Bioframe synthesis of NF–TiO₂/straw charcoal composites for enhanced adsorption-visible light photocatalytic degradation of RhB

Abstract

N–F codoped TiO₂/straw charcoal composites (NF–TiO₂/SC) were synthesized using a simple, bioframe-assisted sol–gel method and confirmed by XRD, SEM, EDX, TEM, N₂ adsorption–desorption, Raman, FT-IR, XPS, and UV-vis DRS measurements. It has been found that the as-synthesized catalysts formed a 3D-hierarchical structure with carbon framework after calcination. The physicochemical properties, such as BET surface area, porosity, crystallite size and pore size distribution, could be effectively controlled by adjusting the calcination temperature. NF–TiO₂/SC calcined at 450 °C exhibited higher surface area, smaller crystallite size and higher absorption in the visible light region. Under visible light irradiation, NF–TiO₂/SC450 exhibited considerably high adsorption ability and photocatalytic activity, and there existed a significant synergistic effect between adsorption and photocatalytic degradation. Furthermore, the as-synthesized catalysts exhibited good mineralization ability, and could be used repeatedly, revealing their great potential for practical applications in environmental cleanup.

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1. Introduction

Semiconductor photocatalysts have been of considerable interest in the past decades owing to their application in water and air purification,^1,2 self-cleaning surfaces,³ antibacterial agents^4–6 and solar cells.⁷ Among the various photocatalysts, titanium dioxide (TiO₂) has attracted extensive interest because of its excellent chemical stability, photodurability, low cost and nontoxicity.⁸ However, conventional TiO₂ is a wide band-gap semiconductor (3.23 eV for anatase TiO₂ and 3.02 eV for rutile TiO₂), which requires ultraviolet (UV) radiation (λ < 388 nm) for photocatalytic activation. This is a technological limitation when aiming at the implementation of large scale sustainable technologies with renewable energy sources such as solar light, because UV radiation accounts only for 5% of the total solar spectrum compared to the visible region (∼45%).^9–11 Many efforts have been made in the last two decades to overcome this limitation. Among these efforts, TiO₂ co-doped with two types of nonmetals, such as S–N,¹² C–N,^13,14 N–F,^15,16 B–F¹⁷ and B–N,¹⁸ is expected to provide the synergistic effect on permitting tune the electronic structure and improving photocatalytic activity and have drawn more and more attention. Usually, photocatalytic behaviors of these materials are investigated in their powdered form.

In the application of wastewater purification, the use of photocatalyst powders for the photo-degradation of organic compounds is known to exhibit a remarkable reaction rate, which is due to the dispersion of nano-sized particles that have a large surface area. However, this method has some limitations, including low light-utilization efficiency caused by the absorption of light and scattering of it into the aqueous suspension of illuminated powder¹⁹ and the necessity of removal of powder particles from the fluid medium after the purification process.²⁰ Because of the above-mentioned problems, bulk photocatalysts were substituted by supported photocatalysts.

For this purpose, a photocatalyst was immobilized on supporters such as perlite,²¹ clay,²² zeolite²³ and carbon-based materials.¹⁹ Various carbon-based materials (active carbon,^24,25 expanded graphite²⁶ and carbon foam²⁷) have been extensively studied as support for catalysts because of their high surface area, high adsorption capacity, suitable pore structure and inert nature in certain rigorous circumstances. In synthesis of catalysts, formation of a three-dimensional porous structure was reported to exhibit an enhanced photocatalytic performance.²⁸ Recently, biostructures, such as green leaves,²⁹ soft rushes,³⁰ butterfly wings,³¹ and diatoms,³² have been used as biotemplates to fabricate different morphologies of catalysts. The cellular structure of these templates was demonstrated to exhibit high surface area and effective photocatalytic reaction rate. Furthermore, the auto-doping of natural elements during the catalyst preparation probably has a synergistic effect for enhancing photocatalytic activity.³⁰ However, these biotemplates could be removed in the process of heating; thus, they have poor mechanical strength for catalyst structure.

Straw is an abundant and inexpensive natural resource in China. It is estimated that the throughput of straw has been approximately 600 million tons a year.³³ However, its utilization has not been fully explored. As a biotemplate, straw has an abundance of pore structures. The “hollow-vascular” structure of straw can act as a supporter for catalyst to form a 3D-porous morphology.³⁴ Besides, it can be carbonized in a furnace at a high temperature in absence of oxygen to produce “carbon framework”. As a consequence, the final catalyst could have good structure and mechanical strength.

