Biomass-derived multifunctional TiO₂/carbonaceous aerogel composite as a highly efficient photocatalyst

Abstract

Recently, the development of porous carbon materials driven by environmentally friendly natural biomass has been attracting tremendous focus. Herein, we present a facile and green approach to fabricate a binary TiO₂/carbonaceous aerogel (TiO₂/CA) composite using wintermelon as the source material via the well-established hydrothermal process. The obtained sponge-like carbonaceous aerogel (CA) has a three-dimensional (3D) porous structure making it a good scaffold for synthesizing the composite. The experimental results show that the TiO₂ nanoparticles were homogeneously anchored on the surface of the CA and the composite exhibited outstanding photodegradation capacities for organic pollutants. The enhanced photodegradation capacity could be ascribed to the synergetic properties of the TiO₂ photocatalyst and the porous CA support. These findings successfully open up a new fabrication strategy to prepare other 3D structure possessing composites combined with carbonaceous aerogels and show the possibility for the preparation of various composites for multiple applications in various other fields.

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1 Introduction

Coincident with the progression of society and technology, a series of environmental problems has been brought to our attention due to the massive amounts of toxic and hazardous organic compounds generated by various industrial production processes.^1,2 Extensive research has been devoted to alleviating fresh water deterioration around the world. Semiconductor photocatalysis is an inexpensive and promising approach to wastewater treatment because it can meet the “green-life” concept and “zero” waste strategy.³ Up to now, among various photocatalysts, TiO₂ is one of the most widely used due to its special physicochemical properties.^4,5 However, TiO₂ is a wide-band-gap semiconductor (3.2 eV for anatase) and absorbs just 3–5% of sunlight in the UV region, which limits its application greatly.^6,7 Therefore, exploring novel TiO₂-based photocatalysts with highly active visible light response and good separation of electron–hole pairs has become an urgent photocatalysis research direction.

Recently, modifying TiO₂ with carbon-based substances on the surface that can induce high visible-light responsive activity have been reported.^8–12 Concentrated efforts have been made to fabricating various new types of carbon-based materials, such as graphene, multiwalled carbon nanotubes, active carbon, carbonaceous aerogels etc.,^12–16 and they have been investigated extensively to be qualified for different various applications. For example, Wang et al.¹⁷ have synthesized a graphene@TiO₂ composite for enhanced photocatalytic performance. Wu et al.¹⁸ applied bacterial-cellulose pellicles as a raw material to develop a carbon nanofiber aerogel as an electrode material for the oxygen reduction reaction and supercapacitors. Thus, carbon-based materials act as a significative and perfect support candidate for other substrates. Among the various carbon-based materials, carbonaceous aerogel is a kind of three-dimensional porous nanostructural solid with air infilling mediums in the interspaces. Inspired by the remarkable and outstanding properties, such as low density, high porosity, and surface hydrophobicity, many researchers have been attempting to find a way to prepare functionalized carbonaceous aerogels from natural and renewable biomass as the raw material and it is a highly attractive subject today. For example, Ren et al.¹⁹ adopted watermelon as the raw material to prepare a MnO₂@CA composite via a hydrothermal method to produce an electrode material; Wu et al.^20,21 have successfully synthesized a multifunctional magnetite Fe₃O₄/carbon aerogel nanocomposite for adsorbents and supercapacitors. Biomass, such as carbohydrates and crude plants, is an excellent raw material for preparing porous carbonaceous materials, as it is a low-cost, sustainable, and environmentally benign material, and it can be directly converted using the hydrothermal carbonization process. As is well known wintermelon is a long seasonal vegetable, grown in most parts of Asian countries, which is an ideal raw material for fabricating carbonaceous substances. However, to the best of our knowledge, it is still unreported to integrate TiO₂ with a carbonaceous aerogel for visible-light driven photocatalytic or photoelectrochemical applications.

Herein, we report for the first time using wintermelon as the raw material to prepare a carbonaceous aerogel and further we developed a template free and environmentally benign route to synthesise a novel binary TiO₂/CA composite via an inexpensive and simple hydrothermal process. The TiO₂/CA composites were utilized as photocatalysts for the degradation of relatively high concentrations of methylene blue and ciprofloxacin, compared with P25, they exhibited a significantly high photodegradation activity. Our study successfully offers a promising fabrication strategy for the rational design of comparatively cheap, high performance carbonaceous aerogel based photocatalysts and then facilitates their practical applications in environmental issues.

