Synthesis of V₂O₅@TiO₂ core–shell hybrid composites for sunlight degradation of methylene blue

Abstract

This study has demonstrated a facile but efficient synthesis method to prepare vanadium oxide@titanium oxide (V₂O₅@TiO₂) core–shell nanostructures using a water bath at mild temperatures (≤100 °C). This method shows a few unique features, including a short reaction time for fabricating core–shell nanostructures, no requirement of high temperature calcination (>500 °C) for TiO₂ crystallization, easily tunable TiO₂ shell thickness, high yield, and good reproducibility. With characterization using several advanced techniques (TEM, BET, XRD, XPS and UV-vis spectroscopy), the as-prepared V₂O₅@TiO₂ nanocomposites were found to exhibit a large surface area, and a good stability. The experimental results show that the V₂O₅@TiO₂ core–shell composites show a superior sunlight photocatalytic activity compared to the pure TiO₂ nanoparticles for the degradation of organic dyes (e.g., methylene blue), probably because of the matched energy bands between V₂O₅ and TiO₂. These findings may bring new insights into the designing of TiO₂-based core–shell and other nanocomposites with enhanced photocatalytic efficiencies for environmental remediation.

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Introduction

The design and fabrication of core–shell nanoparticles have attracted much attention because of their unique shape, size and structure-dependent properties.^1,2 The synergetic effects between the core and shell materials may drastically improve the overall performance of the hybrid composites, which results in a range of potential applications, including biomedical applications,^3,4 catalysis,^5,6 electronics applications⁷ and chemical sensors.⁸ The properties of core–shell nanostructures can be adjusted by changing either the constituting materials or the core to shell ratio.⁹ Among the well-known semiconductors, titanium oxide (TiO₂) nanoparticles have been regarded as one of the most promising nanomaterials because of their abundance, low-cost, long-term stability and nontoxicity.¹⁰ In particular, TiO₂ coating on different metals or metal oxide nanoparticles as TiO₂-based core–shell nanocomposites has shown high potential for research interests.¹¹ However, compared with other hybrid composites with high visible light photocatalytic activity,^12–16 its photocatalytic efficiency is still restricted by a few factors such as fast electron–hole recombination and the limited utilization of sunlight in the visible light range (400–800 nm). The above issues impede its broad applications in energy and environmental areas.

TiO₂ heterojunction materials have been developed as an effective method to improve the photocatalytic performance for pollutant degradation. For example, when TiO₂ combines with V₂O₅ to form a heterojunction structure, the light absorption could be improved by introducing V₂O₅ with a narrow band gap of 2.2 eV (λ = 564). This may allow more visible light absorption, and hence improve the photocatalytic activity.¹⁷ Moreover, V₂O₅ can act as an electron scavenger in the heterojunction structure, which benefits the electron–hole pairs and hence improves the whole photocatalytic performance.¹⁸ To generate such V₂O₅/TiO₂ hybrid nanocomposites, various approaches have been widely explored, however, many of them focused on the surface decoration of V₂O₅ on TiO₂ particles, such as the hydrothermal method,¹⁹ chemical vapour deposition (CVD) method,²⁰ sol–gel method,²¹ solid state reaction,²² and micro-arc oxidation method.²³

Despite many efforts, the synthesis of V₂O₅@TiO₂ core–shell nanostructures has been little reported. Recently, Li et al. reported a hydrothermal method to prepare one-dimensional V₂O₅@TiO₂ core–shell nanobelts.¹⁹ Ajay et al. reported that V₂O₅@TiO₂ core–shell nanorods could be synthesized via hydrothermal methods and then chemically anchored on graphene nanosheets for water remediation.²⁴ However, it is still difficult to coat TiO₂ as a thin film on the surface of V₂O₅ particles through a simple method, with low costs and scale-up potential.

In this study, we aim to develop a facile route to prepare V₂O₅@TiO₂ core–shell nanostructures under mild conditions (aqueous system, <100 °C). The microstructure, composition and surface area will be characterized using various advanced techniques, such as transmission electron microscopy (TEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) and ultra violet-visible absorption spectroscopy (UV-vis). The photocatalytic performance in the degradation of methylene blue (MB) molecules using sunlight will be tested and evaluated. Finally, a possible mechanism to improve the photocatalytic performance of the heterojunction V₂O₅@TiO₂ core–shell nanostructures will be discussed.

