Fabrication of TiO₂ rod in tube nanostructure with enhanced photocatalytic activity: investigation of the effect of the states of the precursor on morphology

Abstract

Here, we report a special rod in tube nanostructured TiO₂ prepared by a moderate esterification reaction. The morphology evolution with reaction time was observed by FSEM and FETEM. Comparing to the as-reported sphere in sphere nanostructured TiO₂, the formation of a rod in tube nanostructure was attributed to the use of a solid precursor. The nucleation-dissolution-anisotropic grain growth-recrystallization mechanism is responsible for the formation of the rod in tube nanostructure. Moreover, the photocatalytic properties of different morphologies of samples were also compared, indicating that the rod in tube nanostructured TiO₂ possessed the best photocatalytic activity. The improvement of photocatalytic activity is mainly attributed to the enhancement of light-harvesting and the separation of electron–hole pairs. Finally, the present work also provided a template-free method to synthesize hierarchical porous TiO₂ nanotube through prolonging reaction duration.

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Introduction

For years, TiO₂ as a promising photocatalyst has been investigated due to its low cost, chemical inertness, nontoxicity and excellent physical properties. As is well known, the factors which affect the photocatalytic properties include crystallinity, microstructure, specific surface area, phase composition, light harvesting, separation rate of photogenerated charges and energy band structure. Hence, several strategies have been developed to improve the photocatalytic activity of TiO₂ by controlling the influencing factors mentioned above.^1–5

For the synthesis of TiO₂ nanomaterial, researchers have developed many effective methods, such as hydrothermal, sol–gel, chemical vapour deposition and magnetron sputtering et al. The chemical synthesis method is the most economical and extensively used tool. However, most of chemical synthesis protocols need to use precursor. The widely used precursor contains titanium alkoxide and inorganic substance with titanium. So far, lots of publications have reported that the use of different kinds of precursor induce the formation of TiO₂ with various morphology, size and crystal phase. For example, the porous, sheet, tubular and complex hierarchical structure (anthoid, yolk, dendritic et al.) TiO₂ have been prepared by adjusting pH value, solvent composition, reaction duration and temperature.^6–10 In addition, demand for high efficient photocatalyst determines the development of many convenient and valid preparation methods. These methods must effectively enhance specific surface area and improve the separation of electron–hole pairs. Obviously, the one-dimensional tubular structure achieves the requirements. Nonetheless, most of the synthesis of tubes needs to use templates or toxic chemicals, even for a tedious process.^11–19 Hence, it is necessary to search an eco-friendly, template-free and simple protocol to prepare TiO₂ nanotube.

In the present work, a mesoporous titania nanotube with tunable chamber structure has been generated by using a template-free method which is inspired by Li.²⁰ To our surprise, the sphere in sphere structure is not generated whereas the rod in tube structure forms since the precursor is TiOSO₄ powders. Hence, the effect of different states of precursor on the morphology of TiO₂ and the relative formation mechanism is also investigated. In addition, the comparison of the as-prepared samples with different morphologies shows that the special rod in tube structure TiO₂ possesses the best photocatalytic activity towards degradation of gaseous benzene, and the relative reason is also explained.

Experimental

Synthetic procedures

For rod structure, TiOSO₄ (Aladdin, China) 2.4 g, glycerol 18 mL, ethanol 36 mL and ethyl ether 16.5 mL was added into a 100 mL autoclave (working pressure ≤ 3 MPa) at 110 °C for 2 h. The morphology varied with solventhermal reaction time. When reaction time prolonged to 48 h, the rod in tube structure formed. When reacted for 192 h, the tube structure was produced. After solventhermal reaction was over, the precipitated powders were filtered, washed with absolute ethanol several times, dried at 100 °C and calcined at 550 °C in muffle furnace for 3 h. (The samples are also notated as T-x, where x represents the solventhermal reaction time.) For sphere in sphere structure, all chemicals and condition were same except that precursor was 2 mL tetrabutyl titanate and reacted for 48 h. (This sample is notated as ST-48.)

Characterization

The crystal phases of samples were analyzed by X-ray diffraction with CuKα radiation (XRD: PANalytical B.V., Almelo, Netherlands, CuKα radiation with λ = 1.5406 Å). The morphology, structure and grain size of the samples were examined by transmission electron microscopy (TEM: FEI Tecnai G² F30, Netherlands) and field emitted scan microscopy (FSEM: FEI Nova 450, Netherlands). The specific surface areas (SSA) of the powders were determined in a Flow Sorb ASAP 2020 apparatus (Micromeritics) by using the single-point BET method. The PL spectra were gained by using a LabRAM HR spectrometer (HORIBA Jobin Yvon, France) with a laser excitation of 325 nm.

