Facile Preparation Method of TiO2/Activated Carbon for Photocatalytic Degradation of Methylene Blue

The development of nanocomposite photocatalysts with high photocatalytic activity, cost-effectiveness, a simple preparation process, and scalability for practical applications is of great interest. In this study, nanocomposites of TiO2 Degussa P25 nanoparticles/activated carbon (TiO2/AC) were prepared at various mass ratios of (4:1), (3:2), (2:3), and (1:4) by a facile process involving manual mechanical pounding, ultrasonic-assisted mixing in an ethanol solution, paper filtration, and mild thermal annealing. The characterization methods included XRD, SEM-EDS, Raman, FTIR, XPS, and UV-Vis spectroscopies. The effects of TiO2/AC mass ratios on the structural, morphological, and photocatalytic properties were systematically studied in comparison with bare TiO2 and bare AC. TiO2 nanoparticles exhibited dominant anatase and minor rutile phases and a crystallite size of approximately 21 nm, while AC had XRD peaks of graphite and carbon and a crystallite size of 49 nm. The composites exhibited tight decoration of TiO2 nanoparticles on micron-/submicron AC particles, and uniform TiO2/AC composites were obtained, as evidenced by the uniform distribution of Ti, O, and C in an EDS mapping. Moreover, Raman spectra show the typical vibration modes of anatase TiO2 (e.g., E1g(1), B1g(1), Eg(3)) and carbon materials with D and G bands. The TiO2/AC with (4:1), (3:2), and (2:3) possessed higher reaction rate constants (k) in photocatalytic degradation of methylene blue (MB) than that of either TiO2 or AC. Among the investigated materials, TiO2/AC = 4:1 achieved the highest photocatalytic activity with a high k of 55.2 × 10−3 min−1 and an MB removal efficiency of 96.6% after 30 min of treatment under UV-Vis irradiation (120 mW/cm2). The enhanced photocatalytic activity for TiO2/AC is due to the synergistic effect of the high adsorption capability of AC and the high photocatalytic activity of TiO2. Furthermore, TiO2/AC promotes the separation of photoexcited electron/hole (e−/h+) pairs to reduce their recombination rate and thus enhance photocatalytic activity. The optimal TiO2/AC composite with a mass ratio of 4/1 is suggested for treating industrial or household wastewater with organic pollutants.

(4:1), (3:2), and (2:3) possessed higher reaction rate constants (k) in photocatalytic degradation of methylene blue (MB) than that of either TiO 2 or AC.Among the investigated materials, TiO 2 /AC = 4:1 achieved the highest photocatalytic activity with a high k of 55.2 × 10 −3 min −1 and an MB removal efficiency of 96.6% after 30 min of treatment under UV-Vis irradiation (120 mW/cm 2 ).The enhanced photocatalytic activity for TiO 2 /AC is due to the synergistic effect of the high adsorption capability of AC and the high photocatalytic activity of TiO 2 .Furthermore, TiO 2 /AC promotes the separation of photoexcited electron/hole (e − /h + ) pairs to reduce their recombination rate and thus enhance photocatalytic activity.The optimal TiO 2 /AC composite with a mass ratio of 4/1 is suggested for treating industrial or household wastewater with organic pollutants.

Introduction
Water is an essential resource for the growth of the natural environment on earth and people.Water pollution with organic dyes, heavy metals, and other contaminants (i.e., pesticides, steroid hormones, and antibiotics) is a current environmental issue [1].Synthetic organic dyes are omnipresent in many application areas of the textile, tannery, cosmetic and food industries, and medicine [1].The dying process in the textile and tannery industries can release a vast majority of organic dyes due to the low efficiency of the dying process, with approximately 15% of dye lost [1,2].Organic dye pollution in the aquatic environment poses a threat to animal or human health and causes adverse effects on aquatic biota and water ecosystems [1,3].In 1995, M.R. Hoffmann et al. [4] presented the underlying principles governing semiconductor photocatalysis and its potential applications as an environmental control technology, and this field has been greatly developed over the years [5,6].
Titanium dioxide (TiO 2 ) is widely recognized as one of the most practical and prevalent photocatalysts due to its remarkable photocatalytic activity in the ultraviolet (UV) region, cost-effectiveness, chemical stability, abundance, and non-toxic nature [5].However, TiO 2 faces two primary limitations.Firstly, it remains inactive in the visible (Vis) range owing to its wide band gap semiconductor, with Eg = 3.2 eV for anatase and 3.0 eV for rutile [7,8].Secondly, TiO 2 exhibits rapid recombination of photoexcited carriers (electrons and holes) [9], prompting considerable interest in heterocatalyst and composite approaches.
