A core–shell TiO₂@C nano-architecture: facile synthesis, enhanced visible photocatalytic performance and electrochemical capacitance

Abstract

In this work, we elegantly devised a bottom-up solvothermal strategy coupled with subsequent controllable calcination to synthesize a core–shell TiO₂@C nanohybrid with a uniform ultrathin carbon shell of ∼1–3 nm. Physicochemical investigations revealed that Rutin and ethylene glycol played a great role in successful in situ fabrication of the uniform core–shell nano-architecture. Benefiting from the appealing synergetic effect of the mesoporous core–shell structure and composition advantages, the resulting core–shell TiO₂@C with remarkable visible light response exhibited enhanced photocatalytic degradation efficiency and stability for methylene blue under visible light irradiation. Furthermore, the unique core–shell TiO₂@C, thanks to its large surface area, rich mesoporosity and high electronic conductivity, demonstrated excellent electrochemical capacitance with a large specific capacitance of 210 F g⁻¹ at 0.2 A g⁻¹, and ∼2% capacitance degradation over cycling for 1200 times in 0.5 M aqueous H₂SO₄ at a current rate of 1 A g⁻¹.

---

1. Introduction

In last decades, nano-structured anatase titanium dioxide (TiO₂) has attracted considerable attention as a low-cost, nontoxic, green and environmentally-benign functional semiconductor,¹ and has been studied and applied widely in versatile fields, including supports of fuel cell catalysts,¹ Li-ion batteries,² Li–S batteries,^3,4 photoelectrochemical solar cells,⁵ and so on. In particular, the utilization of various carbonaceous materials as the building block towards anatase TiO₂-based nanohybrids has been retrieved extensively for potential photocatalytic degradation of a variety of organic environmental pollutants,^6–9 and low-cost electroactive materials for electrochemical capacitors (ECs)^10–14 recently, in view of the striking synergetic effects of the carbon and TiO₂ phase.

As is well known, the catalytic efficiency of photocatalytic procedures strongly depends both upon the solar energy utilization and photo-generated electron/hole recombination rate.^6–9 As regards to the photocatalytic application, anatase TiO₂ is still limited seriously by its large band-gap energy (E_g, 3.2 eV) resulting in low utilization (just small ultraviolet fraction, <4%) of the total solar spectrum if it is utilized alone.¹⁵ Numerous efforts thus have been devoted to hybridize TiO₂ with carbonaceous materials by forming core–shell TiO₂@C architecture to extend the photo-response to the visible region. In this regards, elegant combination of the intrinsic photocatalytic activity of TiO₂ and the adsorptivity of carbonaceous materials is simultaneously obtained with the final aim to reduce electron–hole recombination rate, increase the light utilization efficiency, enhance the stability of the anatase phase and achieve a narrow band gap meanwhile.^7–9,16,17 Additionally, the non-polar, non-reactive and non-toxic nature of the carbon shell and easy separation of hybrid TiO₂@C with core–shell structure from aqueous solution are attractive greatly from a commercial application of point of view.¹⁷ However, the huge challenges by far inherent in the core–shell approach mainly lie in how to achieve a uniform and ultrathin carbon shell for a compromise between the photo-activity and photo-adsorption.¹⁸ Moreover, the synthetic strategies of core–shell TiO₂@C generally involve in an extra-coating process,^1,9,11,16 which makes the synthesis more sophisticate and increases the fabrication cost meanwhile. Therefore, it is of great significance yet urgent to explore simple but efficient strategy to realize the state-of-the-art design of uniform core–shell TiO₂@C nanohybrid with an ultrathin coating shell.

Furthermore, recent years have witnessed the application of TiO₂ for high-performance ECs as an intriguing electrode.^10–14 Unfortunately, owing to the intrinsically modest electrical conductivity (∼10⁻⁵ to 10⁻² S cm⁻¹) of the TiO₂ itself,^12,19 low specific capacitances (SCs) of <100 F g⁻¹ are commonly observed.^10,13 Charmingly, recent reports explicitly reveal that the carrier densities of nanophase TiO₂ can be enhanced significantly by 3 orders of magnitudes through the introduction of Ti³⁺ sites (i.e., oxygen vacancy) upon hydrogenation at high temperatures.^12,13 However, one should note that the utilization of hydrogen at enhanced temperatures are too dangerous to favor for the practical industrial application. Inspired by this, we hypothesize that it would be a better choice by constructing typical core–shell TiO₂@C nanohybrid, where the carbon shell with excellent electric double-layer capacitance (EDLC) would not only enhance electronic conductivity of the pristine TiO₂ by intimate adhesion to the conducting carbon, but also introduce Ti³⁺ ions via the carbothermal reduction over high-temperature carbonization meanwhile. As a result, appealing electrochemical capacitance would be highly anticipated. Also of note, the newly-formed Ti³⁺ sites would boost the visible light response of TiO₂ meantime,⁸ promoting the visible catalytic performance of TiO₂-based photocatalysts.

Bearing these comprehensive considerations in mind, in the study, we purposefully designed and explored an efficient bottom-up solvothermal strategy coupled with following calcination to in situ fabricate core–shell TiO₂@C nanohybrid with a uniform ultrathin carbon shell. The tentative underlying formation mechanism of uniform core–shell nano-architecture was also proposed herein. Thanks to the intriguing mesoporous core–shell feature and composition advantages, the resulting core–shell TiO₂@C nanohybrid presented remarkable photocatalytic degradation efficiency and stability of methylene blue (MB) under visible light irradiation, and attractive electrochemical capacitance in aqueous H₂SO₄ electrolyte when evaluated as a promising electrode for advanced ECs.