Experimental

Preparation of V₂O₅ particles

A glycothermal approach was used in this study for the synthesis of V₂O₅ nanosheets with a slight modification, based on our previous report.²⁵ In a typical procedure, 0.55 g Na₃VO₄ was added into a three-necked flask containing 20 mL ethylene glycol (EG), and the mixture was stirred to make sure all of the sodium orthovanadate was dissolved completely. Then, a few droplets of concentrated HCl were gradually added to adjust to pH = ∼1. It was found that the colour of the solution changed from opaque light yellow to brick red with the addition of HCl. The brick-red solution was heated in an oil bath at 120 °C; after ∼20 min, the colour of the solution became greenish black. When continuously heated for another 10 min, some blue precipitate was formed in the reaction system. The solution was continuously heated for several hours and then cooled down to room temperature naturally. The precipitate was collected using centrifugation and washed with ethanol 2–3 times, followed by drying in an oven at 60 °C for a few hours.

Synthesis of V₂O₅@TiO₂ nanoparticles

In a typical protocol, a few steps were involved. First, 0.05 mL titanium(IV) butoxide 97% (TBT) was added to 10 mL EG. The mixture was magnetically stirred for 8 h at room temperature to become a clear solution, noted as solution A. Second, 3 mL of the newly synthesized V₂O₅ colloidal suspension (0.01 M) was poured into 10 mL acetone under stirring for around 5 min, noted as solution B. Third, 0.5 mL of solution A was added to solution B with strong vibration for a few seconds and then left to age for around 1 h. The precipitate, titanium precursor coated V₂O₅, was harvested using centrifugation and then dispersed in a boiling water bath for ∼2 h. Finally, the white grey precipitate was collected using centrifugation at 3000 rpm for ∼10 min, following by washing with alcohol, and then dried under vacuum at 60 °C for a few hours to generate the target V₂O₅@TiO₂ core–shell particles.

Characterizations

Particle characteristics and properties were examined using various advanced microscopic and spectroscopic techniques, as demonstrated below:

The shape and microstructure was confirmed using TEM (JEOL 1400), operated at an accelerated voltage of 100 kV. The TEM specimen was prepared by dropping solution onto a copper grid and drying in air naturally. The crystalline structure of the composite colloids was further characterized using high-resolution TEM (HRTEM), Philips CM200, at an accelerating voltage of 200 kV. To observe surface structure, SEM (Hitachi 900) was used and operated at 20 kV. To further identify the elemental composition, XPS analysis was performed with a Physical Electronics PHI 5000 Versa probe spectrometer with Al Kα radiation (1486 eV). Analysis of the spectra was conducted using the Physical Electronics Multipack software package. Composition analysis of the final products was conducted using XRD (Philip MPD diffractometer) with Cu Kα radiation with a scanning step of 2θ = 0.02° s⁻¹. The optical properties were obtained on a Shimadzu UV-2600 UV-vis spectrophotometer (Varian) with a 1 cm quartz cell. The surface area of the as-synthesized powders and pore size distribution were measured using BET equipment (TriStar 3000) via nitrogen gas adsorption and desorption isotherms.

Photocatalysis test

The photocatalytic behaviour of the TiO₂-based nanocomposites was determined by measuring the decolourization of MB under simulated sunlight irradiation in a batch reaction. The reactor volume was 250 mL. A 300 W xenon lamp (PLS-SXE300C) equipped with AM 1.5G total reflection filters was used to obtain simulated sunlight to trigger the photocatalytic reaction, and the lamp was positioned about 15 cm away from the reactant solution meniscus. The luminance of the light source over the reactant solution was 0.7 W cm⁻². A 100 mL solution of 30 ppm MB was injected into the reaction system, while 0.020 g of the photocatalyst was added. Prior to irradiation, the suspension was magnetically stirred in the dark for ∼30 min to ensure the adsorption–desorption equilibrium of the dye molecules on the surface of TiO₂ nanocomposites. At a given time interval, 3 mL of the solution was sampled and filtrated/centrifuged to remove the catalysts. The suspension was analysed by recording the variations of the absorption band maximum (657 nm) of methylene blue using a Shimadzu UV-2600 UV-vis spectrophotometer. After various time intervals, the remaining MB concentration could be estimated using the following equation:

MB concentration = C/C₀ × 100% (1)

where C₀ is the initial MB absorbance at 657 nm and C is the absorbance obtained after various time intervals. The reaction rate (k) could be estimated as a linear relationship between ln(C/C₀) and time (t):

ln(C/C₀) = kt (2)

To eliminate the effect of heating from irradiation, the reactor was equipped with a reflux condenser. After decolourization, the supernatant of the solution was obtained via centrifugation, and then characterized using UV-vis spectroscopy.

