Effects of Calcination Temperatures on Photocatalytic Activity of Ordered Titanate Nanoribbon/SnO2 Films Fabricated during an EPD Process

Ordered titanate nanoribbon (TNR)/SnO2 films were fabricated by electrophoretic deposition (EPD) process using hydrothermally prepared titanate nanoribbon as a precursor. The formation mechanism of ordered TNR film on the fluorine-doped SnO2 coated (FTO) glass was investigated by scanning electron microscopy (SEM). The effects of calcination temperatures on the phase structure and photocatalytic activity of ordered TNR/SnO2 films were investigated and discussed. The X-ray diffraction (XRD) results indicate that the phase transformation of titanate to anatase occurs at 400◦C and with increasing calcination temperature, the crystallization of anatase increases. At 600◦C, the nanoribbon morphology still hold and the TiO2/SnO2 film exhibits the highest photocatalytic activity due to the good crystallization, unique morphology, and efficient photogenerated charge carriers separation and transfer at the interface of TiO2 and SnO2.


Introduction
A large number of investigations have focused on the semiconductor photocatalyst for its applications in solar energy conversion and environmental purification since Fujishima and Honda discovered the photocatalytic splitting of water on the TiO 2 electrodes in 1972 [1][2][3][4][5][6][7][8][9].Among various oxide semiconductor photocatalysts, titania is a very important photocatalyst for its strong oxidizing power, nontoxicity, and long-term photostability [10][11][12][13].However, TiO 2 acting as a photocatalyst has an inherent and significant shortcoming: the fast recombination of the photogenerated charge carriers (hole-electron pairs).Thus, it is of great importance to reduce the recombination of photogenerated charge carriers in TiO 2 to enhance its photocatalytic activity for practical and commercial use.Coupling TiO 2 with other semiconductors can provide a beneficial solution for this drawback [14][15][16][17][18][19][20].For example, Tada et al. [14,16,17] and our previous work [19] conducted a systematic research on the SnO 2 as a cou-pled semiconductor and confirmed that the photogenerated electrons in the TiO 2 /SnO 2 system can accumulate on the SnO 2 and photogenerated holes can accumulate on the TiO 2 because of the formation of heterojunction at the TiO 2 /SnO 2 interface, which can result in lower recombination rate of photogenerated charge carriers and higher quantum efficiency and thus better photocatalytic activity.Moreover, conventional powder photocatalysts have serious drawbacks such as the need for posttreatment separation in a slurry system and their easy aggregation, resulting in the low photocatalytic activity [14].Therefore, the development of twodimensional (2D) film photocatalysts with efficient electronhole utilization and favorable recycling characteristics is a challenge for practical applications [21].
Various approaches, such as vacuum evaporation, sputtering, chemical vapor deposition, and sol-gel methods, have been contributed to the fabrication of 2D thin film.However, these approaches have some disadvantages for industry applications.Vacuum evaporation, sputtering, and chemical vapor deposition methods require special apparatuses for deposition of films, and sol-gel method needs coating repeatedly in order to get thick films.Recently, a versatile and facile method, electrophoretic deposition (EPD) method, was successfully utilized to fabricate thin film materials [19,20].The EPD method exhibits many advantages, such as higher deposition rate, reproducibility, and efficient control over thickness and morphology of the films through tuning the applied current or potential.Moreover, the EPD method can be used to deposit films on different shaped and sized substrates, which can be extended to a large scale and commercial applications.
Our recent work indicates that titanate nanotube films could be obtained by the EPD method [20].However, there is no report on the preparation of ordered titanate nanoribbon/SnO 2 films for photocatalytic application.Herein, we present a facile and effective approach for the preparation of ordered titanate nanoribbons/SnO 2 films by the EPD process using hydrothermally prepared titanate nanoribbon as a precursor.The formation mechanism of ordered titanate nanoribbons films during the EPD process was investigated by SEM results.Moreover, the effects of calcination temperatures on the phase structure and photocatalytic activity of ordered TNR/SnO 2 were investigated and discussed.

Experimental
2.1.Preparation of Titanate Nanoribbon.Titanate nanoribbon was synthesized by a hydrothermal method using commercial TiO 2 powder (P25, Degussa, Germany) as a starting material according to our previous reported method [22].In a typical preparation, 1.5 g P25 was mixed with 140 mL of 10 M NaOH solution followed by hydrothermal treatment of the mixture at 200 • C in a 200 mL Teflonlined autoclave for 48 h.After hydrothermal reaction, the precipitate was separated by filtration and washed with a 0.1 M HCl solution and distilled water until the pH value of the rinsing solution reached ca.6.5, approaching the pH value of the distilled water.The washed sample was dried in a vacuum oven at 60 • C for 8 h.