2. Experimental

2.1 Materials synthesis

All the aqueous solutions were prepared with Milli-Q water from a Milli-Q Plus system (Millipore). In a typical synthesis, 1.2 mmol of Rutin trihydrate (C₂₇H₃₀O₁₆·3H₂O, Sigma-Aldrich, >99%) was dissolved in 120 mL of ethylene glycol (EG, Sigma-Aldrich, >99.5%), which was denoted as the solution A. 0.2 mmol of Ti(OBu)₄ (TBOT) was added into 20 mL of EG under stirring vigorously at ambient temperature, which was designed as the solution B. The solution B was added dropwise into the solution A. Then, a mixed solution was obtained, and transferred to a Teflon-lined stainless steel autoclave for solvothermal reaction at 180 °C for 10 h in an electric oven. After cooled, the final product (denoted as Rutin–Ti) was collected by centrifugation, and washed with water and absolute ethanol in order, and further calcinated at 700 °C for 3 h in N₂ atmosphere with a ramp rate of 5 °C min⁻¹. After this, the sample was further annealed at 440 °C for 5 min in air with a heating rate of 5 °C min⁻¹. Accordingly, a black product (designed as TiO₂@C) was obtained. For comparison, white TiO₂ sample was fabricated by directly annealing the Rutin–Ti at 700 °C for 3 h in air with a ramp rate of 5 °C min⁻¹.

2.2 Materials characterizations

The samples were examined by powder X-ray diffraction (XRD) (Max 18 XCE, Japan) using a Cu Kα source (λ = 0.154056 nm) at a scanning speed of 3° min⁻¹ over a 2θ range of 10–80°. The morphologies and structures were observed by using field-emission scanning electron microscopy (FESEM, JEOL-6300F, 15 kV), transmission electron microscope (TEM) with energy dispersive X-ray (EDX) analyzer, high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) (JEOL JEM 2100 system operating at 200 kV). Fourier transform infrared (FT-IR) spectra were obtained on 360 Nicolet AVATAR. X-ray photoelectron spectra (XPS) measurement was performed on a VG ESCALAB MKII X-ray photoelectron spectrometer with Mg Kα excitation source (1253.6 eV). N₂ adsorption/desorption isotherms were determined by using an ASAP-2010 surface area analyzer. The Raman analysis of the sample was recorded by Laser Raman (T6400, Jobin yvon corp. France). Thermogravimetric (TG) analysis was carried out by using a TA instruments Q 500 analyzer under air flow with a temperature ramp of 10 °C min⁻¹.

2.3 Photocatalytic experiments

Photocatalytic activities were investigated in aqueous solution in a water-cooled quartz cylindrical cell. The reaction mixture in the catalytic cell was maintained at 20 °C by a continuous flow of water and magnetic stirring, and illustrated with an internal xenon lamp (XHA 150 W) equipped with a cut-off glass filter transmitting λ > 400 nm. A MB solution was prepared with an initial concentration of 5 mg L⁻¹ in the presence of solid photocatalyst (0.03 g). The solution was stirred in dark for 30 min to obtain a good dispersion, reached adsorption/desorption equilibrium between the organic MB molecule and the photocatalyst surface, and then subjected to visible light irradiation. At given irradiation time intervals, a series of reaction solutions (∼3 mL) were taken out and remove the suspended catalyst particles for photocatalytic analysis. The concentration of MB was analyzed on UV-vis spectrophotometer 2500 by using its characteristic absorption at 660 nm. For comparison, the photocatalytic activities of commercial catalyst P25 and as-obtained TiO₂ synthesized by calcination in air were also evaluated under the same conditions.

2.4 Electrochemical evaluation

The working electrode was fabricated with the as-prepared TiO₂@C or TiO₂, acetylene black (AB) and polytetrafluoroethylene (PTFE) in a weight ratio of 7 : 2 : 1. A small amount of 0.5 M H₂SO₄ solution was added to make a homogeneous mixture, which was then smeared onto the graphite substrate (1 cm²) for subsequent electrochemical measurement by cyclic voltammetry (CV) and chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) tests performed on an IVIUM electrochemical workstation (the Netherlands). The typical loading of the electroactive TiO₂@C or TiO₂ is 7 mg cm⁻². All experiments were carried out in a three-electrode cell with a working electrode, a platinum plate counter electrode (1 cm²) and a saturated calomel electrode (SCE) reference electrode. The electrolyte was 0.5 M H₂SO₄ solution. Cycling performance was carried out with a CT2001D tester (Wuhan, China). The SCs were calculated by using the following equation:where I, t and ΔV denoted the discharge current (A g⁻¹), discharge time (s), and the discharge potential interval (V), respectively.