Results and discussion

Morphology of V₂O₅@TiO₂ core–shell nanocomposites

Fig. 1 shows the morphologies of pure V₂O₅ nanoparticles, V₂O₅@titanium glycolate core–shell nanostructures and V₂O₅@TiO₂ core–shell nanocomposites. The V₂O₅ nanoparticles generated by the glycothermal approach²⁵ have two-dimensional (2D) irregular shapes with lengths and widths from 100 to 500 nm (Fig. 1A). Through adding titanium glycolate into the V₂O₅ nanoparticles containing acetone suspension with a molar ratio of V₂O₅ to titanium glycolate of 1 : 100, the TiO₂ precursor (titanium glycolate) can be uniformly coated onto the surface of all V₂O₅ nanoparticles. As shown in Fig. 1B, the thin shell of the titanium glycolate is around 15 nm in thickness, which can be controlled by adjusting the concentration of the titanium glycolate in the acetone. The TiO₂ shell thickness increased with the amount of titanium glycolate in the composites. Through using a boiling water bath for ∼2 hours, the shell of amorphous titanium glycolate could be transferred to crystallized TiO₂ (Fig. 1C), based on our previous work.²⁶

Fig. 1 TEM images of (A) V₂O₅ nanoparticles; (B) titanium glycolate (TG) coated V₂O₅ nanoparticles; (C) TiO₂ coated V₂O₅ nanoparticles; (D) SEM images of TiO₂ coated V₂O₅ nanoparticles; (E) HRTEM images of TiO₂ coated V₂O₅ nanoparticles; and (F) electron diffraction pattern of TiO₂ coated V₂O₅ nanoparticles.

As a further confirmation, the HRTEM image (Fig. 1F) shows that the TiO₂ shell is composed of tiny crystals with sizes of less than 10 nm, and the TiO₂ shell is well connected with V₂O₅. The SEM image of the as-prepared V₂O₅@TiO₂ core–shell heterojunction shown in Fig. 1D confirms that the surface of the TiO₂ shell is rough and composed of many tiny TiO₂ particles. This is good for potential applications which need large surface areas. The electronic diffraction pattern of the V₂O₅@TiO₂ core–shell composites is shown in Fig. 1F. The cycles (200 and 400 crystalline planes) may originate from anatase TiO₂ crystals, in agreement with our previous studies.²⁷

Composition analysis

To confirm the composition of the V₂O₅@TiO₂ core–shell nanostructures, the X-ray diffraction (XRD) technique was used and conducted. As shown in Fig. 2, for pure TiO₂ nanoparticles prepared under the same conditions without V₂O₅ cores, the XRD pattern shows that there were diffraction peaks centered at 2θ = 25.3°, 38.5°, 48°, 53.8°, and 62.6°, consistent with those from anatase TiO₂; while the peak centered at 2θ = 30.8° could be assigned to that from the brookite phase.²⁸ The composition of the V₂O₅@TiO₂ core–shell nanocomposites was also confirmed by the XRD pattern, compared with pure TiO₂ nanoparticles as shown in Fig. 2. The orthorhombic phase of V₂O₅ (JCPDS 01-089-0612) could be indexed by referring to the previous study.²⁵ It was found that no diffraction peaks were identified as the TiO₂ phase, as the shell. This may be caused by the very small weight fraction of the TiO₂ shell, compared with that of the V₂O₅ cores in the whole core–shell nanocomposites.

Fig. 2 XRD patterns of the pure TiO₂ nanoparticles and the V₂O₅@TiO₂ core–shell nanocomposites.

The surface composition of the as-prepared V₂O₅@TiO₂ core–shell structures was further investigated using X-ray photoelectron spectroscopy (XPS). Fig. 3A shows the characteristic energy peaks of the elemental titanium, oxygen and vanadium, with binding energies of E_b = 459.08 eV (Ti 2p), E_b = 530.64 eV (O 1s) and E_b = 517.88 eV (V 2p3), respectively. The XPS survey indicates that the TiO₂ shell exists on the surface of the V₂O₅ particles. The photoelectron peaks of Ti 2p, V 2p and O 1s in the core–shell structure were narrowly scanned. For the purpose of better comparison, the XPS photoelectron peaks of Ti 2p, V 2p and O 1s in pure V₂O₅ or TiO₂ particles are also presented in Fig. 3B–D. The binding energies of the corresponding peaks are summarized in Table 1.