In this study, N–F codoped TiO₂/straw charcoal (NF–TiO₂/SC) was synthesized by a bioframe-assisted sol–gel method. Morphological, structural and electronic properties of the produced composites were thoroughly characterized. The application of NF–TiO₂/SC composites for remediation of water contaminated with rhodamine B (RhB), a common pollutant in textile waste, was further explored under visible light irradiation. The effect of calcination temperature was investigated to optimize the conditions for synthesizing NF–TiO₂/SC with the highest visible light photocatalytic activity for RhB degradation.

2. Experimental section

2.1 Preparation of NF–TiO₂/SC composites

In this study, straw was purchased from a local farmhouse in Shanghai. Before use, it was dried at room temperature for seven days and then milled into power.

A bioframe-assisted sol–gel method was used to synthesize NF–TiO₂/SC composites (Fig. 1). 35 mL tetrabutyl titanate and 0.36 g urea were dispersed in 100 mL absolute ethanol containing 4 mL acetic acid and 1 mL hydrofluoric acid; this solution was named A. Subsequently, pretreated straw (4 g) was added and dispersed with ultrasound for 0.5 h, then stirred for 1 h to make sure precursors were absorbed on the surface of straw. Then, 28 mL absolute ethanol, 3 mL acetic acid and 18 mL deionized water were mixed to obtain solution B. Solution B was then added drop by drop to solution A with stirring to obtain a mixture. The resultant mixture was stirred until a white gel was obtained. The gel was aged at room temperature for 24 h then dried at 105 °C for 12 h to obtain a xerogel. Then, the xerogel was crushed into fine powder and calcined at 350–650 °C in a flow of nitrogen for 2 h to obtain NF–TiO₂/SC composite catalyst. Catalysts are named as NF–TiO₂/SCx, where x (350, 450, 550 and 650 °C) corresponds to calcination temperature. P25 purchased from Degussa Company, catalyst synthesized without straw (NF–TiO₂) and with straw powder calcined at 450 °C (SC) were produced and used as control materials.

Fig. 1 Bioframe synthetic process of NF–TiO₂/SC catalyst.

2.2 Characterization of NF–TiO₂/SC composites

For crystal structure analysis of the as-prepared catalysts, X-ray diffraction (XRD) analysis was carried out on a Bruker D8 ADVANCE (German) X-ray diffractometer with Cu Kα radiation (40 kV, 40 mA) with a 0.01° step and 2.5 s step time over the range of 10° < 2θ < 90°. Nitrogen adsorption–desorption isotherms were used to determine BET surface area and pore size distribution (Micrometrics, ASAP 2020). The morphology of the synthesized catalysts was observed initially using scanning electron microscopy (SEM, Hitachi S4700) together with an energy dispersion X-ray (EDX) analysis. A high resolution-transmission electron microscope (HR-TEM, JEOL JEM-2010F) with a field emission-transmission gun at 200 kV was utilized to investigate the morphology and crystallinity of the samples. IR spectrum was recorded using KBr pellets at room temperature on a Fourier transform infrared (FTIR) spectrophotometer (Nicolet Instrument Corporation, USA). Raman analysis was carried out on a Renishaw InVia Raman spectrometer using a high power near infrared (near-IR) diode laser (λ = 785 nm, 500 mW) as the excitation source. For the characterization of the light absorption features and band-gap determinations, diffuse reflectance spectra (DRS) of the particles were measured in the range of 200–800 nm on a UV-vis-NIR scanning spectrophotometer (Shimadzu UV-2550, Japan) equipped with an integral sphere using BaSO₄ as a reference. The photocatalyst powder was placed in the sample holder on an integrated sphere for the reflectance measurements. For the surface properties and elemental composition, XPS measurements were conducted by a Thermo-VG Scientific ESCALAB 250 XPS system with Al Kα X-ray radiation. The binding energies were corrected by C 1s level at 284.6 eV as a reference to reduce the relative surface charging effect.