3. Results and discussion

3.1 Physicochemical, textural and structural characterizations

In the typical synthetic protocol, as depicted in Fig. 1a, we herein used the EG instead of DI water to manipulate the hydrolysis and condensation rates of the TBOT. After mixed with EG, TBOT would react with EG to form colorless solution of titanium glycolate or mixed alkoxide/glycolate derivatives, as demonstrated by the following two eqn (2) and (3).^20,21

Ti(OBu)₄ + HOCH₂CH₂OH → Ti(OCH₂CH₂O)(OBu)₂ + 2HOBu (2)

Ti(OBu)₄ + 2HOCH₂CH₂OH → Ti(OCH₂CH₂O) + 4HOBu (3)

Fig. 1 Schematic illustration for synthetic process of the core–shell TiO₂@C (a); wide-angle powder XRD patterns of the Rutin–Ti (b) and the resulting TiO₂ and TiO₂@C (c); Raman spectra of the TiO₂@C (d and e) and TiO₂ (e).

When the colorless solution is added dropwise into the light-yellow EG solution of Rutin (the step I in Fig. 1a), a scarlet solution is obtained accordingly, which should be ascribed to the strong interaction between titanium alkoxide/glycolate and Rutin. After following solvothermal treatment at 180 °C for 10 h (the step II in Fig. 1a), latericeous Rutin–Ti powder was obtained. Fig. 1b displays typical wide-angle XRD pattern of the resulting Rutin–Ti sample. Obviously, a very low and broad signal located at 2θ = 25.3° indicates the formation of TiO₂ nanocrystallites. However, it should be still pointed out that the existence of amorphous phase cannot be excluded under this circumstance.

To further investigate the specific composition and structure of the Rutin–Ti product, FT-IR measurements were carried out accordingly. Corresponding spectra were recorded for the Rutin–Ti and Rutin (Fig. S1, ESI†), and specific assignments of the bands are also comparatively collected (Table S1, EIS†). Evidently, the FT-IR spectrum of Rutin–Ti is similar to that for Rutin just with the exception of another two bands emerging at 638 and 499 cm⁻¹, which are generally ascribed to typical Ti–O stretching bend associated with titanium glycolate/amorphous materials,²⁰ and the anatase TiO₂,²² respectively. As reported previously,²⁰ a little water in EG system is one key reagent for the formation of anatase nano-TiO₂. Therefore, we believe that the observation of nanophase TiO₂ here should be ascribed to the contribution from the crystal water in Rutin trihydrate over solvothermal process. After careful examination, it is inspected easily that the stretching vibrations of ν(C=O) and ν(C–O–C) both shift to lower wave number in the spectrum of Rutin–Ti, when compared to those for the Rutin (Fig. S1 and Table S1, EIS†), which reveals the evident coordination between the Rutin and titanium glycolate/anatase TiO₂. It is owing to the strong interaction that a scarlet solution is unexpected obtained when the colorless EG solution of TBOT is mixed with the light yellow EG solution of Rutin, as discussed above. Also for the same reason, the titanium glycolate/anatase TiO₂ would be highly expected to be dispersed uniformly in the as-fabricated Rutin–Ti at the nanoscale. Such unique nanohybrid can efficiently restrain the growth of the formed nanosized TiO₂, and facilitate the in situ coating the nano-TiO₂ core with uniform carbon shell meanwhile during the annealing in N₂ atmosphere at enhanced temperature. With further calcination in air, the carbon species is oxidized partially after the step III in Fig. 1a, and black core–shell TiO₂@C is obtained with a low carbon content of ∼3 wt% (Fig. S2, EIS†).

Fig. 1c shows the representative powder XRD pattern of as-synthesized core–shell TiO₂@C nanohybrid. Apparently, nine reflection peaks appear at 2θ = 25.3° (101), 48.0° (112), 37.8° (200), 53.9° (105), 55.0° (211), 62.7° (204), 68.8° (116), 70.3° (220) and 75.0° (215), respectively, which can be successfully indexed to the anatase TiO₂ with tetragonal space group I4₁/amd (141), compared to the standard pattern (JCPDS no. 21-1272) marked by the blue vertical lines. In general, the transition temperature of the anatase to rutile phase is at ∼600 °C.²³ Interestingly, no any rutile TiO₂ is found in our study after calcinated even at 700 °C, which suggests that the carbon species involved in the hybrid can effectively suppress the anatase–rutile transition.^24,25 Of note, the broader diffraction peaks with lower intensity are observed for the TiO₂@C sample, suggesting the smaller size of the TiO₂ core in the hybrid, compared to the white TiO₂ (the inset in Fig. 1c) obtained directly by annealing in air, which strongly confirms that the existence of carbon shell can inhibit the growth and/or aggregation of the nano-scaled TiO₂ during high-temperature annealing to a certain extent.

Furthermore, it is worthy of noting that typical peaks related to the carbon phase cannot be discerned in Fig. 1c. To further verify the carbon species in the core–shell sample, corresponding Raman spectrum of the TiO₂@C from 1080 to 2450 cm⁻¹ is demonstrated in Fig. 1d. Two typical vibrational modes of carbonaceous materials, that is, the D-band centered at ∼1370 cm⁻¹ (A_1g) and the G-band sitting at ∼1596 cm⁻¹ (E_2g),^26,27 are distinctly presented. According to the intensity ratio (∼1.1) of the D- to G-band and the XRD result (Fig. 1c) above, the amorphous form of the carbon shell in the nanohybrid is convincible here. As a consequence, the low content and amorphous nature of the carbon in the nanohybrid should be responsible well for the absence of diffraction peaks for carbon species, as seen from Fig. 1c.

