He Jie; Shi Chaojie; Yang Zhengchun; Hou Qiang;Zhang Rui ; Zhu Tianjia; Pan Peng; Zhang Ping

(1. School of Integrated Circuit Science and Engineering, Adνanced Materials and Printed Electronics Center, Tianjin Key Laboratory of Film Electronic & Communication Deνices, Tianjin Uniνersity of Technology, Tianjin 300384;2. Department of Chemical Industry, SINOPEC Tianjin Company, Tianjin 300271;3. Department of Electronics, Nankai Uniνersity, Tianjin 300350;4. School of Electrical and Information Engineering and Key Laboratory of Adνanced Ceramics and Machining Technology of Ministry of Education, Tianjin Uniνersity, Tianjin 300072)

Abstract: Fluorine-doped tin oxide (FTO)/TiO2 seed layer/TiO2 nanorods were prepared by ion-beam deposition and hydrothermal methods. Under UV light, the photocurrent density of these nanorods was found to reach 1.39 mA/cm2, which was higher than that without the seed layer and nanorod structures. Furthermore, the FTO/TiO2 seed layer/TiO2 nanorods can also absorb visible light, overcoming a notable problem with standard TiO2. The photocurrent density of the FTO/TiO2 seed layer/TiO2 nanorods was found to reach 0.21 mA/cm2 under visible light. This high-performance results from the deposition of the TiO2 seed layer, which reduces the band gap of TiO2. The FTO/TiO2 seed layer/TiO2 nanorods also exhibited high photodegradation ability for the organic pollutant methylene blue (MB). Within 120 min, 77.3% of the MB was found to have been degraded, and the degradation rates remained almost unchanged after four cycles with the same catalyst sample. Additionally, compared with powdered photocatalysts, the FTO/TiO2 seed layer/TiO2 nanorod sample is easy to recover, requiring only rinsing with water and natural drying after the reaction.

Key words: TiO2 nanorods; photocatalysis; visible light; photodegradation

1 Introduction

Water pollution is a long-standing issue that has arisen alongside the development of global industrialization;it is a serious environmental problem and a threat to people’s health[1]. Organic pollutants in particular, such as organic dyes[2-3]and derivatives of petroleum[4-6], are a major cause of many public health problems. Among the various physicochemical techniques for removing organic contaminants from polluted aquatic environments,photocatalytic technology has drawn increasing research interest due to its high efficiency and low cost, and it shows great potential[7-8]. Furthermore, this technology does not require the additional consumption of raw materials and only requires solar energy, meaning it has become a competitive candidate because of its cleanness and sustainability[9]. Semiconductor photocatalytic materials can produce photogenerated carriers under the excitation of sunlight, and these photogenerated carriers can be used to generate different free radicals[10-12].The photogenerated carriers and free radicals can then jointly degrade organic pollutants. Additionally,due to the resulting potential difference between the two photoelectrodes, photogenerated electrons are spontaneously transferred to the cathode through an external circuit to generate electrical energy[13-15].

Titanium dioxide (TiO2) is the main representative of n-type metal-oxide photocatalyst, and it is one of the most used photoanode materials[16-17]. There are abundant reserves of TiO2on the earth, and it is non-toxic. It also has a high carrier mobility, high stability, and a suitable energy band position. Nevertheless, its wide band gap(~3.2 eV) means that it is unable to absorb visible light[18].In this work, fluorine-doped tin oxide (FTO)/TiO2seed layer/TiO2nanorods were prepared for photoelectric conversion and treatment of organic pollutants. It was found that the existence of the seed layer is conducive to the regular and dense growth of TiO2nanorods. More importantly, the structure of FTO/TiO2seed layer/TiO2nanorods narrows the band gap of TiO2, which helps it to absorb visible light and effectively overcomes the noted disadvantage of pure TiO2. These advantages further improve the photocatalytic activity of TiO2under visible light. The nanorods also exhibit a high photodegradation ability and have high stability and reusability with organic pollutants.

2 Experimental

2.1 Materials

Tetrabutyl titanate (C16H36O4Ti, 99.99%) was provided by Shanghai Macklin Biochemical Co., Ltd., China.Hydrochloric acid (HCl, 36.5% - 38.0% by weight) was provided by Tianjin Jiangtian Chemical Technology Co.,Ltd., China. A titanium dioxide sputtering target (TiO2,99.99%) was provided by Zhongnuo Advanced Material(Beijing) Technology Co., Ltd. FTO substrates were provided by Yingkou OPV New Energy Technology Co.,Ltd., China.