Preparation of
Ordered TNR/SnO 2 Films.The ordered TNR films were deposited on FTO glass (Sheet resistance 14-20 ohm/sq) using an EPD method.The electrolyte solution was obtained by adding 1.5 g titanate nanoribbons powder to 200 mL mixed solution of 60 mL ethanol and 140 mL distilled water and then ultrasonicated for 20 min.The pH value of the electrolyte was adjusted to about 9.0 by addition of tetra-methyl-ammonium hydroxide for controlling the surface charge of TNR.The isoelectric point of TNR was reported to be about 5.5 [23,24].Therefore, TNR in the pH 9 electrolyte solution had a negatively surface charge density and would be attracted to positive electrode.During the EPD, the cleaned FTO glass was kept at a positive potential while pure silver foil was used as the counter electrode.The linear distance between the two electrodes was about 4 cm.The applied voltage was 15 V.The coated substrates were rinsed with distilled water, dried in air, and calcined at 300, 400, 500, and 600 • C in air for 2 h, respectively.

Characterization. X-ray diffraction (XRD) patterns were
obtained on a D/MAX-RB X-ray diffractometer (Rigaku, Japan) using Cu Ka irradiation at a scan rate of 0.05 • 2θ s −1 and were used to determine the identity of any phase present.The accelerating voltage and the applied current were 15 kV and 20 mA, respectively.Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) analyses were conducted with an H-600 STEM/EDX PV9100 microscope, using 200 kV accelerating voltage.Morphology observation was performed on a JSM-6700F field emission scanning electron microscope (FESEM, JEOL, Japan).

Measurement of Photocatalytic Activity. Rhodamine B
(RhB), one of the N-containing dyes, which are resistant to biodegradation and direct photolysis, is a popular probe molecule in the heterogeneous photocatalysis reaction.It is often used as a tracer dye within water to determine the rate and direction of flow and transport.Rhodamine dyes fluoresce and can thus be detected easily and inexpensively with instruments called fluorometers.Rhodamine dyes are used extensively in biotechnology applications such as fluorescence microscopy, flow cytometry,and fluorescence correlation spectroscopy.In USA, RhB is suspected to be carcinogenic, and thus products containing it must contain a warning on its label.Therefore, we chose it as a model pollutant compound to evaluate the photocatalytic activity of the as-prepared calcined TNR/SnO 2 films [25].Photocatalytic activity of the calcined TNR/SnO 2 films was evaluated and compared by the photocatalytic decolorization of RhB aqueous solution at ambient temperature, as reported in the previous studies [25,26].The calcined TNR/SnO 2 films were settled in a 20 mL RhB aqueous solution with a concentration of 1.0 × 10 −5 mol L −1 in a dish with a diameter of 9.0 cm.A 15 W 365 nm UV lamp (Cole-Parmer Instrument Co.) was used as a light source.The absorbance of RhB at 554 nm was measured by a UV-Vis spectrophotometer (UV2550, Shimadzu, Japan).According to the Lambert-Beer Law [27,28], the absorbance (A) of RhB is proportional to its concentration (c), which generally followed the following equation: where ε is the molar absorption coefficient and b is the thickness of the absorption cell.In our experiment, all the testing parameters were kept constant, so the ε and b can be considered as a constant.Therefore, the changes of the concentration (c) of RhB aqueous solution can be determined by a UV-Vis spectrophotometer.As for the RhB aqueous solution with low concentration, its photocatalytic decolorization is a pseudo-first-order reaction and its kinetics may be expressed as follows [29]: where k is the apparent rate constant and A 0 and A t are the initial and reaction absorbance of RhB aqueous solution, respectively.