As is established before,⁴ the carbothermal reduction commonly occurs over high-temperature carbonization in N₂ atmosphere. Raman spectroscopy in the wavenumber range of 300–800 cm⁻¹ is next carried out to investigate the effect of carbon shell upon the Ti species in the final TiO₂@C product. As observed in Fig. 3e, typical bands of anatase-phase TiO₂ are both displayed clearly for the TiO₂ and TiO₂@C. In specific, the peaks at 393.5/400.1, 510.9/519.7, and 632.3/635.2 cm⁻¹ should be attributed to the B_1g, A_1g or B_1g, and E_g modes of TiO₂ with anatase structure, respectively. Of particular note, these characteristic peaks are both blue shifted for the TiO₂@C, when compared to those for phase-pure TiO₂, suggesting the existence of oxygen vacancies, i.e., Ti³⁺ sites in the as-fabricated hybrid TiO₂@C.^4,12,28,29

More detailed information about the specific chemical and bonding environment of the resulting TiO₂@C is further ascertained by using XPS measurements, and corresponding results are summarized in Fig. 2. The full spectrum of the TiO₂@C is shown in Fig. 2a, and Ti, O and C species are all detected on the surface of the nanohybrid. By using a Gaussian fitting method, the Ti 2p emission spectrum is best fitted, and the Ti 2p high-resolution XPS spectrum is analyzed in Fig. 2b, characteristic of Ti⁴⁺ and Ti³⁺. Specifically, the typical binding energies (BEs) of Ti⁴⁺ at 458.6 (Ti 2p_3/2) and 464.5 (Ti 2p_3/2) eV are evident obviously. Besides, another fitted peak at BE of 463.4 eV is the contribution from the Ti³⁺ species in the sample. The ratio of trivalent to tetravalent Ti is calculated to be ∼3.1 : 96.9. Fig. 2c shows the C 1s fine XPS spectrum. Clearly, the main C 1s peak is dominated by elemental carbon at BE of 284.6 eV, attributed commonly to extensively delocalized alternant hydrocarbon. The peak at 283.8 eV can be assigned to the representative Ti–C bond.^30,31 The shift of BE for Ti–C (∼282.0 eV) to a higher energy should be reasonably related to the decreased electron density around the Ti atom in the O–Ti–C bond, compared to that in the C–Ti–C bond.^30,32 Thus, the C doped into the TiO₂ lattice is confirmed by forming an O–Ti–C bond. Moreover, the observed peak at 285.8 eV should be ascribed to the contribution of C–O species, thus it can be inferred that a carbon atom may also occupy the site of the titanium atom, to form a Ti–O–C bond.^30,33 As for the O spectrum (Fig. 2d), the peaks at BEs of 530.1 and 531.7 eV correspond to Ti–O–Ti (lattice O) and O–H (ref. 30) on the surface of the core–shell TiO₂@C, respectively.

Fig. 2 Full survey spectrum (a), high-resolution Ti 2p (b), C 1s (c) and O 1s (d) XPS spectra for the core–shell TiO₂@C nanohybrid.

Fig. 3 demonstrates the representative morphology of the as-obtained TiO₂@C specimen. As seen in Fig. 3a and b, simple loose aggregation of nanoparticles (NPs) is explicitly evident. It is easy to note the uniform distribution of Ti, O and C elements in the resulting nanohybrid, as observed from the EDX mapping images (the inset in panel a). To reveal the specific micro-structures of the TiO₂@C more clearly, (HR)TEM measurements are carried out accordingly, and typical images are demonstrated in Fig. 4a–f. A spherical aggregation of ∼400 nm in size, as displayed in Fig. 4a, which are constructed evidently with lots of NPs. Corresponding EDX data (Fig. S2, EIS†), obtained from the red rectangular region in Fig. 4a, suggests the co-existence of Ti, O and C elements in the hybrid TiO₂@C. Furthermore, the EDX mapping images (Fig. 4b) clearly reveal the uniform distribution of the three species in the as-fabricated nanohybrid at the nanoscale once more. As seen from the higher-magnification TEM image (Fig. 4c), detected from a sampling area indicated by the green rectangle region in Fig. 4a, the unique loose structure is composed of primary NPs subunits, and presents a typical wormhole meso-framework. The interlinked mesopores between the NPs would allow light scattering inside their pore channels and thus enhance the light harvesting, as reported previously,^8,34 and also render the electrolyte rapid transportation in the hybrid meanwhile. HRTEM results are presented in Fig. 4d–f. Apparently, amorphous carbon phase is deposited either on the surface and/or between TiO₂ NPs with discernable crystalline fringes, as shown in Fig. 4d. TEM image of a single TiO₂@C particle is also shown in Fig. 4e, where a typical core–shell structure consisting of a crystalline TiO₂ core of ∼13 nm in size and carbon shell with a thickness of ∼1–3 nm is observed. Similar structure is also evident in Fig. 4f, which is taken from the blue rectangle region in Fig. 4c. The clear fringes observed in Fig. 4e and f correspond to the interplanar distances of ∼0.19 and ∼0.23 nm, respectively, which agree well with the lattice spacings of the (202) and (112) planes for the anatase TiO₂. Fig. 4g demonstrates the SAED pattern with a series of concentric rings along with a spot pattern, revealing the polycrystalline characteristics of the TiO₂ core, which matches well with (101), (112), (200), (105), (204), (116) and (215) planes of the anatase TiO₂, respectively. This is in good correspondence with the XRD analysis above (Fig. 1c).

