Effect of Isopropanol on Microstructure and Activity of TiO 2 Films with Dominant { 001 } Facets for Photocatalytic Degradation of Bezafibrate

Titanium dioxide (TiO 2 ) films with dominant {001} facets coated on a titanium sheet (Ti) were synthesized with the simple hydrothermal method by using Ti as the precursor and substrate. The effect of addition of isopropanol into the hydrothermal solution on the structure, photocatalytic activity, and stability of as-synthesized TiO 2 films was investigated. The presence of isopropanol obviously influenced the microstructure of as-synthesized TiO 2 films, which was converted frommicrospheres into irregular close stack of truncated tetrahedrons. And the percentage of exposed {001} facets calculated from the Raman spectra increased from 48.2% to 57%. Accordingly, the TiO 2 films prepared with addition of isopropanol showed high and stable photocatalytic activity, which is nearly 2.6 times as that of the conventional P25 TiO 2 coated on Ti-substrate. In addition, the photocatalytic activity of as-synthesized TiO 2 films was greatly enhanced after calcination treatment at 600C, which can be attributed to removal of fluoride ions and organic residuals adsorbed on the surface of the catalyst. Photoluminescence (PL) technique was used for the detection of produced hydroxyl radicals (OH) on the surface of UV-illuminated TiO 2 using terephthalic acid as probe molecule. The photocatalytic degradation intermediates of bezafibrate were analyzed by an ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS), and accordingly the degradation pathways were proposed.

In addition to the {001} facets TiO 2 in the powder form, the immobilized films with exposed {001} facets were also reported in order to minimize the cost of separating powders from water after treatment and thus to make the process easy for practical applications.Wang et al. [17] for the first time fabricated the TiO 2 -film with exposed {001} facets by 2 International Journal of Photoenergy hydrothermal treatment of Ti-foil at 140 ∘ C in 40 mL of 0.01 M HF aqueous solution in a 80 mL Teflon lined stainless steel autoclave for 10 h.Zhang et al. [18] synthesized {001} facets TiO 2 crystals on titanium foil by hydrothermal treatment of titanium foil in a 60 mL of 0.5% (v/v) HF solution at 180 ∘ C for 3 h.Wu and Tang [19] fabricated {001} exposed TiO 2 film by immersing Ti-plate in a Teflon-lined autoclave for 1 to 24 h at 180 ∘ C. However to the best of our knowledge, there is little research on the role of isopropanol or other alcohols on the fabrication and photocatalytic stability of {001} facets exposed TiO 2 films immobilized on Ti-substrate.In this study, we found the presence of isopropanol in the hydrothermal solution makes {001} facets exposed TiO 2 films have sheet-like morphology, which possess higher activity and stability towards removal of organic contaminants as compared to TiO 2 microspheres with exposure of {001} facets.
Recently, pharmaceuticals and personal care products (PPCPS) have drawn increasing attention due to their pres ence in waters or soils [20].Bezafibrate, that is, 2-(4-{2-[(4chlorobenzoyl)amino]ethyl}phenoxy)-2-methylpropanoic acid, is a widely used lipid regulator drug throughout the world, and consequently it has been frequently detected in the environment [21,22].For example, in Germany this drug has been found in sewage treatment plants at a concentration of ∼4.6 g L −1 [21,22].In rivers and water streams, its concentration is nearly 0.35 g L −1 [22].In Italian rivers it has been found that bezafibrate concentration is ∼57.15 ng L −1 [23].In surface waters, this drug has been detected at median concentrations of ∼3.1 g L −1 [21].The removal of bezafibrate has been investigated using different advanced oxidation processes (AOPs) such as photo-Fenton [24], photodegradation using solar-simulated irradiation [25], ozonation, and UV/H 2 O 2 [26]; however no studies have been reported for the photocatalytic removal of bezafibrate by using TiO 2 films with dominant {001} facets.