Fig. 3 (A) XPS spectra of the as-prepared TiO₂ coated V₂O₅ nanocomposites and pure V₂O₅, and (B–D) XPS elemental spectra for O 1s, Ti 2p3, and V 2p3 in pure V₂O₅ and TiO₂ coated V₂O₅ nanocomposites.

Table 1 Binding energies for Ti, V, and O in core–shell composites (C) and pure phase (P)

	V 2p3	Ti 2p3	O 1s
Binding energy (C) (eV)	517.88	530.64	459.08
Binding energy (P) (eV)	515.95	530.58	458.18

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Fig. 3B displays the XPS spectra of Ti 2p3 in V₂O₅@TiO₂ core–shell composites and pure anatase TiO₂, with binding energies of 459.18 eV and 459.08 eV, which agree well with the values reported in the literature.²⁹ The Ti 2p3 peak in the composites slightly shifts towards a lower binding energy compared with those from the pure anatase TiO₂. This suggests that the chemical bond of the Ti atoms in the core–shell nanocomposites is different from those in the pure anatase TiO₂. The V atom could induce the electrons in the Ti–O bond and then affect the electron density of the Ti–O bond at the interface, thus causing a little decline of the Ti 2p3/2 binding energy.²³ The V 2p3 binding energy in the pure V₂O₅ was measured as 515.95 eV, observed in the XPS spectra in Fig. 3C, while the binding energy for the V species in the core–shell composites is 517.88 eV, which indicates that the V species are probably in the form of V⁵⁺ ions.³⁰ The obvious rise in the binding energy is an indication of an increase in the oxidation state of the V species which is probably due to the formation of crystalline TiO₂ at the interface.³¹ The O 1s high-resolution profile, as shown in Fig. 3D, is due to the overlapping contribution of oxygen from V₂O₅ and TiO₂ in the case of the V₂O₅@TiO₂ core–shell composites. There is a small increase in the binding energy compared with oxygen in the pure V₂O₅ particles (from 530.58 to 530.63 eV), which again confirms the change of electron density of the Ti–O bond or V–O bond.

Optical properties of the nanoparticles

The optical properties of the nanoparticles prepared in this study, including pure TiO₂, V₂O₅, V₂O₅@titanium glycolate core–shell colloids, and V₂O₅@TiO₂ core–shell nanostructures were measured and analyzed. Fig. 4 shows the UV-vis spectra of different nanoparticles and that there is no obvious surface plasmon resonance for those pure V₂O₅ particles in the range from 200 to 600 nm, probably because the particle size is too large in comparison with the small V₂O₅ nanoparticles (∼20 nm) in other TiO₂/V₂O₅ heterostructures.³² An absorption peak appears at ∼273 nm when the V₂O₅ particles are coated with titanium glycolate, which is mainly caused by the composites. After water boiling for around 2 h, the absorption peak of the V₂O₅@TiO₂ core–shell particles (200–250 nm in size) was mainly centered at ∼369 nm. In comparison, the absorption peak of the pure TiO₂ particles appeared at ∼320 nm, consistent with our previous study.²⁷ It was found that there is an obvious red shift in the absorption peak (from 320 to 369 nm) when combining V₂O₅ with TiO₂ particles, suggesting that the photocatalytic activity of V₂O₅@TiO₂ core–shell hybrid particles could be different from pure TiO₂ and V₂O₅ particles. Fig. 4B and C shows the absorption coefficient of the TiO₂ and V₂O₅@TiO₂ core–shell structure, which was calculated by Tauc’s relation.³³ The solid lines are linear extrapolations of the absorption coefficient to estimate the optical band gap energy. The variation in the optical band gap energy can generally be associated with the structural and carrier concentration changes.³⁴ By hybridizing with the semiconductor V₂O₅, the optical band gap energy of TiO₂ is blue shifted from 3.05 eV to 2.78 eV in this study, which is beneficial in enhancing the photocurrent performance and photocatalytic activity of V₂O₅@TiO₂ core–shell composites.

Fig. 4 (A) UV-vis spectra of TiO₂ nanoparticles, V₂O₅ nanoparticles, V₂O₅@titanium glycolate core–shell nanostructures and V₂O₅@TiO₂ core–shell nanostructures; Tauc plot of (B) the pure TiO₂ nanoparticles and (C) V₂O₅@TiO₂ core–shell nanostructures.