Fig. 3 FESEM images (a and b) and the corresponding EDX mapping (the inset in panel a) of the Ti, O, and C phases for the TiO₂@C. The image in (b) taken from the red rectangle region in the panel (a).

Fig. 4 TEM images (a and c), elemental mapping images (b) taken from the red rectangle region in the panel (a), HRTEM images (d–f), and SAED pattern (g) of the TiO₂@C nanohybrid. The image in (b) is taken from the green rectangle region in panel (a). The images in (e) and (f) are the magnified ones of the red and blue rectangle regions in panel (d), respectively.

The mesoporosity and textural properties of the unique TiO₂@C product are further studied by N₂ adsorption–desorption measurement. Typical sorption isotherms are presented with typical IV sorption behavior, as illustrated in Fig. 5a, according to the IUPAC classification. A distinct hysteresis loop sitting in the larger pressure range of >0.8 P/P₀ suggests the inherent mesoporous feature of the resulting TiO₂@C nanohybrid, which can be further verified by the pore size distribution (PSD) data in Fig. 5b, where the pore size is mainly ranged from 2.0 to 50 nm. The textural properties of the resultant TiO₂@C are quantitatively summarized as follows: the Brunauer–Emmett–Teller specific surface area (SSA), mesoporous volume, and average pore size are ∼130 m² g⁻¹, ∼0.43 cm³ g⁻¹, ∼12.5 nm, respectively. With the crystallite size of the TiO₂@C and corresponding TEM images (Fig. 4a and c) in mind, the mesopores are believed to be produced by the aggregation and connection of the adjacent TiO₂@C NPs with the core–shell nano-architecture.

Fig. 5 N₂ adsorption isotherms (a) and PSD (b) of the core–shell TiO₂@C nanohybrid.

3.2 Photocatalytic characterization

The optical absorption property, corresponding separation and migration of the light induced electrons and holes of a semiconductor photocatalyst are recognized as the vital aspects in determining its photocatalytic activity.^35,36 Diffuse reflectance spectroscopy is often utilized to characterize the optical absorption properties of the photocatalysts, and corresponding electron states in the photocatalysts. Fig. 6a presents the UV-vis diffuse reflectance spectra of the as-prepared TiO₂ and TiO₂@C samples. As expected, strong absorption property in the UV light region is observed for the TiO₂ powders. In contrast, the core–shell TiO₂@C shows an enhanced absorption capacity in the range of 200–800 nm. And the strong absorption region is extended to the visible region from the UV light region, owing to the synergetic effect of the carbon shell coating upon the TiO₂ core,^7–9,16,17 the Ti³⁺ sites,^8,37 and the C-doping.^18,30,37

Fig. 6 UV-vis adsorption spectra (a) of the resultant TiO₂@C and TiO₂; photocatalytic degradation profiles (b) of MB over different photocatalysts under visible light irradiation as indicated.

Since the TiO₂@C nanohybrid possesses remarkable light absorption in visible region, we expect its appealing photochemical activity in the visible light region. Accordingly, photocatalytic activities of the TiO₂ and TiO₂@C nanohybrid are systematically evaluated by photocatalytic degradation of the MB as a test reaction under the visible light irradiation, where the commercial P25 is also applied for comparison. Fig. 6b collects the photocatalytic degradation rate of the dye MB over the resulting photocatalysts as a function of the irradiation time, where C is the absorption of MB at the wavelength of 664 nm after irradiation time of t, and C₀ is the absorption after the adsorption equilibrium on the samples before irradiation. For comparison, the direct photolysis test (that is, just MB without any catalyst) under the visible light irradiation, and the dark adsorption experiment in the presence of the core–shell TiO₂@C are also performed. Apparently, the visible light induced self-degradation of MB is negligible, as indicated by the photolysis behavior in Fig. 6b. The dark experiment with the as-prepared TiO₂@C sample demonstrate its strong adsorption capacity for MB, and the adsorption de-colorization rate of MB over the TiO₂@C is even close to ∼40%, which should be reasonably attributed to its typical mesoporosity and large SSA, as mentioned above. Strikingly, assisted by the visible light irradiation, the degradation rate of MB is up to near 100% just after ∼3 h of visible light irradiation, much higher than those of pure TiO₂ (11.5%) and the commercial P25 (23.6%). Except for the adsorption removal, both of TiO₂ and P25 almost show the activity comparable with the self-degradation of MB under the same irradiation conditions, suggesting their extremely weak photocatalytic activities under the visible light irradiation, which is consistent with the report before.⁷

