11 In this study, consequently, the pH value increased with an increase in the UV irradiation time from ca. 6.46 to ca. 6.53, indicating that the surface became more basic. The titania surface charge became more positive by the UV
dye molecules. The interaction is caused by the charges on the titania surface and the dye molecules. Figure S4 shows the UV−vis absorption spectra of rhodamine 6G and rhodamine B deposited from their acidic solutions on the titania films UVirradiated as a function of time. Figure S5 shows the dependence of their relative deposition amounts onto the titania films on the UV irradiation time compared to that of the relative adsorption amount of water. The amount of rhodamine B deposited from its acidic solution was greater than that deposited from the basic solution because rhodamine B easily formed hydrochloric salt and precipitated during dipcoating under the acidic condition. However, their deposition 5946
DOI: 10.1021/acsomega.9b00151 ACS Omega 2019, 4, 5944−5949
ACS Omega
Article
Table 1. Absolute Adsorption and Deposition Amounts (/10−5 mol) of the Dye Molecules before and after the 5 h UV Irradiation erythrosin B adsorption deposition
before after before after
1.0 1.8 20.3 62.3
± ± ± ±
eosin Y
0.1 0.1 0.8 0.2
1.1 1.7 7.3 27.9
± ± ± ±
0.1 0.1 0.2 0.3
fluorescein 1.4 2.0 11.6 41.4
± ± ± ±
0.1 0.1 1.6 1.2
rhodamine B 1.4 1.5 2.6 3.7
± ± ± ±
0.1 0.1 0.2 0.3
irradiation. It is presumed that the concentration of basic OH groups increased on the titania surface, originally having the acidic and basic OH groups, during the UV irradiation based on the reported mechanism.3,4 This phenomenon was utilized for the reaction of surface modification with silica, zinc oxide, etc.11,23−25 The concentration of the active species and basicity slowly increased at limited sites because its efficiency is not very high during the photocatalytic reaction on the titania. This property is suitable to control the reaction forming very small particles or thin layers. The present results supported the fact that the titania surface dissociates hydroxyl ions into the water phase and has a positive charge.
■
CONCLUSIONS In this study, the amounts of several xanthene dyes adsorbed and deposited on the titania films after the UV irradiation were evaluated as a function of the irradiation time. The UV−vis absorption spectra were observed to evaluate the dye amounts and titania surface acidity. The increase in the adsorption and deposition amounts of the more negative dye versus the UV irradiation time was greater. These results indicated that the titania surface charge became more positive by the UV irradiation because the surface dissociated hydroxyl ions into the water phase.
Figure 3. (a) Absolute and (b) relative increases in the adsorption and deposition amounts of the dye molecules after the 5 h UV irradiation.
■
EXPERIMENTAL SECTION Titanium tetraisopropoxide, ethanol, acetone, nitric acid, hydrochloric acid, and sodium hydroxide (Wako, S or Reagent Grade) were used without further purification. Water was ionexchanged and distilled using a distiller (Yamato WG23). The glass plates (Matsunami S-1111) were individually washed with a detergent and acetone, soaked in 0.10 mol dm−3 hydrochloric acid for 2 h, and washed with ethanol. The xanthene dyes, erythrosin B, eosin Y, fluorescein, rhodamine 6G, and rhodamine B (Wako, S grade) were used as probe molecules without further purification. The glass substrates (76 mm × 26 mm) were coated with a thin titania film by the sol−gel method. The titania sol was prepared by mixing 37.5 cm3 of ethanol, 0.32 cm3 of nitric acid, and 0.30 cm3 of water and then adding 7.50 cm3 of titanium tetraisopropoxide under a dry nitrogen atmosphere. The anatase-type titania films were prepared by 10 dip-coatings using the sol and then heating at 773 K for 30 min. The thickness of the 10-layered film was ca. 300 nm, as previously reported.11 The UV−vis absorption spectra were measured using a spectrophotometer (Shimadzu UV-3150). The FTIR-RAS for the film samples was conducted using an FTIR spectrophotometer (Shimadzu IRPrestige-21) with an RAS accessory. The XPS was conducted by Al Kα radiation using an X-ray photoelectron spectrophotometer (VG Scientific ESCALAB 250). The titania films were irradiated with UV light for 0−5 h. Pure water, 1.0 cm3, was spread on the titania plates and dried
Figure 4. (a) Dependence of the relative deposition amounts of the dyes onto the titania films on the UV irradiation time compared to that of the relative adsorption amount of water and (b) the surface acidity estimated from the dianion/anion ratio of fluorescein.
