Abstract

Crystal facet engineering and surface modifification of semiconductor have become important strategies to improve photocatalytic activity by optimizing surface charge carrier separation/transfer and extending solar spectrum utilization. In this work, we report anatase single-crystalline TiO2 hollow tetragonal nanocones with large exposed (1 0 1) facets by a facile liquid-phase interfacial synthetic strategy, using the hydrolysis of tetrabutyltitanate with adscititious water in the organic solvent of cyclohexane and a capping agent of 1, 6-hexanediamine. The specifific surface area of these TiO2 hollow tetragonal nanocones is as high as 331.3 m2 /g. Thanks to large exposed (1 0 1) facets and high surface area, these TiO2 hollow tetragonal nanocones exhibited excellent full-arc photocatalytic activities for the degradation of organic pollutants. Remarkably, the butoxy group could be modifified onto TiO2 hollow tetragonal nanocones through post-synthesis treatment in tetrabutyltitanate glycol solution, which brought about eximious visible light photocatalytic activities for the degradation of colored dyes of RhB and MO, especially for RhB, by virtue of much improved electron trapping ability of the Ti-O group from the excited dye due to the strong electronegativity of the oxygen atom in the butoxy group. This work advances us to rationally tailor the atomic and electronic structure of the photocatalyst for outstanding photocatalytic properties in various environmental and energy-related applications.

1. Introduction

Titanium dioxide (TiO2) is one of the most widely investigated metal oxides in photocatalysis of renewable energy creation and cal stability and photoactivity as well as its nontoxicity and low cost. Generally, the properties and performance of TiO2 depend on its crystal phase, crystallinity, morphology, surface area, etc [6]. Among four naturally occurring polymorphs of TiO2, i.e., anatase, rutile, brookite and TiO2 (B), anatase TiO2 has been acknowledged as the most active phase in photocatalysis for environmental applications [7]. In the crystal structure of anatase, the primary TiO6 octahedron building units are signifificantly distorted and interconnected via corner and edge sharing to form zigzag chains with a screw axis, which offers a relatively loose atomic stacking and low density that is correlated with its greater absorption capability and more abundant active sites than other three polymorphs, therefore, more effificient charge separation and a lower carrier recombination rate could be achieved. The photocatalytic activity and preference of TiO2 crystals is heavily dependent upon the crystallographic orientations, maximizing the surface of TiO2 photocatalyst to reactive facets is feasible to realize the high activity and preference of photocatalytic reactions [1,8]. Increasing research attention is now being directed toward engineering the surface structure of TiO2 at the most fundamental and atomic level control of exposed active facets to optimize its physicochemical properties. Anatase TiO2 crystals have been increasingly investigated with a high percentage of high-energy {0 0 1} and small particle size for improving the photocatalytic activity [9–12]. On the other hand, on the basis of the calculated results, the order for the relative photooxidation and photoreduction activities of the three surfaces of anatase TiO2 is {1 0 0} > {1 0 1} > {0 0 1} [13]; the (1 0 1) facets have a higher activity on CO2 photoreduction than (0 0 1) facets [14]. Moreover, it has been experimentally revealed that the true photoreactivity order of low index facets of anatase TiO2 is {0 0 1} < {1 0 1} < {0 1 0} by comparing photocatalytic hydrogen evolution,
OH radical generation and photoreduction reactions from the crystals with a predominance of {0 0 1}, {1 0 1}, and {0 1 0} [15]. Achieving TiO2 crystals with dominant low-energy {1 0 1} deserves more attention. Equally important to acquiring the above reactive facets for promoting surface charge carrier separation/transfer, decreasing serious bulk carrier recombination will defifinitely contribute to the photocatalytic activity. The most widely used strategy is to shorten the bulk diffusion length of carriers by decreasing particle size particularly to nanoscale. Many substantial efforts have been devoted to achieve hollow nanostructures of anatase TiO2 with high surface area by various synthetic strategies for obtaining more active sites in photocatalysis [16], however, most of which are usually polycrystalline and lack faceted surfaces, where a large number of grain boundaries will increase electron-hole recombination rate. Being singlecrystalline hollow nanostructure with exposed reactive facets will be an excellent alternative choice, however, which is rarely realized through various chemical processes involving HF corrosion or a solid state precursor strategy [17–21].