Morphology and Phase Structure of the As-Prepared TNR.
Figure 1(a) shows the SEM image of well-dispersed TNR with a length range from several micrometers to several tens of micrometers and a width of 30-300 nm.As shown in Figure 1(b), a typical image of belt-like structure was obtained, indicating a high aspect ratio (the width to the thickness) of 3-15 for the as-prepared TNR, similar to the previous reported work [30].Moreover, the XRD pattern of as-prepared TNR (Figure 1(d)) exhibited a feature similar to that of alkali or hydrogen titanates such as H 2 Ti 3 O 7 [31], Na x H 2−x Ti 3 O 7 [32], or Na y H 2−y Ti n O 2n+1 •xH 2 O [33] due to a similar structure of layered titanate family.TEM and HRTEM were further used to observe the morphology and microstructures of the as-prepared TNR.Figures 1(b) and 1(c) exhibit the TEM and HRTEM images of TNR, respectively.As shown in Figures 1(b) and 1(c), an obvious layered structure in the titanate nanoribbon could be observed, and the layer spacing was about 0.8 nm, corresponding to the diffraction peak located at ca. 11 • in Figure 1(b).Moreover, the selected area electron diffraction (SAED) pattern (inset in Figure 1(b)) also reveals that the titanate nanoribbon is single crystalline in structure according to the features of the diffraction pattern.

Formation Mechanism of the Ordered TNR Film
Deposited on FTO Glass.The formation mechanism of ordered TNR film is investigated by SEM.As shown in Figure 2(a), before deposition (0 min), FTO glass exhibits relatively rough surfaces, and the SnO 2 grains with size of 50-150 nm can be clearly observed.The clear grain boundary indicates that SnO 2 is well crystallized.After deposition for 1 min, a large amount of TNRs are randomly deposited on FTO glass (Figure 2(b)).As the deposition time increases, more TNRs are further deposited on the surface of substrate.Interestingly, as the deposition time increased to 3 min, ordered TNR film is obtained (Figure 2(c)).However, why the early formed TNR film appears disorder structure or morphology?This is ascribed to the fact that FTO film contains different sized grains, and thus its surface is not smooth.After deposition for certain time, the surface roughness of FTO film decreases, the electric field near the surface becomes uniform, especially, the negative charged TNRs repel each other, and their strong Brownian motion causes restructure and rearrange of deposited TNR [25].Therefore, it is not surprising that ordered TNR film can be easily obtained due to the synergistic effects of the above factors [34].along the same direction, which are parallel to the substrate, and the thickness of calcined TNR film is about 4 μm (Figure 4(b)).Moreover, Figure 4(b) shows that the SnO 2 film exhibits a thickness of about 500 nm.This SEM observation confirms the intimate contact between TiO 2 film and SnO 2 film, which could facilitate the interfacial transfer of photogenerated electrons and holes.

Photocatalytic Activity of Calcined TNR/SnO 2 Films.
The photocatalytic activity of the TNR/SnO 2 film calcined at various temperatures was evaluated by photocatalytic decolorization of RhB aqueous solution at room temperature under UV irradiation.However, under dark conditions without light illumination, the concentration of RhB almost does not change for every measurement in the presence of calcined TNR/SnO 2 film.Illumination in the absence of calcined TNR/SnO 2 film does not result in the photocatalytic decolorization of RhB.Therefore, the presence of both UV illumination and calcined TNR/SnO 2 film is necessary for the efficient degradation.Figure 5 shows the apparent rate constants (k) of TNR/SnO 2 film calcinated at various temperatures.It can be seen that the calcination temperature has a great effect on the photocatalytic activity of the TNR/SnO 2 films.The TNR/SnO 2 films calcined at 300 • C show weak photocatalytic activity probably due to absence or low crystallization of anatase phase in the calcined TNR film.However, when the calcination temperature increases to 400 • C, the calcined TNR/SnO 2 film shows a decent photocatalytic activity, and the corresponding k value reaches 2.8 × 10 −3 K/min.This can be attributed to the formation of anatase phase and the bilayer structures of TiO 2 /SnO 2 films in favor of separation of photogenerated charge carrier [14,[16][17][18][35][36][37]. Figure 6 show the charge separation and transfer mechanism of TiO As the temperature further increases, the photocatalytic activity of the calcined TNR/SnO 2 films increased obviously due to the enhancement of crystallization of anatase (Figure 3).At 600 • C, the highest photocatalytic activity is observed, and the k value is about 4.8 × 10 −3 K/min, which can be ascribed to good crystallization and the fast photogenerated charge carriers separation and transfer at the interface of the calcined TNR/SnO 2 films.
Figure 7 shows the change of absorption spectra of RhB aqueous solution during photocatalytic decolorization using the TNR/SnO 2 films calcined at 600 • C as the photocatalyst.It can be seen that the intensity of absorption peak gradually decreases with increasing UV irradiation time.After UV irradiation for ca.300 min, the intensity of absorption peak of RhB aqueous solution is very weak, and it becomes colorless, indicating that the calcined TNR films can completely decolorize RhB aqueous solution under UV irradiation.Therefore, the ordered TNR/SnO 2 films prepared by the EPD method could be useful for environmental protection such as air purification, water disinfection, and hazardous waste remediation due to their cheap preparation process, controllable structure, strong adhesion, and good photocatalytic activity.

