Characterization, photoelectrochemical properties, and surface wettabilities of transparent porous TiO2 thin films

Accepted Manuscript Title: Characterization, Photoelectrochemical Properties, and Surface Wettabilities of Transparent Porous TiO2 Thin Films Authors: Li-Heng Kao, Ya-Ping Chen PII: DOI: Reference:

S1010-6030(16)31038-3 http://dx.doi.org/doi:10.1016/j.jphotochem.2017.03.011 JPC 10562

To appear in:

Journal of Photochemistry and Photobiology A: Chemistry

Received date: Revised date: Accepted date:

13-11-2016 5-3-2017 9-3-2017

Please cite this article as: Li-Heng Kao, Ya-Ping Chen, Characterization, Photoelectrochemical Properties, and Surface Wettabilities of Transparent Porous TiO2 Thin Films, Journal of Photochemistry and Photobiology A: Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.03.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

Characterization, Photoelectrochemical Properties, and Surface Wettabilities of Transparent Porous TiO2 Thin Films

Li-Heng Kao* and Ya-Ping Chen

Department of Chemical and Materials Engineering National Kaohsiung University of Applied Science Kaohsiung 80778, Taiwan

March 5, 2017 Manuscript submitted for review and publication in Journal of Photochemistry and Photobiology A: Chemistry

*Corresponding author Tel: +886-7-3814526*5141 Fax: +886-7-3830674 E-mail: biny@ kuas.edu.tw

Graphical abstract TiO2 thin films fabricated using CTAB showing superhydrophilicity, visible-light photoactivity, and transmittance were prepared using a simplified sol–gel method.

HIGHLIGHTS 

CTAB-modified TiO2 thin films are formed on FTO glass by spin coating.



Effects of CTAB addition on characteristics of the thin films are investigated.



Increasing the amount of CTAB used results in formation of rutile TiO2.



This increases the TiO2 crystalline grain size.



Film morphology and surface roughness can be controlled in this manner.

Abstract 1

1

FTO: Fluorine-doped tin oxide; CTAB: cetyltrimethylammonium bromide; IEP: isoelectric point; TTIP: titanium isopropoxide; AcAc: acetylacetone; XRD: X-ray diffraction; SEM: scanning electron microscopy, FTIR: Fourier-

In this study, a simplified sol–gel method was used for the cetyltrimethylammonium bromide (CTAB)-assisted fabrication of transparent and porous TiO2 thin films on fluorinedoped tin oxide glass substrates by spin coating. The competing effects of the surface morphology, electron–hole recombination rate, and crystal phase of CTAB on the synthesis and photoelectrochemical (PEC) performance of the thin films were investigated. Increasing the CTAB content in the precursor solution not only resulted in the formation of the rutile phase and pores, but also increased the TiO2 crystalline grain size and roughness. Consequently, the film crystal phase could be controlled and the surface roughness increased by varying the CTAB concentration; this resulted in increases in the photocurrent response and hydrophilicity of the films. The as-obtained thin film with a [CTAB]/[TTIP] molar ratio of 1 exhibited the highest photocatalytic activity as well as the highest textured surface area and porosity, because it had a rougher surface and contained more surface defects. Furthermore, the CTAB-assisted fabrication method exhibited rapid superhydrophilicity conversion after visible-light irradiation, owing to the synergistic effects of the rutile and anatase particles during the PEC reaction.

Keywords: titania, cetyltrimethylammonium bromide, photoelectrochemical, photocurrent, superhydrophilicity, synergistic effect

1. Introduction TiO2 exhibits photocatalytic properties and is regarded as an advanced oxidation process material that shows a strong oxidizing ability upon exposure to ultraviolet light [1].

transform infrared; AFM: atomic force microscopy; UV-vis: ultraviolet-visible; PL: photoluminescence; FTIR: Fourier transform-infrared spectroscopy; PEC: photoelectrochemical

