高光催化性能的二氧化鈦納米管
—氧化石墨烯雜化材料

賴　奇，　羅學(xué)萍，　朱世富

(1.攀枝花學(xué)院 材料工程學(xué)院 攀枝花石墨烯工程技術(shù)研究中心，四川 攀枝花617000；2.攀枝花學(xué)院 四川省石墨深加工高校重點(diǎn)實(shí)驗(yàn)室，四川 攀枝花617000；3.四川大學(xué) 材料科學(xué)與工程學(xué)院，四川 成都610065)

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高光催化性能的二氧化鈦納米管
—氧化石墨烯雜化材料

賴奇1，2，羅學(xué)萍2，朱世富3

(1.攀枝花學(xué)院 材料工程學(xué)院 攀枝花石墨烯工程技術(shù)研究中心，四川 攀枝花617000；2.攀枝花學(xué)院 四川省石墨深加工高校重點(diǎn)實(shí)驗(yàn)室，四川 攀枝花617000；3.四川大學(xué) 材料科學(xué)與工程學(xué)院，四川 成都610065)

摘要：以金紅石型TiO2、氧化石墨烯和氫氧化鈉為反應(yīng)劑，通過水熱合成法制備了TiO2納米管-氧化石墨烯復(fù)合材料，并研究復(fù)合材料制備過程中水熱溫度、煅燒溫度對制備材料結(jié)構(gòu)性能的影響關(guān)系。結(jié)果發(fā)現(xiàn)， TiO2納米管-氧化石墨烯復(fù)合材料比純TiO2納米管具有更好的光催化性能。石墨烯有利于TiO2納米管的形成，而且存在一個(gè)優(yōu)化的加強(qiáng)光催化性性能的煅燒溫度。TiO2納米管-氧化石墨烯復(fù)合材料在煅燒過程中，氧化石墨烯的還原與逐漸損失同TiO2納米管的結(jié)晶動態(tài)平衡，可能是其中存在優(yōu)化煅燒溫度的原因。同時(shí)分析了TiO2納米管-氧化石墨烯復(fù)合新材料的復(fù)合機(jī)理。

關(guān)鍵詞：納米結(jié)構(gòu)； 表面； 電子顯微； 光電子能譜； 光學(xué)材料

English edition available online ScienceDirect ( http:www.sciencedirect.comsciencejournal18725805 ).

1Introduction

Titania (TiO2) is a wide band-gap semiconductor possessing specific chemical properties and a high stability. The band gap of anatase TiO2～3.2 eV enables it photocatalytically active under UV irradiation. Among TiO2nanoparticles[1], TiO2nanotubes (TiO2NTs) are known to be effective photocatalysts in the degradation of environmental contaminants. TiO2NTs have high specific surface areas and improved photocatalytic performances compared with bare TiO2[2, 3]. TiO2NTs can be synthesized via various methods, including electrochemical anodization, sol-gel and hydrothermal methods[4, 5]. In comparison with other techniques, hydrothermal synthesis is an inexpensive and environment-friendly method with the ability to control chemical composition and morphology of the synthesized products. The synthesis conditions affect the phase composition and morphology of hydrothermally synthesized TiO2.

A series of strategies have been developed to synthesize TiO2NT based nanocomposites for the inhibition of high intrinsic electron-hole pair recombination as well as a further modification of the band gap of the composite, such as doping with quantum dots, semiconductors and carbon materials. In particular, there is a growing interest in the combination of carbon based materials and TiO2to enhance photocatalytic performance. Graphene is one of the carbon nanomaterials, which is unique and has been intensely studied because of its excellent conductivity and high transmittance[6，7]. Previous reports have shown that the addition of graphene improves the supercapacitor performance of MnO2[8], photocatalytic performance of ZnO[9], et al[10]. Especially, graphene improves the photocatalytic activity of TiO2[11，12], and enhances energy conversion performance in homogeneous and heterogeneous semiconductors, because of its effective electron transfer and interaction effects[13，14].

There have been several reports highlighting the improvements in the photocatalytic activity of TiO2NT-reduced GO for the degradation of organic molecules and photocatalytic splitting of water. However, TiO2nanoparticles tend to agglomerate and have poor interfacial contact with the graphene surface because of the nanoparticle’s nearly spherical shape. And their formation process and mechanism is not well elaborated. In this paper, we present a practical hydrothermal reaction technique, in which TiO2NTs self-assemble on GO. The self-assembly of TiO2NTs on GO leads to a high interfacial contact and offers potential electron-transfer capabilities that improve the photocatalytic properties of TiO2.

