Junwei Sha a,b,c , Naiqin Zhao a,b,c,* , Enzuo Liu a,b , Chunsheng Shi a,b , Chunnian He a,b, Jiajun Li a,b 

a School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China 

b Tianjin Key Laboratory of Composite and Functional Materials, Tianjin 300072, China 

c Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China

ABSTRACT

Two kinds of TiO2 with novel structures, interpenetrating anatase TiO2 tablets (IP-TiO2), and overlapping anatase TiO2 nanosheets (OL-TiO2) with exposed {001} facets, are synthesized. The graphene oxide (GO) supported ultrathin TiO2 nanosheets (OL-TiO2/GO) is also prepared by one-pot hydrothermal method. The microscopic feature, morphology, phase, and nitrogen adsorption–desorption isotherms are characterized. The performance of photocatalytic degradation of methyl blue is also measured. Compared with IP-TiO2, the OLTiO2 with GO possess higher photocatalytic effificiency. The GO can improve the photocatalytic property by increasing specifific surface area, accelerating the separation of electron– hole pairs, as well as extending the electron life. The growth process of TiO2 nanosheets on graphene oxide layers probably follows a step-growth mechanism with F as morphology controlling agent. The steps on the surface can improve the photocatalytic activity further due to the increase of dangling bonds of 5-coordinated Ti (Ti5c) which are considered to be the active sites in the photocatalytic reaction.

1. Introduction

Semiconductor photocatalysts have attracted intensive attention and wide study in order to solve the problems of natural environmental pollution because of the easy way to utilize the inexhaustible clean energy of sunlight and artifificial indoor illumination [1–4]. Titanium dioxide, which has the advantages of chemical stability, low cost and non-toxic, is considered as a suitable photocatalyst material in the fifield of photocatalytic water purifification, photocatalytic water splitting into H2, CO2 photoreduction and so on [5–15]. However, the wide bandgap (about 3.2 eV) [16] and high recombination rate between electrons and holes restrict the application of TiO2 in the fifield of photocatalysis

Generally, the effective methods to improve the photocatalytic performance of TiO2 are adjusting the electronic band structure by doping [1], decorating with co-catalyst and controlling the structure of TiO2 with exposed high energy facets [15]. For accelerating the separation of electron–hole pairs, carbon materials, such as carbon nanotubes [17–20], are usually used as the support of semiconductor photocatalysts. After the fifirst report of the synthesis of graphene loading TiO2 composite by Williams et al. [21], graphene loading semiconductor composite attracts more attention in the fifield of photocatalysis [22], because graphene has the advantages of high thermal conductivity ( 5000 W m 1 K 1 ), high mobility of charge carriers (200,000 cm2 V 1 s 1 ) and large specifific surface area (calculated value, 2630 m2 g 1) [23]. It can be utilized as an excellent electron acceptor to improve the separating rate of electron–hole pairs when decorated with other photocatalysts.

The morphology controlling exposure of high energy facets is of signifificance for improving the photocatalytic properties. Because of the fact that many chemical reactions take place on the surface of photocatalyst, the photocatalytic reactions are easier to take place on high energy facets where the reaction activation energy can be reduced. The {001} facets of anatase TiO2 crystals with a high surface energy of 0.90 J m 2 [1] are considered to have high activity in photocatalytic reaction. Lu and Qiao [24] reported the decahedral anatase TiO2 crystals with exposed {001} facets were synthesized by a hydrothermal method with hydroflfluoric acid as the morphology controlling agent. Wang et al. [25] reported that the large ultrathin anatase TiO2 nanosheets with exposed {001} facets on graphene were synthesized by a solvothermal process with TiF4 as Ti source. However, the growth mechanism was not mentioned and the evidence for forming the Ti–O–C bonds was obscure.

In this work, two kinds of TiO2, IP-TiO2 and OL-TiO2 with steps on {001} facets, are synthesized by a hydrothermal reaction with TiCl4 as Ti source and hydroflfluoric acid as a morphology controlling agent. To improve the efficiency of photocatalytic activity, GO was used as the support and dispersant of TiO2. The growth mechanism of IP-TiO2, OL-TiO2 and OL-TiO2/GO during the hydrothermal process is discussed and the effects of a stepped structure on the photocatalytic activity are also analyzed.

2. Experimental

2.1. Preparation of graphite oxide

GO was synthesized by a modified Hummers method [26] from natural flflake graphite (Sigma–Aldrich, cat #332461,  150 lm flakes) as previously reported [27] by our group.

