Vol.:(0123456789)1 3

J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7346-z

Preparation of superlong TiO2 nanotubes and reduced graphene oxide composite photocatalysts with enhanced photocatalytic performance under visible light irradiation

Sijia Lv1 · Junmin Wan1,2 · Yuewei Shen1 · Zhiwen Hu1 

Received: 8 May 2017 / Accepted: 10 June 2017 © Springer Science+Business Media, LLC 2017

1 Introduction

Since the 20th century, the continuous progress of science and technology and the rapid development of industry have brought the comfort and convenience to mankind, but also caused the pollution and deterioration of the environment, which have brought the potential threat to human health and life. Among all kinds of environmental pollution, the chemical pollution to be effectively controlled was the most influential and important. Now, the titanium dioxide (TiO2), which was employed for the degradation of toxic organic pollutants as a photocatalyst, has had broad application prospects because of its low cost and outstanding photo-catalytic activity [1]. Nevertheless, TNT was considered to be an excellent semiconductor photocatalyst because of its non-toxicity, high catalytic activity, good stability and low cost. What’s more, the TNT was a semiconductor material with the narrow absorption band between the valence and conduction band [2]. When the photon energy of irradia-tion light was equal to or greater than band gap of the TNT, the electrons (e−) of valence band would be excited into the conduction band across the gap, and the holes (h+) of valence band would arise, which were called electron–hole pairs. In the process of photocatalysis, the holes (h+) and electrons (e−) of the interface of water and TNT brought forth free radicals, which had strong chemical activity and could react with a variety of inorganic and organic pol-lutants. However, there was still serious limitation for the application of nano-TNT materials, because its band gap determined that it could only be excited by ultraviolet light. So the difficulty in recovery and low utilization ratio of vis-ible light restricted its practical application [2, 3].

Up to now, a great deal of research has been done to broaden the range of the absorption spectrum of TNT by means of doping, surface sensitization and complex

Abstract A novel composite photocatalyst, reduced gra-phene oxide (rGO) modified superlong TiO2 nanotubes (LTNTs) with length of about 500 nm, has been success-fully synthesized by improved hydrothermal process and heating reflux method. The prepared rGO–LTNT catalysts have been characterized and analyzed by transmission elec-tron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, powder X-ray dif-fraction (XRD), photoluminescence spectroscopy (PL) and electron paramagnetic resonance (EPR). The results from these investigations provided a deep insight into the physi-cal structure and chemical composition of the rGO–LTNT nanocomposites and pure LTNT. Furthermore, the photo-catalytic activity of rGO–LTNT nanocomposites for degra-dating methylene blue (MB) solution was evaluated under visible light irradiation. The obtained 1.5% rGO content of rGO–LTNT photocatalyst showed a purification of more than 50% MB in MB solution for an hour, which was about five times higher than that of the pure TNT. The results confirmed that the prepared rGO–LTNT nanocomposite photocatalysts showed excellent co-photocatalytic ability. That’s because rGO played a critical role in utilizing solar light and increasing separation of the electron–hole pairs more efficiently, and which greatly accelerated the decom-position of organic pollutants in waste water or air.

* Junmin Wan wwjm2001@126.com; wanjunmin@zstu.edu.cn

1 National Engineering Lab of Textile Fiber Materials & Processing Technology, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China

2 Engineering Research Center for Eco-Dyeing and Finishing of Textiles, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou 310018, People’s Republic of China

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structure [3]. In order to further improve the photocatalytic properties, scientists have made many explorations of TNT-based composites, such as the introduction of the reduced graphite oxide (rGO) [4, 5]. The rGO possessed high sur-face area, high chemical stability, great adsorptive property and efficient electrons and holes separation capacity. There-fore, the prepared rGO–TiO2 nanocomposites could pro-vide more active adsorption sites, and the catalytic activity of the catalytic reaction centers was significantly enhanced. In addition, the rGO–TiO2 composite catalysts were pre-pared with using four butyl titanate and graphene oxide (GO) by water hydrothermal method, which indicated that the addition of rGO not only improved the quantum effi-ciency, but also made the TiO2 absorption edge red shift, and increased the absorption of visible light [6–9].