2Experimental

2.1Synthesis of GO

GO was synthesized from flake graphite (Pingdu BEISHU Graphite Co. Ltd., Shandong Province, China) using a modified Hummers method[15]. First, 10 g natural graphite was put in a flask (500 mL), then concentrated sulfuric acid (300 mL) was added slowly while the flask was placed in an ice water bath. After ten min, 45 g potassium permanganate was gradually added into the flask over a period of 1 h and the mixture was stirred for another 4 h in the ice water bath. Then 5 wt% H2O2(150 mL) was added within 30 min. The final suspension was filtered to collect solid product that was washed with deionized water until a pH value of 6-7 was obtained. The solid product was dried for 24 h at 80 ℃. The resulting graphite oxide product was sonicated for 20 min in distilled water, centrifuged at 4 000 r/min for 20 min to produce a GO aqueous dispersion (0.5 mg/mL). Using this method, we previously showed that the synthesized GO consists of mainly 1-4 layers[16].

2.2Synthesis of TiO2NT-GO hybrid

38 g NaOH was added to the GO solution (0.5 mg/mL, 62 mL) and heated until the NaOH was dissolved. Rutile-phase TiO2was added and after the solution was stirred for 5 min. The resulting solution was poured into a 100 mL Nalgene flask and stored in an oven at 130 ℃ for 40 h. The resulting dehydrated powder was put in a HCl aqueous solution (0.1 mol/L) for 10 h and washed with distilled water to a pH value of 6-7. Then the powder was dehydrated and heated at 60 ℃ for 24 h. The resulting product was named as TiO2NT-GO. TiO2NT-GO was calcined at different temperatures in a furnace for 1 h at a heating rate of 1 ℃/min. Bare TiO2NTs were prepared using the same procedure without adding the GO solution.

2.3Characterization

The structures of the samples were observed using a scanning electron microscope (SEM, Model S4800, Hitachi) and a high-resolution transmission electron microscope (TEM, Model Tecnai G2F20, FEI), at 15±25 kV and 200 kV acceleration voltages, respectively. For the TEM observations, the samples were ultrasonicated in ethanol and then placed on a copper grid. X-ray photoelectron spectroscopic (XPS) measurements were taken using a Quantum 2000 Scanning ESCA Microprobe (Physical Electronics) using AlKαradiation (1486.6 eV). In the XPS data analysis, peak deconvolution was performed using Gaussian components after a Shirley background subtraction. Solution absorbance was measured using a ultraviolet-visible (UV-vis) spectroscope.

2.4Spectroscopy and photocatalytic reactions

Photocatalytic degradation of methylene blue (MB) was carried out at ambient temperature in a homemade photocatalytic reactor. The UV-light source was a commercial 8W black-light tube with a spectral peak at 365 nm. 300 mL methylene blue solution (20 mg/L) and 0.05 g photocatalyst were fed into the reactor, then air was blown into the reactor from the bottom via a gas distributor at a flowrate of 4 L/min. After 30 min of premixing, the reactants were irradiated for 80 min. Samples (10 mL), taken before and after irradiation were centrifuged and supernatants were analyzed by recording the absorption peak of methylene blue at 666 nm. The intensity of the main absorption peak was converted to the residual dye concentration (C).

3Results and discussion

Fig. 1 shows the SEM images of the raw material of rutile-phase TiO2(Fig. 1(a)), intermediate product (Fig. 1(b)) obtained at 130 ℃ for 20 h through the alkali treatments and acid treatments，and TiO2NT-GO hybrid (Fig. 1(c)). The structure of intermediate product, is markedly different from that of the raw material, more specifically, the nanoparticles present in the raw material are no longer observed and became plate shaped. In Fig. 1(c) of the TiO2NT-GO hybrid, needle-shaped crystals with a length of ～2 μm were observed in a large amount, which are TiO2NTs. After calcination at 550 ℃, the TiO2NT-GO hybrid is gradually changed into granular (Fig. 1(d)). In order to compare the effect of GO on the growth of TiO2NTs, GO solution without centrifugation was used to prepare another sample (Fig. 1(e)). From SEM image, we cannot find TiO2NTs for this sample. We also find the surface of layers are cracked, this is possibly due to the fact that the graphite is too thick to curl up. This suggests that GO can facilitate the formation of TiO2NTs.