TiO 2 P25 (a powder-form photocatalyst) offers relatively high photocatalytic degradation of organic dyes [10] and pharmaceuticals [10][11][12][13] and a high possibility of practicalscale application.However, TiO 2 P25 use in the form of suspension (slurry) may pose an ecological risk to aquatic organisms [14], which should require an expensive filtration process to separate the suspension catalyst from the treated water.Therefore, many nanostructured TiO 2 films developed on rigid substrates by the anodizing method (e.g., TiO 2 nanotubes [15], TiO 2 nanowires on TiO 2 nanotube arrays [16][17][18]) have been studied for photocatalyst, solar energy conversion, and other applications [15].In addition, nanoparticular TiO 2 films can be synthesized successfully on various substrates (Si, quartz, or sapphire) using a gas-phase method of supersonic cluster beam deposition (SCBD) for studying the photodegradation of salicylic acid [19] and propane oxidation under 375 nm UV-LED illumination (8 mW/cm 2 ) [20].Also, Ag and Ag/TiO 2 nanocomposite films were grown by SCBD to investigate the effects of structure morphology on the optical properties of the films [21].Photocatalytic activity in film forms of TiO 2 nanomaterials faces a big challenge in scaling up to the practical scale of water treatment.Another approach is that TiO 2 nanomaterial is loaded on/in another bigger nano/micro-scale structure to form a heterostructure or a composite.Heterostructure and composite photocatalysts can offer activity enhancement due to some mechanisms related to manipulating the photoresponse region and the fate of the electron/hole pairs [8].In this way, TiO 2 has been impregnated on several porous supports such as silica, alumina, zeolite, and carbon materials [e.g., activated carbon (AC)] [22][23][24].TiO 2 /AC has been found to possess higher photocatalytic activity than bare TiO 2, as it combines the good photocatalytic activity of TiO 2 with the high surface area of AC [23][24][25][26][27][28][29][30].In addition, TiO 2 nanoparticles loaded in the AC immobilization can resolve the limitation of using TiO 2 suspension, while AC is a very economical and useful adsorbent and is characterized by a high surface area, micro-to macro-pore structures, and a high degree of surface reactivity [31].
Many studies focused on developing TiO 2 /AC with structural, compositional, and surface functional modification, which usually requires the use of more specialized chemicals and laboratory equipment for the material synthesis via the sol-gel methods [23,24,28,29], hydrothermal and reflux methods [30], hydrothermal-impregnation-carbonization-activation processes [27], ultrasonic-assisted sol-gel treatment and solvothermal treatment combined with microwave-assisted heating [26], and hydrolytic precipitation of TiO 2 from tetrabutylorthotitanate and following heat treatment [25].Noticeably, a complicated preparation process of TiO 2 /AC can hinder the practical application at the household level, where non-specialized people will be the operators of their household water treatment tank or plant.In this study, we prepared TiO 2 P25/AC using the two commercial raw materials by a facile method by mixing TiO 2 /AC mechanically at different mass ratios.Differing from the previous studies, the compositional weight portions of TiO 2 /AC were in some specific range, such as TiO 2 /(1-15 wt.%)-AC [30], TiO 2 /(5-75 wt.%)-AC prepared by the sol-gel method [23], while the weight ratios of TiO 2 /AC in this study were TiO 2 /AC = 5:0, TiO 2 /AC = 4:1, TiO 2 /AC = 3:2, TiO 2 /AC = 2:3, TiO 2 /AC = 1:4, and TiO 2 /AC = 0:5.It is worthy of mention that the mass ratio unit for the composite should be easier and more convenient for normal people to apply in practice when they implement TiO 2 /AC material into their household water treatment plant.Moreover, this study has two reference samples (i.e., TiO 2 , AC) to obtain insight into the role of either TiO 2 or AC in the photocatalytic degradation of methylene blue, which is a typical organic dye and considered a good model for studying photocatalyst activity and process.This study provides the effect of mixing weight ratios of TiO 2 /AC on the morphological, structural, compositional, and photocatalytic properties of the composite.