2 Experimental sections

2.1 Materials

Crude wintermelon was bought from the local fruit market in Zhenjiang, China and thoroughly washed with deionized water before use. Titanium sulfate Ti(SO₄)₂ (CP) and absolute ethanol (C₂H₅OH), were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used for all experiments.

2.2 Photocatalyst preparation

The monolithic carbonaceous aerogel was prepared using a one-pot hydrothermal process directly from the soft tissue biomass of the wintermelon according to the previous report.¹⁷ First, the rind and soft pulp of the wintermelon was totally removed. The soft tissue of the wintermelon was cut into sizes about 2 × 2 × 3 cm³ and put into the corresponding Teflon-lined stainless steel autoclave at 180 °C for 12 h. After that, the carbonaceous hydrogel was washed using a mixture of deionized water and absolute ethanol (volume ratio is 1 : 1) to remove the soluble impurities. Carbonaceous aerogel was obtained using a freeze-drying process at −51 °C for 24 h. The laboratory freeze drying machine is a multi-manifold freeze dryer FD-1C-50, bought from Beijing Boyikang Laboratory Instruments Co., Ltd.

The TiO₂/CA composites were synthesized using a well-established hydrothermal process, as illustrated in Fig. 1. Typically, 1 mmol of Ti(SO₄)₂ was dissolved in 20 mL of H₂O for a few minutes, then a stoichiometric amount of carbonaceous aerogel was put into the solution, the mixture was stirred for 30 min. After that, the mixture was transferred into a 25 mL Teflon-lined autoclave, and following this it was held at 180 °C for 12 h. The obtained sample was washed with deionized water and absolute ethanol each three times. Finally, the sample was dried under vacuum at 60 °C. The mass ratios of TiO₂ in the composites were 20, 33, 50, 67 and 80 wt%. For comparison, bare TiO₂ was prepared similarly but without the addition of CA.

Fig. 1 Schematic illustration of the formation of the TiO₂/CA composite.

2.3 Characterization

Power X-ray diffraction (XRD) data were measured by using D8 Advance X-ray diffraction (Bruker AXS Company, Germany). Fourier transform infrared spectroscopy analysis (FTIR) was performed on Nicolet Nexus 470 equipment (Thermo Electron Corporation, USA) using the standard KBr disks. Scanning electron microscopy (SEM) measurements were carried out using Field-emission scanning electron microscopy on a JSM-7001F (JEOL, Japan) equipped with an energy-dispersive X-ray spectroscope (EDS) operating at an acceleration of 10 kV. Transmission electron microscopy (TEM) micrographs were recorded on a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. Raman spectra were obtained using a DXR Laser Raman spectrometer (Thermo Fisher Corporation, USA) with a 532 nm laser source at room temperature. X-ray photoelectron spectroscopy (XPS) was examined using a Thermo ESCALAB 250XI spectrometer (Thermo Fisher Corporation, USA). UV-vis diffuse reflectance spectra (DRS) were acquired on a Shimadzu UV-2450 spectrophotometer (Shimadzu Corporation, Japan). The photoluminescence (PL) spectra were examined using a QuantaMaster⁴⁰ (Photon Technology International, Inc., USA). N₂ adsorption–desorption isotherms were carried out at 77 K with a NOVA-2000e analytical system (Quantachrome Corporation, USA). The electron spin resonance (ESR) analysis was conducted using an electron paramagnetic resonance A300-10/12 spectrometer (Bruker AXS Company, Germany). Photocurrent measurements and electrochemical impedance spectroscopy (EIS) were performed on a CHI 760B electrochemical analyzer (Chenhua Instruments Company, China).

2.4 Photocatalytic experiments

The photocatalytic activities of the as-prepared TiO₂/CA composites were assessed using the decoloration of MB and CIP aqueous solutions. All the experiments were performed in a self-made quartz photoreactor fitted with a water circulation system to maintain a constant temperature and under visible-light irradiation of a 500 W tungsten lamp with a 420 nm cutoff filter to remove the UV irradiation. 10 mg of photocatalyst was ultrasonically dispersed in 40 mL of MB (40 mg L⁻¹) or CIP (40 mg L⁻¹) solution and then the solution was stirred magnetically for 30 min in the dark to obtain the establishment of an absorption–desorption equilibrium between the catalyst and the simulated pollutant. At regular intervals, 4 mL aliquots were collected and subsequently centrifuged to remove the particles. The concentration of the solution at the different times was monitored quantitatively using the absorption peak generated by the UV-vis spectrophotometer (UV-2450, Shimadzu) in terms of its characteristic absorption.