2.3 Photocatalytic evaluation with rhodamine B under visible light

The adsorption-photocatalytic experiments were carried out in the PHCMIII photochemical reactor (NBeT Group Corp., Beijing). 20 mg of catalyst was immersed in 50 mL of an aqueous RhB (C₀ = 20 mg L⁻¹) solution with vigorous and constant stirring. For adsorption experiments, the suspension containing catalyst and RhB was stirred for 3 h with a magnetic stirrer in the dark to obtain adsorption equilibrium. The photocatalytic experiments were performed in a photochemical reactor with a 500 W Xe lamp, which was used with a UV cut-off filter (1 M sodium nitrite solution, λ ＞ 400 nm), as a visible light source. After irradiation, the RhB solution was filtered through a membrane filter (pore size = 0.45 μm). Changes in RhB concentration during adsorption or photodecomposition were characterized by absorbance at 554 nm using a UV-vis spectrophotometer (UV-vis 4802, China). The filtrate was used for TOC measurement with a Shimadzu TOC-VCPN analyzer. To examine the durability of NF–TiO₂/SC composites, the NF–TiO₂/SC450 composite was reused five times for the photocatalytic degradation of RhB in an aqueous solution.

3. Results and discussion

3.1 Catalyst characterization

To investigate the effect of heat treatment on crystal structure, the crystalline phase of synthesized catalysts (NF–TiO₂/SC350, NF–TiO₂/SC450, NF–TiO₂/SC550, and NF–TiO₂/SC650) and reference sample (SC) was determined by XRD (Fig. 2). All the peaks in the XRD patterns of different catalysts were designated to anatase crystal phase with no indication of other crystalline phases (rutile or dopant related) under the experimental conditions. With increase in calcination temperature, diffraction peaks became sharper and more intense, indicating the formation of larger crystallites and higher crystallinity.³⁵ NF–TiO₂/SC calcined at 650 °C showed the appearance of clear diffraction peaks at 36.9° and 38.5°, corresponding to the characteristic peaks of crystal plane (103) and (112) of anatase, respectively. The carbon bioframe of catalysts did not show any crystallized phase, while the reference SC sample showed sylvite crystal phase, which is due to residual K in crop straw.

Fig. 2 XRD spectra of synthesized catalysts.

The morphology and structure of synthesized catalysts were further investigated by SEM. Fig. 3A shows that the original morphology of straw charcoal is cylinder-shaped with a diameter of about 1.55–5.69 μm. The morphology of NF–TiO₂/SC (calcined at 450 °C) basically retains the shape of SC and displays a three-dimensional (3D) hierarchical porous column (Fig. 3B). It can be observed that co-doped TiO₂ is coated on the surface of carbon bioframe with a thickness of about 95.3–256 nm (Fig. 3C). The C, N and F changes in NF–TiO₂/SC with calcination temperature were identified using the EDX (Fig. 3D). The content of C is 10.09%, 10.21%, 9.21% and 4.53% at 350, 450, 550 and 650 °C, respectively. The result indicates that there is a sharp decrease in C content when calcination temperature is higher than 550 °C. The weight ratio of N/F is in the range of 2.8–3.5 for all the synthesized catalysts.

Fig. 3 SEM images of (A) SC and (B) and (C) NF–TiO₂/SC450 catalyst. The inset in (C) shows the EDX spectrum of NF–TiO₂/SC450. (D) The main elemental changes in synthesized catalysts at different calcination temperature.

HR-TEM analysis was performed to further confirm the crystallite size and morphology of synthesized catalysts. As shown in Fig. 4, the particle size of catalysts calcined at 650 °C increased to 33–57 nm compared to 16–18 nm at 350–550 °C. This is also reflected by the peak shape changes in XRD. Smaller crystal size suggests more catalytic active surface sites per unit catalyst mass.³⁶ Consequently, NF–TiO₂/SC550 was proved to have the highest surface area with the measured S_BET of 121.03 m² g⁻¹ (Table 1). Nitrogen adsorption–desorption isotherms, shown in Fig. 5A, are consistent with type IV of hysteresis loops, which is typical for the mesoporous structure of the materials according to the IUPAC classification.³⁷ Barrett–Joyner–Halenda (BJH) pore size distribution is shown in Fig. 5B. Apart from NF–TiO₂/SC350, it was evidenced that the pore size was approximately 2–5 nm. Higher calcination temperatures resulted in smaller pore size and narrower pore size distribution. Selected area electron diffraction (SAED) pattern was obtained to confirm the phase content of NF–TiO₂/SC as single TiO₂. Besides, clear lattice fringes with an interlayer distance of 0.35 nm (Fig. 4) were invariably resolved in the HR-TEM images for all catalysts, which were assigned to the anatase TiO₂ facet (101) using the corresponding XRD pattern.³⁵

Fig. 4 HR-TEM images of (A) and (B) NF–TiO₂/SC350, (C) and (D) NF–TiO₂/SC450, (E) and (F) NF–TiO₂/SC550 and (G) and (H) NF–TiO₂/SC650. The insets in (A), (C), (E) and (G) show their SAED pattern.