Materials
Commercial Degussa TiO 2 P25 was purchased from Merck (Darmstadt, Germany), and activated carbon (AC) was a product of Tra Bac Company, Tra Vinh City, Vietnam, which was made from coconut shell.First, TiO 2 Degussa P25 was activated in a NaOH 5 M solution at room temperature for 100 min, then it was filtered and cleaned with deionized water before drying in an oven at 100 • C for 3 h.Then, AC (~5 g) was placed in a clean mortar, then crushed with a pestle for 30 min (Figure 1a).Next, AC was mixed with TiO 2 nanoparticles (P25) at different weight ratios, while the total weight of the material (TiO 2 + AC) was fixed at 0.5 g.Six types of samples were prepared, including S1 (TiO 2 ), S2 (TiO 2 /AC = 4:1), S3 (TiO 2 /AC = 3:2), S4 (TiO 2 /AC = 2:3), S5 (AC/TiO 2 = 1:4), and S6 (AC).Specifically, for example, we mixed 0.3 g TiO 2 with 0.2 g AC to prepare the S3 sample.To obtain a uniform mixture and surface functionalize for the TiO 2 /AC composite, the 0.5 g powder was placed in a beaker filled with 10 mL of 95% ethanol under sonication for 30 min (Figure 1b).Next, the powder mixture was filtered using a paper filter (Figure 1c).Finally, the material was annealed in an oven at 160 • C for 3 h.The powder products were stored in glass containers for later use.

Photocatalytic Activity in Degradation MB of TiO 2 /AC
Photocatalysis was carried out by placing 10 mg of the material powder in a 30 mL MB (at an initial concentration of 10 mg/L; the catalyst dosage of 333.3 mg/L) beaker and irradiating with ultraviolet-visible (UV-VIS) radiation from a Xenon lamp (Prosper OptoElectronic Co., Ltd., Yilan County, Taiwan; power 100 W and power density of 120 mW/cm 2 ).The temperature of all QXT reaction beakers was kept at 32-33 • C.After each given photocatalytic reaction time (15,30,45, or 60 min), 1 mL of MB solution was extracted for qualitative and quantitative study of the relative concentration of MB by UV-VIS spectrophotometer (Hitachi U-2900, Hitachinaka, Japan) through the characteristic absorption peak of MB at 655 nm.

Characterization Methods
The orientation and crystallinity of the materials were determined by X-ray diffraction (XRD, Bruker D2, Bruker, Billerica, MA, USA) using Cu K α radiation (λ = 1.5406Å) and θ-2θ configuration.The morphology of the samples was determined by a scanning electron microscope (SEM, Hitachi S-4800, Tokyo, Japan).The chemical composition was analyzed by X-ray energy dispersive spectroscopy (EDS, Oxford probe, Oxfordshire, UK) and integrated SEM (JEOL JSM-IT700HR, Tokyo, Japan).The Raman spectrum of a selected TiO 2 /AC was obtained using a Horiba Evolution (Labram HR model, Kyoto, Japan) spectrometer operated with an excitation wavelength of 514 nm.The FTIR-ATR spectra of TiO 2 and selected TiO 2 /AC powders were acquired using the PerkinElmer FT-IR/NIR spectrometer (Spectrum Two TM , Perkin Elmer Inc., Waltham, MA, USA), which was equipped with a Universal ATR (attenuated total reflectance) sampling accessory containing a diamond/ZnSe crystal.The surface chemistry of TiO 2 /AC was analyzed using X-ray photoelectron spectroscopy (XPS, ThermoVG 350, Waltham, MA, USA) with a Mg Kα X-ray source (power of 300 W, photon energy of 1253.6 eV).Calibration of the XPS spectra was performed using the C1s peak at 284.7 eV.Curve fitting analysis was conducted using the XPSPEAK 4.1 software, assuming a Gaussian-Lorentzian peak shape and employing Shirley background subtraction.
The crystallite size (D) in TiO 2 P25 and AC was estimated using the Scherrer equation and TiO 2 (101) and G(200) peaks, respectively.The Scherrer equation is given as D = 0.9λ/βcosθ, where λ, β, and θ are the X-ray wavelength, full width at half maximum of the diffraction peak, and Bragg diffraction angle, respectively [18,23,34].The D value for TiO 2 P25 in S1-S5 samples was a range of 20.6-21.0 nm, while the D of AC was 48.9 nm (Figure 2b).The D of TiO 2 in this study is comparable with that of pristine TiO 2 synthesized by the sol-gel method (D = 17.5 nm), smaller than the D values of 43.4-109.0nm for the TiO 2 annealed thermally at 200-500 • C for 120 min [28].Noticeably, the crystallite size (48.9 nm) for the present AC is very close to the D of 50 nm for the commercial AC used in ref. [28].