3 Results and discussion

The monolithic 3D sponge-like CA was produced via a hydrothermal process. This process contained a series of reactions, involving dehydration of the carbohydrate, polymerization–polycondensation to form the polyfurans and carbonization via further intermolecular dehydration.^19,22,23 Through a further freeze-drying process, the CA was obtained. Fig. 2a shows that the CA monolith with a dark brown color could easily stand on a soft feature. The mass density of the CA was measured to about 18.9 mg cm⁻³. The ultra-lightweight properties made it float beneath the surface after immersion into water (as shown in Fig. 2b), suggesting the porous CA possessed extreme ultralight weight and hydrophobicity. The unique properties made it a promising platform to load a range of functional nanomaterials. In addition, the CA monolith can be easily divided into different sizes to match the requirements of varied practical applications (Fig. 2c).

Fig. 2 Digital photographs of CA monolith (a) showing lightweight nature, (b) floating beneath the surface of water and (c) with different sizes.

Morphology observations of the resulting CA and TiO₂/CA composite were verified using SEM, TEM, HRTEM and EDS. The obtained CA consisted of carbonaceous nanofibers with a well-defined cross-linked network, which constituted the 3D porous structure, as shown using SEM in Fig. S1a.† The TEM image further shows that the interconnected network of the CA had many irregular ligaments and branches consisting of cross-linked nanofibers (Fig. S1b†).²⁰ Benefiting from the surface functional groups and the defects in the carbon structure of the CA, the porous multilayer networks provided an additional surface for the loading of TiO₂ nanoparticles. From Fig. 3a it can be clearly observed that the TiO₂/CA composite maintained the porous architectures of the original CA, and TiO₂ nanoparticles with a relatively uniform size distribution were anchored on the surface of the CA. The porous structure was of vital importance for allowing the pollutants into the photocatalytic active sites. The TEM image of the composite further proved the interconnected porous structure of the CA and the ultradispersed distribution of the TiO₂ nanoparticles (Fig. 3b). A HRTEM image in Fig. 3c confirmed the high crystallinity of TiO₂ by showing the clear lattice fringes. The observed spacing distances between the lattice planes of the as-fabricated composite were 0.238 and 0.350 nm, which were assigned to the most stable and frequently observed in anatase TiO₂ (001) and (101), which was consistent with the following results revealed using XRD.²⁴ Electron dispersive X-ray spectra (in Fig. S2†) and EDS elemental mapping (in Fig. 3d) based on the TiO₂/CA composite presented the distribution of Ti, O, and C in the whole testing area, indicating that the TiO₂ nanoparticles were homogeneously distributed on the surface of the CA. Particularly, even after the long time ultrasonication during the TEM sample preparation, TiO₂ nanoparticles on the surface of the CA were still tightly attached, suggesting the strong intimate interaction between the CA and TiO₂ nanoparticles. Consequently, combining the SEM, TEM, HRTEM and EDS results, it was obvious that the TiO₂ nanoparticles were successfully distributed on the surface of the CA with a 3D interconnected network during the hydrothermal treatment. Such 3D carbon-based aerogels embedded with nanoparticles have been reported to show enhanced interface contact with suppressed dissolution and a reduction in the agglomeration of nanostructures, as well as improved photo-electrochemical activity and hybrid stability.^25,26

Fig. 3 (a) SEM, (b) TEM, (c) HRTEM, and (d) EDS mapping images of the 50 wt% TiO₂/CA composite.

The XRD patterns of the CA and the TiO₂/CA composites are given in Fig. 4a. The peaks at 25.2°, 37.8°, 47.9°, 53.9° and 62.6° can be ascribed to the (101), (004), (200), (105) and (204) plane of anatase TiO₂, respectively. All the sharp diffraction peaks of the samples could be readily indexed to the anatase crystalline phase of TiO₂, matching well with the data from JCPDS card no. 21-1272, and the intensity and sharpness of the diffraction peaks increased with increasing Ti(SO₄)₂ content.^6,27 The broad characteristic peaks located at around 22° indicated that the CA consists of graphitized carbon, similar to the previous report.²² Furthermore, no other characteristic peaks of impurities were observed, which demonstrated the high purity of the as-prepared composites. From the XRD patterns, it could be seen that the incorporation into the CA had little influence on the phase structure of TiO₂.

