Preparation and photocatalytic performance of TiO2/lignin-based carbon composited photocatalyst

In this study, the lignin-based carbon/TiO2 (LCT) nanocomposite photocatalyst was prepared by the Sol-gel-carbonization method. The raw material of carbon precursor was lignin extracted from coconut by the solvent heating method. The optimal conditions for lignin extraction, such as material ratio, temperature and time, were investigated. The optimum extraction conditions for lignin were determined as follows: material ratio of 1:7, heating time of 4 hours, and heating temperature of 110 °C. The photocatalytic results demonstrated that the prepared LCT exhibited efficient degradation of MO, achieving a degradation efficiency of up to 91.8% within 2 hours, whereas pure TiO2 showed negligible degradation ability under visible light. Moreover, the LCT composite exhibited good stability during the photodegradation process.


Introduction
Large amounts of wastewater containing dyes are generally not biodegradable because of their intricate molecular composition and high solubility [1] [2] .The photocatalytic process has the ability to transform photon energy into chemical energy, which degrades the toxic pollutants completely [3][4] .Thus, the photocatalysis has extensive potential for its application in the treatment of organic wastewater [5] [6] .
Photocatalysis relies on the ability of semiconductors to generate a hole-electron pair (h + -e -) via a photon with sufficient energy, causing electrons to move from the valence to the conduction band.Semiconductors encourage redox events, which promote photodegradation reactions by producing hydroxyl radicals and other reactive oxygen species (ROS) [7] [8] .The semiconductor photocatalysis method is an environmentally friendly, sustainable, and low-cost treatment solution [9] .However, pure semiconductor photocatalysis suffers from easy aggregation and compounding of photogenerated electrons.Incorporating carbon materials can effectively address these issues through their extensive surface area, electrical conductance and porous nature.Sana Akir et al. [10] prepared carbon-ZnO nanocomposites with glucose as the carbon precursor, which demonstrated improved photocatalytic activity compared to pristine ZnO.These findings suggest that incorporating an appropriate amount of carbon material can enhance the photocatalytic activity of semiconductor materials.
Lignin is an aromatic molecule and the second-most prevalent natural polymer material after cellulose [11][12] [13] .Furthermore, lignin is a by-product largely produced in the pulp, paper and bioethanol industries [14] .Because of the large number of aromatic groups with many active groups [15] , lignin possesses high carbon content.As a result of its high carbon content, potential abundance, renewable nature, and inexpensive price, it is widely regarded as an ideal precursor for the synthesis of various carbon materials [16] .
In recent years, lignin carbon/metal oxide composites have been widely reported [17][18] .Wang et al. [19] effectively synthesized a lignin-based carbon/ZnO composite with a straightforward one-pot carbonization approach.The resulting hybrid composite demonstrated exceptional photocatalytic performance.However, the photoinstability in an aqueous solution of ZnO was one of the primary drawbacks of this catalyst due to the corrosion by light, which dramatically decreased the activity of the photocatalyst [20] .TiO2 was observed to have better optical performance and higher photocatalytic activities compared with ZnO.The combination of lignin and TiO2 into composite photocatalysis was significant for preparing the photocatalyst with high efficiency.
This study investigated the optimal extraction conditions for lignin from coconut shells.As the bond between lignin and negatively charged TiO2 was weak, quaternized lignin (QEL) incorporating positively charged functional groups was prepared with lignin through a straightforward quaternization procedure prior to the fabrication of the photocatalyst.The TiO2/lignin-based carbon nanocomposite photocatalyst was prepared by the Sol-gel-carbonization method with lignin-based carbon and TiO2.The composite demonstrated exceptional photocatalytic performance compared to pure TiO2 alone under simulated solar light conditions, as assessed by conducting the photodegradation reaction of organic pollutants in water.The improvement in performance was attributed to lignin-based carbon effectively inhibiting the photogenerated electron-hole pairs from recombination, thereby enhancing the overall photocatalytic capacity of LCT.
The combination of TiO2 and lignin-based carbon offered an efficient, scalable, and low-cost strategy for enhancing the practical applications of photocatalysis, particularly in wastewater treatment and environmentally sustainable processes.Additionally, the method utilized in this study, which was both convenient and scalable, introduced an environmentally friendly and economically viable approach for the synthesis of photoelectric conversion materials.