However, the large band gap of anatase TiO2 (
3.2 eV) renders it inactive under visible light and consequently ineffificient in the utilization of solar energy. Intentional modifification of TiO2 for utilizing visible light of the solar spectrum in photocatalysis has been carried out by manipulating its electronic structure, such as nonmetal doping [22–26], transition metal incorporation [27], noble metal decorating [28–30], carbon modifification [31–35], narrow bandgap semiconductor coupling [36–39], organic-inorganic hybrid [40–42] and surface adsorbates (or complexes) sensitizing [43,44]. The modifified substance on TiO2 critically inflfluences the photo-induced charge transfer behaviors at the interfacial region. It will be very attractive coupling a simple and environmentally friendly organic substance on faceted TiO2 hollow nanostructure crystals without reconstructing the solid lattice structure for improving the visible light photocatalytic activity of TiO2, yet, rarely explored. In this work, we report a facile liquid-phase interfacial synthetic strategy for fabricating novel anatase singlecrystalline TiO2 hollow tetragonal nanocones with large exposed (1 0 1) facets, which exhibit much promoted full-arc photocatalytic activities toward the degradation of certain chemical species. Moreover, through a low-temperature solution-phase posttreatment, the butoxy group was modifified on TiO2 hollow tetragonal nanocones, which made the product exhibit excellent visible light photocatalytic activities. This work is important for the rational design and fabrication of TiO2 crystals with tailored facets and modifified organic group, providing a valuable platform to explore enhanced performances in various photocatalytic applications.

2. Experimental

2.1. Synthesis of bare TiO2 hollow tetragonal nanocones 

All chemicals were of analytical purity and used without further purifification. In a typical synthesis, 5 mL of 1, 6-hexanediamine was dissolved in 25 mL of cyclohexane. Then, 1 mL of tetrabutyltitanate was added under stirring. After dissolution, 1 mL of deionized water was added and stirred for 10 min. The as-obtained mixture was loaded into a 50 mL Teflflon-lined stainless steel autoclave and heated at 180  C for 12 h, then allowed to cool to room temperature naturally. The resulting white precipitate was collected by centrifugation and washed with deionized water and ethanol several times to remove residual ions, then dried at 60  C for 6 h for further characterization. For convenience, the product was simplified as TiO2-HT NCs.

2.4. Photocatalytic and photo-electrochemical measurements

Photocatalytic activities of as-prepared bare and BO modifified TiO2-HT NCs were evaluated by measuring the photodegradation of a cationic dye of Rhodamine B (RhB) under irradiation of a 300 W full-arc Xe lamp (CEL-HXF300, Beijing, China) or with a cut  off filter of k > 420 nm. For comparison, an anionic dye of Methyl Orange (MO), and a colorless organics of phenol were also examined. Typically, 10 mg catalyst was dispersed into 100 mL of 10  5 M pollutant aqueous solution by constant stirring in the dark for 60 min to reach an adsorption/desorption equilibrium of the pollutant on the catalyst surface. At a given time interval, about 3 mL of the suspension was withdrawn under irradiation and cen trifuged to remove the photocatalyst for UV–vis absorption spectrum measurement (Shimadzu UV-2550, Japan). The concentration of the pollutant was determined through checking the characteristic absorption.

To conduct the photo-electrochemical measurements, the products were fabricated into fifilms on FTO glasses. The detailed fabrication process of the films was carried out as follows: 5 mg catalyst was initially ultrasonically dispersed in 1 mL of deionized water, then 0.01 mL dispersion was uniformly dropped on the 0.28 cm2 FTO-coated glass, afterwards, the FTO-coated glass was heated at 80  C for 30 min to volatilize the solvent. The photoelectrochemical test systems were composed of a standard three-electrode confifiguration with the product deposited on FTO-coated glass as the working electrode, the Pt wire electrode and the Ag/AgCl electrode as the counter and the reference, respectively, which were immersed in 0.2 mol L 1 Na2SO4 solution containing 10 5 M pollutant. The photo-electrochemical properties were measured on a CHI 660B electrochemical workstation (Chenhua Corp., Shanghai, China) in ambient conditions under illumination using a 300 W full-arc Xe lamp (CEL-HXF300, Beijing, China) or with a cutoff filter of k > 420 nm. The potential was swept from  0.1 to +0.8 V (versus Ag/AgCl) at a sweep rate of 50 mV s  1.