Experimental Section
2.1.Synthesis of TiO 2 Films with Dominant {001} Facets.Tisheet (99.5% in purity, Baoji Shengrong Titanium Corporation, China), with a size of 180 mm × 120 mm × 0.15 mm, was first pretreated in an oxalic acid solution for 40 min at 100 ∘ C. The pretreated Ti-foil was then hydrothermally treated at 180 ∘ C in aqueous solution of 0.03 M HF in a Teflonlined autoclave with a total capacity of 160 mL.To study the role of isopropanol on the stability and formation of {001} facets, two different ratios of isopropanol were tried, that is, 0 and 50 mL, keeping all the other experimental conditions the same.After hydrothermal treatment, the obtained Tisheets with {001} facets were washed with distilled water for 3 times and then dried in air at room temperature.Finally, the Ti-sheets with {001} facets were calcined in air at 600 ∘ C for 90 min.For simplicity we named the two samples as S0 (without isopropanol) and S1 (with isopropanol).

Characterization.
The morphology of as-synthesized TiO 2 film was observed with an ultrahigh resolution fieldemission scanning electron microscope (FESEM, S-5500, Hitachi) performed at an accelerating voltage of 5.0 kV.The high resolution transmission electron microscopy (HRTEM) analysis was conducted using a JEM-2011F electron microscope (JEOL, Japan).To prepare the TEM specimen, the TiO 2 films on the Ti-sheet were first ultrasonically peeled in absolute ethanol for 40 min and then the suspension was dropped on carbon film supported on copper grid and dried in air before analysis.X-ray diffraction (XRD) analysis was carried out with a Rigaku D/max-RB using Cu K radiation ( = 0.15418 nm), operated at 40 kV and 100 mA.Chemical states of surface elements were investigated by Xray photoelectron spectroscopy (XPS, PHI-5300, ESCA) at a pass energy of 50 eV, using Al K as an exciting X-ray source.Raman spectra were recorded on an inVia Raman microscope (Renishaw) at the range of 100-800 cm −1 .The diffuse reflectance absorption spectra (DRS) of the samples were recorded on a UV-Vis spectrophotometer (Hitachi UV-3010) by using BaSO 4 as the reference standard.

Photocatalytic Procedures.
Photocatalytic activity was measured by taking 10 mg/L of bezafibrate (Sigma-Aldrich) solution in a 120 mL cylindrical vessel into which TiO 2 /Tisheet with dominant {001} facets was inserted.A 10 W lowpressure mercury lamp ( max = 254 nm) was used as the irradiation source.Oxygen gas was supplied from the bottom of the reactor at the flow rate of 30 mL/min.Aliquots were taken out every 15 min and the concentration of bezafibrate was determined with a high performance liquid chromatography (HPLC, Shimadzu, LC-10AD) with a UV detector (SPD-10AV) at 230 nm and a Kromasil C18 column (250 mm × 4.6 mm) for separation.The mobile phase was a mixture of methanol and water (70 : 30, v/v) at a flow rate of 1 mL min −1 .TiO 2 (Degussa P25) was coated on Ti-sheet by dipping the Ti-sheet 15 times in TiO 2 aqueous suspension (10 g/L) and then calcining at 400 ∘ C for 90 min.P25 coated Ti-sheet was adopted as the reference with which the photocatalytic activity under the same experimental conditions is compared.
To check the stability of as-synthesized TiO 2 films (after calcination at 600 ∘ C), the photocatalytic activity of S0 and S1 samples to degrade bezafibrate was continuously tested for 6 times, and each test lasted for 120 min.
Degradation intermediates of bezafibrate were identified by an ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS).The optimized mass parameters were as follows: capillary voltage, 2.1 kV; source temperature, 120 ∘ C; desolvation temperature, 280 ∘ C; and desolvation gas flow rate, 650 L/h.The parent ion, daughter ion, cone voltage (V), and collision energy (eV) for bezafibrate detection were 360.22 > 274.12, 22, and 15.In addition, the full scan mode was used to detect parent ions for acquiring more information about intermediates.Samples were directly injected into the mass spectrometer ion source by pure methanol with the cone voltage of 10 V.The reaction of terephthalic acid with produced • OH by means of UV-irradiation on the surface of {001} facets TiO 2 /Ti films gives the formation of a fluorescent product 2hydroxyterphthalic acid, whose PL intensity is in proportion to the formation of • OH in water [27].In this method a small sheet (2.5 cm × 1 cm) of our as-synthesized film was placed in a mixed aqueous solution of 2 × 10 −4 M terephthalic acid and 2 × 10 −3 M NaOH under the UV-irradiation of a 15 W 254 nm lamp.Then after every 15 mins of UV-irradiation, the PL spectrum of produced 2-hydroxy terephthalic acid was taken on Hitachi F-7000 fluorescence spectrophotometer.