The crystallite size (D) in TiO2 P25 and AC was estimated using the Scherrer equation and TiO2 (101) and G(200) peaks, respectively.The Scherrer equation is given as D = 0.9λ/βcosθ, where λ, β, and θ are the X-ray wavelength, full width at half maximum of the diffraction peak, and Bragg diffraction angle, respectively [18,23,34].The D value for TiO2 P25 in S1-S5 samples was a range of 20.6-21.0 nm, while the D of AC was 48.9 nm (Figure 2b).The D of TiO2 in this study is comparable with that of pristine TiO2 synthesized by the sol-gel method (D = 17.5 nm), smaller than the D values of 43.4-109.0nm for the TiO2 annealed thermally at 200-500 °C for 120 min [28].Noticeably, the crystallite size (48.9 nm) for the present AC is very close to the D of 50 nm for the commercial AC used in ref. [28].Figure 3 shows the surface morphology of the studied materials.TiO2 P25 exhibited uniform nanoparticles (NPs) with a size of 25-35 nm, while AC contained micron-and sub-micron particles.For TiO2/AC composites, TiO2 nanoparticles were well dispersed and decorated tightly with AC particles (Figure 3).The SEM images also reflect the contents of TiO2 and AC in the composites; e.g., TiO2 content decreases while AC content increases when we observe the SEM images from S2 to S5 (Figure 3).The elemental composition and distribution were illustrated by the EDS result (S4) in Figure 4.This composite has 18.04 at.%C, 52.18 at.%O, and 19.00 at.%Ti, which indicates a composition Figure 3 shows the surface morphology of the studied materials.TiO 2 P25 exhibited uniform nanoparticles (NPs) with a size of 25-35 nm, while AC contained micron-and sub-micron particles.For TiO 2 /AC composites, TiO 2 nanoparticles were well dispersed and decorated tightly with AC particles (Figure 3).The SEM images also reflect the contents of TiO 2 and AC in the composites; e.g., TiO 2 content decreases while AC content increases when we observe the SEM images from S2 to S5 (Figure 3).The elemental composition and distribution were illustrated by the EDS result (S4) in Figure 4.This composite has 18.04 at.%C, 52.18 at.%O, and 19.00 at.%Ti, which indicates a composition of AC/TiO 2 .Notably, small contents of Na (5.5 at.%),Si (5.25 at.%), and Pt (0.03 at.%) were observed due to the remaining after the surface activation process for P25 using NaOH 5 M, the Si substrate, and the Pt coating for taking SEM images, respectively.In addition, the EDS mapping in Figure 4 suggests a uniform elemental distribution, uniform decoration of TiO 2 on AC, and possibly loading of TiO 2 NPs inside the micro-pores and micro-channels of AC.The SEM and EDS results indicate the tight binding between TiO 2 and AC that should support carrier transport for enhancing photocatalytic activity.
To gain insight into the crystalline structure of TiO 2 /AC composites, a typical Raman spectrum of TiO 2 /AC was examined.In Figure 5a, the spectrum exhibited characteristic peaks of the TiO 2 predominant anatase phase at 146 cm −1 (E g (1) ), 200 cm −1 (E g (2) ), 396 cm −1 (B 1g ), 516 cm −1 (A 1g ), and 635 cm −1 (E g ) as well as two graphite carbon peaks at 1327 cm −1 (D-band) and 1588 cm −1 (G-band) (Figure 5a).The D-band is associated with asymmetric lattices and bond-angle disorders in graphitic structures [27], while the G-band comes from the doubly-degenerate iTO and LO phonons with E 2g symmetry at the Brillouin zone center [35].This Raman result for TiO 2 /AC agreed well with the Raman result for TiO 2 /AC in ref. [27].