Fig. 4 (a) The XRD patterns and (b) FTIR spectra of the as-prepared samples.

To further reveal the structural properties of the as-prepared composites, FTIR spectra were measured and displayed in Fig. 4b. Characteristic absorption peaks for the pure CA were observed at 3450, 2890, 1692, 1631, 1442, 1224, and 1120 cm⁻¹, which were attributed to the O–H, C–H, C=O in COOH, aromatic C=C, carboxyl C–O, expoxy C–O and alkoxy C–O, C–C stretches, respectively.^28–31 These results indicate that the surface of the CA is rich in substantial oxygen-containing functional groups, which enhances the accessibility for further reactions with other materials. Compared with the CA, the TiO₂/CA composites exhibited a broad peak at the 500–800 cm⁻¹range which was assigned to the Ti–O–Ti groups.³² Moreover, after the hydrothermal process, the oxygen-containing functional groups persisted and then facilitated the hybridizing of the TiO₂ nanoparticles on the surface of the CA.³³

Raman spectroscopy is also one of the most sensitive and informative techniques to characterize disorders in sp² carbon materials. In order to further affirm the intimate interaction between TiO₂ and the CA, Raman spectra of the CA and the 50 wt% TiO₂/CA composite are recorded in Fig. 5a. The recorded spectrum of the CA shows two typical features at the well-documented D band at 1386 cm⁻¹ and the G band at 1595 cm⁻¹, respectively. As is well known, the D band is common for the vibrations of carbon atoms with dangling bonds in disordered graphite planes and the defects incorporated into pentagon and heptagon graphite-like structures.³⁴ The G band corresponds to a splitting of the E_2g stretching mode of graphite and reflects the structural intensity of the sp²-hybridized carbon atom, which is characteristic of graphitic-like materials.³⁵ In addition, the I_D/I_G ratio was used to measure the structural disorder, which changed slightly from 0.61 to 0.59 because of the sp² domain size of the carbon atoms during the hydrothermal process.³⁶ For the 50 wt% TiO₂/CA composite, the four distant peaks located at 150 cm⁻¹ (E_g), 400 cm⁻¹ (B_1g), 512 cm⁻¹ (A_1g), and 634 cm⁻¹ (E_g) were identical to the characteristic peaks of anatase TiO_2, which agreed with the previously reported results.^37–39 Notably, all the characteristic peaks of the two components existed in the composite, further suggesting the coexistence of the TiO₂ and CA, and no peak assigned to a rutile or brookite phase was observed, which was consistent with the above results.⁴⁰

Fig. 5 (a) Raman spectra, and (b) UV-vis diffuse reflectance spectra of the as-prepared samples.

The optical properties of the as-prepared samples were further investigated using the UV-vis diffuse reflectance spectra (Fig. 5b). TiO₂ showed a characteristic spectrum with a fundamental absorbance threshold of 380 nm. Comparing with the DRS spectra of TiO₂, it is easy to observe that the absorption edge of the TiO₂/CA composites undergo a red shift and an enhanced absorption, indicating a narrowing of the band gap of TiO₂. The red shift has been assigned to the unique 3D interconnected network porous structure of the CA and the positive synergetic effects between the two components.^27,41 Similar results were observed for TiO₂ upon doping with other carbon-based materials (graphene,⁴² carbon nanotube⁴³ etc.). The result suggested the binary TiO₂/CA composite exhibited efficient charge separation and enhanced light absorption, as well as effective interfacial contacts among the TiO₂ and CA components, which would display excellent photocatalytic activities.

XPS has been carried out to acquire further information about the surface chemical composition and chemical valence states of the elements. The typical survey spectrum in Fig. 6a further confirmed the existence of Ti, O and C elements. The main C 1s spectrum in Fig. 6b is dominated by elemental carbon at 284.6 eV, attributed mainly to the sp² hybridized carbon (BE = 284.6 eV). Several binding energies located at 286.4 and 288.4 eV, imply that different carbon groups exist in the sample, which were attributed to the C=C, C–O and C=C–O groups, respectively.^44–46 With respect to the XPS of Ti 2p in Fig. 6c, two strong peaks located at approximately 458.7 and 464.47 eV have been fitted, which are ascribed to Ti 2p_3/2 and the Ti 2p_1/2 spin–orbital splitting photoelectrons in the Ti⁴⁺ chemical state, respectively. The splitting value between these two bands is about 5.7 eV, further confirming that in the composite the Ti mainly existed in the Ti⁴⁺ state.^29,47–49 Fig. 6d demonstrated O 1s peaks around 530.0 and 531.7 eV, which are attributed to Ti–O in the TiO₂ lattice, adsorbed H₂O, and hydroxyl –OH.^32,50,51

Fig. 6 XPS spectra of 50 wt% TiO₂/CA composite: (a) survey, (b) C 1s (c) Ti 2p and (d) O 1s.