2.2 Synthesis of FTO/TiO2 seed layer

The FTO substrates were cleaned with ethanol and deionized water, and they were then put into an ion-beam deposition system for deposition of the TiO2seed layer.A mechanical pump and a molecular pump were engaged in turn, and when the vacuum level of the ion-beam sputtering chamber had reached 2.0 × 10?5Pa, ion-beam sputtering was performed. The flow rate of argon (Ar,99.9%) used for the ion-beam deposition was 3 cm3, the beam intensity was 25 mA, the acceleration voltage was 250 V, and the sputtering energy was 1000 eV. The FTO/TiO2seed layers were obtained after sputtering deposition for 40 min.

2.3 Synthesis of FTO/TiO2 seed layer/TiO2 nanorods and FTO/TiO2 nanorods

First, 30 mL HCl solution was mixed with 30 mL deionized water and stirred for 5 min. Then, 1 mL of tetrabutyl titanate was dropped into the mixed solution,and stirring was continued for a further 5 min. Next,the mixed solution was transferred to a 100-mL stainless-steel autoclave, with the surface of the TiO2seed layer deposited on the FTO placed obliquely downward in the chamber, which was then sealed and heated at 150 °C for 12 h. After the autoclave had cooled, the FTO/TiO2seed layer substrate on which the TiO2nanorod arrays were grown was taken out, and this was washed with deionized water several times.Finally, the naturally dried TiO2nanorod arrays were put into a tubular furnace and annealed in air at 500 °C for 5 h. The FTO/TiO2seed layer/TiO2nanorods were obtained by natural cooling after annealing. FTO/TiO2nanorods were prepared by the same method, but the FTO/TiO2seed layer was replaced with ordinary FTO.A schematic of the preparation process for the FTO/TiO2seed layer/TiO2nanorods is shown in Scheme 1,and the designated names of the different samples are listed in Table 1.

Table 1 Designated names of different samples

2.4 Characterization

An X-ray diffractometer (XRD; Ultima IV, Rigaku, Japan)with Cu Kα radiation was used to measure the crystal structures of the photocatalysts, and X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Scientific)was used to analyze the composition and valence states of the photocatalysts. Scanning electron microscopy (SEM;Verios 460L, FEI) was used to observe the morphology of the nanorod arrays. An ultraviolet-visible (UV-vis)spectrophotometer (UV-2600 PC, Shimadzu) was used to determine the optical absorption spectra.

2.5 Photoelectrochemical (PEC) measurements

On an electrochemical workstation (VersaSTAT 3,Princeton, USA), the photocurrent and the cyclic voltammetry (CV) curves were measured in a 0.5 mol/L Na2SO4electrolyte with pH 6.8 using the three-electrode method. In this system, T1, T2, and T3 were used as working electrodes, a Pt wire was used as the counter electrode, and an Ag/AgCl (3.5 mol/L KCl) electrode was the reference electrode. Chronoamperometry was used for the photocurrent tests. During these tests, a 100 mW/cm2Xe-arc lamp was used as the simulated light source,and the working electrodes were irradiated with light of different wavelengths. The photocurrent response and photocurrent can be obtained by testing under light and dark conditions at a constant potential. Linear Sweep Voltammetry (LSV) was also used in the tests of the working electrodes. The LSV tests were carried out under light and dark conditions, and the scanning range was from ?0.4 to +2.2 V vs. a reversible hydrogen electrode(RHE). CV tests were also carried out with scan rates of 20, 30, 40, 50, and 60 mV/s, and the voltage range was 0 - 0.1 V vs. RHE. Electrochemical impedance spectroscopy (EIS) was also conducted using the threeelectrode configuration. This was carried out under light and dark conditions in the frequency range 0.1 Hz to 100 kHz. The EIS tests can effectively evaluate the carrier transmission of the working electrodes.

2.6 Photodegradation performance measurements

In this work, the photodegradation performance of T3 was tested by photodegradation of organic pollutants. In the photodegradation experiments, a 100 mW/cm2Xearc lamp was again used as a UV light source, and a solution of the organic pollutant methylene blue (MB,C16H18N3ClS) was used as the target pollutant. During the tests, 4 mL MB solution was extracted every 20 min to measure the UV-Vis absorption spectrum (UV-5100B,Metash). By testing the intensity change of the strongest absorption peak, the photodegradation rate of the catalyst was calculated according to:

where,C0andCtare the initial concentration and the concentration at timet, respectively; andA0andAtare the initial absorbency and the absorbency at timet,respectively.