Morphology and Phase Structures.
Figure 1 shows surface morphologies of samples S0 and S1.Truncated tetrahedrons with exposure of {001} facets can be clearly observed on the as-synthesized TiO 2 films.As shown in Figure 1(a), when no isopropanol was added in the hydrothermal solution, the TiO 2 films generally consisted of microspheres assembled with nanosheets, which is in agreement with the literatures [4,28].However, in the case of isopropanol addition, the TiO 2 films almost consisted of irregular stack of truncated tetrahedrons without formation of microspheres (Figure 1(b)).
Based on geometrical symmetry of anatase TiO 2 , the top and lateral surfaces are {001} and {101} facets, respectively.These results demonstrate that while keeping other hydrothermal conditions the same, the microstructure of TiO 2 film can be affected by addition of isopropanol into the hydrothermal mixture.Figures 1(c) and 1(d) show the cross-sectional SEM images of samples S0 and S1.The film thickness was ∼ 30 m without addition of isopropanol, while it was reduced to ∼0.8 m when isopropanol was added to hydrothermal solution.
Generally the surface of TiO 2 in aqueous media exists in one of the possible species, that is, TiOH 2 + , TiO − , or TiOH depending on the pH values [29].In the solution of 0.03 M HF with pH 2.3, nearly 94.8% of previously dominated TiOH would be replaced by TiF through ligand exchange between surface hydroxyl (-OH) groups on TiO 2 and fluoride ions (F − ) in the aqueous solution, as illustrated by It is well known that HF acts as a capping agent for the synthesis of anatase TiO 2 single crystal with {001} facets.In the presence of isopropanol along with HF, the isopropoxy ions (iPrO − , (CH 3 ) 2 CHO − ) will bind partially to Ti-atoms on the surface and accordingly more stable {001} facets are formed.Similar results were also reported by Yang et al. [6] where they synthesized single-crystal nanosheets with dominant {001} facets with a solvothermal method by using titanium tetrafluoride (TiF 4 ) as a precursor.
Solvents usually influence the growth and morphology of metal crystals due to difference in the viscosities of various  solvents [30].The present results show that an addition of isopropanol provided a good environment to strengthen the stabilization effects associated with fluorine adsorption on the {001} surface and thus enhanced its preferred growth and maximized the exposure of {001} facets.[9,17], respectively.The rutile character may be due to the Ti-substrate to which the TiO 2 film was directly attached [17].

Raman Spectra Analysis of {001} Facets TiO 2 Films.
Raman spectroscopy was used for surface characterization and accordingly the percentage of exposed {001} facets for samples S0 and S1 was calculated.Figure 4 shows that all the samples were of similar peaks appearing at 145.8, 191 cm −1 , 399, 520, and 634 cm −1 indicating the anatase TiO 2 phase [33,34].It can be noticed that as the percentage exposure of {001} facets increases, the intensity of   peak at 634 cm −1 reduced gradually, while the intensity of  1 peak at 399 cm −1 and  1 peak at 520 cm −1 increased simultaneously.It is well known that   peak mainly originates from stretching vibration of O-Ti-O in TiO 2 , the  1 peak is caused by symmetric bending vibration of O-Ti-O, and  1 peak is mainly due to antisymmetric bending vibration of O-Ti-O [33].The higher the percentage of exposed {001} facets, the less the number of stretching vibration modes of O-Ti-O, and consequently the intensity of   peak in the Raman spectra becomes decreased.On the other hand, when the exposed {001} facets become lowered, the number of symmetric bending vibrations and the antisymmetric bending vibrations of O-Ti-O increases.In the same way the higher the percentage of exposed {001} facets is, the more the number of symmetric bending vibrations and the antisymmetric bending vibrations of O-Ti-O will be, accordingly, and the intensity of  1 and  1 peaks in the Raman spectra becomes increased.
Based on the ratio of peak intensity of   at 145.8 cm −1 and peak intensity of  1 at 511.7 cm −1 the percentage of exposed {001} facets in samples S0 and S1 was calculated to be 48.2% and 57.0%, respectively, according to a well-established method reported by Tian et al. [33].
Furthermore, the appearance of five active Raman modes confirms the formation of anatase TiO 2 film and supports our previous statement that the rutile character in our XRD may be due to Ti-substrate on which the TiO 2 film is grown [17].On the basis of different detecting depths of XRD and Raman techniques, it can be suggested that the synthesized TiO 2 films may have layered structure, that is, Ti-foil, TiO  , rutile TiO 2 , and anatase TiO 2 with oriented {001} facets [17].Besides, slight shifting of   peaks (145.8 cm −1 and 634 cm −1 ) towards lower wave number was also observed for samples S0 and S1 which may be due to changes in the thickness of TiO 2 films or the increased particle size of TiO 2 films [35,36].