Furthermore, TiO2 is the most commonly used photocatalytic material because it is chemically stable, nontoxic, easy to prepare, and cheap [2]. However, during the degradation of pollutants, TiO2 is generally used as a photocatalyst in the form of high-specific-surface-area powders, whose recovery is not easy and whose use can cause secondary pollution. Recently, the preparation of thin films of TiO2, which is also a semiconductor oxide, has been investigated, and such films have been used in photoelectric cells, sensors, and solar cells [3-8]. The lightinduced generation of charge carriers is the basic requirement for the use of semiconductors in photocatalysis and photoelectrochemical (PEC) reactions. These charge carriers are trapped in defects and vacancies and can react with the electron acceptors or donors adsorbed on the surface of the photocatalyst. The equilibrium between the trapping and recombination of these charge carriers and interfacial charge transfer determines the overall quantum efficiency [911]. In the case of photocatalytic systems for which the rate-limiting step is interfacial charge transfer, improved charge separation and the inhibition of charge-carrier recombination are essential for enhancing the overall quantum efficiency of the photocatalytic process [12]. By using TiO2 thin films, secondary pollution can be avoided. This increases the applicability of TiO2 as a photocatalyst. However, the specific surface area of thin films is significantly lower than that of powder particles. As a result, the degradability of thin films is lower. Therefore, the thin films must be modified, in order to prevent these negative effects. In addition to exhibiting a high interaction area, a thin-film material must also show high optical transmittance, good light absorption, and a low electron–hole recombination rate in order to be an efficient photocatalyst. Herein, we used a cationic surfactant to modify TiO2 thin films, based on the charge interactions between the surfactant and TiO2, to ensure that the thin films exhibited a porous structure, so that their specific surface area was high. When the pH value of the precursor solution used was controlled to be close to or higher than the isoelectric point (IEP) of TiO2 (pH = 5.8) [13], deprotonation occurred on the TiO2 surface, resulting in

the

surface

becoming

negatively

charged.

The

cationic

surfactant

used,

cetyltrimethylammonium bromide (CTAB), can be employed as a templating reagent to synthesize porous TiO2 thin films by the sol–gel process. Further, TiO2 produced with the assistance of CTAB exhibits higher net surface charges, resulting in an increase in the flow of electrons and holes. A higher charge-separation rate can improve photocatalytic performance [14, 15]. The synthesis of porous TiO2 thin films with surfactant modification is not a new technology but is nowadays extremely popular. However, the increase in the number of publications related to the CTAB-assisted fabrication of TiO2 thin film has not kept pace with our increased knowledge regarding the comprehensive and systematic elucidation of the competing effects of the surface morphology, electron–hole recombination rate, and crystal phase of CTAB on the synthesis and PEC performance of TiO2 thin films have not been investigated previously. In this study, the sol–gel method was used for the CTAB-assisted fabrication of transparent and porous TiO2 thin films by the spin-coating method. In addition, the crystal phases, morphologies, material properties, structural defects, optical transmittances, PEC properties, and surface wettabilities of the thin films formed for different CTAB concentrations were clearly elucidated through a comprehensive comparative study.

2. Materials and methods 2.1 Materials Titanium isopropoxide (TTIP, 99%) was used as the titanium source, acetylacetone (AcAc, 99%) was employed as the chelating agent, and CTAB (99%) was used as the modifying agent. The solvent used was absolute ethanol (99.5%), while methylene blue (MB, reagent grade) was used as the pollutant. All the materials were used as-received without further purification.

2.2 CTAB-assisted fabrication of TiO2 films In a typical synthesis procedure, two solutions were prepared separately. Solution I was made by adding TTIP to AcAc (molar ratio = 1:1) in a dropwise manner under constant stirring. To form solution II, specific amounts of CTAB ([CTAB]/ [TTIP] ratio denoted by R; R = 0, 0.1, 0.5, and 1.0) were dissolved in 0.3 mole absolute alcohol. Next, solution I was added dropwise to the well-stirred solution II to induce the sol–gel reaction; the mixture was blended at 25 C for an additional 24 h after all of solution I had been added to solution II. The pH values of the final clear solutions were 7.78 (R= 0), 7.43 (R= 0.1), 7.38 (R= 0.5), and 7.26 (R= 1). The pH values of all the prepared solutions were higher than the IEP of TiO2, which ensured that the surfaces of the TiO2 films were negatively charged [13]. A 2 cm  2 cm piece of fluorine-doped tin oxide (FTO) glass was used as the substrate. The FTO glass was cleaned in an ultrasonic bath for 5 min. It was then placed in absolute alcohol and subjected to ultrasonic cleaning for 10 min. The cleaned FTO glass was then dried in an oven. The titanium precursor solution was coated on the cleaned FTO glass by spin coating, first at a preliminary speed of 100 rpm and then at a final speed of 3000 rpm. The total spincoating duration was 30 s. Next, the TiO2 thin film was prepared by the calcination of the coated glass substrate for 1 h at a heating rate of 1 °C/min to a maximum temperature of 500 °C. This yielded TiO2 films with a thickness of approximately 1.3 μm.