Chemicals and reagents
The coconut shells were purchased from Hainan Wenchang Coconut Products Factory.(3-Chloro-2hydroxypropyl) Trimethylammonium chloride solution (65wt%), titanium tetra butoxide, sodium hydroxide (NaOH), ethanol, methyl orange (MO), etc. were supplied by Shanghai Titan Technology Co., Ltd.The purity of all reagents was analytical grade and the chemicals were applied without further purification.

Extraction of lignin
The lignin was extracted from coconut shells using a heating method.Acetic acid and sulfuric acid were employed as the heating liquids.A specific quantity of coconut shells, acetic acid, and sulfuric acid were added into a three-necked flask with a condenser tube.Subsequently, the mixture was agitated within a thermostatically controlled water bath, maintaining a consistent temperature throughout the process.Following a designated duration, the filtrate was concentrated under heat, followed by multiple washes and filtrations with water until the pH of the aqueous wash reached a range of 6 to 7. The resulting lignin filtrate was subsequently dried in an oven.The optimal extraction condition for lignin was determined based on the quality of the obtained lignin.

Preparation of LCT
The quaternized lignin (QEL) powder was achieved according to the method of Li [21] before the preparation of LCT. 10 g of butyl titanate was suspended in 30 mL of ethanol while being stirred magnetically at a speed of 800 r/min, which resulted in the formation of an ethanolic dispersion of butyl titanate.Afterward, a quantity of 5 g of QEL powder was solved into 10 mL of distilled water, followed by the adjustment of pH to 3. The QEL solution was then added to the ethanolic dispersion of butyl titanate.After 20 minutes, the mixture was filtered and heated at 50℃ within an oven for 6 hours.The dried LCT precursor obtained from the above step was further subjected to calcination at 600°C for 2 h in a furnace under a nitrogen atmosphere.This process led to the successful preparation of LCT photocatalyst.Additionally, pure TiO2 was synthesized using the same procedure mentioned above without QEL.

Characterization methods
To assess the functional groups present in lignin, NEXUS-670 type Fourier transform infrared (FT-IR) spectrometer (Nicolet, America) was employed.The carbonation process of the prepared composite was evaluated using a micro-Raman spectrometer (LabRAM Aramis, France).N2 adsorption (BEL Japan, BELSORP 18) was applied for Brunauer-Emmett-Teller (BET) analysis to measure the specific surface area (SSA).while N2 desorption was employed to estimate pore structures based on the Barrett-Joyner-Halenda (BJH) method under vacuum conditions at 300℃.Using a S4800 scanning electron microscope (SEM, HITACHI, Japan) and FEIG2F30 transmission electron microscope (TEM, HITACHI, Japan), the micromorphology and microstructure of the produced samples were examined.The lignin yield with different material ratios (heating temperature: 60℃, heating time: 3h).The experimental results depicted in Figure 1(a) displayed a prominent relationship between the yield of lignin and the duration of the heating process.The yield of lignin steadily increased as the reaction time was extended, culminating at a maximum of 78.9% after 4.2 hours.After this point, the yield was not significantly increased by prolonging the duration of the heating process.The acid present during the reaction process broke the chemical bonds that linked cellulose, hemicellulose, and lignin, leading to their dissociation.As lignin was more resistant to hydrolysis than cellulose and hemicellulose, the increased duration of the reaction led to further separation of lignin and thus increased the yield.However, it was crucial to consider that acid could also decompose lignin.At a reaction time beyond 4.2 hours, the production of lignin and its degradation reached equilibrium, and the yield remained constant.Further prolongation of the reaction time led to the decomposition of lignin into other byproducts, which obstructed the preparation of subsequent porous carbon.Therefore, the optimal reaction time was 4.2 hours.