Conclusions
We have successfully fabricated ordered TNR/SnO 2 via an EPD method using hydrothermally prepared TNR as a precursor.The formation mechanism of ordered TNR film on FTO glass substrate was investigated by SEM.The calcination temperature has a great effect on the phase structure and photocatalytic activity of TNR/SnO 2 films.When the calcination temperature increases to 600 • C, the highest photocatalytic activity was obtained on the calcined TNR/SnO 2 film due to the formation of well-crystallized anatase phase, the unique morphology, and the fast charge carrier separation and transfer at the interface of TiO 2 and SnO 2 .This ordered TNR/SnO 2 should also have many potential applications in photocatalysis, catalysis, solar cell, and so on.

3. 3 .
Phase Structure and Morphology of the Calcined TNR/SnO 2 Film.XRD was used to identify and determine the phase structures of the calcined TNR/SnO 2 film.Figure3shows the XRD patterns of the FTO substrate and the TNR/SnO 2 film calcined at various temperatures.As shown in Figure3(a), for pure FTO substrate, strong and sharp diffraction peaks can be observed, and all peaks are indexed to SnO 2 (space group: P4 2 /mnm (136); a = 4.750 Å, c = 3.198 Å, JCPDS no.46-1088).Figures 3(b)-3(e) show the XRD patterns of the TNR/SnO 2 film calcined at 300 to 600 • C. At 300 • C, it can be seen that only SnO 2 phase is identified in the calcined TNR/SnO 2 film and no other diffraction peaks are observed, indicating the amorphous states of TNR film.However, as the calcination temperature increases to 400 • C, a small peak at 2θ= 25.5 • appears, indicating the formation of anatase phase (space group: I4 1 /amd (141); a = 3.785 Å, c = 9.514 Å, JCPDS no.21-1272).With further increase in calcination temperature from 400 to 600 • C, the peak intensities of anatase increase, implying the enhancement of crystallization of the anatase phase and the growth of crystallites.Further observation on the morphology and microstructure of the calcined TNR/SnO 2 film was performed with SEM.Figures 4(a) and 4(b) show the surface and cross section SEM images of the TNR/SnO 2 film calcined at 600 • C for 2 hours.As shown in Figure 4(a), the calcined TNR still exhibits relatively uniform and ordered structure.The nanoribbons are densely packed and deposited almost

Figure 6 :
Figure 6: Schematic diagram of the charge-transfer process in the TiO 2 /SnO 2 films.
[19]37]2 films.Both TiO 2 and SnO 2 are n-type semiconductors with bandgap energies greater than 3.0 eV and strongly absorb UV light.Upon bandgap excitation, charge carriers (electron-hole pairs) are generated in each semiconductor film.The conduction band (CB) edges of anatase TiO 2 and SnO 2 are locatedFigure 7: Absorption changes of RhB aqueous solution at room temperature in the presence of the TNR/SnO 2 film calcined at 600 • C under UV irradiation.at−0.34 and +0.07 V versus normal hydrogen electrode (NHE) at pH 7, respectively.The valence band (VB) edge of SnO 2 (+3.67 V) is more positive than that of anatase TiO 2 (+2.87 V)[36,37].In terms of the energetics, electrons flow into the SnO 2 layer, while holes oppositely diffuse into the TiO 2 layer.Consequently, more holes reach the TiO 2 surface to cause oxidation reaction, whereas electrons are probably consumed for reduction of O 2 at the edge of the SnO 2 film.Therefore, the interfacial electrons transfer from TiO 2 to SnO 2 can explain the high photocatalytic activity of the TiO 2 /SnO 2 films.That is to say, a better charge separation in the composite film is enhanced by a fast electron-transfer process from the conduction band of TiO 2 to that of SnO 2 .Levy et al.[38]and Zhou et al.[19]also reported the same results that photoelectrons and holes transferred toward the reverse direction on the interface of TiO 2 /SnO 2 , resulting in a good photocatalytic activity.