The BET surface area is one of the important factors for the photocatalytic properties, as it provides more opportunities for the photocatalyst to come into contact with light and reactants, resulting in the enhancement of photocatalytic activity. The N₂ adsorption–desorption isotherms and pore-size distribution curves of the as-fabricated samples are illustrated in Fig. S3.† Both the CA and 50 wt% TiO₂/CA composite exhibit a type IV isotherm according to the IUPAC classification, with the distinct hysteresis loop in the range p/p₀ = 0.5–1.0, indicative of the mesoporous structure.⁶ The surface area and pore size of the CA are about 10.101 m² g⁻¹ and 2 nm, similar to the previous report.¹⁹ The surface area and pore size of the 50 wt% TiO₂/CA composite are about 113.33 m² g⁻¹ and 7 nm, respectively. The high BET surface area may results from the porous structure of the CA and the 3D structure inhibiting the aggregation of the TiO₂ nanoparticles as well as the formed synergetic effect of the CA and TiO_2. This large surface area allowed more light to be adsorbed and also provided more reactive sites for dye photodegradation.

3.1 Photocatalytic activity of the as-prepared composites

Organic dyes and antibiotic drugs are extensively applied in the multitudinous industrial applications in human life. However, the frequent occurrence of dyes and antibiotics in the aqueous environment has led to the emergence of environmental issues, which pose a threat to human health. Therefore, many studies have chosen them as the representative organic pollutants to evaluate photocatalytic degradation over as-prepared composites,^52–54 and have generally chosen concentrations of 10 mg L⁻¹. In this work, we chose a relatively high simulated pollutant concentration of 40 mg L⁻¹ to confirm the photocatalyst performance.

The photodegradation properties of the as-synthesized photocatalysts for MB (40 mg L⁻¹) and CIP (40 mg L⁻¹) solutions were measured and presented in Fig. 7a and c. In order to eliminate the influence of the physical adsorption, the adsorption experiments on the various photocatalysts were first conducted in the dark for 60 min. As shown in Fig. S5 and S6,† all the samples could reach adsorption–desorption equilibrium within 30 min, indicating the photocatalytic degradation experiment could effectively remove the effect of physical adsorption. A blank test confirmed negligible self-photolysis of MB, as shown in Fig. 7a. The P25 showed weak activity with only about 20% of MB degraded after irradiation for 210 min, while pure TiO₂ exhibited a higher activity of more than 40% for MB degradation within the same irradiation time. With respect to the TiO₂/CA composites, when the two components were coupled together, it resulted in enhanced ability for the degradation of MB. The 50 wt% TiO₂/CA composite has the highest photocatalytic activity, on which MB was degraded by about 82%. At the same time, CIP was another model pollutant used to evaluate the photocatalytic activity of the TiO₂/CA composite. The photodegradation results indicated that for the photodegradation of CIP the TiO₂/CA composite was much better than the pure TiO₂ (Fig. 7c), after the irradiation time was extending to 210 min, about 65% of the CIP solution was photodegraded. The time-dependent absorption spectra of the MB and CIP solutions in the presence of the 50 wt% TiO₂/CA composite, show an evident decrease in the characteristic absorptions (Fig. S4†).

Fig. 7 (a and c) The relationship between C_t/C₀ and reaction time (t) in the photodegradation of MB and CIP, (b) MB degradation curves of ln(C₀/C) versus time for the different catalysts, and (d) the four runs test of photocatalytic activity using 50 wt% TiO₂/CA composite.