2.3 Characterization of TiO2 films Qualitative analyses were performed based on X-ray diffraction (XRD) measurements (Rigaku D/max-2200), which were made using Cu kα (λ =0.15418 nm) radiation and a Ni filter operating at 40 kV/30 mA. The patterns were collected for 2θ values of 20–80° at a rate of 1.5°/min in the continuous-scan mode. The surface morphologies of the synthesized films were examined using a scanning electron microscopy (SEM) system (Phenom Pro) operated at 20

kV. An atomic force microscopy (AFM) system (NT-MDT Solver P47) with a probe with a tip curvature radius of less than 7 nm (Nanosensors PointProbe Plus-RT-NCHR) was used to determine the two- and three-dimensional morphologies, step profiles, and surface roughnesses of the films. Fourier transform-infrared spectroscopy (FTIR) measurements were performed for wavenumbers of 400–4000 cm-1 with a resolution of 4 cm−1 using a Perkin-Elmer Spectrum One spectrometer. Ultraviolet-visible (UV–vis) spectrophotometry performed using a system (UV-Vis., JASCO V-670) equipped with an integrating sphere (JASCO ISN-923) was employed to measure the transmittances, absorption spectra, and band gap energies of the films. Photoluminescence (PL) measurements were performed with a HITACHI F-4500 spectrophotometer at an excitation wavelength of 320 nm. The surface wettabilities of the prepared thin films were evaluated through contact-angle measurements performed using pure water droplets (approximately 1 μl); a Digidrop contact-angle meter from GBX Instruments (model R&D) was employed for the purpose. The PEC measurements were performed at room temperature using a three-electrode PEC cell containing a 0.5 M Na2SO4 solution as the electrolyte; the cell had a quartz window, and a 450 W halogen lamp was used as the visible-light source. A TiO2 film sample with an area of 2 cm  2 cm was used as the working electrode, while a Ag/AgCl electrode (3 M KCl) and a Pt sheet acted as the reference and counter electrodes, respectively. The measurements were carried out using a potentiostat (Biologic SP150), and the photocurrents were determined under a potential bias of +0.5 V. The photocatalytic activity of the fabricated thin films was evaluated with respect to the photocatalytic degradation of MB in an aqueous solution. A batch photoreactor system was used for the experiments. It consisted of a cylindrical quartz reactor and an external light source that allowed for vertical irradiation. A 450 W halogen lamp with an average radiation density of 17.5 mW/cm2 was used as the light source. The system was cooled by air and water, so that

it remained at room temperature. A set of photocatalytic degradation experiments was performed as follows: a piece of the TiO2 film being tested (2 cm  2 cm) was dipped into 10 ml of an MB solution with an initial concentration of 10 mg/l. Prior to the photoreaction, air was pumped into the reactor under dark conditions for 30 min to ensure adsorption–desorption equilibrium had been reached. Then, the reaction was performed, with the film being irradiated vertically from the top by the halogen lamp. During the photoreaction, a sample was collected from the degraded solution after every hour, and the corresponding UV−visible spectrum was recorded, in order to monitor the progress of the degradation of the MB solution.

3. Results and discussion 3.1 Phase identification The results of the crystal-phase analyses of the synthesized TiO2 thin films are shown in Figure 1. Diffraction peaks related to anatase (tetragonal, I41/amd (No. 141)) and rutile (tetragonal, P4/mnm (No. 136)) phases (corresponding to JCPDS PDF 21-1272 and 21-1276, respectively) can be observed unambiguously. In the spectrum of the film corresponding to R = 0.5, characteristic rutile peaks can also be seen. These peaks appear because, when CTAB is selectively absorbed onto the TiO2 surface, it induces the transformation of the face-sharing anatase polyhedra into those of edge-sharing rutile [16]. This irreversible transition into the rutile phase is accompanied by significant grain growth [17, 18], resulting in large rutile grains and small anatase grains. This is what causes the observed changes in the ratios of the XRD peak intensities. The results of the XRD analyses are shown in Table 1. To calculate the crystallite size, the Debye-Scherrer equation was used [19]. In addition, the Spurr-Myers method was used to estimate the crystal phase ratios [20] (see Table 1). During the hydrolysis and condensation of the precursor solution and its interaction with CTAB, the CTA+ cations produced by CTAB and the TiO- anions experience attraction forces and form Ti-O-ATC,

hindering the growth of the TiO2 crystal particles [16, 21]. Consequently, in the case of the film corresponding to R = 0.1, the crystallites were smaller. However, for R ≥ 0.5, the TiO2 crystal particles increased in size because, in this case, the metastable anatase in the TiO2 thin film was converted into particles of stable rutile. As the amount of rutile formed increased, the average size of the crystal particles increased as well.