5947
DOI: 10.1021/acsomega.9b00151 ACS Omega 2019, 4, 5944−5949
ACS Omega
Article
Scheme 2. Parts of Acid−Base Equilibria of Fluorescein
■
at 293 K and a relative humidity of 40% for 1 h to evaluate the hydrophilicity of the titania surface. The relative amount of the adsorbed water was estimated by measuring the FTIR-RAS spectra of the titania films. The UV irradiation was conducted using a black lamp (500 μW cm−2). The dye molecules were adsorbed and deposited on the titania films from their aqueous solutions, in which the pH values were adjusted to around 8.0 by adding sodium hydroxide. However, rhodamine 6G was insoluble under this condition. Rhodamine 6G and rhodamine B were also dissolved in water, in which the pH values were around 3.0. The titania films were irradiated with UV light for 1−5 h. The titania plates were immersed in a 1.0 × 10−2 mol dm−3 aqueous solution of each dye for 80 min. The amounts of the adsorbed dyes were estimated by measuring the UV−vis absorption spectra of the dyes remaining in the solutions. The titania plates adsorbing the dyes were withdrawn from the solutions and then dried at 293 K and a relative humidity of 40% for 1 h. The amounts of the deposited dyes were estimated by measuring the UV−vis absorption spectra (absorbance areas) of the withdrawn titania plates based on analytical curves obtained using the absorbance areas of known amounts of the deposited dyes shown in Figure S6.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +81-26-269-5536. Fax: +81-26-269-5531. ORCID
Hiromasa Nishikiori: 0000-0002-6398-310X Katsuya Teshima: 0000-0002-5784-5157 Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI under Grant numbers JP15K05472 and JP16KK0110. REFERENCES
(1) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium Dioxide Photocatalysis. J. Photochem. Photobiol., C 2000, 1, 1−21. (2) Chatterjee, D.; Dasgupta, S. Visible Light Induced Photocatalytic Degradation of Organic Pollutants. J. Photochem. Photobiol., C 2005, 6, 186−205. (3) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Enhancement of the Photoinduced Hydrophilic Conversion Rate of TiO2 Film Electrode Surfaces by Anodic Polarization. J. Phys. Chem. B 2001, 105, 3023−3026. (4) Sakai, N.; Fujishima, A.; Watanabe, T.; Hashimoto, K. Quantitative Evaluation of the Photoinduced Hydrophilic Conversion Properties of TiO2 Thin Film Surfaces by the Reciprocal of Contact Angle. J. Phys. Chem. B 2003, 107, 1028−1035. (5) Fujii, T.; Ishii, A.; Takusagawa, N.; Anpo, M. Fluorescence Spectra and Chemical Species of Fluorescein Molecules Adsorbed on A Calcinated Porous Vycor Glass. Res. Chem. Intermed. 1992, 17, 1− 14. (6) Fujii, T.; Ishii, A.; Kurihara, Y.; Anpo, M. Multiple Fluorescence Spectra of Fluorescein Molecules Encapsulated in the Silica Xerogel Prepared by the Sol−Gel Reaction. Res. Chem. Intermed. 1993, 19, 333−343. (7) Nishikiori, H.; Nagaya, S.; Tanaka, N.; Katsuki, A.; Fujii, T. Acid−Base and Monomer−Dimer Equilibria of Methylene Blue in Dip-Coated Thin Films. Bull. Chem. Soc. Jpn. 1999, 72, 915−921. (8) Nishikiori, H.; Tanaka, N.; Isowaki, Y.; Tanno, Y.; Nomoto, T.; Toriumi, K.; Mishima, S.; Fujii, T. Proton Donor−Acceptor Property of Matrix during the Sol−Gel Reaction. Res. Chem. Intermed. 2009, 35, 227−240. (9) Nishikiori, H.; Tanaka, N.; Minami, Y.; Katsuki, A.; Fujii, T. Molecular Forms and Fluorescence Processes of 9-Aminoacridine in Thin Sol−Gel Films. J. Photochem. Photobiol., A 2010, 212, 62−67. (10) Islam, S.; Bidin, N.; Riaz, S.; Naseem, S. Synthesis of Optically Active Bromophenol Blue Encapsulated Mesoporous Silica−Titania Nanomatrix: Structural and Sensing Characteristics. J. Sol-Gel Sci. Technol. 2018, 85, 231−242. (11) Nishikiori, H.; Matsunaga, S.; Iwasaki, M.; Zettsu, N.; Yamakawa, M.; Kikuchi, A.; Yamakami, T.; Teshima, K. Formation of Silica Nanolayer on Titania Surface by Photocatalytic Reaction. Appl. Catal., B 2019, 241, 299−304. (12) Mchedlov-Petrossyan, N. O.; Kleshchevnikova, V. N. Influence of the Cetyltrimethylammonium Chloride Micellar Pseudophase on