3. Results and discussion

The crystal structure and phase composition of the bare and BO modifified TiO2-HT NCs were investigated by XRD analysis, as shown in Fig. 1. The XRD pattern of the bare TiO2-HT NCs shows the sharp diffraction features, which clearly indicate the high crystallinity of the product, and the 2h diffraction positions could be perfectly indexed to the tetragonal anatase phase (JCPDS No. 71- 1166). No impurity peaks such as rutile and brookite were detected, indicating the high purity of the product. The XRD pattern of BO modifified TiO2-HT NCs was similar with that of the bare TiO2-HT NCs, revealing the butoxy group modification did not change the phase of the product.

T NCs were characterized by the FESEM. Fig. 2a is a lowmagnifification panoramic FESEM image, which shows the product is large-scale nanocones, about 200 nm long from the top to the bottom of the nanocones, 100 nm wide of the lateral size at the bottom of the nanocones. The high-magnifification FESEM image in Fig. 2b reveals the product is hollow tetragonal cone-like nanostructure composed of nanoplates with average thickness of 15 nm. The crystal structure and morphology of the product are further examined by TEM. A low-magnifification TEM image in Fig. 2c clearly shows the product is cone-like nanostructure. The highmagnifification TEM image in Fig. 2d further reveals the hollow nanocones are consisted of nanosheets. The corresponding selected area electron diffraction (SAED) pattern (Fig. 2d inset) on a single nanocone indicates its single-crystalline nature, and suggests the presence of low-energy {1 0 1} facets and high-energy {0 0 1} facets. The HRTEM image in Fig. 2e taken from the edge of a TiO2 hollow tetragonal nanocone exhibits good crystallinity, where the clear continuous lattice fringes with interplanar lattice spacings of 0.35 nm and 0.48 nm match well with the (1 0 1) and (0 0 2) atomic planes of tetragonal anatase TiO2, of which the intersection angle is 68.3 , identical to the theoretical value between {1 0 1} and {0 0 1} facets in an anatase crystal. According to above structure analysis, along with the crystallographic symmetry of anatase TiO2, it can be concluded that the four lateral surfaces of the hollow tetragonal nanocones are low-energy {1 0 1} facets and the thickness part of the nanocones is high-energy {0 0 1} facets. Therefore, the results indicate TiO2-HT NCs are singlecrystalline with large exposed low-energy {1 0 1} facets and small exposed high-energy {0 0 1} facets. After modifification with the butoxy group, the morphology and size of the product was unchanged as shown in the FESEM images in Fig. S1.

The morphology evolution of the intermediates reveals TiO2-HT NCs were formed by preferential growth of {1 0 1} facets (Fig. S2). The XRD investigations showed the product was evolved from low- to high-crystalline TiO2 (Fig. S3), which is a common evolution feature of {1 0 1} dominated TiO2 crystals [1]. In the synthesis of TiO2-HT NCs, tetrabutyltitanate gradually hydrolyzed in the interface of a small quantity of deionized water and a large amount of cyclohexane at elevated temperature, forming primary lowcrystalline TiO2 nanoparticles. The {1 0 1} facets of the anatase TiO2 was preferentially grown owing to its lowest surface energy in comparison with other facets. Then, 1, 6-hexanediamine molecules effificiently capped on the TiO2 {1 0 1} facets through surface ions, passivated the surface atoms, makes the thickening rate much lower than the lateral growth rate. While the crystal growth predominantly occurred along the four energetically equivalent {1 0 1} directions, the hollow tetragonal TiO2 nanocones with exposed (1 0 1) facets were formed. Herein, 1, 6-hexanediamine is chosen as a capping/shape-controlling agent that plays dual functions, as a bridging ligand facilitating the formation of Ti-O-Ti skeleton in a particular direction, and as a capping agent inhibiting the formation of the Ti-O-Ti skeleton three dimensionally. As the surface atomic arrangement and surface affinity for the solvent to each orientation of TiO2 crystal is different, the solvent can affect the growth rates to the crystal surfaces and hence the fifinal morphology of TiO2 crystal [23]. Herein, a small quantity of deionized water and a large amount of cyclohexane were chosen as the solvents to suppress the quick hydrolyzation of tetrabutyltitanate that is highly reactive toward moisture. Moreover, the difference of the surface tension between water and cyclohexane prompts the TiO2 crystals predominantly grew along the four energetically equivalent {1 0 1}