BET surface analysis of V₂O₅@TiO₂ nanoparticles

The surface area and structure of nanomaterials are critical for their functional properties and applications in catalysis, gas sensing and those applications related to the contact surfaces. In this study, BET surface analysis was conducted for V₂O₅@TiO₂ core–shell structures and V₂O₅ nanoparticles, respectively. The N₂ adsorption–desorption isotherms and the pore size distribution for the V₂O₅@TiO₂ core–shell structures are shown in Fig. 5. The isotherms can be ascribed to type IV (BDDT classification), indicating the presence of mesoporous pore structures. The BET surface area of the as prepared V₂O₅@TiO₂ core–shell nanoparticles was estimated to be ∼151 m² g⁻¹, and the average pore size is ∼6.4 nm. But without the TiO₂ shell, for pure V₂O₅ nanoparticles, as shown in Fig. 6, the BET surface area is only ∼16 m² g⁻¹ with a pore size of 12.9 nm. Therefore, coating TiO₂ small nanocrystals on the surface of V₂O₅ nanoparticles can significantly increase the surface area of nanocomposites, which may be beneficial for the proposed functional applications.

Fig. 5 BET measurements for the V₂O₅@TiO₂ core–shell structures with a surface area of S V₂O₅@TiO₂ = ∼151 m² g⁻¹ and an average pore size of 6.4 nm.

Fig. 6 BET measurements for the V₂O₅ particles with a surface area of S(V₂O₅) = ∼16 m² g⁻¹ and an average pore size of 12.9 nm.

Photocatalysis assessment

The photocatalytic performances of mixed or hybrid TiO₂/V₂O₅ nanocomposites have been widely studied.^17,35,36 The presence of another oxide may result in extending the absorbance of TiO₂ to the visible light range or prolonging the life of the separated electrons and holes, and finally increasing the photocatalytic activity. V₂O₅ has been regarded as a catalyst to facilitate reactions with organics in the gas phase.³⁷ It has been concluded that doping V₂O₅ on a TiO₂ substrate could produce a visible light response of the substrate, leading to highly enhanced visible light photocatalytic efficiency.^21,38,39 However, the effect of V₂O₅ on the photocatalytic activity of TiO₂ in V₂O₅@TiO₂ core–shell structures is rarely reported.

In this study, the photocatalytic performance of the V₂O₅@TiO₂ core–shell nanostructures was evaluated by monitoring the degradation of methylene blue (MB) in aqueous solution, under photocatalytic reaction conditions illuminated with a 300 W lamp that simulated sunlight. An organic dye suspension with the photocatalyst was kept in dark conditions for 30 minutes to ensure the establishment of the adsorption–desorption equilibrium of MB on the sample surfaces.

Fig. 7 shows the corresponding concentration changes of the MB solution and the reaction rate (k) as a function of light exposure time, using the as-prepared pure TiO₂, V₂O₅, and V₂O₅@TiO₂ core–shell structures as photocatalysts. MB solution alone without photocatalysts was also used to test the self-photodegradation of MB. Fig. 7A shows that, in the case of V₂O₅ nanoparticles, after being exposed to sunlight for 120 min, the trace change in MB decolouration indicates that there was almost no self-degradation in the case of the MB solution alone. The degradation rate of MB with V₂O₅ nanoparticles as a photocatalyst was less than 15%. This indicates that the pure V₂O₅ nanoparticles themselves have a poor photocatalytic activity. In comparison, the as-prepared TiO₂ nanoparticles show that the degradation of ∼50% MB could be achieved under similar conditions. Nearly 100% degradation of MB could be obtained if the photocatalyst is replaced with V₂O₅@TiO₂ core–shell composites. The results demonstrate that by hybridizing V₂O₅ nanoparticles, the V₂O₅@TiO₂ core–shell nanostructure enhances the photocatalytic activity of TiO₂ under sunlight irradiation. Moreover, the reaction rates of the V₂O₅ nanoparticles, V₂O₅@TiO₂ core–shell composites and pure TiO₂ nanoparticles are calculated to be 0.0017, 0.0314 and 0.0073 min⁻¹ respectively (Fig. 7B), that is, V₂O₅@TiO₂ core–shell nanostructures show a superior photocatalytic activity compared to the pure TiO₂ particles. These results are in agreement with other reported V₂O₅–TiO₂ hybrid nanostructures.^17,21,40

Fig. 7 (A) Concentration change of MB in the presence of pure TiO₂ nanoparticles, V₂O₅@TiO₂ core–shell nanocomposites and V₂O₅ nanoparticles under sunlight irradiation. (B) Photodegradation rates of the pure TiO₂ nanoparticles, V₂O₅@TiO₂ nanocomposites and V₂O₅ nanoparticles. Reaction rates (k) in the inset are calculated from the linear relationship between ln(C₀/C) and time.