3.2 Compositional analysis The FTIR spectra were used to identify the functional groups of the TiO2 thin films. The spectra for the films corresponding to R = 0 (prepared without CTAB), R = 0.1, R = 0.5, and R = 1.0 are shown in Figure 2. The characteristic bands at 500–800 cm-1 can be attributed to the Ti-O stretching vibrations [15]. At 1640 cm-1, a peak related to the O-H bending vibrations attributable to the water vapor present on the film surface is observed; this can be ascribed to the water absorbed by TiO2 after calcination [20, 22]. Furthermore, a small peak can be seen near 2340 cm-1; this is attributable to the characteristic stretching vibrations of CO2. The presence of this peak can probably be ascribed to the decomposition of AcAc, CTAB, and the other organic components into CO2. Some of the CO2 was probably absorbed by the TiO2 film [23].

3.3 Morphological characterization The nanostructural morphology of a photocatalyst plays an important role in determining its photocatalytic efficiency and PEC activity [14, 23]. In this study, using AcAc, which served as a stabilizer, and CTAB, which acted as a pore-forming agent, flat and thin TiO2 films could be obtained, owing to the fact that AcAc acts only as an inhibitor and complexes with Ti(OPri)4, such that the rate of hydrolysis and precipitation of the titanic alkoxides can be controlled. It was observed that the TiO2 film prepared without CTAB (R = 0, Figure 3a) was flat and

nonporous. However, after the addition of CTAB, different types of porous structures developed in the films. In the case of the films corresponding to R = 0.1–1.0, increasingly larger pores appeared on the surface, and numerous aggregates of the nanoparticles were formed (Figures 3b-3d). The pore size and porosity increased with an increase in the amount of CTAB used. The reason for this is primarily phase separation owing to the polycondensation of the hydrolyzed titanium isopropoxide and the aggregation of CTAB micelles in the solvent mixture during the sol–gel process. The regions where the CTAB micelles aggregated were the original sites of pores, which were formed after the high-temperature calcination process. As the amount of CTAB added is increased, more of these regions are formed and their area increases [21]. The results of the AFM analyses are shown in Figure 4. The average roughness was calculated by selecting certain areas on the surfaces of the films. The film corresponding to R = 0 was smooth with an average roughness (Ra) of 1.51 nm (Figure 4a). However, the surface roughness increased with an increase in the CTAB concentration. For R = 0.1, the Ra value increased only slightly (Figure 4b), to approximately 4.41 nm, and small pores appeared on the film surface. However, in the case of the films corresponding to R = 0.5 and 1.0 (Figures 4c and 4d), a large number of pores and particles formed on the film surface, and Ra increased to 56.20 nm and 113.54 nm, respectively; these results are consistent with those obtained using SEM. Therefore, it can be concluded that the pore diameter and depth of the TiO2 films increased with the CTAB concentration.

3.4 Transmittance measurements and energy gap analysis TiO2 films should exhibit a high optical transmittance, in order to be suitable for use in optical devices and products, because transparent TiO2 thin films allow for improved light penetration and absorption. Figure 5 shows the appearances of the TiO2 thin films prepared in

this study. We found that, as a greater amount of CTAB was added, the synthesized films exhibited a greater degree of misting, with the transparency of the films decreasing. By comparing these observations with the SEM images, the origin of the reduced transparency could be ascertained. Particles with large surface holes are formed when the CTAB concentration is increased, resulting in an increase in the surface roughness; this not only affects film transparency but also alters film hydrophilicity because the rougher the surface, the greater is the degree of light scattering, which decreases the transparency [24]. Figure 6a shows the transmittance spectra of the films for wavelengths of 300–800 nm; this range includes the ultraviolet and visible wavelengths. The films (R = 0 and 0.1) exhibited relatively high transparencies and transmittances similar to those of the FTO glass substrate in the visible spectrum (400–800 nm); this was indicative of the uniformity and smoothness of the films. It should be noted that, for the films corresponding to R = 0 and 0.1, the curves contained interference fringes similar to those seen in the transmittance spectrum of the FTO glass substrate. These oscillations are primarily caused by the multiple reflections occurring at each interface. This phenomenon was also indicative of the homogeneity and uniform thickness of the films. If the films had not had a homogeneous surface morphology or did not exhibit uniform thickness, all the interference effects in the transmittance spectra would have been smoothened out [25, 26]. Thus, these results were also consistent with those of SEM and AFM. Figure 6b shows the absorption spectra of the TiO2 films, while the energy gaps as determined from the absorption spectra are shown in Table 2. It is interesting to note that the band gap energy of the synthesized TiO2 thin films was constant regardless of the amount of CTAB added, even though rutile TiO2 was formed when R 0.5. This indicated that the wavelength of the absorption edge of the fabricated TiO2 thin films extended into the visible-light region. The absorbance of the TiO2 films in the visible region can be attributed to the existence of a small amount of Ti3+ species; it is likely that the Ti3+ species extended the optical absorption

range of TiO2 into the visible-light region [27]. The enhanced absorbances of the TiO2 films are in accordance with the bands at 446 nm and 498 nm (charge is transferred from the Ti3+ ions to the oxygen anions in the TiO68− complex) in the PL emission spectra. Enhanced light absorption in the visible-light region can improve the photoactivity of TiO2 by increasing the numbers of photogenerated electrons and holes. This, in turn, would increase its applicability.