of AC/TiO2.Notably, small contents of Na (5.5 at.%),Si (5.25 at.%), and Pt (0.03 at.%) were observed due to the remaining after the surface activation process for P25 using NaOH 5 M, the Si substrate, and the Pt coating for taking SEM images, respectively.In addition, the EDS mapping in Figure 4 suggests a uniform elemental distribution, uniform decoration of TiO2 on AC, and possibly loading of TiO2 NPs inside the micro-pores and micro-channels of AC.The SEM and EDS results indicate the tight binding between TiO2 and AC that should support carrier transport for enhancing photocatalytic activity.   of AC/TiO2.Notably, small contents of Na (5.5 at.%),Si (5.25 at.%), and Pt (0.03 at.%) were observed due to the remaining after the surface activation process for P25 using NaOH 5 M, the Si substrate, and the Pt coating for taking SEM images, respectively.In addition, the EDS mapping in Figure 4 suggests a uniform elemental distribution, uniform decoration of TiO2 on AC, and possibly loading of TiO2 NPs inside the micro-pores and micro-channels of AC.The SEM and EDS results indicate the tight binding between TiO2 and AC that should support carrier transport for enhancing photocatalytic activity.To explore the interaction between TiO 2 and AC, we recorded the FTIR spectra of TiO 2 (S1) and TiO 2 /AC (S2, S4).As depicted in Figure 5b, the band observed in the range of 500-700 cm −1 corresponds to the FTIR band of TiO 2 , indicative of the Ti-O-Ti stretching vibration at approximately 593 cm −1 and the Ti-O bond at around 666 cm −1 [36].Notably, all TiO 2 and TiO 2 /AC samples exhibited a minor peak at approximately 3424 cm −1 , attributed to the O-H stretching of hydroxyl groups resulting from moisture adsorption [36].Moreover, the FTIR spectra of TiO 2 /AC materials revealed additional peaks at approximately 1050 cm −1 and around 1620 cm −1 .These peaks are attributed to the ring vibration in aromatic compounds, primarily present in carbonaceous materials such as AC, and the C=O vibration, respectively [36].
peaks of the TiO2 predominant anatase phase at 146 cm (Eg ), 200 cm (Eg ), 396 cm (B1g), 516 cm −1 (A1g), and 635 cm −1 (Eg) as well as two graphite carbon peaks at 1327 cm −1 (D-band) and 1588 cm −1 (G-band) (Figure 5a).The D-band is associated with asymmetric lattices and bond-angle disorders in graphitic structures [27], while the G-band comes from the doubly-degenerate iTO and LO phonons with E2g symmetry at the Brillouin zone center [35].This Raman result for TiO2/AC agreed well with the Raman result for TiO2/AC in ref. [27].To explore the interaction between TiO2 and AC, we recorded the FTIR spectra of TiO2 (S1) and TiO2/AC (S2, S4).As depicted in Figure 5b, the band observed in the range of 500-700 cm −1 corresponds to the FTIR band of TiO2, indicative of the Ti-O-Ti stretching vibration at approximately 593 cm −1 and the Ti-O bond at around 666 cm −1 [36].Notably, all TiO2 and TiO2/AC samples exhibited a minor peak at approximately 3424 cm −1 , attributed to the O-H stretching of hydroxyl groups resulting from moisture adsorption [36].Moreover, the FTIR spectra of TiO2/AC materials revealed additional peaks at approximately 1050 cm −1 and around 1620 cm −1 .These peaks are attributed to the ring vibration in aromatic compounds, primarily present in carbonaceous materials such as AC, and the C=O vibration, respectively [36].
XPS spectra analysis of TiO2/AC (S2) was conducted to examine the chemical composition and bonding environment within TiO2/AC nanomaterials.In Figure 6a, the C1s spectrum can be deconvoluted into four Lorentzian-Gaussian peaks.The primary peak observed at 284.7 eV corresponds to C-C bonds, while three minor peaks appear at 283.7 eV for C=C bonds, 286.0 eV for C-OH due to -OH groups chemisorbed on the surface of TiO2, and 288.7 eV for C=O/COOH bonds [26].This finding is consistent with previous studies on TiO2/AC [26] and TiO2-graphene systems [37].In Figure 6b, the O1s spectrum is well-fitted with three peaks.Notably, a dominant peak at 530.5 eV corresponds to oxygen within the crystal lattice of TiO2, while two additional peaks at 531.7 and 533.0 eV are associated with Ti-OH and C-O/-OH, respectively.The present O1s spectrum result agrees well with that of TiO2/AC [26].XPS spectra analysis of TiO 2 /AC (S2) was conducted to examine the chemical composition and bonding environment within TiO 2 /AC nanomaterials.In Figure 6a, the C1s spectrum can be