Mesoporous titanium dioxide coating on gold modified silica nanotubes: a tube-in-tube titanium nanostructure for visible-light photocatalysts

Abstract

A novel tube-in-tube structured titanium dioxide (TiO₂) based visible-light photocatalyst with non-metal doping and plasmonic metal decoration was fabricated using a well-controlled programmed synthesis method and was characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray powder diffraction (XRD), N₂ adsorption/desorption, X-ray photoelectron spectroscopy (XPS), and UV-vis diffuse reflectance spectroscopy. The as-obtained tube-in-tube structure is composed of inner mesoporous silica nanotubes with high specific surface area and an outer layer of anatase TiO₂ nanocrystals with considerable visible-light activity. For the photocatalytic degradation of rhodamine B (RhB) in aqueous solution, the photocatalyst showed superior photocatalytic activities compared with commercial TiO₂ and nanometer-sized photocatalyst Degussa P25. The strategy is simple, but efficient, and can be extended to the synthesis of other multifunctional composites. It has opened a new pathway for the construction of hetero-nanocomposites with high activity and durability, which would serve as excellent models in catalytic systems of both theoretical and practical interest.

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Introduction

Nanomaterials are currently receiving a great deal of attention due to their fantastic physical, chemical, and biological properties that potentially benefit catalysis, biomedicine, sensors, environmental remediation, and so on.^1–4 Because of the rapid progress of nanoscience and nanotechnology, structure–property correlations of nanomaterials have drawn tremendous interest in both theoretical and technological research in recent years. In this regard, tubular and porous materials containing both interconnected nanometer-sized hollow cavity and nanopore channels have attracted increasing attention in the natural science and materials science fields from the viewpoint of their potential applications.^5–8 Comparing with other nanostructures, such multifunctional composite nanomaterials present several advantages to the corresponding bulk materials. First, tubular architectures can increase capabilities of mass transport through the material.⁹ Second, mesostructured nanotubes maintain a specific surface area on the level of fine pore systems.¹⁰ Third, the multifunctionalities can be introduced on the inner and outer surfaces independently.¹¹

In recent years, appreciable efforts have been made to develop nanostructured multifunctional composite materials to degrade organic pollutants (e.g., azo dyes) in waste water.^12,13 The whole process is generally divided into two parts, one part is physical adsorption, and the other part is chemical degradation. For adsorption, high specific surface area and porous structures are two critical features that enhance the interaction between the pollutants and adsorption sites for high adsorption capacities.^6,14,15 In particular, silicate nanomaterials can be used as a highly efficient adsorbent to adsorb organic pollutants. So far, hierarchical hollow nanospheres and nanotubes are two of the most promising nanomaterials, compared to silicate hollow nanospheres, one-dimensional (1D) silicate nanotubes with a mesoporous shell have better mechanical strength and shorter diffusion path for mass transportation, and are thereby more suitable for adsorption.^16–18 However, most work on silicates focus on preparing hollow structures in the past few years. Apart from high adsorption capacity, the ability of chemical degradation is also very important. Since Honda and Fujishima first achieved hydrogen production from water by photocatalysis with TiO₂ under UV irradiation, this material has been widely used as a photocatalyst for the photooxidation of different kinds of hazardous organic pollutants in wastewater, drinking water, and air.^19–21 Owing to the wide band gap (3.2 eV), pristine TiO₂ can only utilize UV light, so that it is important to develop visible-light-active TiO₂ with enhanced photocatalytic activity. Many strategies, such as nonmetal doping,²² metal decoration,²³ selecting the most appropriate crystalline phases by calcination,²⁴ have been proposed to extend the absorption of TiO₂ to the visible spectrum, it is therefore expected that a composite of small doped TiO₂ nanocrystals decorated with metal nanoparticles may be a powerful photocatalyst. However, the incompatibility among the synthesis, doping, and decoration procedures means that the production of such nanocomposites has remained a great challenge and rare work succeeded in designing tube-structured TiO₂ nanomaterial with high adsorption capacities and prominent degradation ability.