Inspired by those discoveries, the GO modified LTNT with length of about 500  nm has been synthesized via a hydrothermal method, and the rGO–LTNT catalysts were applied to the photocatalytic degradation of MB solu-tion under visible light irradiation. The novel photocata-lysts have been investigated and analyzed by TEM, FT-IR, Roman, XRD, PL, XPS and EPR in order to systemati-cally study the physical structure and chemical composi-tion of rGO–LTNT and pure LTNT. The LTNT modified with rGO not only improved the quantum efficiency, and also made the LTNT absorption edge red shift, for a more accurate, increased the visible light absorption [5, 6, 10]. Thus, the rGO–LTNT has been successfully proposed for the enhanced photocatalytic activity and stability under vis-ible light irradiation based on the experimental results.

2 Experiment

2.1 Chemicals and materials

TiO2 nanoparticles (P25, d = ai.30  nm) were bought from EVONIK-DEGUSSA Co., Germany. Methylene blue, ana-lytical reagent grade quality, were purchased from Beijing Entrepreneur Science & Trading Co., China, and used with-out further purification. Natural graphite, Sodium hydrox-ide solution (NaOH), hydrogen nitrate (HNO3, 99.9%, AR), ethanol (C2H5OH, 99.7%, AR) and other chemicals were analytical reagent grade quality. They were used without further purification in the process of experiment. And the solutions were prepared with deionized (DI) water.

2.2 Preparation of GO

An improved Hummers’ method was used to synthesize GO. 1.0  g of natural graphite and 1.0  g of NaNO3 were slowly added into 46 mL of concentrated H2SO4 (18.4 M) with continuously stirring in an ice bath for 10 min. Then

8.0 g of KMnO4 was transferred to an ice bath until the sys-tem became a purple hybrid solution, again continuously stirring for another 20 min. Then, put the hybrid solution into a water bath at 40 °C with stirring for 80 min. The dark brown colored adhesive paste was diluted with the slow addition of 96 ml of DI water with continuously stirring for another 15 min. 10 M H2O2 (24 mL) was slowly injected to quench the solution, and yielded a golden–brown collosol. Then 50 mL of DI water was slowly added, and the result-ants were centrifuged at a speed of 8000 rpm, and washed repeatedly with DI water until the pH value of the filtrate was neutral. The dried product (GO) was obtained at 80 °C under vacuum condition for 24 h.

2.3 Preparation of LTNT and rGO–LTNT catalysts

The pure LTNT has been prepared as below: 0.1 g of P25 power was dispersed into 15 mL of NaOH solution (10 M) with continuously stirring for 5  min, and then transferred into 25  mL Teflon-lined stainless-steel autoclave with a magnetic stirrer. The autoclave was put inside a silicon oil bath and the reaction temperature was set at 130 °C for 24 h. The mechanical disturbance condition could be con-trolled by adjusting the stirring rates. After reaction, the autoclave was taken out from oil bath and cooled to room temperature. The product was collected by centrifugation, and washed with DI water for several times to reach a pH value of 9. Then, the wet centrifuged sodium titanate mate-rials were subjected to a hydrogen ion exchange process in a diluted HNO3 solution (0.1  M) for three times. Finally, the suspension was centrifuged again, and washed with DI water for several times until a pH value of 7 was reached, and then the hydrogen titanate nanotube materials were generated [7].

The 1% rGO of rGO–LTNT nanocomposites were pre-pared as the following: 15 mg of GO was put into DI water, under sonication for 1 h to be 1 mg/mL GO dispersion liq-uid. 1.5 g of as-made titanate nanosheet was added to the GO dispersion liquid. Then the mixture displayed a homo-geneous light gray color by full agitation. Then the suspen-sion was moved to a Teflon-lined stainless steel autoclave and maintained at 120 °C for 3 h. After natural cooling, the suspension was washed with DI water for several times. Finally, the product was dried in an oven at 50 °C for 12 h and grind into powder. The 0.5 and 1.5% rGO content of rGO–TNT nanocomposites were prepared by the same way (Scheme 1).

2.4 Characterizations

TEM is characterized by the Tecnai G2 instrument, which operates under 120  kV conditions. XRD characterized by Rigaku Ultima IV X ray diffraction at RT, and operated at

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40 kV and 44 mA condition, and scanned with a step length of 0.02 at 1° min−1 in the scope of 2θ = 10°–90°. Diffuse reflectance UV–Vis spectrum (DRS) was characterized by a Varian Cary-500 UV–Vis NIR spectrophotometer (USA). Photoluminescence (PL) spectra was carried out on Horiba Jobin Yvon-Fluoromax-4 equipment. The excitation wave-length was 325 nm. The emission was detected in the range of 400–700 nm. The EPR signals was acquired by A Bruker A300 spectrometer at RT under the following parameters: center field of 3400 G, sweep width of 2000 G, microwave frequency of 9.87 GHz, modulation frequency of 100 kHz and power of 20 mW.