3.5 Photoluminescence measurements PL emissions arise from the recombination of free carriers and thus can be used to evaluate the charge-capture efficiency of defects within the semiconductor material and analyze the migration and transition behaviors of light-excited electron and holes [26]. It is well known that a high emission intensity indicates a high recombination rate [28]. Figure 7a shows the PL emission spectra of the synthesized TiO2 thin films. As can be seen from the figure, the film corresponding to R = 0 exhibits the lowest PL intensity. Furthermore, it can be seen that the emission intensity increases with an increase in the amount of CTAB used. Because anatase TiO2 is an indirect band gap semiconductor material, the PL intensity is only related to the number of oxygen vacancies present [29, 30]. After the addition of CTAB, the film surface becomes porous and rough, implying a discontinuity in the atomic arrangement. Porous structures have a higher number of surface defects. Thus, the presence of a higher number of oxygen vacancies would lead to a higher PL intensity [30-32]. The figure also shows a broad band in the region of 450–500 nm. For a detailed analysis, the PL spectra of the synthesized TiO2 thin films were deconvoluted into three peaks (Figures 7b-7e), which were centered at 412, 446, and 498 nm, by using Gaussian functions. The PL band at 412 nm can be ascribed to self-trapped excitons, oxygen vacancies, and surface states, which are the physical origins of the PL emission of anatase TiO2 [33]. Further, the bands at 446 nm and 498 nm can be attributed to the surface oxygen vacancies. It is known that, when

charge is transferred from Ti3+ ions to oxygen anions in a TiO68− complex, it results in the formation of oxygen vacancies on the surface [30].

3.6 Photoelectrochemical activities To further investigate the ability of the TiO2 thin films to generate electron-hole pairs under visible-light irradiation, the linear sweep voltammograms and transient photocurrent responses of the films were obtained under intermittent irradiation (on/off switching of irradiating light). A comparison of the I–V characteristics of the TiO2 thin films is shown in Fig. 8a. The film corresponding to R = 0 shows a higher photocurrent density compared to those of the other films (R = 0.1–1.0) for the entire potential region, indicating that its visible-light harvesting ability was greater, on account of its smooth surface and low electron–hole recombination rate. The saturated photocurrent density of this TiO2 thin film (R = 0) photoanode was 43.64 μA/cm2. Figure 8b shows a comparison of the I–t curves of the various films. When the films are irradiated using visible light, a large number of electron-hole pairs are generated at the titanium dioxide/sodium sulfate interface, resulting in a large and instantaneous spike in the photocurrent in the initial stage. The photocurrent starts to decay immediately and subsequently plateaus, owing to the recombination of the electrons and holes. This phenomenon can be attributed to two main factors: (1) holes accumulate on the surface of the TiO2 thin film and combine with the electrons in the TiO2 conduction band and (2) the electrons in the TiO2 conduction band start to reduce the photogenerated oxidized species in the electrolytic solution. After the recombination of the excessive holes with electrons, the generation and transfer of the electron–hole pairs reaches a stage of equilibrium, resulting in a stable current [32, 33]. The photocurrent decreases rapidly to zero as soon as the irradiating light is turned off. Further, the photocurrent increases again to a constant value when the light is turned back on, confirming the high reproducibility of the phenomenon.

Figure 8c shows the results of the stability tests performed to evaluate the photocurrent responses of the synthesized TiO2 thin films when used as photoanodes. The film corresponding to R = 0 exhibits the best light response as well as the highest photocurrent (35.52 μA/cm2), owing to its smooth surface and low electron–hole recombination rate. Moreover, the photocurrent density increases with the increase in the amount of CTAB added (the photocurrents were 24.98, 20.95, and 19.07 μA/cm2 for the films corresponding to R = 1.0, 0.5, and 0.1, respectively). Photocurrent generation depends on the efficiency of transport of the electron-hole pairs to the electrode surface. On the one hand, this behavior is related to the ability of the material to prevent the recombination of the electrons and holes. On the other hand, it is also affected by the structure and crystallinity of the material [33-35]. Thus, the more complete the structure, the higher the photocurrent is likely to be. The film corresponding to R = 0 was very dense and defect-free. Consequently, the separated electrons and holes could readily move to the electrode surface and complete the circuit. On the other hand, the films correspond to R = 0.1, 0.5, and 1 contained a large number of defects on their surfaces. These defects resulted in carrier accumulation during the migration process, increasing the probability of recombination and resulting in a lower photocurrent. However, this mechanism alone cannot explain the observed phenomenon, and we suggest another mechanism to explain it. The results of the XRD analyses showed that, in the films corresponding to R = 0.5 and 1.0, a rutile phase was present. Furthermore, the higher the amount of CTAB added, the greater the volume extent of the rutile phase was. Mixed-phase photocatalysts with rutile–anatase compositions have been reported to exhibit enhanced photoactivity relative to single-phase titania, owing to the synergistic effects between the two phases [36-38]. Rutile has a higher absorbance than does anatase [34, 39, 40]. Consequently, a large number of electron-hole pairs would be generated in it under visible-light irradiation. These charge carriers would then transport charge to the anatase phase before undergoing