In this article, we report the design and synthesis of a novel tube-in-tube structured titanium nanomaterial with the desired properties mentioned above through a step-by-step strategy. This unique multicomponent nanostructured material, designated as mSiNTs-N/Au@TiO₂, was fabricated by combining the template approaches, interfacial deposition and versatile sol–gel process through a multistep approach involving producing mesoporous silicon nanotubes (mSiNTs) from multiwalled carbon nanotubes (MWCNTs), depositing a layer of gold nanoparticles (AuNPs), and finally introducing a doped TiO₂ nanocrystalline shell by calcination processes. Compared to traditional Au/TiO₂ composites, in which AuNPs are loosely attached to the surface of TiO₂ such that they are unstable during calcination and subsequent photocatalysis,²⁵ the novel tube-in-tube structure with the AuNPs embedded inside a TiO₂ matrix protects the former from moving together and coagulating. As discussed later, only a relatively small quantity (0.06 wt%) of AuNPs are required for optimal catalytic performance, thus making this catalyst feasible for large-scale practical applications. Herein we demonstrate the excellent performance of the new photocatalyst in degradation reactions of a number of organic compounds under visible light.

Experimental

Synthesis of silica-coated MWCNTs (MWCNTs@SiO₂)

The MWCNTs@SiO₂ nanoparticles were prepared by using a published method with a slight modification.²⁶ In a typical synthesis, 50 mg of MWCNTs and 500 mg of cetyltrimethyl ammonium bromide (CTAB) were dispersed into 15 mL of deionized water and the mixture was sonicated for 30 min. Next, the above mixture was added to 65 mL of anhydrous ethanol and sonicated for 10 min to form a stable dispersion. This was followed by the immediate addition of 1 mL of NH₃·H₂O (28%) to the as-prepared MWCNTs dispersion. Then, tetraethyl orthosilicate (TEOS) solution (0.3 mL TEOS in 2 mL ethanol) was added dropwise to the mixture under ultrasonication, and the reaction mixture was magnetically stirred for another 12 h at room temperatures. Finally, the obtained solution was centrifuged, washed with ethanol several times and then dried at 30 °C in a vacuum oven.

Synthesis of mSiNTs and amino groups modified mSiNTs (mSiNTs-NH₂)

The MWCNTs@SiO₂ nanoparticles were heat-treated at 560 °C for 6 h in air at a rate of 3 °C min⁻¹ to remove the MWCNTs and CTAB, resulting in mSiNTs.⁶ For the surface modification, the obtained mSiNTs (50 mg) was further diluted with isopropyl alcohol (50 mL) containing 3-aminopropyl-triethoxysilane (3-APTES, 0.1 mL) under ultrasonication and heated to 80 °C for 2 h to functionalize the mSiNTs surface with –NH₂ groups. The surface modified particles were centrifuged, washed with ethanol several times and then dried at 30 °C in a vacuum oven.

Synthesis of AuNPs

AuNPs were prepared according to Frens's method.²⁷ An aqueous solution of HAuCl₄·4H₂O (0.205 mL, 24 mM) was added to deionized water (10 mL) and then stirred rapidly in a clean and new three-neck round-bottom flask equipped with a condenser. The flask was immerged in an oil bath for heating at 100 °C. When the solution began to be boil, 0.5 mL of trisodium citrate solution (1.0 wt%) was added by injection. After further stirring for about 30 min, the dark-red solution containing AuNPs was obtained. The solution was cooled down by magnetically stirring at room temperature for further uses.

Deposition of AuNPs on mSiNTs-NH₂ (mSiNTs-NH₂–Au)

In an optimized deposition process, a certain amount of the AuNPs solution was mixed with 50 mL of aqueous dispersion of mSiNTs-NH₂ by ultrasonication. The mixture was subjected to magnetically stirring for 3 h, and the mSiNTs-NH₂–Au was centrifuged, washed with ethanol several times and then dried at 30 °C in a vacuum oven.