XPS Analysis.
To investigate the surface environment of as-synthesized {001} facets TiO 2 films, XPS spectrum survey was carried out for sample S1.As shown in Figure 5, before calcination treatment at 600 ∘ C, sharp photoelectron peaks appear at the binding energies (BE) 459.1 (Ti 2p 3/2 ), 464.9 (Ti 2p 1/2 ), 530.9 (O1s), 684.5 (F 1s), and 284.7 (C 1s) eV.The BE of Ti 2p 3/2 and Ti 2p 1/2 is slightly higher than the earlier reported values in fluorine terminated anatase TiO 2sheet powder [7].The F 1s peak at 684.5 eV is a typical value for the fluorinated TiO 2 , and no signal of fluorine in the lattice of TiO 2 (688.5 eV) is observed [6,27,37].As reported in literature [7], surface-bonded fluoride ions can be easily removed via being calcined at 600 ∘ C without affecting the crystal structure or morphology.It is confirmed in the present work that, as shown in Figure 5, F 1s peak at 684.5 eV was completely ruled out from as-synthesized {001} facets TiO 2 film after calcination treatment at 600 ∘ C. UV-irradiation times.It can be seen that a gradual increase in PL intensity is observed at 425 nm with increase in UV-irradiation times.When there was no UV-irradiation supplied to the terephthalic acid sample solution, then no PL peak was observed, which means that fluorescence is only caused by product (2-hydroxyterephtahlic acid) formed by the reaction of terephthalic acid with • OH radical.The PL results also suggest that • OH radicals are produced on the surface of {001} facets TiO 2 /Ti films when exposed to UVirradiation.

Photocatalytic Activity and Stability.
The photocatalytic activity of as-synthesized {001} facets TiO 2 films was examined by decomposing a pharmaceutical pollutant, that is, bezafibrate.were heat treated or not showed higher photocatalytic activity than P25, which indicates TiO 2 films with dominant {001} facets have higher activity.In addition, it can be observed that the addition of isopropanol in the hydrothermal mixture significantly affects the photocatalytic activity of as-synthesized samples.The photocatalytic decomposition of bezafibrate by TiO 2 films with dominant {001} facets follows pseudofirst order kinetics with a rate constant of 0.036 min −1 as shown in Figure 7(b).The sample S1 shows the highest activity with a rate constant of 0.0318 min −1 (removal ratio of 93.7% after calcination treatment at 600 ∘ C), nearly 2.6 times higher than that of P25 films coated on Ti-substrate.This can be attributed to the fact that the exposed percentage of {001} facets owned by sample S1 is higher than that of sample S0, as revealed from Raman spectra (Figure 4).
The role of fluorine in stabilizing {001} facets of anatase TiO 2 has been widely accepted.However, the influence of fluorine on the photocatalytic activity of anatase TiO 2 is still controversial.Some authors reported that it is good for enhancement of photocatalytic activity [16], while others have reported its negative impacts on photocatalytic activity [2].As shown in Figure 7(a), after removal of surfacebonded fluoride ions via calcination treatment at 600 ∘ C, both samples S1 and S0 show better activity.The reason for its negative impact on fluorine element may be partly due to the strong binding between fluoride ions (F − ) and Ti 4+ ions.In addition, the residual organic compounds on the surface of as-synthesized samples before calcination may also influence their photocatalytic activity.Figure 8 shows the diffuse reflectance UV-absorption spectrum of sample S1 before and after calcination.It can be seen that there is an obvious absorption tail at 400 nm before calcination which is probably due to the existence of colored organic compounds on the surface of TiO 2 films, while it almost disappears after calcination treatment at 600 ∘ C. Photocatalytic stability over a large period of time is an important aspect of developing a suitable material for an industrial photocatalysis process.Figure 9 shows that more than 90% of bezafibrate was degraded within 120 min by sample S1, which was synthesized with addition of isopropanol into the hydrothermal solution, and its activity remained almost the same during the six cycles.However, sample S0, which was synthesized without addition of isopropanol and had spherical morphology, degraded 80.3% of bezafibrate in the first run within 120 min.Furthermore, its activity gradually decreased in the subsequent runs, and only 65% of bezafibrate degraded at the sixth run.The deterioration of photocatalytic activity of sample S0 may be due to the loose