Extraction of lignin under different conditions
It could be seen from Fig. 1(b) that the experimental results demonstrated a direct correlation between increasing temperature and improved yield.However, it was important to note that the boiling point of acetic acid was 117.9°C, making the conditions challenging to manage and susceptible to material loss.Therefore, it was considered optimal to maintain the extraction temperature at 110°C.This temperature ensured effective process control and minimized the risk of material wastage.
As shown in Fig. 1(c), there was a clear correlation between lignin yield and the mass of the coconut shells.This could be attributed to two factors.Firstly, the increased mass of the coconut shells resulted in a higher concentration of lignin within the experimental system.Consequently, the larger quantity of available lignin facilitated a greater yield.Secondly, the augmentation in coconut shells mass concurrently led to an enlargement in the total contact area between acetic acid and the coconut shells.This expanded contact area enhanced the efficiency of the extraction process, further contributing to the increased lignin yield.
During the extraction experiment, several factors could influence the quality of the extracted lignin, such as the different parts of the coconut shells, variations in coconut shells sizes, and losses during the washing process.Based on the current experimental results, the optimal extraction conditions could be summarized as follows: maintaining a material ratio of 1:7, conducting the heating process for 4 hours, and utilizing a heating temperature of 110 °C.These conditions had been determined to yield the highest quality of extracted lignin based on the present experimental findings.The spectrum of the extracted lignin was shown in Fig. 2. The FT-IR spectra demonstrated that the peaks of phenolic and alcoholic hydroxyl groups exhibited broadening and shifting toward the range of 3550-3200 cm -1 .This suggested that a significant number of hydroxyl groups were present in the form of -OH in lignin.