Synthesis of TiO₂-coated mSiNTs-NH₂–Au (mSiNTs-N/Au@TiO₂)

The mSiNTs-NH₂–Au (50 mg) particles were dispersed in a mixture of CTAB (125 mg), ethanol (75 mL), and deionized water (4.8 mL). After stirring for 20 min, titanium tetrabutyl titanate (TBOT, 0.35 mL) in ethanol (4 mL) was added drop by drop to the mixture. After injection, the temperature was increased to 85 °C under refluxing conditions for 4 h. The precipitate was isolated by using centrifugation, washed with ethanol several times and then dried at 30 °C in a vacuum oven. The mSiNTs-NH₂–Au@TiO₂ was calcined in air at 500 °C for 2 h to remove all organic compounds and crystallize the amorphous TiO₂.

Characterization

Transmission electron microscopy (TEM) images were obtained using a Tecnai G2 F30 electron microscope operated at 300 kV. The samples for TEM analysis were prepared by dipping carbon coated copper grids into ethanolic solutions of samples and drying at ambient condition. Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet NEXUS 670 FT-IR spectrometer with a DTGS detector, and samples were measured with KBr pellets. X-ray powder diffraction (XRD) measurements were carried out at room temperature and performed using a Rigaku D/max-2400 diffractometer with Cu-Kα radiation as the X-ray source in the 2θ range of 10–80°. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method and pore sizes by the Barrett–Joyner–Halenda (BJH) methods using the Tristar II 3020 instrument. X-ray photoelectron spectroscopy (XPS) was performed using a PHI-5702 instrument (America Electrophysics Corporation) and the C 1s line at 291.4 eV was used as the binding energy reference. UV-vis spectroscopy measurements were conducted with a Cary 5000 UV-vis-NIR spectrophotometer (Agilent Technologies). Inductive coupled plasma atomic emission spectrometric (ICP-AES) analysis was conducted with a Perkin-Elmer Optima-4300DV instrument.

Photocatalytic activity measurement

The aqueous solution of RhB was used as the target pollutants to evaluate the photocatalytic performance of our samples. Typically, 50 mL RhB (1 × 10⁻⁵ mol L⁻¹) and 4 mg mSiNTs-N/Au@TiO₂ were firstly dispersed in a 100 mL home-made beaker, which contained a cooling water system. The distance between the bottom surface of the lamp and the top surface of the solution was kept at a constant of 10 cm. Prior to irradiation, the solution was magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption–desorption equilibrium between mSiNTs-N/Au@TiO₂ and RhB. For the UV irradiation experiment, a 400 W high-pressure mercury lamp was used as the UV radiation source. For the visible light irradiation experiment, a 200 W tungsten lamp was used as the light source, and a cut-off filter was used to block the UV light (<420 nm). For the natural sunlight reaction, the reaction flasks were exposed to natural sunlight directly.

At given time intervals, 2 mL suspensions were sampled and centrifuged twice at 10 000 rpm for 5 min to remove photocatalyst powders, and then the filtrates were transferred to a quartz cuvette for measuring their absorption spectra in a wavelength range of 300–800 nm by diffusive reflective UV-vis spectroscopy. In the recycling experiments, after the mSiNTs-N/Au@TiO₂ composites were separated from solution by centrifugation at 10 000 rpm for 5 min, we removed the clear upper solution, and then redispersed in the RhB solution (50 mL) for another cycle. To ensure the RhB was completely mineralized and had no influence on the next cycle, we increased the irradiation time to 1 h for each cycle. The mass of the TiO₂ layer was determined by measuring the weight difference before and after TiO₂ coating, and the concentration of TiO₂ in the reaction solution was about 40 mg L⁻¹ for all the runs.