Run time
Presence of iso-PrOH Absence of iso-PrOH adherence of microspheres to the Ti-substrate, and the assynthesized TiO 2 microsphere films more easily peel from the Ti substrate.
3.6.Degradation Intermediates and Pathway.It has been observed that during advanced oxidation processes (AOPs), including TiO 2 photocatalysis, various degradation intermediates can be generated.The exact identification of all the degradation products (DPs) present in the sample solution is a difficult task, as sometimes the concentrations at which they are generated are too low to be identified.Semipolar and polar degradation products were analyzed by UPLC-MS/MS with the electrospray ionization mode.Figure 10 shows the total ion-chromatogram (TIC) of bezafibrate solution after being photocatalytically treated for 90 min by sample S1.The detailed examination of all the peaks present in the chromatogram gives the identification of bezafibrate DPs on the basis of their accurate mass measurements and the information provided by the system software, which calculates the elemental composition and empirical formula, generating a list of best-match hits.All the identified products are eluted before bezafibrate, except one (DP1), involving the formation of smaller and more polar products as compared to the parent compound.Table 1 shows data related to degradation products including their calculated and observed masses, chemical formula, and structures.
Various degradation products identified with UPLC-MS/MS were the consequence of • OH radical attack on the aromatic moiety.Since there are two benzene rings in the bezafibrate molecule, various hydroxylated intermediates were detected as summarized in Figure 11.There are two possibilities for the attack of • OH radical, either hydroxylation on the 4-cholorobenzoyl ring or hydroxylation of the phenoxy moiety [38].
Since • OH radical has strong electrophilic character, it will attack the carbon atoms with highest electron density, that is, the phenoxy ring.On the contrary, the chlorine containing ring has lower reactivity as compared to unchlorinated one due to the electron withdrawing effect of chlorine itself.So, on the basis of structural information of the target compound, • OH radical preferably attacks at the phenoxy ring and the attack at 4-chlorobezoyl moiety may be the minor route.
The reaction product with MW of 343 (DP2) results from oxidative dechlorination of bezafibrate molecule by • OH radical attack at the ipso-chloro position.Similarly, the product with MW of 275 (DP3) results from the attack of • OH at phenoxy side of bezafibrate molecule, which further gives hydroxy-substituted products with MW of 291 (DP4), 257 (DP5), and 138 (DP7).Degradation product with MW of 157 (DP6) shows the formation of 4-chlorobenzoic acid, which further favors decarboxylation instead of hydroxylation of the aromatic ring resulting in the formation of 4-cholorphenol (DP1) via the photo-Kolbe reaction mechanism [39].Further degradation of 4-chlorophenol results in the degradation products with MW of 144 (4-chlorocatechol, 4-chlororesorcinol, and hydroquinone) and then to hydroxyhydroquinone (DP10), which are in agreement with previous results [40].Formation of lactic acid (DP11) shows that target compound can be converted to short chain carboxylic acids via the breaking of the aromatic ring.