Table 1 Structural characteristics of synthesized catalysts with different calcination temperatures and reference materials

Sample	BET specific surface area (m^2 g^−1)	Average pore size (nm)	Total pore volume (cm^3 g^−1)
SC	8.04	16.056	0.032
NF–TiO2/SC350	106.66	11.663	0.311
NF–TiO2/SC450	109.51	11.605	0.318
NF–TiO2/SC550	121.03	11.315	0.342
NF–TiO2/SC650	62.33	19.671	0.307

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Fig. 5 Nitrogen adsorption–desorption isotherms (A) and pore size distribution of catalysts at different calcination temperatures (B).

The NF–TiO₂/SC catalysts and reference materials were also characterized by Raman spectroscopy for their structural properties (Fig. 6). The results of Raman analysis are consistent with XRD and SAED results that all catalysts contain single anatase TiO₂. Anatase belongs to the tetragonal space group D¹⁹_4h (I4₁/amd) leading to six Raman-active modes (A_1g + 2B_1g + 3E_g).¹⁶ According to the spectra of P25, anatase TiO₂ active vibrations are located at 141 cm⁻¹ (E_g), 196 cm⁻¹ (E_g), 401 cm⁻¹ (B_1g), 517 cm⁻¹ (A_1g, B_1g), and 638 cm⁻¹ (E_g). When the sample was calcined at 350 °C, the E_g peak at around 150 cm⁻¹ appeared, which is mainly caused by symmetric stretching vibration of O–Ti–O in TiO₂. This means that the crystal nucleus of anatase TiO₂ began to form. With increase in calcination temperature, E_g peak first had a blue shift and then a red shift. The blue shift is probably because of the enhancement of the bending vibration of O–Ti–O in the transformation from amorphous to anatase TiO₂, whereas the red shift is due to the quantum size effect caused by crystal growth. The excess broadening for the E_g mode at lower heating temperatures indicates the reduction of anatase crystalline size upon N–F co-doping and the deviations from stoichiometry caused by oxygen vacancies or interstitials. When the calcination temperature was increased to 650 °C, other active modes at 200 cm⁻¹ (E_g), 390 cm⁻¹ (B_1g), 511 cm⁻¹ (A_1g, B_1g), and 633 cm⁻¹ (E_g) were identified. This indicated the accomplishment of transformation from amorphous form to anatase.^38,39 In addition, a high fraction of amorphous carbon was confirmed by the peak around 1588 cm⁻¹ (D-band) and the peak around 1362 cm⁻¹ (G-band) in Raman spectra.

Fig. 6 Raman spectra of NF–TiO₂/SC catalysts and reference materials. The inset displays details of NF–TiO₂/SC650.

FT-IR spectra were obtained to investigate the functional groups on the surface of NF–TiO₂/SC. FT-IR transmittance spectra of NF–TiO₂/SCs are shown in Fig. 7 in comparison with the reference material. The bands at 3768 cm⁻¹ and 3693 cm⁻¹ are assigned to the surface O–H stretching vibration.⁴⁰ The presence of the band at 3082 cm⁻¹ was attributed to C–H stretching vibrations. With increase in calcination temperature, the peak position shifted to a lower wavenumber, indicating that the C–H bond is connected with the doping atoms.^41,42 The bands at 1589 cm⁻¹ and 1490 cm⁻¹ are assigned to C–C stretching and carboxyl C–O. Moreover, the strong absorption bands in the range of 400–1000 cm⁻¹ correspond to Ti–O–Ti bonding, and the bands remarkably shifted toward the lower region with the increase in temperature. This suggests the presence of Ti–O–Ti associated with Ti–O–N (C or F) in the catalysts, indicating the chemical interaction between surface hydroxyl groups of TiO₂ and functional groups of dopants.^40,43

Fig. 7 FT-IR spectra of NF–TiO₂/SC catalysts and reference materials.