On the basis of experimental results above, the appreciable visible photocatalytic performance observed here for the core–shell TiO₂@C should be rationally ascribed to the following contributions, as proposed in Fig. 7. Firstly, the carbon incorporation into the crystalline lattice of the TiO₂ modifies the electronic band structure of TiO₂, leading to an extra C-doping level above the O 2p valance band (VB), which results in the narrowing of the band gap of TiO₂ and shifts the optical absorption edge to the visible light region.^{18,30,37–40} Secondly, the 3d orbital of the formed Ti³⁺ states (i.e., oxygen vacancies) generated during carbothermal reduction provides a donor energy below the conduction band (CB), improving the visible light response of TiO₂ itself.^8,37 Thirdly, the surface-coating carbon species can enhance the visible light absorption and reduce the reflection of light as a photosensitizer. Moreover, the carbon shell generates electrons under visible light irradiation, thereby transferring the donated electrons to the CB of TiO₂ through Ti–O–C bonds,^41,42 while the generated holes stay electronically and structurally near the VB of TiO₂, which can greatly enhance the transfer efficiency of photo-generated carriers. Furthermore, the low carbon content is manifested in a thin coating shell layer, enabling the MB solution to reach the active sites easier and faster. Fourthly, the dye of MB itself also partially absorbs visible light due to its photosensitization effect,^43,44 and the photo-generated electrons are transferred to the excited state from the ground state of the dye ascribing to intramolecular π–π* transition, then the photoelectrons of the excited state are immediately injected into the CB of TiO₂. Finally, an easier mass transport, remarkable confinement of the target MB and enhanced light harvesting, which result from the mesoporous feature, are also beneficial to the achieved photocatalytic capacity of the core–shell nanohybrid with large SSA.^8,30,34,45 Hence, the collected electrons in the CB of TiO₂ can react with O₂ (dissolved or adsorbed oxygen molecules) to produce active species such as superoxide radicals, and the photo-generated holes in the VB of TiO₂ can react with water to generate the active species such as hydroxyl radicals. The photo-generated holes and active radicals transfer easily to the nearby MB, and take part in the oxidation degradation.^46,47 With the decomposition of the MB, the adsorption equilibrium is broken and more and more MB moves from the aqueous solution to the TiO₂@C interface and, subsequently, the dye of MB is wholly degraded to CO₂ and water through a series of reactions.

Fig. 7 Schematic illustration of electron transition and photocatalytic process of the core–shell TiO₂@C.

3.3 Electrochemical characterization

To further evaluate the potential application of the core–shell TiO₂@C for ECs, CV measurements were carried out between 0.0 and 1.0 V (vs. SCE) at various sweeping rates ranging from 2 to 100 mV s⁻¹, and corresponding curves have been shown in Fig. 8. Attractively, all the curves exhibit quasi-rectangular shapes, similar to that for the hydrogenated TiO₂,¹² which remains little change as the scan rate increases from 2 to 100 mV s⁻¹, suggesting good electrochemical capacitance of the core–shell TiO₂@C in 0.5 M aqueous H₂SO₄. Also, a couple of Faradaic redox peaks are evidently discernable in Fig. 8a, revealing the inherent pseudo-capacitive nature of the TiO₂@C electrode, which is generally ascribed to the cation intercalation and de-intercalation processes according to the following equation:^48–50

(TiO₂)_surface + H₃O⁺ + e⁻ ↔ (TiO₂⁻H₃O⁺)_surface (4)

Fig. 8 Electrochemical performance: CV curves with various scan rates (a), CP plots (b), SC vs. current density (c) and cycling behavior for the core–shell TiO₂@C.

This can be visibly verified by the non-linear charge–discharge plots at a series of mass-normalized currents, as shown in Fig. 8b. Furthermore, the observation of nearly symmetric potential–time plots at all current rates from 0.2 to 4.0 A g⁻¹ suggests the high charge–discharge coulombic efficiency of the TiO₂@C nanohybrid for electrochemical storage in aqueous H₂SO₄ solution. Fig. 8c shows the calculated SCs as a function of the applied current density. Strikingly, the unique core–shell TiO₂@C electrode exhibits remarkable SCs of ∼210, ∼186, ∼156, ∼122 and ∼100 F g⁻¹ at current densities of 0.2, 0.6, 1.0, 2.0 and 4.0 A g⁻¹. While the TiO₂ just delivers a SC of ∼30 F g⁻¹ at 0.2 A g⁻¹, and only ∼6 F g⁻¹ at a high current rate of 4 A g⁻¹. More attractively, the SCs estimated here are much higher than other TiO₂-based electrodes, such as, Fe-doped TiO₂/C nanofibers (∼137 F g⁻¹),⁵¹ TiO₂-G-PPy (∼57–202 F g⁻¹),⁵⁰ TiO₂@C cloth (∼197 F g⁻¹),¹³ TiO₂@C composites (∼70 F g⁻¹),¹⁰ Co-functionalized TiO₂ nanotubes (∼124 F g⁻¹),⁵² and so on. What is more, calculation of the pure EDLC by using an average value of 20 μF cm² (ref. 53) renders an EDLC of ∼26 F g⁻¹ for such core–shell TiO₂@C, which is even lower than the SC of 210 F g⁻¹ at 0.2 A g⁻¹. As a result, it is inferred reasonably that the SCs calculated here should result mainly from the pseudo-capacitive contribution, and the Faradaic SC is estimated as ∼184 F g⁻¹, corresponding to a area SC of ∼1.42 F m⁻², which verifies its high electrochemical utilization. The electrochemical stability of the core–shell TiO₂@C electrode is evaluated at a current density of 1.0 A g⁻¹, as shown in Fig. 8d. Obviously, the SC retention of the core–shell TiO₂@C electrode gradually reduces, whereas the SC degradation is as small as ∼2% over continuous 1200 cycles, suggesting its stable long-term cycling ability at large current density. The appealing electrochemical performance for the core–shell TiO₂@C should be rationally related to its large SSA, rich mesoporosity, smaller charge-transfer resistance and higher electronic conductivity originating from the carbon shell and Ti³⁺ sites, as confirmed by the EIS data (Fig. S4, EIS†).