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00151. Change in the contact angle of a water droplet, FTIRRAS spectra of the adsorbed water, and the relative amount of the adsorbed water on the titania film UVirradiated as a function of time; XPS spectra for the O 1s electrons of the titania film and percentages of elements on the titania film surface before and after the UV irradiation; UV−vis absorption spectra of rhodamine 6G and rhodamine B deposited from their acidic solutions on the titania films UV-irradiated as a function of time; percentages of elements on the titania film surface before and after the UV irradiation; UV−vis absorption spectra of rhodamine 6G and rhodamine B deposited from their acidic solutions on the titania films UV-irradiated as a function of time; dependence of the deposition amounts of rhodamine 6G and rhodamine B from the acidic solutions onto the titania films on the UV irradiation time compared to that of the adsorption amount of water; and analytical curves obtained using the absorbance areas of known amounts of the deposited dyes (PDF) 5948
DOI: 10.1021/acsomega.9b00151 ACS Omega 2019, 4, 5944−5949
ACS Omega
Article
the Protolytic Equilibria of Oxyxanthene Dyes at High Bulk Phase Ionic Strength. J. Chem. Soc., Faraday Trans. 1994, 90, 629−640. (13) Sjö back, R.; Nygren, J.; Kubista, M. Absorption and Fluorescence Properties of Fluorescein. Spectrochim. Acta, Part A 1995, 51, L7−L21. (14) Batistela, V. R.; Pellosi, D. S.; de Souza, F. D.; da Costa, W. F.; de Oliveira Santin, S. M.; de Souza, V. R.; Caetano, W.; de Oliveira, H. P. M.; Scarminio, I. S.; Hioka, N. pKa Determinations of Xanthene Derivates in Aqueous Solutions by Multivariate Analysis Applied to UV−Vis Spectrophotometric Data. Spectrochim. Acta, Part A 2011, 79, 889−897. (15) Vanzin, D.; Freitas, C. F.; Pellosi, D. S.; Batistela, V. R.; Machado, A. E. H.; Pontes, R. M.; Caetano, W.; Hioka, N. Experimental and Computational Studies of Protolytic and Tautomeric Equilibria of Erythrosin B and Eosin Y in Water/DMSO. RSC Adv. 2016, 6, No. 110312. (16) Calzada, M. L.; Delolmo, L. Sol-Gel Processing by Inroganic Route to Obtain a TiO2−PbO Xerogel as Ceramic Precursor. J. NonCryst. Solids 1990, 121, 413−416. (17) Amor, S. B.; Baud, G.; Besse, J. P.; Jacquet, M. Structural and Optical Properties of Sputtered Titania Films. Mater. Sci. Eng., B 1997, 47, 110−118. (18) Ivanda, M.; Musić, S.; Popović, S.; Gotić, M. XRD, Raman and FT-IR Spectroscopic Observations of Nanosized TiO2 Synthesized by the Sol−Gel Method Based on an Esterification Reaction. J. Mol. Struct. 1999, 480−481, 645−649. (19) Trimboli, J.; Mottern, M.; Verweij, H.; Dutta, P. K. Interaction of Water with Titania: Implications for High-Temperature Gas Sensing. J. Phys. Chem. B 2006, 110, 5647−5654. (20) Nishikiori, H.; Sato, T.; Kubota, S.; Tanaka, N.; Shimizu, Y.; Fujii, T. Preparation of Cu-Doped TiO2 via Refluxing of Alkoxide Solution and Its Photocatalytic Properties. Res. Chem. Intermed. 2012, 38, 595−613. (21) Nishikiori, H.; Setiawan, R. A.; Miyamoto, K.; Sukmono, G.; Uesugi, Y.; Teshima, K.; Fujii, T. Photoinduced Electron Transport in Dye-Containing Titania Gel Film. RSC Adv. 2012, 2, 4258−4267. (22) Gusmano, G.; Montesperelli, G.; Nunziante, P.; Traversa, E.; Montenero, A.; Braghini, M.; Mattogno, G.; Bearzotti, A. HumiditySensitive Properties of Titania Films Prepared Using the Sol-Gel Process. J. Ceram. Soc. Jpn. 1993, 101, 1095−1100. (23) Nagaya, S.; Nishikiori, H. Deposition of ZnO Particles by Photocatalytic Reaction. Chem. Lett. 2012, 41, 993−995. (24) Nishikiori, H.; Nagaya, S.; Takikawa, T.; Kikuchi, A.; Yamakami, T.; Wagata, H.; Teshima, K.; Fujii, T. Formation of ZnO Thin Films by Photocatalytic Reaction. Appl. Catal., B 2014, 160, 651−657. (25) Nishikiori, H.; Fujiwara, S.; Miyagawa, S.; Zettsu, N.; Teshima, K. Crystal Growth of Titania by Photocatalytic Reaction. Appl. Catal., B 2017, 217, 241−246.
5949
DOI: 10.1021/acsomega.9b00151 ACS Omega 2019, 4, 5944−5949