To further explore the nature of doped elements, the sample of NF–TiO₂/SC450 was analyzed by XPS (Fig. 8). The survey spectra of NF–TiO₂/SC450 predominantly contain Ti, O, and C elements and a low amount of N and F elements. The measured binding energies in the N 1s range are at 394–408 eV and three peaks were discovered by fitting the curves. The peak at 399.5 eV is attributed to substitutional N for O in the form of O–Ti–N bonding. The peak at 400.6 eV indicates the surface adsorption of N₂. The peak at 401.5 eV is attributed to nitrogen species bound to various surface oxygen sites.⁴⁴ Asahi et al.⁴⁵ suggested that nitrogen doped into substitutional sites of TiO₂ could narrow the band gap and improve photocatalytic activity. The F 1s XPS spectra were obtained in the range of 680–700 eV. The peak at 684.2 eV can be assigned to the physical adsorption of F ions on the surface of TiO₂.⁴⁶ The peak at 686.4 eV is assigned to the substitutional F atoms that occupy the oxygen sites in the TiO₂ crystal lattice and then form the Ti–O–F bonds.⁴⁷ The peak at 693.0 eV originated from the reaction between F atoms and carbon substrate to form the C–F bond. Other studies have reported that F substitution could inhibit the recombination of the electron hole pair due to charge compensation between F⁻ and Ti⁴⁺.^15,48,49 The oxygen vacancies can be created, which act as active sites to produce more oxidizing species. Three constituents are found in C 1s XPS spectra at 284.8, 286.7, and 288.4 eV. The peak at 284.8 eV can be attributed to carbidic carbon and sp³-hybridized C–C bonds, whereas the peak at 286.7 eV is typically assigned to C–O in surface species. The peak at 288.4 suggests the substitution of Ti atom by C and formation of the Ti–O–C structure. Substitution of oxygen atoms by carbon atoms seems not to appear, due to the missing signal around 282 eV.^30,35 O 1s XPS spectra were obtained in the range of 526–544 eV. The binding energies at 530.6, 532.1, and 533.1 eV can be ascribed to Ti–O, surface –OH, and adsorbed H₂O, respectively.^50,51 Two peaks are detected in the binding energy region of Ti 2p, which are centered at 460.2 eV and 465.5 eV. The results can be ascribed to Ti 2p3/2 and Ti 2p1/2 of the Ti⁴⁺ chemical state, respectively.^30,44

Fig. 8 XPS spectra of NF–TiO₂/SC450 and high resolution XPS of N 1s, F 1s, C 1s, O 1s and Ti 2p of NF–TiO₂/SC450.

The UV-vis absorption spectra of NF–TiO₂/SC catalysts and reference materials are shown in Fig. 9. It is noted that the carbonic substrate (SC) has strong absorption in both UV and visible light region. Compared with SC, NF–TiO₂/SC catalysts calcined at 450 and 550 °C have a stronger absorption in visible light region. This result indicates that the doping of elements into TiO₂ lattice could improve photo-absorption in the visible light region and facilitate a higher visible light photocatalytic activity. When a higher calcination temperature was applied, the visible light absorbance of NF–TiO₂/SC650 was significantly reduced. The decrease in visible light absorbance may be attributed to the fact that the heating process can remove the dopants from the doped TiO₂.³⁵ On the other hand, the significant loss of carbon content, as shown in Fig. 3D, is also responsible for the low-visible light absorption.

Fig. 9 UV-vis absorption spectra of NF–TiO₂/SC catalysts and reference materials.

3.2 Adsorption and photocatalytic activity

The photocatalysis performance is significantly dependent on the surface adsorbability and transportation of electron/hole pairs.⁵² Therefore, 20 mg of catalyst was used to adsorb RhB solution with the initial concentration of 20 mg L⁻¹ at room temperature in the dark. The effect of calcination temperature on the adsorption capacity of NF–TiO₂/SC is displayed in Fig. 10. Compared to the adsorption of SC sample, RhB adsorption capacity strongly increased in the presence of NF–TiO₂/SC catalysts. The adsorbed quantities of RhB at time t, q_t (mg g⁻¹), are calculated according to eqn (1):where C₀ and C_t (mg L⁻¹) are the initial concentration and concentration at time t of RhB, respectively; V₀ is the volume of aqueous RhB solution (L); and w_c is the mass of catalyst (g).^40,53

Fig. 10 Kinetics of RhB adsorption on the surface of NF–TiO₂/SC and SC (A) and second-order kinetic plots for RhB removal (B).