Measurement of photocatalytic activity

Evolution of the photocatalysis was carried out in a self-designed airtight aluminum alloy reactor. The volume of reactor is 2 L. The mass of photocatalyst was 0.10 g and well dispersed to form a thin layer over an aluminum slice (50 × 50 mm). The initial concentration of benzene was kept at 300 ppm for all the experiments. The concentration of benzene was monitored every 10 min by GC 9560 gas-phase chromatogram equipped with FID detector. A 300 W Xe lamp was put over the reactor to irradiate the samples. The conversion was calculated by (C₀ – C)/C₀, where C is the concentration of the reactant after irradiation, C₀ is the concentration of the reactant after adsorption equilibrium and before the irradiation in the presence of catalyst.

Results and discussion

The TiOSO₄ powders as Ti source mixed with alcohol, glycerol and diethyl ether were transferred into autoclave and reacted at 110 °C for a certain period of time. The reasonable choice of reaction time afforded the synthesis of tube with adjustable morphology, size and interior structure that is tunable from rod, rod in tube, to tube. The morphology evolution with reaction time of all samples is observed by FSEM and FETEM, depicting in Fig. 1. A solid clubbed structure is obtained after 2 h reaction (denoted as T-2) and a smooth surface is also observed. The nanorod is demonstrated intuitively by TEM. It is obvious to see that the smooth surface is consisted of uniform nanoparticles, and the average size of nanoparticle is around 26.2 nm, the size distribution is 21–50 nm (for comparison of TEM micrograph, see Fig. S1 in ESI†). With reaction time prolonging to 48 h (denoted as T-48), an apparently core–shell rod in tube structure is observed. The gap between core and shell is around 140 nm, and the thickness of shell is about 150 nm. Meanwhile, the smooth surface becomes rough and forms the unordered prickly surface. In fact, the prickly protuberance is nanoparticle and the average size is around 24.6 nm. After 192 h reaction (denoted as T-192), the inner clubbed core is finally diminished and creates a hollow spiny clubbed structure. The thickness of shell increases to 215 nm and the size of nanoparticle is 20.3 nm. It is obviously to know that the thickness of outer-shell becomes thicker accompany with reaction time extending (for comparing, see Table S1 in ESI†). Analysis of the data from comparison, an interesting trend is visualized. The size distribution narrows with prolonging the reaction time. It suggests that the particles hydrolyse continuously and rearrange. From the inset of Fig. 1, it is evident to see that these three samples are mainly comprised of highly crystallized anatase with lattice 0.35 nm which is corresponding to the (101) crystallographic plane of anatase.

Fig. 1 The electron micrographs of titania rods synthesized for 2 h (A and D), 48 h (B and E), 192 h (C and F).

Fig. 2 displays the XRD pattern for the above three samples. It is clear to see that all samples show the similar pattern which can be indexed to anatase (JCPDS file no. 21-1272) and rutile (JCPDS file no. 21-1276), which is consistent with TEM. The trace of rutile can be attributed to the calcination at 550 °C.²¹ It is favourable to form rutile at high temperature (>500 °C).

Fig. 2 The XRD pattern for T-2, T-48, T-192 samples.

Interestingly, when liquid tetrabutyl titanate was chose to be precursor and reacted at the absolutely identical condition, the clubbed structure was not obtained whereas the spherical structure appeared. After 48 h reaction (denoted as ST-48), the sphere in sphere structure formed (Fig. 3). An urchin like prickly sphere was also observed, and the diameter was around 600 nm as well as the thickness of shell was about 80 nm. From the inset of Fig. 3, it is apparently to know that the ST-48 was also mainly comprised of anatase. The XRD pattern of ST-48 was demonstrated in Fig. S2,† indicating that the ST-48 was also consisted of anatase and rutile. The maximum proportion of rutile phase was 8.7 wt% calculated by the widely used equation X = 1/(1 + 0.8I_A/I_R), where I_A and I_R were the diffraction intensities corresponding to the anatase (101) plane and the rutile (110) plane respectively.²² For the T-48 samples, the maximum proportion of rutile phase was 8.1%. The difference of phase composition between ST-48 and T-48 sample was not significant.

Fig. 3 The electron micrographs of titania sphere synthesized for 48 h.

Fig. 4 N₂ sorption isotherms (A) and pore-size distribution curves (B) for T-2, T-48 and ST-48 samples. The isotherms (A) for T-48 and ST-48 are offset vertically by 100 and 300 cm³ g⁻¹, respectively.

N₂ sorption isotherms display type-IV curves which the capillary condensation phenomenon appears at relative high pressure for T-2, T-48 and ST-48 sample, characteristic of mesoporous solids with uniform pore sizes (Fig. 4).^23–25 This is coincident with TEM results. The pore-sizes distribution curves with a mean value of about 15 nm are calculated from the adsorption branches based on the BJH mode for T-2 sample. After 48 h reaction, the average value of pore size is still around 15 nm whereas the pore-size distribution has narrowed. It might be because the grains grow up and the pore formed by grains stacking becomes smaller. For T-192 sample, the BET surface area cannot measure by using single-point BET method. It may be ascribed to the hollow tubular structure exist in T-192, which is verified by TEM observation. The macropore need be measured by using mercury intrusion method. For ST-48, the pore-sizes distribution is wide. However, the BET surface area of T-2, T-48 and ST-48 is 67.3 cm² g⁻¹, 65.9 cm² g⁻¹ and 25.8 cm² g⁻¹, respectively.