Conclusions
A highly stable and reactive anatase TiO  addition of isopropanol in the hydrothermal solution.Furthermore, the surface morphology changed from nanosheetassembled microspheres into irregular stack of truncated tetrahedrons, with the thickness decreasing from ∼30 m to <1 m.As-synthesized TiO 2 films show excellent photocatalytic activity, far exceeding that of commercially available P25 TiO 2 due to exposure of high percentage of {001} facets.The addition of isopropanol into the hydrothermal solution also plays a crucial role in enhancing the stability of as-synthesized {001} facets TiO 2 film.Different degradation products were identified by UPLC-MS/MS as the consequence of • OH radical attack on the aromatic moiety.The present study motivates us to further explore the role of alcohols in the synthesis of high energy {001} facets TiO 2 film for more promising applications.
Radicals.The production of hydroxyl radical ( • OH) on the surface of UV-irradiated {001} facets TiO 2 /Ti film was analyzed by photoluminescence technique (PL) using terephthalic acid as probe molecule.

Figure 2 :
Figure 2: (a) TEM image of single crystal anatase TiO 2 scratched from Ti-substrate of sample S1; (b) HR-TEM image of anatase TiO 2 single crystal recorded along {001} axis; the inset shows SAED pattern.

Figure 3 :
Figure 3: XRD patterns of the TiO 2 film synthesized with addition of isopropanol (before and after calcination treatment) and without addition of isopropanol.
Figure 2(a) shows the high resolution transmission electron microscopy (HR-TEM) image of sample S1.The large square surface of the nanosheets is the {001} crystal facet and the small isosceles trapezoidal surfaces correspond to {101} crystal facet.Figure 2(b) shows the lattice spacing of 0.221 nm, which corresponds to (001) atomic planes of anatase TiO 2 and thus confirms the formation of {001} facets TiO 2 .Selected area electron diffraction (SAED) pattern in the inset of Figure 2(b) confirms the single crystal nature of this TiO 2 particle and thus favors the migration of photogenerated electron-hole pairs [31, 32].The XRD patterns of samples S0 and S1 are shown in Figure 3.It is clear that the diffraction peaks at 2 = 25.22 ∘ , 36.82 ∘ , 37.67 ∘ , 38.54 ∘ , 47.94 ∘ , 53.79 ∘ , 55.02 ∘ , 62.64 ∘ , 68.72 ∘ , and 70.51 ∘ can be attributed to (101), (103), (004), (112), (200), (105), (211), (213), (204), and (110) of the standard card JCPDS 21-1272 corresponding to anatase, suggesting a high phase purity for the achieved titania.It is also clear from our XRD patterns before and after calcination treatment that there was no change in crystal structure after heat treatment at 600 ∘ C. The synthesized TiO 2 films consist of ca.83% anatase and 17% rutile.Besides TiO 2 , there are other diffraction peaks at 2 = 35.96∘ , 39.95 ∘ , and 52.92 ∘ which correspond to titanium (Ti) sheet (JCPDS number 05-0628).There is also a diffraction peak at 2 = 53.82∘ for sample S1 which belongs to TiO  .The appearance of Ti diffraction peaks in XRD of the TiO 2 films grown on Ti-substrate was also reported by Wang et al. and Alivov and Fan

Figure 7 :
Figure 7: (a) Comparison of photocatalytic activity of P25 TiO 2 with as-synthesized TiO 2 films (S0 and S1) before and after heat treatment at 600 ∘ C. (b) Degradation curve and kinetics of bezafibrate degradation by {001} facets TiO 2 film.

Figure 8 :
Figure 8: UV-Vis diffuse reflectance spectra of as-synthesized TiO 2 films before and after heat treatment at 600 ∘ C.

Figure 9 :
Figure 9: Photocatalytic stability of TiO 2 films synthesized with and without addition of isopropanol.

Figure 10 :
Figure 10: Total ion chromatogram (TIC) of bezafibrate sample after 90 min of UV-irradiation in the presence of {001} facets TiO 2 film (sample S1) showing the bezafibrate peak and its degradation intermediates.

2
film with dominant {001} facets can be synthesized by a simple hydrothermal technique in the presence of isopropanol.The percentage of exposed {001} facets increased from 48.2% to 57% with