Structural characterization of the extracted lignin
In the wavenumber range of 3200-2500 cm -1 , the infrared spectrum of the extracted lignin exhibited peaks corresponding to various hydrocarbon bonds, including methyl, methylene, and benzene ring hydrocarbon bonds.In the range of 2000-1600 cm -1 , there were absorption peaks related to stretching vibrations about the side chains of aromatic rings of lignin, carbonyl groups and carbon-carbon double bonds.The carbonyl peaks conjugated to benzene rings appeared at 1715-1710 cm -1 , while the nonconjugated C=O peaks were detected around 1675-1660 cm -1 .The absorption peaks of the two double bonds in conjugated alkenes were detected at 1715 and 1607 cm -1 , whereas when conjugated to the benzene ring, these peaks shifted to 1512 and 1463 cm -1 .On the basis of the lignin spectra, it could be inferred that carbonyl groups, along with a small amount of carbon-carbon double bonds, were present within the lignin molecule.The vibrational peaks corresponding to the benzene ring backbone were detected in the range of 1607-1369 cm -1 .These peaks were particularly prominent in the case of aromatic compounds.The stretching vibrational peak of the benzene ring was generally less affected by substituent groups, with its position shifting within a wave number range of 10-20 cm -1 .
The peaks associated with phenolic hydroxyl groups generally appeared in the region of 1400-1000 cm -1 .Within this range, the peak at 1250-1150 cm -1 corresponded to the stretching vibration of the benzene ring connected with O-H, while the peak at 1340-1380 cm -1 was attributed to the deformation vibration within the O-H plane connected with the benzene ring.The peak centered at 1240 cm -1 was assigned to the bending vibration of the hydroxyl group in lignin.The peak located at 1229 cm -1 was commonly observed in lignin spectra but could be challenging to assign to a specific functional group.In the Raman spectrum of LCT (Fig. 3(a)), the presence of structural defects in the carbon lattice was indicated by the D-peak around 1345 cm -1 , while the stretching vibration of sp2-hybridized carbon atoms was associated with the G-peak observed at 1583 cm -1 .Additionally, peaks at 146 and 637 cm -1 were observed, which could be attributed to the Eg(1) and Eg(2) structures of TiO2, respectively.These findings provided evidence that lignin carbon was successfully incorporated into the TiO2 composite.The N2 adsorption-desorption curves of LCT were presented in Fig. 3(b), showing a distinct adsorption hysteresis loop during the process.According to the IUPAC classification, the adsorption isotherms exhibited a type IV pattern, demonstrating the existence of the mesoporous structures in the composites.The BET-SSA of the prepared LCT was measured to be 143.453m 2 /g.
The LCT composite was synthesized by the sol-gel carbonization approach, with lignin quaternary ammonium salt playing a crucial role in dispersing TiO2 nanoparticles.The three-dimensional network structure of lignin quaternary ammonium salts, primarily derived from the pore structure of lignin carbon, exhibited an encapsulation effect on TiO2 particles.In the degradation of organic dyes, the rich pore structure and high SSA of LCT greatly enhanced its adsorption capacity for dyes.These features also served as highly reactive sites for the photocatalytic reaction, resulting in an improvement in the photocatalytic performance of LCT.The lignin carbon effectively dispersed TiO2 particles and prevented their agglomeration within the LCT composites.Additionally, the presence of a pore structure in the lignin carbon contributed to an increase in the BET-SSA.As a result, the dispersion behavior of the LCT composites was significantly superior to that of pure TiO2.
Figure 4 displayed the microscopic morphologies of TiO2 (a) and LCT (c, e).The larger particle size observed in LCT composite was due to significant carbon aggregation within sintered lignin, indicating that high temperatures might induce interaction between lignin and carbon, resulting in larger aggregated particles.TEM was used to investigate the microstructures of LCT.The amorphous carbon surface of lignin was observed to be coated with TiO2 crystals after sintering.The LCT structure was comprised by carbon nanospheres and TiO2 nanoparticles.These TiO2 particles were strongly bonded to the carbon nanosheets and showed excellent dispersion.The TiO2 phase in the nanocomposite exhibited notably smaller sizes than pure TiO2.The transfer process of photogenerated electrons and holes was closely associated with the interfacial contact between carbon and semiconductor.LCT, with high SSA and pore capacity, provided ample sites for TiO2, facilitating this intimate interfacial contact.Consequently, it was anticipated that this close interfacial contact would enhance the transfer process of photogenerated electrons and holes, thereby improving the photocatalytic property of LCT.The analyses of X-ray energy dispersive spectroscopy (EDS) in Fig. 4 revealed the existence of carbon (C), oxygen (O), and titanium (Ti) within the LCT composite.Specifically, the atomic percentage of elemental O in LCT was determined to be 49.73%, while the atomic percentage of elemental Ti was 27.46% (Fig. 4(g)).Notably, the atomic ratio of O to Ti was found to be approximately 2:1, providing further evidence that the LCT composites were successfully prepared.

Evaluation of photocatalytic performance
The optical characteristics of the pure TiO2 and LCT composite samples were investigated using UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS), and the UV-Vis absorption spectra were shown in Fig. 5(d).The pure TiO2 exhibited low light absorption capacity in the visible light range (400-700 nm), accounting for only approximately 15% of the incident light.In contrast, the LCT composites demonstrated a significant increase in light absorption intensity in the visible light region, exceeding 40% compared to the TiO2 spectrum.This increased light absorption was advantageous for enhancing the photocatalytic performances of the LCT composites.
Under simulated solar light, the photocatalytic performance of LCT was evaluated by determining the photodegradation rate of MO according to the method of Li [21] .Before proceeding with the experiments, a dark reaction was conducted, and an adsorption-desorption equilibrium curve depicting the interaction of the catalyst with the dye was measured.This curve was illustrated in Fig. 5(a, b).These curves indicated that the dye adsorption reached equilibrium within 30 minutes.Fig. 5(c) presented digital images illustrating the degradation of MO catalyzed by LCT at varied time intervals.The gradual fading of the color of the pollutant over time confirmed the progress of the photocatalytic degradation reaction.When subjected to visible light, pure TiO2 exhibited poor photodegradation of MO.In contrast, the photodegradation performance of LCT was significantly superior to that of pure TiO2.Within a period of 2 hours, LCT achieved a remarkable degradation rate of up to 91.8% for MO.The bandgap energies calculated from the intercepts of the tangent lines in Fig. 5(e) are approximately 3.23 and 2.74 eV for pure titanium dioxide and LCT, respectively.The photodegradation mechanism could be explained as follows (Fig. 5(f)).
LCT + hν → TiO2 (h + ) + LC (e -) • OH or O2 -or h + + MO → CO2 and H2O (4) QEL with positively charged functional groups was combined with TiO2 through electrostatic interaction.The photoelectrons and holes generated by TiO2 could move quickly to the lignin-based carbon through the close interface, inhibiting the effective recombination of holes and electrons.The

