Soft-template synthesis of carbon-doped mesoporous TiO₂ using Citrullus colocynthis fruit shell extract: Characterization and photocatalytic application

2.1. Preparation of TiO₂

Wild Citrullus colocynthis fruits were collected from the Jizan province of Saudi Arabia. The fruits were dried at room temperature and the shells were harvested and ground. 4 g of the obtained powder was added to 100 mL of deionized water and heated at 60°C for 30 mins. The harvested extract will be used as a stock solution for synthesizing TiO₂ samples. The preparation of the TiO₂ nanoparticles was achieved by sol-gel route, as described previously [20,21]. Specifically, 17 mL of Tetra (2-ethylhexyl) titanate (from Sigma Aldrich) was mixed with 17 mL of ethanol and stirred for 30 mins, then 17 mL of the extract stock solution was added to the precursor/ethanol solution while stirring for 30 mins, as a result of which the mixture transformed rapidly to gel. The gel was kept to dry overnight in pre-heated oven at 80°C. The dried powder was ground in a mortar, poured into a ceramic crucible, and annealed at 600°C for 2 hrs.

2.2. Characterization techniques

The crystal structure of the as-synthesized TiO₂ was revealed by a Rigaku Ultima IV diffractometer acquired from Bruker and equipped with Cu-Kα source providing wavelength λ= 0.15406 nm. Diffraction data were performed in the 2θ angle range 10-80° with a step of 0.02°. UV–Vis-NIR spectrophotometer from Agilent Company was used to perform the optical absorption spectra. Infrared spectroscopy investigations were conducted by a Thermo Scientific Nicolet iS50 spectrometer. An Inspect F50 scanning electron microscope (SEM) from FEI Company was used for the morphological studies of the synthesized TiO₂ samples. Thermogravimetric experiment (TGA) was performed by a TGA Shimadzu DTG-60. Photocatalytic degradation experiments of methylene blue (MB) were conducted in a photocatalytic reactor equipped with a 300-W xenon UV lamp (CEL-PF300-16, Beijing China Education AuLight Technology Co., LTD., Beijing, China) providing an intensity of ∼125 mW/cm².

2.3. Catalytic activity

The photocatalytic activity of synthesized TiO₂ towards the decomposition of methylene blue (from Sigma Aldrich) under UV excitation was evaluated on a UV-visible spectrophotometer. 10 mg of MB dye was dissolved in 1L of deionized water to form a dye stock solution. Next, 200 mL of the stock solution was mixed with 5 mg of the synthesized TiO₂ under vigorous stirring. Afterwards, the solution was illumined by a UV source, and every 15 mins, 2 mL were taken and subjected to a UV absorbance experiment. The methylene blue removal efficiency was calculated by the following formula (Eq. 1):

Where, $A_0$ and $A_t$ represent the initial maxima of the absorbance of the methylene blue solution and that of the solution after being irradiated under UV during a given time interval t.

3.1. Crystallographic analysis

XRD explored the crystallographic structure of the as-synthesized TiO₂. As shown in Figure 1, diffraction peaks can be identified at 25.3°, 36.95°, 37.79°, 38.57°, 48.05°, 53.88°, 55.07°, 62.69°, 68.75°, 70.3°, and 75.05°, attributed to the crystallographic diffraction planes (101), (013), (004), (112), (020), (015), (121), (024), (116), (220), and (125), respectively. All these diffraction peaks are assigned to the anatase TiO₂ polymorph (JCPDS card no. 96-900-8214). The absence of any unidentified diffraction peaks in this pattern confirmed the high purity and crystallinity of the synthesized sample. Moreover, a thermodynamically stable anatase phase was achieved despite the fact that the synthesized TiO₂ was calcined at 600°C for two hrs. It is likely that the reason for such thermal stability is the complex composition of the Citrullus colocynthis extract, including versatile phytoelements and biomolecules that not only have reducing and capping effects beneficial for the growth of the TiO₂ nanoparticles but also promote the inhibition of anatase-to-rutile phase transition via carbon self-doping. Although the X-ray diffraction (XRD) pattern does not display any peaks revealing the existence of doping impurities in the TiO₂ structure, its thermal stability could be associated with the doping of lattice by carbon native to the plant extract as will be confirmed below by X-ray photoelectron spectroscopy (XPS) analysis. In fact, many authors suggested that anatase–to-rutile transformation can delayed by carbon doping [22,23].

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Figure 1.

XRD diffraction pattern of the synthesized TiO₂.

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The crystallite size of the synthesized TiO₂ was estimated by application of the Debye-Scherrer equation to the more intense diffraction peak located at 25.52° and assigned to the hkl crystallographic plane (101) [23]. The calculated mean size of crystallites was about 17.87 nm.