Typically, the overall efficiencies of the photocatalysts for MB solution degradation can be quantitatively evaluated by comparison of the apparent rate reaction constant (k), hence a pseudo-first-order reaction model was used to describe the experimental data as follows: −ln(C/C₀) = kt, where k is the apparent rate reaction constant and C₀ and C are the adsorption equilibrium and MB concentrations at reaction time t. Linear relationships were obtained as depicted in Fig. 7b, indicating that the MB photodegradation process can be well described using the pseudo-first-order model. The 50 wt% TiO₂/CA composite has the maximum rate constant of 0.00775 min⁻¹, which is almost 3.28 times higher than that of pure TiO₂. The pseudo-first-order constants and relative linear regression coefficients are summarized in Table 1.

Table 1 Pseudo-first-order rate constants for MB photocatalytic oxidation with different photocatalysts

Series	Photocatalyst	The first order kinetic equation	k (min^−1)	R^2
1	P25	ln(C0/C) = 0.00109t	0.00109	0.97012
2	TiO2	ln(C0/C) = 0.00236t	0.00236	0.99094
3	20 wt% TiO2/CA	ln(C0/C) = 0.0062t	0.00620	0.99749
4	33 wt% TiO2/CA	ln(C0/C) = 0.00646t	0.00646	0.99450
5	50 wt% TiO2/CA	ln(C0/C) = 0.00775t	0.00775	0.99314
6	67 wt% TiO2/CA	ln(C0/C) = 0.00602t	0.00602	0.99724
7	80 wt% TiO2/CA	ln(C0/C) = 0.00567t	0.00567	0.99699

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The stability and recyclability of the photocatalysts were significant factors in their practical application. The 50 wt% TiO₂/CA composite was repeatedly confirmed by recycling four times without any significant reduction of the photocatalytic activity (Fig. 7d). After four cycles of MB photodegradation, the efficiency could still be well maintained. The photocatalyst exhibited a slight loss of activity after four cycles, suggesting that the TiO₂/CA nanocomposite is a stable photocatalyst for the degradation of MB. As a result, this suggested that the TiO₂/CA composite has outstanding catalytic activity for photodegradation and could be readily recycled for potential application in environmental protection.

The transient photocurrent responses and electrochemical impedance spectroscopy (EIS) measurements are the fundamental tools used to reveal the interfacial charge transfer properties of materials.^55,56 The transient photocurrent responses of the as-prepared TiO₂ and TiO₂/CA modified ITO electrodes were recorded for several on–off cycles under visible light irradiation (Fig. 8a). When the light was switched on, the photocurrent increased sharply, reaching a stable value, and as soon as the lamp was turned off, the photocurrent rapidly returned to its initial status, which was repeatable. The photocurrent density of the 50 wt% TiO₂/CA composite was approximately 2 times as high as that of TiO₂. The enhanced photocurrent of the 50 wt% TiO₂/CA composite indicated the enhanced separation efficiency of the photo-induced electrons and holes. Fig. 8b displays the Nyquist plot for the prepared samples. The 50 wt% TiO₂/CA composite showed a smaller diameter of the Nyquist circle than that of pure TiO₂, demonstrating a lower charge transfer resistance and a more effective electron–hole pair after the TiO₂ nanoparticles were hybridised with the CA. Both electrochemical investigations demonstrated the increased participation of the photogenerated electrons and holes in the photodegradation process by the interfacial transfer and separation of the photogenerated charge.

Fig. 8 (a) Transient photocurrent response and (b) electrochemical impedance spectroscopy for TiO₂ and the 50 wt% TiO₂/CA composite.

To gain deeper insight into the transfer and recombination of the electron–hole pairs in the photocatalysis process, PL measurements were collected and are shown in Fig. 9. TiO₂ and the TiO₂/CA composites exhibited a broad PL emission band, which was similar to the earlier study.^17,57 The emission band centered at the peak position of ∼540 nm is for anatase.^58,59 Obviously, compared with the pure TiO₂, the emission intensity of the TiO₂/CA composites became significantly feeble. The above characterization results of the photocurrents and EIS measurements are consistent with the PL analysis, which indicated that the 50 wt% TiO₂/CA composite had a lower electron–hole recombination rate, the enhanced efficiency of separation of the photoinduced electrons and holes might be ascribed to the CA because of the unique 3D interconnected network structure and the synergetic effect of the CA and TiO₂ semiconductor.

Fig. 9 PL emission spectra of TiO₂ and the TiO₂/CA composites.