4. Conclusion

In conclusion, in the study, we purposefully designed and explored a bottom-up solvothermal strategy coupled with subsequent calcination to in situ fabricate core–shell TiO₂@C nanohybrids. The plausible underlying formation mechanism of uniform core–shell nano-architecture was also tentatively proposed herein. Physicochemical, textural and structural characterizations demonstrated the unique core–shell TiO₂@C with a uniform ultrathin carbon shell of ∼1–3 nm was endowed with rich mesoporosity and large specific surface area. Benefiting from the appealing structure and composition advantages, the resulting core–shell TiO₂@C exhibited remarkable visible light photocatalytic efficiency and stability for degradation of methylene blue under visible light irradiation, and attractive electrochemical capacitance in aqueous H₂SO₄ electrolyte when evaluated as a promising electrode for high-performance ECs. More significantly, the design and approach we devised herein will benefit future commercialization efforts of the core–shell TiO₂@C in visible light photocatalytic degradation of pollutants, next-generation ECs and even for advanced Li-ion batteries.

Acknowledgements

The authors acknowledge the financial support from National Natural Science Foundation of China (no. 51202004). Anhui Province Funds for Distinguished Young Scientists (no. 1508085J09), the Natural Science Foundation of Anhui Province (no. 1508085ME106, KJ2013A051), the Foundation for Young Talents in College of Anhui Province, and the Opening Project of CAS Key Laboratory of Materials for Energy Conversion (no. 2014001).