Both the pseudo-first-order (eqn (2)) and pseudo-second-order (eqn (3)) Lagergren kinetic models are used to estimate the equilibrium adsorption capacity (q_e) and adsorption rate constant (k). The correct values are chosen based on higher correlation coefficient, r².

where q_e and q_t are the adsorption capacity (mg g⁻¹) at equilibrium and at time t, respectively, and k₁ (min⁻¹) and k₂ (g mg⁻¹ min⁻¹) are the rate constants of the pseudo-first-order and pseudo-second-order sorption, respectively.⁵⁴

The parameters fitted to Lagergren equations are presented in Table 2. The pseudo-second-order adsorption kinetics are adopted based on the higher r² values of 0.97779–0.99312. This result indicated that the adsorption of RhB was controlled by the adsorption mechanism and not by diffusion or mass transfer inside the substrate particles. q_e and k₂ values were determined from the slope and intercept of the plot, respectively. The adsorptive capacity of RhB is directly influenced by calcination temperature, and NF–TiO₂/SC450 exhibited the best adsorptivity, ca. 44.68275 mg g⁻¹, along with the lowest k₂ value, about 0.00084 g mg⁻¹ min⁻¹. The adsorptivity of catalyst is closely related to the value of specific surface area. Compared with that of SC, the enhancement in adsorption performance of NF–TiO₂/SC can be attributed to the hierarchical morphology obtained by immobilizing catalysts on the carbon bioframe and the increase in functional groups.

Table 2 Adsorption and photodegradation kinetic parameters of NF–TiO₂/SC catalysts

Catalyst	Lagergren pseudo-first-order			Lagergren pseudo-second-order			Langmuir–Hinshelwood
	qe (mg g^−1)	k1^a (min^−1)	r^2	qe (mg g^−1)	k2^b (g mg^−1 min^−1)	r^2	kapp^c (min^−1)	R^d
a Equilibrium adsorption capacity and pseudo-first-order adsorption rate constant, determined from parameters of the plot of qt = f(t).b Equilibrium adsorption capacity and pseudo-second-order adsorption rate constant, determined from the slope and intercept of the plot of (t/qt) = f(t).c Apparent first-order degradation rate constant, evaluated from the slope of the plot of ln(C/C0) = −kappt.d Synergy factor, R = kapp(NF–TiO2/SCx)/kapp(NF–TiO2x).
SC	6.06946	0.02909	0.92169	6.60633	0.00975	0.96490	—	—
NF–TiO2/SC350	24.80055	0.00873	0.97826	36.54971	0.00101	0.98281	0.00801	7.41667
NF–TiO2/SC450	30.641	0.01556	0.96023	44.68275	0.00084	0.97779	0.01363	10.17164
NF–TiO2/SC550	28.38385	0.01351	0.99571	43.91743	0.00098	0.99219	0.01163	10.76852
NF–TiO2/SC650	11.93176	0.02409	0.94976	23.71354	0.00258	0.99312	0.00635	7.83951

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The photocatalytic degradation of RhB on the synthesized catalysts was evaluated under visible light irradiation and compared with the adsorption process of the catalysts in the dark (Fig. 11). The previous studies have already demonstrated that RhB could hardly be diminished under visible light irradiation alone. Therefore the removal of RhB is attributed to adsorption and photocatalysis. It is clearly shown that the synthesized catalysts displayed more efficient removal of dye pollutant under visible light irradiation than the single adsorption process in the dark. This is due to the synergistic effects of adsorptive properties of SC and photocatalytic activity of NF–TiO₂ in the composite. RhB can be enriched on the substrate by surface adsorption, which causes a concentration effect in chemical reaction; this could enhance the photocatalytic activity. On the other hand, the photodegradation of RhB could set free the adsorption sites on the surface of catalyst and enhance RhB adsorption. Under identical experimental conditions, NF–TiO₂/SC450 exhibited the best performance for the photodegradation of RhB with the removal rate of 92.15% under visible light irradiation for 180 min. This result is attributed to the combined effect of specific surface area and crystallinity. Lower calcination temperature resulted in poor crystallinity, while higher calcination temperature demonstrated a significantly lower specific surface area.

Fig. 11 (A) Adsorption-photocatalytic degradation of RhB with NF–TiO₂/SC. (B) Comparison of the photocatalytic degradation and TOC removal efficiency using NF–TiO₂/SC450. (C) The pseudo-first-order reaction kinetics of RhB degradation under visible light irradiation by NF–TiO₂/SC and NF–TiO₂.