3.2. Thermal analysis

Thermogravimetric analysis (TGA) of the biosynthesized TiO₂ nanoparticles was performed at a heating rate of 10°C/min. The recorded TGA curve reveals a gradual reduction in mass that can be divided into three regions (Figure 2). The first, for temperatures below 200°C associated with the release of physisorbed water generated a mass loss of 5.9%. The second region between 200°C and 450°C, with a mass loss 8.9%, was likely attributed to the degradation of the organic compounds originating from the Citrullus colocynthis extract. After 450°C, the TGA curve consisted of a plateau, indicating the thermal stability of the synthesized TiO₂ beyond that temperature.

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Figure 2.

Thermogravimetric Analysis (TGA) curve of the synthesized TiO₂.

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3.3. Morphologic analysis

Field emission scanning electron microscopy (FESEM) images of Figure 3(a) and 3(b) indicate that the synthesized TiO₂ exhibited an agglomerated morphology formed by spherical-shaped nanoparticles having an average diameter of 18 nm. The image also exhibits the presence of pores presumably associated with the removal of biomolecules included in the plant extract during the calcination process.

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Figure 3.

(a) and (b) SEM micrographs captured at two magnifications (x5000, x50000), (c) and (d) TEM images, (e) HRTEM of the synthesised TiO₂, (f) Inverse fast Fourier Transform (IFFT) of the yellow dashed delimited grain and (g) the schematic diagram of an edge dislocation. SEM: Scanning electron microscopy, TEM: Transmission electron microscopy, HRTEM: High-resolution transmission electron microscopy, IFFT: Inverse fast fourier transform.

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Furthermore, transmission electron microscopy (TEM) (Figure 3c and 3d) and high-resolution transmission electron microscopy (HRTEM) Figure 3(e) experiments were conducted to delve deeper into the structure and morphology of the synthesized TiO₂. The lattice arrangement of the synthesized TiO₂ with distinct grains and grain boundaries (marked with dashed lines) was observed, which revealed its polycrystalline nature. The high-resolution TEM image presented in Figure 3(e) exhibits the lattice fringes of the TiO₂ nanoparticles. Specifically, the interplanar lattice spacing of 0.352 nm refers to the typical (101) crystallographic plane of anatase polymorph in agreement with the XRD findings. In addition, the sample crystal lattice exhibits a certain stacking fault evidenced in inverse Fast Fourier transform (IFFT) performed on the yellow-dashed delimited region of (Figure 3e) and can be associated with edge dislocations (Figure 3f). Edge dislocation is a crystallographic defect that appears whenever an extra half-plane of atoms is introduced into the crystal lattice, triggering the distortion of nearby planes (Figure 3g). They are typically activated by the insertion of point defects in the crystal lattice, such as vacancies and heteroatoms, thereby acting as trap sites for the photogenerated charge carriers, which, in turn, improve photoactivity efficiency [24,25].

3.4. Infrared spectroscopy analysis

As can be seen in Figure 4, the FTIR spectrum of the synthesized TiO₂ exhibits a prominent band near 643 cm⁻¹ ascribed to the Ti-O stretching vibration, confirming the formation of TiO₂ [26]. The absence of vibration bands specific to any organic groups confirmed the effective removal of the organic phase after the calcination step and indicated the purity of the synthesized TiO₂.

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Figure 4.

FTIR spectrum of the synthesised TiO₂.

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3.5. Elemental analysis

The surface chemical composition of the prepared TiO₂ was probed by X-ray photoelectron spectroscopy. The survey curve points out the presence of O, Ti, and C (Figure 5a). Further insightful investigations of the as-synthesized TiO₂ were achieved by recording the high-resolution peak spectra of the various elements revealed in the survey spectrum to check whether the growth approach generates any point defects such oxygen vacancy and doping in the synthesized TiO₂ structure (Figure 5b-d).

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Figure 5.

(a) XPS survey of the synthesised TiO2, high-resolution spectra of: (b) O1s, (c) C1s, (d) Ti2p and (e) valence energy. XPS: X-ray photoelectron spectroscopy, O1s: Oxygen 1s, C1s: Carbon 1s, Ti2p: Titanium 2p.

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The asymmetric O 1s peak is fitted with three sub-peaks centered at 530.08 eV, 530.8 eV, and 531.9 eV and ascribed to the lattice oxygen, the oxygen vacancy, and the adsorbed oxygen, respectively, in agreement with several previous reports [27,28].

C1s spectrum was fitted into three peaks, where those situated at 284.68 eV, 286.26 eV, and 288.56 eV reflected that carbon exists in three different chemical environments. Indeed, the peak at 284.68 eV is attributed to the residual carbon element, whereas peaks at 286.26 eV and 288.56 eV are attributed to C=O and Ti-O-C, respectively [29]. The existence of a Ti–O–C bond associated with the oxygen substitution by carbon, together with the absence of any peaks around 282 eV characteristic of the Ti-C bond support that the carbon doping of the synthesized TiO₂ is either insertional or substitutional to O [30]. Previous experimental and theoretical studies suggest that such kind of doping induced many localized mid-bandgap states, improving the photoactivity potential of the synthesized TiO₂ by promoting the electron-hole separation [31].