recombination. The lifetime of photogenerated electrons and holes is higher in the anatase phase; this would increase the photocurrent generation efficiency. Another possibility is that the application of an external voltage (+0.5 V) to the TiO2 photoanode limited electron–hole pair recombination. The photogenerated carriers were sufficiently separated because the electrons moved towards the conducting photoanode (at which the potential was applied). Therefore, the total number of holes initiating the oxidation reaction at the surface of the TiO2 anode increased, resulting in an enhancement of the PEC process [35].

3.7 Photocatalytic activity In order to investigate the photocatalytic activities of the TiO2 films, degradation experiments involving MB dye were performed under visible light; the results are shown in Fig. 9a. The kinetics of degradation of MB dye in the TiO2 batch reactor were modeled using the Langmuir-Hinshelwood (L-H) model. For low substrate concentrations (approximately <10-3 M), the equation describing the kinetics may be simplified to a pseudo-first-order kinetics relationship represented by Eq. (1) [41]: -ln(C/C0) = kapp t

(1)

where the kapp is the apparent rate constant of the degradation process and is determined from the slopes in Fig. 9b (the values are listed in Table 2) The TiO2 films fabricated with and without CTAB effectively decomposed the MB dye, with the rate constants being 0.02–0.17 h-1. The rate constants of the TiO2 films fabricated using CTAB were significantly higher. The highest apparent degradation rate constant (0.17 h-1), which was more than eight times higher than that of the pristine TiO2 film (R = 0), was obtained in the case of the film corresponding to R = 1.0. This enhancement in the photocatalytic activity may be attributed to the synergistic effect arising from the mixed-phase (rutile-anatase) nature of the film, to the presence of surface defects in the film, its higher surface roughness, and to the fact that a greater number

of charge carriers were available because they were transported from the rutile phase to the anatase phase [35, 42]. Next, we compared the PL emission and photocatalytic properties of the as-obtained TiO2 films. There was an obvious difference in the photocatalytic activity. Even though the pristine TiO2 film (R = 0) exhibited the lowest electron–hole recombination rate, it showed the lowest photocatalytic activity. This suggests that the rate of electron–hole recombination is not the primary factor determining the photocatalytic performance. In other words, the pristine TiO2 film obviously did not exhibit the mixed-phase (rutile-anatase) related synergistic effects or surface defects or a high enough surface roughness and this was reflected in its poor photocatalytic activity. Furthermore, rutile has a higher absorbance than does anatase. Further, a greater number of charge carriers are available before recombination in the case of the mixedphase films, owing to the transport of carriers from the rutile phase to the anatase phase. This phenomenon explains why the photocatalytic activity of the films corresponding to R = 0.5– 1.0 was higher.

3.8 Surface wettabilities of TiO2 thin films Figure 10 shows the changes in the contact angle of water droplets on the TiO2 films before and after visible-light irradiation for 5–15 min. The measurement results are listed in Table 3. Before visible-light irradiation, the water contact angle on the uncoated FTO glass substrate was 59.40°. The contact angle of water on the TiO2 thin films before light irradiation decreased with the increase in the amount of CTAB added; this was due to the increase in the surface roughness of the TiO2 thin films with the increase in the CTAB concentration. As was seen in the SEM and AFM images, on increasing the amount of CTAB used, the structure of the thin films became increasingly porous, with the surface area also increasing. The film corresponding to R = 0 exhibited a very smooth surface. Consequently, the contact angle for