To investigate the effective mineralization of contaminants throughout the photocatalytic process, total organic carbon (TOC) was selected as the mineralization index for this system and NF–TiO₂/SC450 was selected as the tested catalyst. The time dependence of TOC in the RhB solution during the photoreaction is displayed in Fig. 11B. It is clearly seen that the value of TOC decreased with time. After irradiation for 180 min, 88% of TOC was eliminated, indicating that RhB was effectively mineralized by NF–TiO₂/SC450 under visible light irradiation.

The degradation kinetics of RhB using synthetic NF–TiO₂/SC and reference NF–TiO₂ catalysts were investigated by fitting the experimental data to Langmuir–Hinshelwood (L–H) model. Here, the mass of NF–TiO₂ used in photocatalytic experiments is calculated according to the weight percent excluding carbon by EDX spectra (Fig. 3D). The mass ratio of (NF–TiO₂x)/(NF–TiO₂/SCx) (x is the calcination temperature) is 89.91%, 89.79%, 90.79% and 92.29% when x is 350, 450, 550 and 650 °C, respectively. The pseudo-first-order kinetics equation (eqn (4)) was used as follows:

where C₀ and C_t are the reactant concentration at t = 0 and at t, respectively, and k_app (min⁻¹) is the apparent reaction rate constant determined by plotting −ln(C/C₀) vs. the reaction time (t).⁵⁵

Accordingly, the results presented in Fig. 11C show that the photodegradation of RhB on NF–TiO₂ and NF–TiO₂/SC obeys the L–H kinetic model. By fitting to the logarithmic expression, the apparent rate constant (k_app) was estimated and summarized in Table 2. A synergy factor (R)⁴⁰ defined as R = k_app(NF–TiO₂/SCx)/k_app(NF–TiO₂x) was also estimated to quantify the extent of the synergistic effect of NF–TiO₂/SC composites compared to neat NF–TiO₂. The result further demonstrates that the optimum calcination temperature is 450 °C with a k_app value of 0.01363 min⁻¹. The incorporation of SC resulted in an increase of the rate constant by a factor of 10.17164 and 10.76852 for NF–TiO₂/SC450 and NF–TiO₂/SC550 composites, respectively. The lower synergy factor was observed for catalysts calcined at 350 °C and 650 °C. The results are similar to the adsorption performance, indicating adsorption played an important role in photocatalytic degradation.

To investigate the recyclability of the synthesized hierarchical catalysts, NF–TiO₂/SC450 was selected to test for reusability. Catalyst particles were collected by filtration after the photocatalytic reactions and reused four times under the same conditions. As is shown in Fig. 12, the NF–TiO₂/SC450 catalyst is stable and maintained high photocatalytic performance over four reaction cycles, which is very important for its practical application.

Fig. 12 Recycling properties of photocatalytic degradation of RhB over NF–TiO₂/SC450.

4. Conclusions

NF–TiO₂/SC catalysts with different calcination temperatures were synthesized using a simple, bioframe-assisted sol–gel method and confirmed by XRD, SEM, EDX, TEM, N₂ adsorption–desorption, Raman, FT-IR, XPS, and UV-vis DRS measurements. The as-synthesized catalysts exhibited active anatase phase and 3D-hierarchical structure. Small crystal size of 16–18 nm and high surface area of 106–121 m² g⁻¹ were observed in a calcination temperature range of 350–550 °C. The doping elements decreased with the increase of calcination temperature, and the presence of doped N and F were determined to be substitutional atoms for O. Moreover, the calcination process could cause auto-doping of carbon from straw into the titania lattice. Among all NF–TiO₂/SC samples, NF–TiO₂/SC450 (calcined at 450 °C) exhibited stronger photo-absorption in the visible light region. The investigation of adsorption indicated a coincidence to the pseudo-second-order adsorption kinetics. The adsorption-photocatalytic degradation process was in accordance with the L-H model and the adsorption synergy factor was also calculated. NF–TiO₂/SC450 exhibited the best performance for the photodegradation of RhB with a removal rate of 92.15% and a k_app of 0.01363 min⁻¹ under visible light irradiation for 180 min. The synergy factor of NF–TiO₂/SC450 was 10.17164.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 21277097 and 51179127) and the Fundamental Research Funds for the Central Universities (No. 0400219270).

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