The PL spectrum is an effective way to investigate the electronic structure, optical and photochemical properties of semiconductor materials as well as the efficiency of charge carriers trapping, immigration and transfer can be also obtained. In our experiment, the PL spectrum of four samples is shown in Fig. 5. All samples can exhibit a strong and wide PL signal at the range from 380 to 750 nm, with the excitation light of energy higher than the bandgap energy, having two obvious PL peaks at about 425 and 575 nm, respectively, which are attributed to excitionic PL. The stronger the PL signal, the higher the combination rate of photo-induced charges is.

Fig. 5 The PL spectrum for T-2, T-48, T-192 and ST-48 with excitation wavelength of 325 nm.

The result from Li's experiment show that the formation of titania spheres with sphere in sphere structure is not affected by the kinds of precursor. Comparing to that, the present work imply that the morphology is dependent on state of precursor. When liquid precursor is used in this experiment, the titania spheres with sphere in sphere structure tend to form. If solid precursor is used in the reaction, it is conductive to obtain the titania tubes with rod in tube structure. The formation mechanism is illustrated in Fig. 6. During the solventhermal reaction, the water was produced continuously by etherifying reaction between alcohol and glycerol. TiO₂ building clusters was generated by hydrolysis–condensation reactions of precursor (titanium alkoxide or titanium inorganic salt). Then the as-formed clusters aggregated and self-assembled. When liquid precursor is used, the generated TiO₂ crystal nucleus is tiny. Moreover, the tiny crystal tends to aggregate to cluster in order to minimum the surface energy. More importantly, the anisotropic grain growth is inhabited. Hence, the spherical structure is formed. For the solid precursor, it can be explained by nucleation-dissolution-anisotropic grain growth-recrystallization mechanism. The solid precursor powders suspend in the solvent as seed crystals. It is not restricted to anisotropic grain growth for the large scale crystal. As the reaction progress, the grains stack each other and self-assemble to form a rod. Meanwhile, water is generated continuously and reacts with the spheres and rods, leading to the dissolution and rearrangement of the surface building clusters. As a result, the core–shell structure appears. Continuation of this process, the core dissolves gradually. The hollow structure creates at last.

Fig. 6 The formation mechanism of morphology evolution with reaction time.

The photocatalytic activity toward decomposing benzene under UV-light is demonstrated in Fig. 7. Apparently, the blank experiment shows benzene cannot be decomposed without using photocatalyst under UV light illumination. It is evident that the sample T-48 is the most active photocatalyst among these samples. The concentration of benzene has reduced to 32.3%, 28.6%, 82.7% and 44.3% for T-2, T-48, T-192 and ST-48, respectively. In addition, the trend of photocatalytic activity is not absolutely identical to that of specific surface area and is not consistent with the result from Li's experiment, either. The specific surface area of T-2 is larger than T-48 while the photocatalytic activity of T-2 is worse. Importantly, the outside surface area can be obtained by BET measurement. The surface of T-2 is comprised of nanoparticle. The porous formed by the stacking of nanoparticles have a great contribution to the surface area. More importantly, the enhancement of photocatalytic activity of T-48 is ascribed to the multiple reflections of UV light within the tube interior voids, viz. the improvement of light-harvesting of T-48 (ref. 26–28) and the high separation rate of photo-induced charges. Thus, T-48 samples possess the best photocatalytic activity among all samples.

Fig. 7 Catalytic oxidation of benzene (left) and CO₂ generation (right) using four samples under UV light illumination.

Conclusions

In the present work, a unique rod in tube core–shell hierarchical structure TiO₂ is prepared by a mild solventhermal reaction. The effect of states of precursor on the morphology is discussed. The liquid precursor is favourable to the formation of sphere structure whereas the solid precursor prefers to form clubbed structure. It may be ascribed different growth mechanism. The homogeneous solution saturated precipitation mechanism and nucleation-dissolution-anisotropic grain growth-recrystallization mechanism can be responsible for the formation of sphere and tube, respectively. The enhancement of photocatalytic activity is mainly attributed to harvesting more light and improvement of separation of photo-induced charges. The present work investigates the effect of states of precursor on morphology and the relationship between the morphology and the photocatalytic activity as well as presents a free-template method to prepare tubular structure materials, too.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (nos. 51071073 and 50927201), and the National Basic Research Program of China (Grant nos. 2009CB939705). The authors are also grateful to Analytic and Testing Center of Huazhong University of Science and Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06086e

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