The Ti2p high-resolution spectrum resolved in two peaks centered at 458 eV and 464 eV and assigned to the Ti2p_3/2 and Ti2p_1/2 spin-orbit splitting, respectively, indicating the presence of Ti⁴⁺ [32].

The valence band energy spectrum of the synthesized TiO₂ shown in Figure 5(e) was fitted by two Gaussian peaks centered at 3.73 eV and 6.25 eV, associated with the electron emission from π (nonbonding) and σ (bonding) O2p orbitals, respectively. These values correlated well with previous results on anatase TiO₂ [33,34]. Moreover, the valence band maximum (VBM) was determined by linear extrapolation of the valence band to the baseline to be 2.08 eV_, being barely equal to that previously reported for intrinsic carbon-doped TiO₂ prepared by solvothermal method (2.05 eV) [35].

3.6. UV-visible absorption

The optical absorption spectrum of the synthesized TiO₂ was conducted by scanning the wavelength range 200 nm - 800 nm, and the recorded data have been shown in Figure 6. The inset of Figure 6 represents the Tauc plot [36].

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Figure 6.

UV-visible absorption of the synthesised TiO₂, the inset represents the Tauc plot.

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The optical bandgap of the as-synthesized TiO₂ derived from the Tauc plot was about 3.15 eV, nearly equal to the reported value for pristine anatase polymorph (3.2 eV) in line with previous studies stating that carbon-doping does not alter the band gap energy of TiO₂ significantly but promotes the separation of the electron-hole pairs [35,37].

3.7. Dye degradation

Photocatalysis is a process widely used for the degradation of dyes. This process mainly involves the use of heterogeneous catalysts, such as metal oxides and metal sulphides, metal–organic frameworks (MOFs), and appropriate light irradiation [38-40].

The photocatalytic potential of the as-synthesized TiO₂ nanostructures towards the degradation of methylene blue dye was evaluated via UV-visible absorption measurements. Figure 7(a) highlights a gradual decay of the absorption of methylene blue solution with UV irradiation time, reflecting the effectiveness of the synthesized TiO₂ in speeding up the degradation of MB. The maximum optical absorption of MB was achieved at a wavelength of 661.5 nm. A degradation rate of 71.5% was achieved after 150 mins of UV irradiation. This compared well with a recent study carried out using cellulose nanocrystals (CNCs) as a template to synthesize TiO₂, achieving an MB degradation rate ranging from 65.3% to 79.9% according to the used mass concentration of CNCs [41].

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Figure 7.

(a) Degradation of methylene blue by the synthesized TiO₂ under UV excitation, (b) Photocatalytic reaction’s kinetics study by plotting $\ln \left(\frac{{\text{C}}_{\text{t}}}{{\text{C}}_0}\right)$ against irradiation time for the first order reaction degradation, (c) Plausible degradation mechanism. UV: Ultraviolet, CB: Conduction band, VB: Valence band, MB: Methylene blue, TiO₂: Titanium dioxide.

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To find out the photocatalytic reaction’s kinetics model, $\ln \left(\frac{C_t}{C_0}\right)$ was plotted versus the irradiation time (Figure 7b). The obtained curve is fairly linear (R² = 0.95) with a slope $k$ and can be described by the following Eq. (2):

Where $A_0$ and $A_t$ represent the absorption intensity at the maximum absorption wavelength (661.5 nm) for the MB solution without TiO₂ at (t=0) and that after being UV-irradiated in the presence of TiO₂ for a time interval t, respectively. The constant $k$ that represents the reaction rate constant was found to be -0.0775 min^-1. The good linearity of the curve suggests that the degradation process follows a pseudo-first-order reaction’s kinetics.

Conceptually, when TiO₂ is irradiated by a UV source, electrons jump from the valence band to the conduction band, leaving behind holes in the valence band. Therefore hole-electron pairs are created. Excited electrons in the conduction band react with molecular oxygen in air to produce superoxide anions, while holes in the valence band react with water molecules adsorbed on the photocatalyst surface to form hydroxyl radicals. The produced OH^* and ${\text{O}}_2^{-\text{*}}$ radicals are highly reactive and can oxidize organic compounds near the surface of the photocatalyst. This oxidation process breaks down complex organic molecules into simpler substances, primarily CO₂ and H₂O. The presence of mid-gap sub-states acts as trapping sites for the charge carriers, which slows down their recombination to be more reactive to the environing species. The plausible degradation pathway of MB has been shown in Figure 7(c).