2.2. Preparation of TiO2 and TiO2/GO composites

IP-TiO2 and OL-TiO2 were synthesized by a hydrothermal reaction with TiCl4 (Tianjin Weichen Chemical Reagent, China, analytical grade) as Ti source and hydroflfluoric acid (Tianjin Yingda Chemical Reagent, China, analytical grade, 40.0%) as morphology controlling agent. Typically, 0.5 mL hydrochloric acid (1.5 M) (Tianjin Jiangtian Chemical Reagent, China, analytical grade) was added into 15 mL deionized water to adjust the pH to 1–2. A certain amount of TiCl4 (99.0%, 2 mL for IP-TiO2 and 4 mL for OL-TiO2) was dropped into the deionized water slowly under magnetic stirring. Then hydroflfluoric acid (40%, 3.25 mL for IP-TiO2 and 5.5 mL for OL-TiO2) was added into the mixture. After 2 min stirring, a certain amount of deionized water was added into the mixture to get 30 mL mixed solution. Finally, the mixed solution was transferred into a Teflflon-lined stainless steel autoclave with a capacity of 50 mL, and then heated at 200  C for 8 h. After water-cooling to room temperature, the products were collected, washed with deionized water several times, and dried in a vacuum oven at 80  C overnight. To obtain TiO2/GO composites, a certain amount of GO was dispersed in 15 mL deionized water by 2 h ultrasonic treatment, then a certain amount of TiCl4, HF and deionized water were added under stirring before the hydrothermal process mentioned above and the fifinal precipitates were freeze-dried after washing with deionized water several times. The process parameters are shown in Table 1. All reagents were analytical grade and used without any further purification.

2.3. Characterization

The microscopic features and morphology of as-prepared samples were observed by fifield emission scanning electron microscope (SEM, HITACHI S4800) operated at 5 kV and high-resolution transmission electron microscope (TEM, PHILPS TECNAI G2 F20) with an accelerating voltage of 200 kV. All the TEM samples were prepared by depositing a drop of diluted suspension in ethanol on a carbon-film-coated copper grid. The phase analysis was carried out by a powder X-ray diffraction system (XRD, Rigaku D/max-2500, Cu-Ka radiation, wavelength Ka1 0.154056 nm, Ka2 0.154439 nm, average 0.154184 nm) at room temperature. The X-ray photoelectron spectroscopy (XPS) characterization was carried out on a PHI 1600 spectrometer equipped with a monochromatic Al Ka X-ray source. Raman spectra were obtained using DXR Raman Microscope (Thermo Fisher Scientifific). The wavelength of laser is 532 nm. Nitrogen adsorption–desorption isotherms of the samples were measured at 77 K, using an autosorb iQ instrument (Quantachrome, U.S.) with 6 h outgas at 200  C. The specifific surface area was calculated from the Brunauer–Emmett–Teller (BET) method, and the pore volume, pore diameter and pore size distribution data were determined by the Barrett–Joyner–Halenda (BJH) method [28,29] based on the adsorption and desorption data. Photoluminescence (PL) spectra were measured on a Fluorolog-3 spectroflfluorometer (Horiba Jobin Yvon). The samples were excited at the wavelength of 382 nm by a 300 W Xenon lamp under ambient conditions.

2.4. Photocatalytic measurement

Photocatalytic activities were evaluated by the photocatalytic degradation of methyl blue. Typically, 20 mg of photocatalyst was dispersed in 200 mL of 10 mg/L methyl blue aqueous solution. The suspension was magnetically stirred in the dark for 1 h to ensure an adsorption/desorption equilibrium. Then the suspension was illuminated by a 300 W Xenon lamp (CEL-HXF300, without optical fifilter, full spectrum) during magnetic stirring. About 4 mL suspension was sampled by a 10 mL needle tubing with a syringe fifilter (0.22 lm), and monitored with a UV–Vis Spectrophotometer (UV-2700, Shimadzu). The concentration before adsorption/desorption equilibrium was regarded as C0 and blank experiment without photocatalyst was also carried out for comparison. The pH of the methyl blue aqueous solution is  6 before and after the photocatalytic reaction determined by pH test paper universal.