Fig. 1　SEM images of (a) raw material, TiO2 NT-GO at various reaction time of

The N2adsorption and desorption isotherms of the samples were determined at -196 ℃ using an adsorption apparatus. Fig. 2(a) shows the N2adsorption and desorption isotherms of the TiO2NTs calcined at 60 ℃. The sample displays isotherms of type IV according to the IUPAC classification, which is associated with capillary condensation in mesopores. And a hysteresis loop appearing atp/p0> 0.50 was most likely to be attributed to the mesopores formed by aggregation of TiO2NTs. TiO2NT-GO samples calcined at 60 ℃ and 350 ℃ in Fig. 2(b) and 2(c) show the similar isotherms to the TiO2NTs. But their hysteresis loops are different. TiO2NT sample displays a type H1 among the five types of hysteresis loop as IUPAC classification. But TiO2NT-GO sample displays a type H4 loop. And the adsorption step of TiO2NT-GO sample is shifted to a low pressure region ofp/p0< 0.45. With the temperature was increased to 550 ℃, TiO2NT-GO sample shows a hysteresis loop of type H1 in Fig. 2(d).

The type H4 loop is given by TiO2NTs because of their mesopore structures. Due to the addition of graphene oxide, narrow slit-like pores are introduced into the hybrid. As a result, the type H4 loop appears. The BET surface area of the TiO2NT-GO hybrid calcined at 60 ℃ is increased to 356 m2/g upon the deposition of TiO2NTs (212 m2/g). With the temperature increased to 350 ℃ and 550 ℃, BET surface areas of the TiO2NT-GO were decreased to the 165 and 58 m2/g, respectively. The BET surface area of the TiO2NT-GO without centrifugation is 145 m2/g. Both GO addition and calcination changes the pore size of the TiO2NT-GO hybrid, but calcination plays a key role.

TEM images of GO, TiO2NT-GO hybrid, and bare TiO2NTs are shown in Fig. 3. The GO image shows that its planar size is larger than 4 μm2(Fig. 3(a)). In Fig. 3(b), it is found that the TiO2NTs grow along the GO layers. The length of TiO2NTs is more than 400 nm. They are longer than the bare TiO2NTs (Fig. 3(c)). HR-TEM image of GO in TiO2NT-GO hybrid is shown in Fig. 3d. Here, the GO has the similar edge to the pure GO shown in Fig. 3(a). HR-TEM image of TiO2NTs in TiO2NT-GO hybrid is shown in Fig. 3(e). Here, the average external diameter of the TiO2NTs is 8 nm, and their wall thickness is 4 nm. And its inset of the HR-TEM image shows the (101) crystal facet and the 0.35 nm interplane distance of a typical anatase TiO2. Due to the lamellar structure, titanate has high surface energy and is unstable, so it is prone to reduce the surface energy[17].

Fig. 2　N2 isotherms and pore size distributions of samples:

Fig. 4 shows the powder X-ray diffraction (XRD) patterns of the TiO2NTs with and without GO. Here, the wide-angle XRD reveals that all the samples are anatase TiO2. The peak intensities of the TiO2NT-GO hybrid are slightly higher than those of the bare TiO2NTs. The degrees of crystallization in the TiO2NT-GO hybrid calcined at 60 ℃ (Fig. 4(b)) and 350 ℃ (Fig. 4(d)) are higher than the TiO2NTs without GO calcined at 60 ℃ (Fig. 4(a)) and 350 ℃ (Fig. 4(c)), respectively. Therefore, GO induces the formation of anatase TiO2NT crystal structure. The calculated crystallite size of TiO2NT-GO hybrid and the pure TiO2NTs decreases with calcination temperature. The oxygen-containing groups in free and bound states on the surface of GO can move easily and are inserted into the titania structure under hydrothermal reaction conditions, and locate at interstices or occupy some of the titanium lattice sites, forming titanium-oxide. Both of TiO2NTs and TiO2NT-GO hybrid show anatase TiO2when the temperature is increased to 550 ℃.

Fig. 4　XRD analysis of samples at different calcination temperatures:

Fig. 5　C 1s XPS spectra of (a) GO,

and oxygen-containing bonds of GO and the TiO2 NT-GO hybrid at different calcination temperatures.

The UV-vis spectra of GO, the TiO2NT-GO hybrid and the bare TiO2NTs are shown in Fig. 6. The GO shows a peak at 230 nm[16, 18], and the bare TiO2NTs show a peak at 250 nm. With increasing calcination temperature from 150 to 350 ℃, the TiO2NT-GO peak is a red-shifted[19]. This increase into the visible spectral range is favorable for the applications of TiO2NT-GO hybrid as a photocatalyst. When the calcination temperature is increased above 350 ℃, the peak is blue-shifted to 330 nm. We infer that GO decomposition, reduction of GO and formation of titania are in dynamic equilibrium and are strongly affected by temperature. There is an optimum temperature for GO to keep the TiO2NTs attached and maintain charge neutrality.