2.5 Photocatalytic performance

Photocatalytic properties of the samples were investi-gated by degradation of the MB under visible light irra-diation. The photodegradation experiment was using by XPA-VII photocatalytic reactor. The 500 W Xe lamp light source can provide 390 nm cut-off filter. The average value of light intensity on the surface of the sample was about 650 mW cm−2. The reaction system maintained the temper-ature within 25° by the return water, which maintained the balance of the whole system. Reaction suspensions com-posed of 500 mL 10 mg/L MB solution and 50 mg catalysts was stirred with a magnetic rod for 30 min to complete the incorporation before the test the adsorption or photocata-lytic activity. In the intermittent time of 10  min, remove 4 mL from the suspension solution to test its adsorption or photocatalytic activity. Then, the photocatalysts were sepa-rated from the solution by centrifugation at 10,000 rpm for 10 min. Finally, the quantitative determination of the deg-radation of MB was determined by measuring the absorp-tion of UV–Vis at 665  nm. In addition, LTNTs was also tested as a reference.

3 Results and discussion

3.1 Structure and chemical composition analysis

From Fig.  1 a–c, the prepared LTNT showed a tubular structure with uniform size, the inside and outside diam-eters of the LTNT were around 25 and 50 nm, the length of

the LTNT was about 500 nm [10]. The samples possessed good crystallinity and smooth surface as a result of its good lattice stripes and arrangement. The results indicated that the length of the LTNT was longer than that of other TNTs prepared by hydrothermal method [10, 11]. However, the rGO have ever served as a substrate for the densely packed LTNTs because of its gauzelike morphology after reduced with LTNT, as shown in Fig. 1 d. In fact, the part of LTNTs spreading onto the interlayer of the rGO played a enormous role in photocatalysis, which would be reflected in the fol-lowing data [12–14].

FT-IR spectra of different samples were shown in Fig. 2. The two peaks at 3407 and 1620 cm−1 originated from the vibration of Ti–OH groups derived from LTNTs [15]. The spectrum of 1% rGO–LTNT and pure LTNT exhibited a broad and strong light absorption at 3407  cm−1compared with the rGO, which corresponded to the O–H stretch-ing vibration of the LTNT and rGO as the wide absorp-tion range from 3200 to 3600 cm−1 [14, 15]. The peaks at 1250  cm−1 showed that the skeletal vibration of C–O–C owning to the reaction between the hydroxide radicals from LTNT or rGO [18]. Simultaneously, the low frequency bands at around 750 cm−1 were owing to the Ti–O–C skel-etal vibration, and the sharp characteristic peak at approxi-mately 500 cm−1 was attributed to the skeletal vibration of the Ti–O group from LTNT [16, 17]. These phenomena suggested that the rGO–LTNT nanocomposites had an admirable photocatalytic enhancement under irradiation of IR light, and the bonding between them have been chemi-cally prepared [16–20]. Besides, according to these distinct peaks in the IR region, more electron–hole pairs would be efficiently separated, which can subsequently enhanced the photocatalytic activity with a better utilization of the vis-ible light irradiation.

The Raman spectra of the samples were showed in Fig. 3. Obviously, the typical optical modes of LTNT crys-talline phase could be observed as the Raman lines for B1g peak, A1g + B1g(2) peak, Eg(2) peak appeared. The D and G bands were the characteristic peaks of carbon crys-tal in Raman. The D band represented a defect in the car-bon atomic lattice, while the G band provided the in-plane vibration of sp2 bonded carbon atomic information [15]. Furthermore, the outstanding features of graphene were shown in D and G bands located at 1360 and 1601 cm−1, comparing the spectra of the GO with rGO–LTNT shown in Fig. 3. The low but major Raman bands at 209, 403 and 634 cm−1 could be conducive to the anatase phase of LTNT [14–19]. In conclusion, the two characteristic peaks of the rGO appeared and the overall shape of the spectra did not change, which indicated that the rGO–LTNT composite photocatalyst have been successfully prepared.