3. Results and discussion

The morphologies of as-prepared samples were observed by SEM and TEM, as shown in Figs. 1 and 2. It can be seen that, in Fig. 1(A) and (B), TiO2 tablets are interpenetrating analogous decahedral with a size of 2–20 lm in width and length and 0.7–2.5 lm in thickness. The size distribution of the length and thickness are shown in Fig. 1S in the Supporting Information. The steps on the surface of IP-TiO2 can be clearly observed in contrast to the smooth surface in the work of Lu and Qiao [24]. In Fig. 2(A) and (B), the steps are also obvious. Fig. 2(B) clearly shows the (200) facets of anatase TiO2 single crystals with an interplanar distance of  0.198 nm. When increasing the dosage of TiCl4, the morphology of the sample changed from interpenetrating analogous decahedral tablets to overlapping nanosheets, as shown in Fig. 1(C) and (D). The TiO2 nanosheets are only 10–30 nm in thickness but 0.5–5 lm in length and width, as shown in Fig. S1(C), as a kind of two-dimensional materials. OL-TiO2 is overlapping and flflower-like by self-assembly in order to decrease the surface energy. When a certain amount of GO was introduced, shown in Fig. 1(E) and (F), the parallel layer on interpenetrating tablets was well dispersed and distributed as disperse nanosheets without interpenetration. It implies that the growth mechanism of OL-TiO2 is different with or without GO. The observation in this work has proved that introducing GO during TiO2 synthesis cannot change the microstructure and the phase of TiO2 nanosheets, but can disperse the nanosheets well, as shown in Fig. 2(F). Fig. 2(D) with selected area electron diffraction (SAED) pattern shows the (101) facets of anatase TiO2 single crystals with a lattice spacing of  0.35 nm. Obviously, the edges of steps are parallel, and there is a coherent relationship between the different layers when TiO2 grows by a step mechanism, as shown in TEM images and SAED pattern of Fig. 2(C) and (D). Moreover, Fig. 2(E) and Fig. S2 in the Supporting Information indicated that the OL-TiO2 is a kind of mesoporous material.

The results of nitrogen adsorption–desorption isotherms and XRD are shown in Supplementary Figs. S2 and S3. The curves in Fig. S2(A) present type IV isotherms and the average pore diameters of 2.5–3.7 nm indicates that the as-prepared samples are mesoporous with a narrow pore size distribution shown in Fig. S2(B–D). With the morphology changing from interpenetrating analogous decahedral tablets to overlapping nanosheets, the BET specifific surface area increase from 1.853 m2 /g to 4.219 m2 /g, and the pore volume also increase from 0.012 cc/g to 0.029 cc/g. When GO (5 wt.%) was composited with OL-TiO2, the specifific surface area increase to 36.630 m2 /g by about 8.7 times increasing, and the pore volume increase to 0.188 cc/g. The results are in accordance with SEM and TEM. It can be expected that the increase of specifific surface area and pore volume with adding GO is benefificial to the adsorption of dye molecules, and can further improve the performance of photocatalysis. From XRD patterns in Fig. S3, all of the diffraction peaks of IP-TiO2 and OL-TiO2 match well with the anatase phase TiO2. The calculation of average thickness of the samples by Scherrer equation and the average thickness are shown in the Supporting Information.

The phase of titanium dioxide and the crystalline degree of GO were analyzed by Raman spectroscopy to detect the change of molecular polarizability of the samples. There are three types of Raman peaks of GO as shown in Fig. 3. The band around 1352 cm 1 (D band) represents disordered sp3 C, and the band around 1580 cm 1 (G band) is of sp2 C [30] on GO. The 2D band commonly represents a stack of carbon layers. The pure GO without TiCl4 after hydrothermal reaction was also tested as a blank. The ID/IG ratios of GO before and after hydrothermal process are  0.97 and  1.05. The increase of ID/IG ratio indicates that the content of defects on GO increases during the hydrothermal process. But the ID/IG intensity ratios of GO and OL-TiO2/GOs are  0.97 and  1.01. The smaller amplifification reflflects that the nucleation of TiO2 at O-containing defects on GO layers suppress the increase of defects in the hydrothermal process. In both curves of OL-TiO2 and OL-TiO2/GOs, there are peaks at 145 (Eg(1)), 399 (B1g(1)), 637 (Eg(2)) and 516 cm 1 (A1g + B1g(2)), which are the typical modes of anatase TiO2 [31] which are consistent with the results of XRD. It indicates that the GO was decorated by OLTiO2 [24].