3.2 Mechanism of MB photocatalysis degradation

The adsorbed dye molecules can be effectively photodegraded by reactive oxidative species, including h⁺, ˙OH and ˙O₂⁻. In order to distinguish the role of these active species, individual scavengers were added to the photodegradation system, such as ammonium oxalate (AO, holes scavenger), 1,4-benzoquinone (BQ, ˙O₂⁻ radicals scavenger) and tert-butanol (t-BuOH, ˙OH radicals scavenger) (Fig. 10). The addition of AO caused a slight change in the photocatalytic activity, indicating the slight importance of h⁺ species in the photocatalytic process. However, the photocatalytic activity of the TiO₂/CA composite was significantly inhibited when BQ or t-BuOH was introduced into the photocatalytic reaction system. Hence, ˙O₂⁻ radicals and ˙OH radicals should be the two main active species during the photocatalytic degradation of MB by the TiO₂/CA nanocomposites under visible light irradiation.

Fig. 10 Degradation ratio of the 50 wt% TiO₂/CA composite with different scavengers.

In order to gain a deeper understanding of the photocatalytic mechanism, the electron spin resonance (ESR) spin-trap signals of TiO₂/CA with DMPO in methanol solution and aqueous solution were investigated (Fig. 11). From Fig. 11a, four characteristic peaks of the DMPO–˙OH species were detected in aqueous dispersions of the samples under visible light irradiation. Similarly, there are four characteristic peaks with relative intensities corresponding to the ˙O₂⁻ radicals (Fig. 11b) under visible light irradiation, indicating that ˙O₂⁻ radicals are also produced during the photocatalytic process.^60–62 These ESR results further confirmed that the ˙OH and ˙O₂⁻ radicals are the main active species during the photocatalytic process, leading to the improvement of photocatalytic activity.

Fig. 11 ESR spectra of the 50 wt% TiO₂/CA composite with visible light irradiation. (a) DMPO–˙OH in aqueous dispersions. (b) DMPO–˙O₂⁻ in methanol dispersions.

Combined with the discussion presented above and the experimental results, the decolorization of the dye solutions can be supposed to occur due to the formation of the active radicals (superoxide radical and hydroxyl radicals) generated during the photochemical process in the presence of the photocatalyst. The improved photocatalytic performance arises from the enhanced light harvesting and more efficient separation of the photogenerated electron–hole pairs due to the TiO₂/CA composite. These results indicate that the TiO₂/CA system photocatalyst exhibited excellent in situ photodegradation of the pollutants. A possible photocatalytic mechanism was proposed as the following cascade reactions:

TiO₂/CA + hν → TiO₂/CA(e⁻) + TiO₂/CA(h⁺) (1)

H₂O + h⁺ → ˙OH + H⁺ (2)

OH⁻ + h⁺ → ˙OH (3)

O₂ + e⁻ → ˙O₂⁻ (4)

˙OH (or ˙O₂⁻) + pollutant → degradation products (5)

4 Conclusion

In summary, we have demonstrated a simple and cost-effective method for synthesizing a novel TiO₂/CA composite from natural and renewable biomass. Carbonaceous aerogel was first prepared by using wintermelon as the source material through a facile hydrothermal process. Benefiting from the active functional groups and defects of the CA, the TiO₂ nanoparticles were easily grown and immobilized, finally resulting in homogenous distribution loading on the surface of the CA. On the basis of the good dispersibility of TiO₂ with high-loading attached on the surface of the CA, the as-synthesized TiO₂/CA hybrid materials exhibited the synergetic properties of the TiO₂ photocatalyst and the CA support to strengthen its photodegradation activity. The TiO₂/CA composite exhibited higher photocatalytic activity and stability than that of P25 for the degradation of MB and CIP solutions under visible light irradiation. It may be speculated that the porous structure of the CA can separate electron–hole pairs and reduce the photocorrosion of TiO₂ and then remarkably enhance the photocatalytic activity for the degradation of organic pollutants. The developed strategy has the merits of being simple, natural, low-cost, reproducible for utilizing the natural biomass, and meeting the requirements of green chemistry. Furthermore, the hybrid material strategy opens up possibilities for combining carbonaceous aerogels with other novel semiconductors, such as WO₃, ZnO, Fe₂O₃, Bi₂WO₆ etc.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation (21306067, 51402130), the Scientific Innovation Research of College Graduate in Jiangsu Province (KYXX_0025), the Science & Technology Foundation of Zhenjiang (GY2014004 and GY2014028), and the Science & Technology Foundation of Yangzhou (YZ2015019).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra28116d

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