Photoelectrochemical measurements

The fluorine-doped tin oxide (FTO) conductive glasses, as working electrode, were cleaned by sonication in toluene, acetone and ethanol for 30 min, respectively. 5 mg photocatalyst powder was dispersed into 0.5 mL ethanol under sonication for 30 min to get slurry. The as-prepared slurry spread onto the conductive surface of FTO glass to form a photocatalyst film with the area of 1 cm².

Photocurrent was measured by the conventional three electrode electrochemical cell with a working electrode, a platinum foil counter electrode and Ag/AgCl (3 M KCl) as reference electrode. The working electrode was immersed in a Na₂SO₄ electrolyte solution (0.2 M) and irradiated by a 300 W xenon lamp (HSX-F/UV 300, Beijing, China) with a 420 nm cut-off filter. The light/dark short circuit photocurrent response was recorded with electrochemical workstation (CHI 660C, Shanghai, China).

Results and discussion

The novel tube-in-tube structured mSiNTs-N/Au@TiO₂ was prepared in five steps (Scheme 1): firstly, SiO₂ layer was coated onto the MWCNTs. Secondly, mSiNTs was fabricated by means of calcination to remove the template MWCNTs and CTAB. Thirdly, the functionalization of mSiNTs was carried out by 3-APTES, which originally acted as a ligand for immobilization of AuNPs on the surface of the mSiNTs support, but upon subsequent decomposition at high temperature served as a source of both N and/or C for doping. Fourthly, amorphous TiO₂-coated mSiNTs-NH₂–Au was fabricated through versatile sol–gel process. At last, mSiNTs-N/Au@TiO₂ was obtained by calcination at 500 °C.

Scheme 1 Programmed synthesis of mSiNTs-N/Au@TiO₂.

In order to visualize the microstructure of the sample after each step of the catalyst preparation, TEM was used. Fig. 1a represented the TEM image of the templates MWCNTs, using TEOS as the silica source, a thin SiO₂ layer with a thickness of ∼20 nm was successfully coated onto the MWCNTs (Fig. 1b), which were then calcined at 560 °C in air to remove the MWCNTs and CTAB to obtain mSiNTs (Fig. 1c). The tubular architectures was stable even after calcination at 560 °C. To deposit AuNPs efficiently, mSiNTs were modified with 3-APTES to endow abundant amino groups on surface, which could enhance the interaction with AuNPs by coordination bonds.²⁸ FT-IR spectra confirmed the successful surface modification, and the mSiNTs-NH₂ nanotubes were obtained (Fig. S1†). AuNPs were prepared by citrate reduction of HAuCl₄·4H₂O in aqueous solution according to Frens's method.²⁷ From the inset picture of Fig. 1d, it could be seen that the size distribution histograms indicated that more than 90% of the AuNPs fall in the size range 9–13 nm and the mean particle diameter was about 11 nm. In the presence of CTAB, the mSiNTs-N/Au composite colloids were overcoated with a layer of amorphous TiO₂ about 20 nm by hydrolysing TBOT in ethanol solution (Fig. 1e). The reddish composite was finally calcined in air at 500 °C for 2 h to remove CTAB and crystallize the amorphous TiO₂, as shown in Fig. 1f. The silica-protected calcination helped to maintain the integrity of the tubular morphology,^6,24 and more importantly, since the catalytic AuNPs were effectively immobilized between the mSiNTs and TiO₂ layers, they were easily accessible, highly stable and antiaggregation, and thus reusable with extra-long life in practical applications.²⁹ The elemental mapping had been performed to reveal the element distribution in the mSiNTs-N/Au@TiO₂ (Fig. S2†). The mapping results indicated that the elements Si, O, Au and Ti spread evenly in the tubular composite, which confirmed the structure of mSiNTs-N/Au@TiO₂ as obtained by the programmed synthesis. To illustrate the distribution of AuNPs more clearly, TEM of the samples (mSiNTs, mSiNTs-N/Au and mSiNTs-N/Au@TiO₂) were also characterized and shown in Fig. S3.† The two results proved that AuNPs were immobilized on the inner surface of the tube-in-tube structure.