The XRD pattern was shown in Fig.  4. The LTNTs exhibited a series of pattern as that of the standard

GO

GO

GO

P25

rGO

TNT

for hydrothermal treatment

Scheme 1 Preparation of the LTNTs and rGO–LTNT samples

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reflections in 2θ = 25.3°, 27.4°, 36.1°, 37.8°, 41.5°, 48.1°, 53.9°, 64.1°, 69.0° and 73.1° (JCPDS, No. 21-1272 and JCPDS, No. 21-1276) [19]. The results revealed that there were many different crystal planes and its mixed crystal phase. Simultaneously, there were no characteristic peaks

of impurities to be detected, which meant its major crys-tal planes were well-structured [18]. In addition, the nar-row and sharp peaks in the rGO–LTNT composites sug-gested well conversion from the GO to the rGO and the connection between rGO and LTNTs [21, 22].

Fig. 1 TEM images of LTNTs (a, b, c) and rGO–LTNT (d)

Fig. 2 FT-IR patterns of the samplesFig. 3 Raman spectra of the samples

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In order to obtain accurate information about the sepa-ration and recombination of the photogenerated elec-tron–hole pairs, the PL spectra in Fig.  5 reflected the charge transfer at the heterojunction interface [7, 18, 20]. Compared with the LTNT, the fluorescence intensity of the rGO–TNT was strongly attenuated, and the optical absorp-tion was weakened. As could be observed from Fig. 5, the spectra of different samples looked similar to each other, which meant that the rGO in the modified light did not exhibit photoinduced changes. More specifically, the peaks of the rGO–TNT sample were weakened instead of that of P25. The results showed photoinduced interfacial charge transfer and electron–hole pair effective separation pro-duced at the interface of the junction, which played a key role in improving the photocatalytic activity under visible light irradiation. The results also have been proved by other characterization [22].

EPR spectroscopy was employed to distinguish the catalytic mechanism of this bioinspired catalytic system, especially to determine the chemical state of titanium [15]. From Fig. 6, we could see the P25 sample has no charac-teristic EPR resonances, but the peak values increased after being added with the rGO. The results confirmed the rGO played a vital role in enhancing the photocatalytic activity under visible light irradiation [18]. More specifically, the number of free radicals has been increased to some extent, and the recombination of electron–hole pairs has effec-tively decreased, which meant that the co-photocatalytic activity and decomposition of MB was further improved. In addition, the value g = 2.0146 was dissymmetrical, indi-cating a great generation of Ti4+ ions were reduced to Ti3+ during the photocatalytic reduction process owing to the efficient photoinduced interfacial charge transfer observed [8, 19–23, 26]. The Ti3+ ions formed during the photocata-lytic degradation made a great contribution to increase the photocatalytic degradation [26].

3.2 Oxidative removal of MB

There are three significant factors in the photocatalytic pro-cess, namely, the absorption range of light, the adsorption of organic pollutants and the excitation of electron and hole to produce various free radicals [24]. From Fig. 7, we could actually infer that pure LTNT showed rather poor photo-catalytic activity under visible light irradiation, comparing with 1.5% rGO–TNT. More specifically, the LTNTs caused only approximately 15% of the degradation of MB for 1 h, while the degradation rate of 1.5% rGO–TNT was more than 50% of the degradation of MB under the same experi-mental conditions, indicating that the catalytic effect of co-photocatalytic was obviously improved. In the one hand, the LTNTs with its large specific surface area embraced adsorption performance, as well as the rGO with the flaky

Fig. 4 XRD spectra of the samples (A Anatase, R Rutile)

Fig. 5 Photoluminescence spectra of the samples Fig. 6 EPR spectra of the samples

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structure. Moreover, the effective interaction between the rGO and LTNT played a crucial role in causing the MB loss in the solution [22–25]. In the other hand, MB molecules were aromatic structure, which could create a π–π stacking mutual effect with the rGO molecules, resulting from theirs special chemical structure. In conclusion, we confirmed that the rGO could form a steady chemical bridging bonds with LTNTs, which was favorable for the improvement of photocatalytic activity and absorption property for the chemical decomposition of organic pollutants [25].

Besides, the formation of Ti3+, the C–Ti bond, and other chemical bonds were contributed to photoinduced interfacial charge transfer [26, 27]. They further improved the separation of electron–hole pairs among the compos-ites, and more radicals would be formed during the pro-cess. Therefore, we could infer that the chemical structure between the rGO and LTNTs of those samples did not sol-idly established, but have achieved a perfect balance for the 1.5% rGO–TNT nanocomposites, which proposed that organic pollutant purification was efficient.