The recyclability of the V₂O₅@TiO₂ core–shell nanocomposites was investigated to ascertain their stability after several photocatalytic degradation reactions, which is crucial to applying them in environmental technology. As shown in Fig. 8, no significant decrease in the activity was observed when using 20 mg of photocatalyst after four cycles exposed to the same 30 ppm MB solution under the same sunlight irradiation. There was nearly 100% degradation after 120 min, indicating its excellent photocatalytic stability.

Fig. 8 Four times-cycled degradation of 30 ppm MB using 20 mg V₂O₅@TiO₂ core–shell nanoparticles.

The superiority of the V₂O₅@TiO₂ core–shell structures under sunlight benefits from the coupling with the narrow bandgap semiconductor V₂O₅. Interfacial transfer of the charge carrier plays an important role in the photocatalytic processes. Once the system undergoes photo-excitation, a transition of electrons from the valence band to the conduction band occurs, resulting in an equal number of holes at the valence band, and the excited electrons and holes migrate to the surface of the particles. Much more excited electron–hole pairs recombine and dissipate the energy in the form of heat or emitted light. To avoid the recombination rate of electron–hole pairs, heterostructured core–shell metal oxides were incorporated.⁴¹

A schematic illustration of efficient electron–hole separation and transport in the V₂O₅@TiO₂ core–shell structure is proposed in Fig. 9. Theoretically, the band gaps of V₂O₅ and TiO₂ are 2.3 eV and 3.2 eV, similar to the values of their bulk materials separately.^42,43 Both TiO₂ and V₂O₅ have the same Fermi energy level at the composite interface.¹⁷ To maintain the same Fermi energy level during the photocatalytic process, the conduction band and valence band of V₂O₅ lie above the energy band of TiO₂. This may result in the gathering of a large number of electrons in the conduction bands of TiO₂ and holes in the valence band of V₂O₅. So the photogenerated electrons and holes can be separated efficiently at the interface. The oxygen molecules adsorbed on the surface of the photocatalysts could react with the free electrons.²⁴ Accordingly, the energy band matched heterostructures in the V₂O₅@TiO₂ core–shell structure can retard the photogenerated electron–hole recombination and prolong the lifetime of the electron–hole pairs, leading to an improved photocurrent performance and a superior photocatalytic activity.⁴¹

Fig. 9 Schematic illustration of energy band matching and a proposed mechanism of the charge carrier transition of TiO₂/V₂O₅ heterojunctions under light irradiation. E_C (conduction bands), E_V (valence bands), and E_F (Fermi level).

Conclusions

We have demonstrated a simple but efficient synthesis route to obtain V₂O₅@TiO₂ core–shell nanostructures under mild conditions (≤100 °C in water). This synthesis strategy shows a few advantages in generating anatase TiO₂ shells, such as a rapid and effective surface coating, no need for high temperature (>500 °C) treatment, and potential for scale-up production. The as-prepared V₂O₅@TiO₂ core–shell nanocomposites display a large surface area and high stability. In particular, V₂O₅@TiO₂ composites have been proven to show a superior sunlight photocatalytic activity compared to pure TiO₂ nanoparticles. The narrow bandgap V₂O₅ was employed to extend the optical response of TiO₂ to solar irradiation, meanwhile, the matched energy bands of this core–shell structure favor charge transfer and suppress the rapid recombination of photo-induced electrons and holes. This approach could be developed to a general one by replacing V₂O₅ nanoparticles with other semiconductors (e.g. Fe₂O₃, SiO₂, ZnO), metals (e.g. Ag, Au, Pt) and/or polymers (polystyrene) for generating TiO₂ core–shell nanoparticles with highly stable and reusable materials for water splitting and environmental cleaning applications.

Acknowledgements

We gratefully acknowledge the financial support from the China Postdoctoral Science Foundation (No. 2015M581353), the National Natural Science Foundation of China (No. 51404066), the National Basic Research Program of China (N130102001, L1502007), as well as the Australia Research Council (ARC) projects (FT0990942, DP1096185) and others.

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