Fig. 1 TEM images of (a) MWCNTs, (b) MWCNTs@SiO₂, (c) mSiNTs, (d) AuNPs (inset pictures: size distribution histograms image of the AuNPs), (e) mSiNTs-NH₂–Au@TiO₂, and (f) mSiNTs-N/Au@TiO₂.

Fig. 2 showed XRD data used to identify the crystallographic phases of the TiO₂ in the as-prepared tube-in-tube structured titanium catalysts mSiNTs-N/Au@TiO₂. As shown, when the sample was calcined at 500 °C, weak and broad peaks at 2θ = 25.5°, 37.9°, 48.2°, 53.8° and 63.2° were observed. These peaks represented the indices of (101), (004), (200), (105) and (204) planes of anatase phase, respectively.²⁵ No characteristic peaks of crystallized Au could be found in mSiNTs-N/Au@TiO₂ because of its very low concentration.²⁵ As shown in the inset of Fig. 2, in the small-angle XRD range of 2θ = 0.5–5°, there was a 2θ = 0.85 reflection in mSiNTs-N/Au@TiO₂. This proved that the mSiNTs-N/Au@TiO₂ had long-range ordered mesoporous structures.³⁰

Fig. 2 The wide-angle XRD patterns of mSiNTs-N/Au@TiO₂.

Fig. 3 displayed the N₂ adsorption–desorption isotherm and the corresponding pore-size distribution curve for mSiNTs-N/Au@TiO₂. The isotherm exhibited the type IV BET isotherms with a H3-type hysteresis loop (P/P₀ > 0.4), demonstrating the mesoporous characteristics of the mSiNTs-N/Au@TiO₂.^6,31,32 The pore size was calculated by using the BJH method, as shown in the inset of Fig. 3. As a result, average pore size was about 4.9 nm and the BET surface areas and the cumulative pore volumes of mSiNTs-N/Au@TiO₂ were 297 m² g⁻¹ and 0.8 cm³ g⁻¹. Such a high surface area is attributed to the mesoporous tube-in-tube structures and is beneficial for photocatalytic activity by not only adsorbing more RhB molecules but also offering more reaction sites.^6,10

Fig. 3 Nitrogen adsorption–desorption isotherm of the mSiNTs-N/Au@TiO₂ (inset: the corresponding pore diameter distribution).

To illustrate the surface composition of the mSiNTs-N/Au@TiO₂ photocatalyst, the mSiNTs-N/Au@TiO₂ samples were characterized by the XPS analysis, as shown in Fig. 4. The sample surface contained Si, O, C, N, Au and Ti elements according to the XPS survey spectra. The high-resolution XPS spectra of the Si 2p and O 1s of sample (Fig. S4a and b†) were identical with the reported values.²⁵ As previously reported, for the simple TiO₂-coated SiO₂ sample, without doping, Ti exhibited a 2p_3/2 peak at 458.5 eV, which corresponded to the binding energy of Ti⁴⁺ in TiO₂.³³ Once 3-APTES was introduced between the SiO₂ and the TiO₂ shell, however, the Ti 2p_3/2 peak shifted to a lower binding energy of 456.7 eV (see Fig. S4c†). This shift was indicative of the incorporation of N and/or C into the TiO₂ lattice. The existence of additional N and/or C resulting from the decomposition of APTES had also been confirmed by the C 1s and N 1s spectra (see Fig. S4d and e†).

Fig. 4 XPS measurements for the as-obtained mSiNTs-N/Au@TiO₂ photocatalyst.

The diffuse reflectance spectra of the mSiNTs-N/Au@TiO₂ and the reference samples were shown in Fig. 5. It could be apparently observed that the mSiNTs-N@TiO₂ sample strongly absorbed light in the visible light region compared with commercial TiO₂ which strongly absorbed light only in the UV region. This result confirmed doping of TiO₂ with N and/or C as reported by Giamello and co-workers.³⁴ The absorption peak at around 550 nm was mainly ascribed to the characteristic surface plasmon resonance (SPR) peak of AuNPs.³⁵ This ensured the mSiNTs-N/Au@TiO₂ photocatalyst work under visible light irradiation.