this film did not decrease considerably even after visible-light irradiation. For R = 1.0, a large number of pores were present between the particles in the synthesized film, increasing the available contact area. Therefore, owing to their porosity and surface roughness, which increased their surface area, the TiO2 thin films fabricated using CTAB exhibited hydrophilicity even in the absence of visible-light irradiation [24, 43-46]. The increase in the hydrophilicity of the TiO2 thin films fabricated using CTAB after visiblelight irradiation can be explained as follows. The light-induced holes in the valence band diffuse to the TiO2 surface and react with the adsorbed water molecules, forming hydroxyl radicals (•OH). The singly coordinated OH groups produced by light irradiation are thermodynamically less stable and have a high surface energy; this results in the formation of a superhydrophilic surface [47, 48]. Thus, the TiO2 films fabricated using CTAB exhibited rapid superhydrophilicity conversion after light irradiation because of the synergistic effects of the rutile and anatase particles during the reaction [36-38]. This improved charge-carrier separation, most likely owing to the trapping of the electrons in the rutile phase and the resulting reduction in electron–hole recombination [18]. Consequently, by increasing the amount of CTAB added, the reactions occurring in the TiO2 thin films under light irradiation can be enhanced and their hydrophilicity can be increased. The relationship between roughness and wettability was investigated by Wenzel, who stated that increasing the surface roughness enhances the wettability, which is related to the surface chemistry [49]. Recently, researchers have reported that materials having low contact angles can be converted into superwetting surfaces by the manipulation of their surface texture [50]. Wenzel proposed an equation to describe the effect of the surface roughness on the contact angle for homogeneous wetting: cos θ = r cos θY

(2)

where θ is the measured contact angle, θY is the Young contact angle of a flat surface, and r

is the roughness factor. The roughness factor is defined as the ratio of the total surface area to the projected area in the horizontal plane (r = 1 for a smooth surface and > 1 for a rough one) and can be used to calculate the area factor (Sdr). It is important to note that the Wenzel equation describes a homogeneous wetting regime wherein the droplet completely covers the rough surface and no air packets are present. It has been stated that, if the droplet is larger than the roughness scale by two to three orders of magnitude, the Wenzel equation applies. The area factor, Sdr, is expressed by Eq. (3). This parameter is especially useful in wettability studies, because it can be used to calculate the roughness factor using Eq. (4) [51]. 𝑆𝑑𝑟 =

( 𝑡𝑒𝑥𝑡𝑢𝑟𝑒𝑑 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎)−(𝑐𝑟𝑜𝑠𝑠−𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎) (𝑐𝑟𝑜𝑠𝑠−𝑠𝑒𝑐𝑡𝑖𝑜𝑛𝑎𝑙 𝑎𝑟𝑒𝑎)

r = 1 + Sdr/100

∗ 100%

(3)

(4)

The porosity of a surface has a very strong effect on the wettability characteristics of the surface and thus on the affinity of the material for liquids. Some of the liquid spontaneously penetrates the textured features on the porous material through capillary action, while the remaining liquid remains on the surface, forming a solid–liquid patchwork. The measured contact angle of such a system (liquid on top of a porous material) can be described by Eq. (5) [50], where fS is the fraction of the liquid in contact with the solid (1- fS is the fraction of the textured surface that is filled with the liquid and is equal to the porosity of the material). cos θ = fS (cos θY -1)+1

(5)

The values of r and Sdr as well as the textured surface area and porosity corresponding to the contact angle were calculated for the above-described TiO2 thin films using Eq. (2)-(5); the results are listed in Table 4. For the TiO2 films fabricated using CTAB, the porosity was significantly higher. The highest textured surface area and porosity in the case of these films were observed for the TiO2 film with R = 1.0, with the values being 48.47 cm2 and 96.03%,

respectively; these were more than eight times and four times higher than those of the pristine TiO2 film (5.60 cm2 and 21.16%, respectively). The increased porosity is attributable to the surface defects and higher surface roughness. These results confirmed that the roughness and textured surface area increased with an increase in the CTAB concentration. Furthermore, increases in the surface roughness and porosity not only enhanced the wettability but also increased the accessible surface area, which, in turn, increased contact with the reaction media and thus enhanced photocatalytic activity.

4. Conclusions Transparent and porous TiO2 thin films showing superhydrophilicity, visible-light photoactivity, high textured surface area, and high porosity were prepared by a simplified sol– gel method using CTAB. The results of SEM and XRD analyses showed that CTAB not only triggers the generation of the pores and a rutile phase, but also increases the roughness and absorption range of the thin films to the visible-light region. The results of PL spectral analyses showed that the number of surface defects and oxygen vacancies increased with an increase in the CTAB concentration, leading to a high recombination rate of free carriers and enhancing the PL emission intensity. Regarding the PEC properties, the thin film formed at R = 0 generated the highest photocurrent since it had a continuous and smooth surface, which aids carrier transfer. While the photocurrent efficiencies of the thin films containing CTAB were lower than that of the CTAB-free film, the photocurrent increased with an increase in the amount of CTAB used; this could be ascribed to the synergistic effects of the rutile and anatase particles during the PEC reaction, which resulted in an enhancement in the photoreactivity. Hence, TiO2 films fabricated using CTAB should find use in PEC water splitting, dyesensitized solar cells, and photocatalysis. Further, the results showed that the thin film with R=1 exhibited the highest photocatalytic activity, hydrophilicity, textured surface area, and