Notes and references

1. A. Zana, C. Rüdiger, J. Kunze-Liebhäuser, G. Granozzi, N. E. A. Reeler, T. Vosch, J. J. K. Kirkensgaard and M. Arenz, Electrochim. Acta, 2014, 139, 21.
2. G. Q. Zhang, H. B. Wu, T. Song, U. Paik and X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 12590.
3. Z. W. She, W. Li, J. J. Cha, S. S. Hong and Y. Cui, Nat. Commun., 2013, 4, 1331.
4. J. Y. Li, B. Ding, G. Y. Xu, L. R. Hou and X. G. Zhang, Nanoscale, 2013, 5, 5743.
5. L. K. Tsui, J. Huang, M. Sabat and G. Zangari, ACS Sustainable Chem. Eng., 2014, 2, 2097.
6. S. U. M. Khan, M. Al-Shahry and W. B. Ingler Jr, Science, 2002, 297, 2243.
7. L. Zhao, X. F. Chen, X. C. Wang, Y. J. Zhang, W. Wei, Y. H. Sun, M. Antonietti and M. M. Titirici, Adv. Mater., 2010, 22, 3317.
8. D. H. Wang, L. Jia, X. L. Wu, L. Q. Lu and A. W. Xu, Nanoscale, 2012, 4, 576.
9. S. Shanmugam, A. Gabashvili, D. S. Jacob, J. C. Yu and A. Gedanken, Chem. Mater., 2006, 18, 2275.
10. J. L. Liu, Z. W. Cai, Y. K. Lv, Y. J. Zhang, C. Su, M. Ouyang, C. Zhang and D. S. Wright, J. Mater. Chem. A, 2015, 3, 1837.
11. H. M. Zheng, T. Zhai, M. H. Yu, S. L. Xie, C. L. Liang, W. X. Zhao, S. C. I. Wang, Z. S. Zhang and X. H. Lu, J. Mater. Chem. C, 2013, 1, 225.
12. X. H. Lu, G. M. Wang, T. Zhai, M. H. Yu, J. Y. Gan, Y. X. Tong and Y. Li, Nano Lett., 2012, 12, 1690.
13. X. H. Lu, M. H. Yu, G. M. Wang, T. Zhai, S. L. Xie, Y. C. Ling, Y. X. Tong and Y. Li, Adv. Mater., 2013, 25, 267.
14. Z. Zheng, J. J. Chen, R. J. Yoshida, X. Gao, K. Tarr, Y. H. Ikuhara and W. L. Zhou, Nanotechnology, 2014, 25, 435406.
15. A. Linsebigler, G. Lu and J. T. Yates, Chem. Rev., 1995, 95, 735.
16. P. Zhang, C. L. Shao, Z. Y. Zhang, M. Y. Zhang, J. B. Mu, Z. C. Guo and Y. C. Liu, Nanoscale, 2011, 3, 2943.
17. X. Shao, W. C. Lu, R. Zhang and F. Pan, Sci. Rep., 2013, 3, 3018.
18. R. Leary and A. Westwood, Carbon, 2011, 49, 741.
19. X. H. Lu, D. Z. Zheng, T. Zhai, Z. Q. Liu, Y. Y. Huang, S. L. Xie and Y. X. Tong, Energy Environ. Sci., 2011, 4, 2915.
20. P. Wang, T. F. Xie, L. Peng, H. Y. Li, T. S. Wu, S. Pang and D. J. Wang, J. Phys. Chem. C, 2008, 112, 6648.
21. D. M. Puri and R. C. Mehrotra, Indian J. Chem., 1967, 51, 448.
22. I. Ivanova, A. Harizanova and M. Surtchev, Mater. Lett., 2002, 55, 327.
23. Y. T. Lin, C. H. Weng, Y. H. Lin, C. C. Shiesh and F. Y. Chen, Sep. Purif. Technol., 2013, 116, 114.
24. V. Loryuenyong, A. Buasri, C. Srilachai and H. Srimuang, Mater. Lett., 2012, 87, 47.
25. M. Inagaki, Carbon, 2012, 50, 3247.
26. L. Zhou, S. Q. Zhu, H. Cao, L. R. Hou and C. Z. Yuan, Green Chem., 2015, 17, 2373.
27. L. R. Hou, L. Lian, D. K. Li, G. Pang, J. F. Li, X. G. Zhang, S. L. Xiong and S. L. Xiong, Carbon, 2013, 64, 141.
28. X. Pan and X. Ma, J. Solid State Chem., 2004, 177, 4098.
29. A. Golubovic, M. Scepanovic, A. Kremenovic, S. Askrabic, V. Berec, M. Z. Dohevic and Z. Popovic, J. Sol-Gel Sci. Technol., 2009, 49, 311.
30. W. Zhou, Y. Liu, Y. Z. Zhang, G. Yang, S. H. Deng, F. Shen, H. Peng and L. L. Wang, New J. Chem., 2014, 38, 1647.
31. L. Tian, L. Ye, K. Deng and L. Zan, J. Solid State Chem., 2011, 184, 1465.
32. X. Wu, S. Yin, Q. Dong, C. Guo, H. Li, T. Kimura and T. Sato, Appl. Catal., B, 2013, 142–143, 450.
33. D. Wang, L. Xiao, Q. Luo, X. Li, J. An and Y. Duan, J. Hazard. Mater., 2011, 192, 150.
34. J. H. Pan, X. W. Zhang, A. J. Du, D. D. Sun and J. O. Leckie, J. Am. Chem. Soc., 2008, 130, 11256.
35. S. Meng, W. Z. Wang, J. Ren, S. M. Sun, L. Wang and L. Zhang, J. Mater. Chem., 2009, 19, 6213.
36. L. Kong, Z. Jiang, T. C. Xiao, L. F. Lu and M. O. Jones, Chem. Commun., 2011, 47, 5512.
37. Y. C. Liu, M. Y. Xing and J. L. Zhang, Chin. J. Catal., 2014, 35, 1511.
38. H. X. Lin, W. H. Deng, T. H. Zhou, S. B. Ning, J. L. Long and X. X. Wang, Appl. Catal., B, 2015, 176–177, 36.
39. H. Q. Zhuang, Q. Gu, J. L. Long, H. Lin, H. X. Lin and X. X. Wang, RSC Adv., 2014, 4, 34315.
40. Z. Z. Zhang, X. X. Wang, J. L. Long, Q. Gu, Z. X. Ding and X. Z. Fu, J. Catal., 2010, 276, 201.
41. J. Zhou, F. Chen and J. L. Zhang, J. Phys. Chem. C, 2010, 114, 933.
42. J. C. Liu, W. Y. Zhu, S. Y. Yu and X. L. Yan, Carbon, 2014, 79, 369.
43. J. W. Tang, Z. G. Zou and J. H. Ye, Chem. Mater., 2004, 16, 1644.
44. X. T. Pian, B. Z. Lin, Y. L. Chen, J. D. Kuang, K. Z. Zhang and L. M. Fu, J. Phys. Chem. C, 2011, 115, 6531.
45. F. Deng, X. Luo, K. Li, X. Tu, S. Luo, L. Yang, N. Zhou and H. Shu, J. Mol. Catal. A: Chem., 2013, 366, 222.
46. L. M. Zhang, S. Diao, Y. F. Nie, K. Yan, N. Liu, B. Y. Dai, Q. Xie, A. Reina, J. Kong and Z. F. Liu, J. Am. Chem. Soc., 2011, 133, 2706.
47. D. M. Chen, Z. H. Wang, T. Z. Ren, H. Ding, W. Q. Yao, R. L. Zong and Y. F. Zhu, J. Phys. Chem. C, 2014, 118, 15300.
48. Q. H. Huang, X. Y. Wang, J. Li, C. L. Dai, S. Gamboa and P. Sebastian, J. Power Sources, 2007, 164, 425.
49. T. Bordjiba and D. Bélanger, J. Electrochem. Soc., 2009, 156, A378.
50. J. L. Jiang, X. Lu, C. M. Xie, G. J. Wan, H. P. Zhang and Y. H. Tang, J. Phys. Chem. C, 2015, 119, 3903.
51. G. M. K. Tolba, M. Motlak, A. M. Bastaweesy, E. A. Ashour, W. Abdelmoez, M. El-newehy and N. A. M. Barakat, Int. J. Electrochem. Sci., 2015, 10, 3117.
52. L. R. Hou, C. Z. Yuan, D. K. Li, L. F. Shen and X. G. Zhang, Mater. Lett., 2011, 65, 2632.
53. C. Z. Yuan, J. Y. Li, L. R. Hou, J. D. Lin, X. G. Zhang and S. L. Xiong, J. Mater. Chem. A, 2013, 1, 11145.

---

Footnote

† Electronic supplementary information (ESI) available: FT-IR spectrum, TG, EDX and EIS data of the controlled experiments. See DOI: 10.1039/c5ra10109c

---