3 Results and Discussion

3.1 Characterization of photocatalysts

The crystal structure of TiO2nanorods was analyzed using XRD. It can be seen from the XRD spectra of T1,T2, and T3 shown in Figure 1(a) that these three samples all show a rutile-phase structure. The diffraction peaks for the TiO2are located at 2θof 27.1°, 36.5°, 41.7°, 54.8°,61.9°, 63.3°, and 69.3°, corresponding to the (110), (101),(111), (211), (002), (310), and (301) crystal planes of TiO2(JCPDS, 73-2224), respectively[19]. The remaining diffraction peaks at 2θof 24.7°, 34.2°, 38.1°, 51.9°, 65.8°,and 78.4° correspond to the FTO substrate.

XPS was used to characterize the oxidation state of the Ti.In the Ti 2p spectrum in Figure 1(c), the two peaks with binding energies of 464.1 and 458.4 eV are attributed to 2p1/2and 2p3/2peaks of Ti4+in TiO2, respectively, and the satellite peak located at 471.9 eV also represents Ti4+,indicating the same chemical state of Ti atoms in the sample[20]. In the O 1s spectrum of Figure 1(d), two peaks can be observed at 529.6 and 531.5 eV, and these are attributed to lattice oxygen in TiO2and adsorbed water[21-22].Figure 2(a)-(f) show SEM images of T1, T2, and T3.For sample T1 (Figure 2(a) and (b)), the TiO2seed layer presents a film-like morphology. It can be seen from the cross-sectional and top views in Figure 2(c) and (e) that T2 forms a regular nanorod structure. However, from the optical photograph of T2 in Figure 1(b), large numbers of pinholes can be seen, indicating that the nanorods did not form dense growth for without the seed layer. For T3 (Figure 1(b) and Figure 2(d) and (f)), it can be seen that the TiO2nanorods grow densely and have a regular quadrilateral structure. The side length of the TiO2section in T3 is about 100 nm, and its cross-sectional area is larger than that of T2.

Figure 1 (a) XRD spectra and (b) optical photograph of T1, T2, and T3; (c) Ti 2p and (d) O 1s XPS spectra of TiO2 nanorods

Figure 2 (a) Cross-sectional and (b) top views of T1; cross-sectional views of (c) T2 and (d) T3; top views of (e) T2 and (f) T3

3.2 PEC performance under UV light

The PEC test is an important way to evaluate the performance of a catalyst. By testing the turn-on voltage and photocurrent response, the separation and transfer kinetics of photogenerated carriers in the photocatalytic process can be revealed[23]. In this work, LSV, EIS, and photocurrent response tests of T1, T2, and T3 under UV light were carried out to examine the effect of the seed layer and the nanorods on photocatalytic performance.

Figure 3(a) shows the T1, T2, and T3 photocurrent response curves corresponding to on-off cycling of UV light under a constant bias of 1 V vs. RHE. It can be seen that T2 and T3 have stable and rapid photocurrent generation during the UV illumination on-off cycles.The photocurrent densities of T1, T2, and T3 at 1 V vs.RHE were found to be approximately 0.002, 0.120, and 0.460 mA/cm2, respectively. This indicates that, due to the large specific surface area of the TiO2nanorods,T2 and T3 have a better light-trapping effect and good contact area, which greatly improves the photoelectric conversion efficiency. Moreover, compared with T2, the deposition of the seed layer in T3 increases the number of active sites for the photocatalytic reaction, speeds up the carrier transmission efficiency, and further improves the photocurrent density.

In addition to the transient photocurrent, EIS can provide information about the electrical conductivity and interface properties of the photoelectrodes. The arc radius on an EIS Nyquist plot reflects the reaction rate occurring on the surface of a photoelectrode; a smaller arc radius represents a lower charge-transfer resistance and stronger photocatalytic performance[10,16]. As shown in Figure 3(b),EIS tests of T1, T2, and T3 were carried out under dark and UV irradiation conditions. Compared with the other photoelectrodes, the seed layer and nanorod structure of T3 resulted in a significant reduction in the arc radius of the semicircular Nyquist plot. This demonstrates that the electron-transfer efficiency between T3 and the electrolyte in the photocatalytic process was significantly increased[24]. The EIS results further confirm that thesimultaneous presence of the seed layer and the nanorod structure is important for prolonging the charge-carrier life and suppressing e?/h+pair recombination, which can improve the performance of the photoelectrode[25].

Figure 3(c) shows the transient current response test curves of T3 when different voltages were applied under UV light. The photocurrent densities of T3 at 1.0,1.6, and 2.2 V vs. RHE were 0.46, 0.72, and 1.39 mA/cm2, respectively. This is mainly because increasing the external voltage can accelerate the transfer speed of the carriers, reduce the probability of internal recombination of photogenerated carriers, and improve the current density of the photoelectrode[26].