porosity, because it had the roughest surface and the highest number of surface defects. In addition, the increased hydrophilicity of the synthesized TiO2 thin films was correlated with the increase in the CTAB concentration and significantly affected the surface roughness and microstructure. Further, the TiO2 thin films fabricated using CTAB exhibited rapid superhydrophilicity conversion after light irradiation; this was also due to the synergistic effects of the rutile and anatase particles. Therefore, the addition of CTAB can significantly enhance the hydrophilicity of TiO2 thin films. The role of CTAB was clearly elucidated through a comprehensive comparative study, and the findings provide new insights into the competing effects of the surface morphology, electron–hole recombination rate, and crystal phase on the PEC performance of TiO2 thin films.

Acknowledgment This work was supported by the Ministry of Science and Technology of the Republic of China [grant number MOST 102-2221-E-151-057].

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Figure 1. XRD patterns of synthesized TiO2 thin films; patterns were referenced to JCPDS data (●: rutile).

Figure 2. FTIR spectra of synthesized TiO2 thin films.

Figure 3. FE-SEM images of synthesized TiO2 thin films: (a) R = 0, (b) R = 0.1, (c) R = 0.5, and (d) R = 1. Inset figures show magnified SEM image revealing pores and particles formed on film surface.

Figure 4. AFM images of synthesized TiO2 thin films: (a) R = 0, (b) R = 0.1, (c) R = 0.5, and (d) R = 1.

Figure 5. Comparison of transparencies (appearances) of FTO substrate and synthesized TiO2 thin films.

Figure 6. Optical (a) transmittance and (b) absorption spectra of synthesized TiO2 thin films.

Figure 7. (a) PL spectra of synthesized TiO2 thin films, and deconvoluted spectra of the films: (b) R = 0, (c) R = 0.1, (d) R = 0.5, and (e) R = 1.

Figure 8. Comparison of (a) current density vs. bias potential curves, (b) transient photocurrent responses, and (c) stabilities of photocurrent density of synthesized TiO2 thin films in 0.5 M Na2SO4 solution under visible-light irradiation at + 0.5 V (vs. Ag/AgCl).

Figure 9. Comparison of (a) photocatalytic degradation rates of MB and (b) variations in ln(C/C0) as function of irradiation time and corresponding linear fits for synthesized TiO2 thin films under visible-light irradiation.

Figure 10. Photographs of water contact angle measurements (i.e., static contact angles) of FTO substrate and synthesized TiO2 thin films before and after visible-light irradiation.

Table 1.

Phase contents and crystallite sizes of synthesized films anatase

Samples

Crystallite size

rutile Content

Crystallite size Content

a

(nm)a

(%)b

R=0

18.84

100

―

0

R=0.1

15.00

100

―

0

R=0.5

19.39

86.73

35.95

13.27

R=1

21.01

81.43

59.88

18.57

Calculated using Scherrer equation

b

(%)b

Calculated using Spurr-Myers method

(nm)a

Table 2. Basic physical parameters of synthesized TiO2 thin films Samples

Energy gap (eV)

Ra (nm)

kapp (h-1)

R=0

2.84

1.51

0.02

R=0.1 R=0.5 R=1.0

2.89 2.92 2.90

4.41 56.20 113.54

0.10 0.13 0.17

Table 3. Results of water contact angle measurements of FTO substrate and synthesized TiO2 thin films before and after visible-light irradiation Samples

0 min.

10 min.

20 min.

30 min.

FTO R=0 R=0.1 R=0.5

59.40° 52.20° 16.23° 14.90°

59.40° 26.40° 13.50° 2.95°

59.40° 25.00° 3.15° <1°

59.40° 17.40° <1° <1°

R=1.0

11.33°

1.60°

<1°

<1°

Table 4. Roughness factor (r), area factor (Sdr), textured surface area, and porosity values of synthesized TiO2 thin films Sample

Contact angle

r

(°)

Sdr

Textured surface

Porosity

(%)

area (cm2)

(%)

R=0 R=0.1 R=0.5

52.20 16.23 14.90

1.20 1.87 1.90

20 87 90

5.60 27.00 45.26

21.16 91.88 93.15

R=1.0

11.33

1.93

93

48.47

96.03