Conclusions
In this study, the optimal extraction conditions for lignin were determined to be a material ratio of 1:7, a heating time of 4.2 hours, and a heating temperature of 110°C.Under these conditions, the maximum lignin yield obtained was 78.9%.TiO2/lignin-based carbon nanocomposite photocatalysis were prepared using the sol-gel carbonization method.Lignin served as the carbon precursor for the composite photocatalyst, facilitating a favorable contact surface for TiO2 particles and mitigating their agglomeration issue.The abundant pore structures and high SSA of the lignin carbon enhanced the absorption and utilization of visible light, effectively facilitating the separation and migration of the photogenerated electron-hole pairs.The pure TiO2 exhibited minimal degradation ability under visible light, whereas the prepared LCT composite demonstrated the capability to degrade MO by up to 91.8% in 2 h.The LCT composites with low cost were easy to prepare and exhibited good morphological stability, which held significant potential in the field of photocatalysis.

Figure 1 .
Figure 1.(a) The lignin yield with different heating times (heating temperature: 60℃, material ratio: 1:7), (b) The lignin yield with different heating temperatures (heating time: 3h, material ratio: 1:7), (c)The lignin yield with different material ratios (heating temperature: 60℃, heating time: 3h).The experimental results depicted in Figure1(a) displayed a prominent relationship between the yield of lignin and the duration of the heating process.The yield of lignin steadily increased as the reaction time was extended, culminating at a maximum of 78.9% after 4.2 hours.After this point, the yield was not significantly increased by prolonging the duration of the heating process.The acid present during the reaction process broke the chemical bonds that linked cellulose, hemicellulose, and lignin, leading to their dissociation.As lignin was more resistant to hydrolysis than cellulose and hemicellulose, the increased duration of the reaction led to further separation of lignin and thus increased the yield.However, it was crucial to consider that acid could also decompose lignin.At a reaction time beyond 4.2 hours, the production of lignin and its degradation reached equilibrium, and the yield remained constant.Further prolongation of the reaction time led to the decomposition of lignin into other byproducts, which obstructed the preparation of subsequent porous carbon.Therefore, the optimal reaction time was 4.2 hours.It could be seen from Fig.1(b) that the experimental results demonstrated a direct correlation between increasing temperature and improved yield.However, it was important to note that the boiling point of acetic acid was 117.9°C, making the conditions challenging to manage and susceptible to material loss.Therefore, it was considered optimal to maintain the extraction temperature at 110°C.This temperature ensured effective process control and minimized the risk of material wastage.As shown in Fig.1(c), there was a clear correlation between lignin yield and the mass of the coconut shells.This could be attributed to two factors.Firstly, the increased mass of the coconut shells resulted
.1088/1742-6596/2671/1/012013 7 high surface area of LCT promoted the bulk loading of TiO2, and its porous structure could quickly adsorb organic dyes on the surface of the composite material.This further improved the degradation efficiency of organic dyes in wastewater, ultimately achieving 91.8%.

Figure 5 .
Figure 5. (a)Photocatalytic degradation of MO over LCT, (b)adsorption curves of TiO2 and LCT, (c) digital images of the photo-degradation MO over LCT, (d)Diffuse reflectance spectra of TiO2 and LCT, (e) DRS spectra of pure TiO2 and LCT with the corresponding plots of [F(R∞) hv]2 versus hv, (f) Schematic diagram of the principle of LCT degrading organic dyes under sunlight irradiation.