Fig. 5 UV-vis diffuse reflectance spectra for commercial anatase, mSiNTs-N@TiO₂ and mSiNTs-N/Au@TiO₂.

Visible light induced photocatalytic activity

The photocatalytic activities of the mSiNTs-N/Au@TiO₂ were evaluated by degrading RhB under UV light irradiation. The temporal concentration changes of RhB aqueous solution were determined by UV-vis spectra. Fig. 6 showed the absorption spectra of an aqueous solution of RhB exposed to UV light for various time periods using mSiNTs-N/Au@TiO₂ as the catalyst. The typical absorption peak at ∼553 nm gradually diminished as the UV exposure time increased, and completely disappeared after 30 min, suggesting the complete photodegradation of RhB by the mSiNTs-N/Au@TiO₂ composites.³⁶ In addition, the catalyst had similar adsorption capacity in the range of ∼15% of C/C₀ of RhB, owing to the mesoporous tube-in-tube structures.

Fig. 6 UV-vis adsorption spectra showing UV-degradation of RhB using the mSiNTs-N/Au@TiO₂.

To investigate the origin of the enhanced photocatalytic activity, we compared the performance of the mSiNTs-N/Au@TiO₂ to mSiNT, mSiNTs@TiO₂ and mSiNTs-N@TiO₂ composites synthesized under similar conditions by using the same mass of the SiO₂ and TiO₂. As shown in Fig. 7, without the modification of 3-APTES, the mSiNTs@TiO₂ structures showed very low activity under visible light irradiation. However, after N and/or C doping, the mSiNTs-N@TiO₂ became active and shows 56% degradation of RhB in 12 h. Additional decoration of the mSiNTs-N@TiO₂ interface with AuNPs further increased the decomposition rate. The mSiNTs-N@TiO₂ photocatalyst with different Au content were also prepared to examine the effect of Au content on photocatalytic degradation of RhB under visible light irradiation. It is clear that the RhB photodegradation over the catalysts follows pseudo first-order kinetics, and the photocatalytic reaction can be described simply by ln(C₀/C) = kt, in which C and C₀ are the actual and initial concentration of RhB, and k is the apparent degradation rate constant.^36,37 The kinetics of RhB degradation under visible light irradiation for all samples was presented in Fig. S5.† As shown, the corresponding apparent degradation rate constants for mSiNTs, mSiNTs@TiO₂, mSiNTs-N@TiO₂, and mSiNTs-N/Aux@TiO₂ (x = 0.02%, 0.04%, 0.06%, 0.08%) were estimated to be 0.0039 h⁻¹, 0.0253 h⁻¹, 0.0559 h⁻¹, 0.0664 h⁻¹, 0.0981 h⁻¹, 0.1774 h⁻¹ and 0.1338 h⁻¹, respectively. It could be seen that the photocatalytic reaction rate constants followed the order mSiNTs-N/Au_0.06%@TiO₂ > mSiNTs-N/Au0.08%@TiO₂ > mSiNTs-N/Au0.04%@TiO₂ > mSiNTs-N/Au_0.02%@TiO₂ > mSiNTs-N@TiO₂ > mSiNTs@TiO₂ > mSiNTs. According to the previous report, AuNPs supported on TiO₂ have three probable functions in photocatalysis.^38–40 When the loading of AuNPs is low, their primary function is to enhance the charge separation and hence promote the oxidation of organic molecules under visible light. Whereas, superabundant AuNPs may also deteriorate the catalytic performance by increasing the occurrence of exciton recombination, with the reaction rate gradually decreasing when the loading of AuNPs exceeds 0.06 wt%. Only a relatively small quantity (0.06 wt%) of AuNPs were required for optimal catalytic performance, thus making this catalyst feasible for large-scale practical applications.