However, compared with 1.5% rGO–LTNT, the pho-tocatalytic capacity of 0.5% rGO–LTNT and 1.0% rGO–LTNT was weak as same as the pure LTNT. The decomposition of MB solution sharply increased with the content of the rGO reaching up to 1.5%. In 50 min, the dis-parity between the pure LTNT and the degradation of MB was the largest, the proportion was about 1:5 from Fig. 8. These phenomena verified that the amount of the samples used in the experiment was too little, and photocatalytic time was short for accurate observation. In 60 min, a sud-den increase on curve of the 1.5% rGO–TNT was because of the systematic error in the experiment, which could be negligible. Accordingly, following this trend, the above samples could be applied to the treatment of organic pollut-ants in wastewater and achieved well photocatalytic effect.

3.3 Mechanism in the catalytic system

In general, the degradation mechanism of the rGO–TNT was difficult to determine because the electron transport in the binary composites was more complex than that of pure LTNT. However, after being excited by visible light, LTNTs could effectively separate the electron–hole pairs [23]. The rGO sheets would receive electrons from the LTNT to increase the oxygen adsorption on the surface of the contact. Afterwards, photogenerated electrons could make dissolved O2 to form ·O2

−radicals or other radicals. In addition, the hydroxyl free radicals (·OH) was formed by photogenerated hole with water (or surface hydroxyl) [28, 29]. These radicals directly or indirectly oxidize organic pollutants into CO2 and H2O. The main reaction mechanism of photocatalytic deg-radation under visible light irradiation could be summarized by the following equations (1–9), which obviously led to the degradation of MB solution with enhanced photocatalytic performance under visible light irradiation. Furthermore, all the aforementioned characterization results indicated that the rGO in the composites could promote charge separation and enhance the photocatalytic activity (Fig. 9) [28–30]. As was confirmed that efficient heterogeneous interface charge trans-fer would occur; and the recombination of electron–hole pairs would decrease. Therefore, these active radicals had higher catalytic activity that would play a crucial role in reacting with organic pollutants to purify the waste water [30].

(1)TNT + hv → TNT(e− + h+)

(2)TNT(e−) + rGO → TNT + rGO(e−)

(3)TNT(h+) + OH → TNT + ⋅OH

(4)TNT(e−) + O2→ TNT + ⋅O

2

−

(5)rGO(e−) + O2→ rGO + ⋅O

2

−

Fig. 7 The absorption value of MB under visible light Fig. 8 The photocatalytic degradation of MB under visible light

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4 Conclusions

In summary, the different concentration of rGO–LTNT pho-tocatalysts via a facial hydrothermal method have been suc-cessfully proposed, and its photoinduced interfacial charge transfer and photocatalytic behavior were evaluated by dif-ferent characterizations. The nanosized LTNTs modified by rGO nanosheets as the photocatalysts had excellent ionic conductivity and admirable adsorption property, and could efficiently improve the separation of the electron–hole pairs [31]. Meanwhile, a series of rGO–LTNT semiconductor nanocomposites enhanced the photocatalytic activity and improved the stability and efficiency of visible light pho-tocatalyst with the synergistic effects between the rGO and LTNT materials [32, 33]. More specifically, the 1.5% rGO–LTNT with better photocatalytic performance was more stable than pure LTNT, which could act as an indica-tor for the treatment of organic pollutants and be applied to the quality control and analysis of wastewater. All in all, the proposed purification of organic pollutant was a green environmental protection technology for widely use [34].

Acknowledgements We acknowledge financial support from the public technology research plan of Zhejiang Province, Natural Sci-ence Foundation of Zhejiang Province (No. LY13B030009), Sup-ported by Zhejiang Provincial Top Key Academic Discipline of Chemical Engineering and Technology, Science Foundation of Zhe-jiang Sci–Tech University (ZSTU) (No. 1101820-Y) and National Natural Science Foundation of China (No. 21271155 and 20703038).

(6)⋅O2

− + H2O → HO

2⋅ +OH−

(7)⋅OH + Polluants → Degradation Products

(8)⋅O2

− + Polluants → Degradation Products

(9)HO2⋅ +Polluants → Degradation Products

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