LSV tests were performed to evaluate the photocurrent density and the turn-on voltage of the photoelectrodes.Figure 3(d) shows the LSV test curves of T2 and T3 in 0.5 mol/L Na2SO4electrolyte, and the inset shows a comparison of the opening voltages of the two photoelectrodes. It can be seen that the current density of both photoelectrodes was almost 0 in the dark. When UV light was applied, the turn-on voltage of T2 was +0.42 V vs. RHE; however, the turn-on voltage of T3 moved notably in the negative direction, reaching +0.33 V vs.RHE. This movement is mainly due to the deposition of the seed layer, which increases the absorption of light and thus results in the production of more carriers, leading to higher photocatalytic efficiency[27,28].

3.3 PEC performance under visible light

Transient current response tests under visible light were carried out to further understand the photocatalytic performance of T2 and T3. As shown in Figure 4(a), the photocurrent densities of T2 and T3 under visible light were found to be 0.02 and 0.11 mA/cm2, respectively.With the deposition of the TiO2seed layer, the photocatalytic performance of T3 under visible light is much higher than that of T2. Moreover, as shown in Figure 4(b), the greater the applied voltage, the higher the photocurrent density[29].

To reveal the origin of the high photocatalytic efficiency of T3, the optical properties and band gaps of T1, T2, and T3 were studied by analyzing their absorption spectra[30],as shown in Figure 5(a). It can be seen that, due to the nanorod structure having a large specific surface area and a light-trapping effect, the absorption spectra of T2 and T3 are effectively broadened, allowing them to produce a response to visible light. Furthermore, in the visiblelight wavelength range, the absorption intensity of T3 was found to be higher than that of T2, and it can be seen from the band-gap curves fitted in Figure 5(b) that the band gap of T3 is 2.9 eV, which is smaller than 3.1 eV of T2. Taken together, these results show that T3 exhibits stronger photocatalytic activity under visible-light irradiation[31].

To examine the active sites of T3 and T2, their CV curves were obtained at different scan rates (Figure 6). The electrochemically active surface areas (ESCAs) were determined by measuring the double-layer capacitance according to the CV curves. As shown in Figure 7, the ESCA for T3 was higher than that of T2. The increased ESCA indicates that there are a large number of active sites at the surface of T3[32].

Figure 4 (a) Transient current response test curves of T2 and T3 under visible light and 1 V vs. RHE; (b) transient current response test curves of T3 at different voltages under visible light

Figure 5 (a) Absorption spectra of different samples; (b) fitting curves of absorption spectra

Figure 6 CV results for (a) T2 and (b) T3 at different scan rates

Figure 7 Charging current density differences plotted against scan rate

3.4 Photodegradation performance and stability

Photodegradation of organic pollutants is a key step in the field of photocatalysis[33]. In addition to good photoelectric conversion performance, T3 also has strong photodegradation ability. MB is a serious industrial pollution source, among which azo compounds and aromatic amines are known to have carcinogenic and teratogenic properties[34]. In this work, MB was used as an example organic pollutant to establish the photodegradation ability of T3. Figure 8(a) shows absorption spectra of 10 mg/L MB after different UV irradiation times in the presence of T3. It can be seen that MB has its strongest absorption peak at 622 nm, and the peak intensity decreases with increasing illumination time,indicating that MB is effectively degraded. Figure 8(b)shows degradation curves of MB by T3 after repeated uses of the catalyst. After 120 min of UV irradiation,77.3% of the MB was degraded. The degradation rates remained almost unchanged in the three subsequent stability experiments, i.e., 73.8%, 73.6%, and 73.4%,respectively. Furthermore, when compared with the use of nanoparticles, it is much easier to recover the T3 catalyst;T3 can be reused by simply rinsing it with water several times and drying it naturally after the reaction.

Figure 8 (a) Absorption spectra of photodegradation of MB by T3 after different times under UV light; (b) cyclicphotodegradation curves of MB under UV light

4 Conclusions

In summary, FTO/TiO2seed layer/TiO2nanorods with excellent photocatalytic properties were prepared by ion-beam deposition and hydrothermal methods.The experimental and analytical results show that the nanorods based on the seed layer exhibit stronger photocatalytic performance under UV light than the other photoelectrodes. Furthermore, because of their narrow band gap, they also exhibit high photocatalytic activity under visible light. In addition, FTO/TiO2seed layer/TiO2nanorods can effectively and stably photodegrade organic pollutants. This work not only provides a method to enhance the response of TiO2to visible light, but it also provides a feasible approach to efficient treatment of organic pollutants in water.

Acknowledgements:This work was supported by Tianjin Natural Science Foundation (Grant No. 19JCQNJC06200),Major Projects of Science and Technology in Tianjin (Grant No.18ZXJMTG00020).