Fig. 7 Photodegradation of RhB by using mSiNTs (□), mSiNTs@TiO₂ (○), mSiNTs-N@TiO₂ (△), mSiNTs-N/Au0.02%@TiO₂ (▽), mSiNTs-N/Au0.04%@TiO₂ (◁), mSiNTs-N/Au0.06%@TiO₂ (☆) and mSiNTs-N/Au0.08%@TiO₂ (▷) as photocatalyst under visible-light illumination.

The photocatalytic properties of the mSiNTs-N/Au@TiO₂ were studied and contrasted to those of control samples by carrying out chronoamperometry (CA) measurements under an inducing potential of 0.5 V (vs. Ag/AgCl) and a periodic irradiation. As shown in Fig. 8, without light exposure, all samples showed only a small electrochemical current. When they were irradiated by xenon lamp with a 420 nm cut-off filter, however, the current density increased due to the contribution from photo-generated electrons. It could be apparently observed that the mSiNTs-N/Au@TiO₂ showed a significantly higher value (0.32 μA cm⁻²) than P25 (0.17 μA cm⁻²) and mSiNTs-N/@TiO₂ (0.25 μA cm⁻²). The shape of the CA curves was well maintained after many cycles of light illumination, thus implying very good photocatalytic stability.

Fig. 8 Chronoamperometry measurements of P25, mSiNTs-N@TiO₂, and mSiNTs-N/Au@TiO₂ under periodic illumination with visible light.

To explore the photocatalytic activity of mSiNTs-N/Au@TiO₂ for real applications, the photodegradation of organic compounds was investigated under natural sunshine, which contains both UV, visible, and IR light. As shown in Fig. 9a, sunlight could completely decompose RhB within 70 min with the aid of photocatalyst mSiNTs-N/Au@TiO₂, while the conversion only reached approximate values of 70% with P25 and 60% with the commercial TiO₂ during the same period. We also tested the mSiNTs-N/Au@TiO₂-catalyzed degradation of other organic molecules under sunlight, including rhodamine 6G (R6G) and methylene blue (MB) at the same initial concentration. As shown in Fig. 9b, all of those molecules could be decomposed greatly in 60 min. As shown in Fig. 9c, the photocatalyst mSiNTs-N/Au@TiO₂ was also mechanically robust and chemically stable: it could be recovered and reused to catalyze multiple cycles of degradation reactions under direct sunlight.

Fig. 9 (a) Photodegradation of RhB without catalyst (■) and with commercial TiO₂ (●), P25 (▲) and mSiNTs-N/Au@TiO₂ (★) under direct sunlight illumination. (b) Photodegradation of R6G (●), RhB (★) and methylene blue (▲) with mSiNTs-N/Au@TiO₂ under direct sunlight illumination. (c) Nine cycles of the photodegradation of RhB with mSiNTs-N/Au@TiO₂ under sunlight illumination.

Conclusion

In summary, we have prepared tube-in-tube structured mSiNTs-N/Au@TiO₂ photocatalyst that shows high efficiency in catalyzing decomposition of organic compounds under illumination of visible light. The excellent photocatalytic efficiency can be attributed to the interfacial nonmetal doping, which improves visible light activity, to the plasmonic AuNPs decoration, which enhances light harvesting and charge separation, and to the small grain size of anatase nanocrystals, which reduces the exciton recombination rate. Additionally, the design concept for the multifunctional nanomaterials can be extended to the fabrication of other multicomponent nanosystems with integrated and enhanced properties for various advanced applications.

Acknowledgements

The authors are grateful to the Fundamental Research Funds for the Central Universities (Grant no. lzujbky-2015-16), the Projects in Gansu Province Science and Technology Pillar Program (1204GKCA047), and the Key Laboratory of Catalytic engineering of Gansu Province, China Gansu Province for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11934k

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