Fig. 6　UV-vis spectra of the TiO2 NT-GO

The physicochemical properties of samples calcined at different temperatures are shown in Fig. 7. The adsorption of the TiO2NT-GO hybrid calcined at 350 ℃ (Fig. 7(a)) for MB is better than that of bare TiO2NTs calcined at 350 ℃ after they have been stirred in dark for 120 min. The photocatalytic properties of the samples are shown in Fig. 7(b). The TiO2NT-GO hybrid calcined at 350 ℃ has the best photocatalytic activity. An optimized temperature is conducive to maintain charge neutrality of TiO2NTs, as it regulates the formation of surface hydroxyl groups, and enhances the photocatalytic activity of the titania photocatalysts.

Second step, when the layered titanate is treated with the HCl aqueous solution and distilled water, the Ti—O—Na and Ti—OH bonds are believed to react with acid and water to form new Ti—O—Ti bonds. At this stage, the bond distance from one Ti to the next Ti on the surface decreases, resulting in the folding of the sheets and the formation of a tube structure. Due to curl property of GO layer plane, the formation process from a layered titanate structure to a tube structure can be easily finished. The both processes are closely related to the single layer structure of graphene oxide that only single-layer graphene oxide is beneficial to the formation of the titania nanotube. Titanate with a lamellar structure has a high specific surface energy and is in an unstable state. So it is prone to curl to reduce the surface energy.

So, the layered titanate is easily converted to a tubular structure. The last process is closely related to the single layer structure of the GO/reduced GO. Therefore, only a single-layer GO-based solution is possible to prepare the TiO2NT-GO hybrid by hydrothermal synthesis.

Fig. 7　(a) Adsorbance property and (b) photocatalytic degradation behavior of

Scheme 1　Formation mechanism of TiO2 NTs on GO sheets.

Scheme 2　Photocatalytic schematic diagram of TiO2 NT-GO hybrids.

4Conclusions

We have prepared the TiO2NT-GO hybrid by a hydrothermal self-assembly method. Flat GO is conducive to the formation of TiO2NTs. GO reduction occurs in the hydrothermal and calcination processes, and oxygen-containing groups are inserted into the titania layer structure. These processes contribute to the formation of the anatase TiO2NT crystal structure. We found the TiO2NT-GO hybrid has better photocatalytic activity than the pure TiO2NTs. We also found there is an optimal calcination temperature to achieve best photocatalytic activity for methylene blue degradation. This optimal calcination temperature is connected with the dynamic equilibrium of reduction of GO, loss of GO and formation of TiO2NTs.

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Receiveddate: 2015-12-13;Reviseddate： 2016-03-23

Foundationitem: Project of Key Laboratory of Universities and Colleges in Sichuan Province (2013002).

Titania nanotube-graphene oxide hybrids with excellent photocatalytic activities

LAI Qi1, 2,LUO Xue-ping2,ZHU Shi-fu3

(1.PanzhihuaEngineeringTechnologyResearchCenterforGraphene,CollegeofMaterialEngineering,PanzhihuaUniversity,Panzhihua617000,China;2.Deep-processingLaboratoryofGraphite,PanzhihuaUniversity,Panzhihua617000,China;3.CollegeofMaterialScienceandEngineering,SichuanUniversity,Chengdu610065,China)

Abstract:Titania nanotube-graphene oxide(TiO2 NT-GO) hybrids were prepared by a hydrothermal reaction followed by calcination at different temperatures. Results indicate that flat sheets of GO promote the formation of the TiO2NTs. GO is reduced during the hydrothermal and calcination steps, and is inserted into the TiO2 layers by the interaction between oxygen-containing groups on both the GO and TiO2. The rates of GO decomposition, GO reduction and TiO2 NT formation are temperature dependent. The TiO2 NT-GO hybrid calcined at 350 ℃ has a high degree of crystallization, a red-shifted absorption and the highest observed photocatalytic activity for the degradation of methylene blue.

Key words:Nanostructures; Surfaces; Electron microscopy; X-ray photo-emission spectroscopy (XPS); Optical materials

文章編號：1007-8827(2016)02-0121-08

中圖分類號:TB333

文獻(xiàn)標(biāo)識碼:A

基金項(xiàng)目：四川省高校重點(diǎn)實(shí)驗(yàn)室項(xiàng)目(2013002).

通訊作者：賴奇，博士，副教授. E-mail: pzhlaiqi@163.com

Corresponding author:LAI Qi, Ph. D, Associate Professor. E-mail: pzhlaiqi@163.com

DOI:10.1016/S1872-5805(16)60007-0