deconvoluted into four Lorentzian-Gaussian peaks.The primary peak observed at 284.7 eV corresponds to C-C bonds, while three minor peaks appear at 283.7 eV for C=C bonds, 286.0 eV for C-OH due to -OH groups chemisorbed on the surface of TiO 2 , and 288.7 eV for C=O/COOH bonds [26].This finding is consistent with previous studies on TiO 2 /AC [26] and TiO 2 -graphene systems [37].In Figure 6b, the O1s spectrum is wellfitted with three peaks.Notably, a dominant peak at 530.5 eV corresponds to oxygen within the crystal lattice of TiO 2 , while two additional peaks at 531.7 and 533.0 eV are associated with Ti-OH and C-O/-OH, respectively.The present O1s spectrum result agrees well with that of TiO 2 /AC [26].The photocatalytic activities of TiO2, AC, and TiO2/AC with different mixing mass ratios were studied by monitoring the photodegradation kinetics of MB (an organic substance with popular use as an organic pollutant model) under UV-Vis irradiation (120 mW/cm 2 ). Figure 7a shows the evolution of the MB absorption peak (at ~655 nm) over the photocatalytic reaction time (t) using the TiO2/AC (S2).The peak intensity (associated with MB concentration) decreased with t, and this behavior is true for the other photocatalysts in this study (S1, S3, S4, S5, S6).The photodegradation kinetics by photolysis and photocatalytic processes using the materials are shown in Figure 7b, which obeys the Langmuir-Hinshelwood kinetic model with the first-order reaction rate constant (k), Ct = Co × e −kt , where Ct is the concentration of MB at time t (mg/L), and Co is the initial MB concentration (mg/L).The photocatalytic activities of TiO 2 , AC, and TiO 2 /AC with different mixing mass ratios were studied by monitoring the photodegradation kinetics of MB (an organic substance with popular use as an organic pollutant model) under UV-Vis irradiation (120 mW/cm 2 ). Figure 7a shows the evolution of the MB absorption peak (at ~655 nm) over the photocatalytic reaction time (t) using the TiO 2 /AC (S2).The peak intensity (associated with MB concentration) decreased with t, and this behavior is true for the other photocatalysts in this study (S1, S3, S4, S5, S6).The photodegradation kinetics by photolysis and photocatalytic processes using the materials are shown in Figure 7b, which obeys the Langmuir-Hinshelwood kinetic model with the first-order reaction rate constant (k), C t = C o × e −kt , where C t is the concentration of MB at time t (mg/L), and C o is the initial MB concentration (mg/L).photocatalytic reaction time (t) using the TiO2/AC (S2).The peak intensity (associated with MB concentration) decreased with t, and this behavior is true for the other photocatalysts in this study (S1, S3, S4, S5, S6).The photodegradation kinetics by photolysis and photocatalytic processes using the materials are shown in Figure 7b, which obeys the Langmuir-Hinshelwood kinetic model with the first-order reaction rate constant (k), Ct = Co × e −kt , where Ct is the concentration of MB at time t (mg/L), and Co is the initial MB concentration (mg/L).The solid lines in Figure 7b are the fitting curves using the kinetic model.The fittings yield the k values of the photolysis (P) and photocatalytic processes using the S1-S6 materials (see Figure 7c).The k of the photolysis reaction was a small value of 1.8 × 10 −3 min −1 , while the k values were 32.1 × 10 −3 min −1 (S1), 55.2 × 10 −3 min −1 (S2), 38.5 × 10 −3 min −1 (S3), 37.5 × 10 −3 min −1 (S4), 28.9 × 10 −3 min −1 (S5), 18.7 × 10 −3 min −1 (S6).This indicates that MB degradation is much faster and more effective by using the photocatalysts as compared to the photolysis process.In addition, the composites of TiO 2 /AC (S2, S3, S4) exhibited an enhancement in the photocatalyst activity over either the pristine TiO 2 (S1) or AC (S6).Among the investigated TiO 2 /AC composites with various mass mixing ratios (S2-S5), S2 possessed the highest photocatalyst activity, suggesting that TiO 2 /AC = 4:1 is the optimal mass mixing ratio between TiO 2 P25 and micron-and sub-micron AC.Further increased AC and decreased TiO 2 contents to TiO 2 /AC = 3:2 and TiO 2 /AC = 2:3 also exhibited higher photocatalytic activities than that for TiO 2 .Meanwhile, the sufficient low TiO 2 and high AC contents for the TiO 2 /AC = 1:4 (S5) lead to a decrease in activity as compared to pristine TiO 2 (see the dashed line in Figure 7c).Noticeably, the observed decrease in MB concentration over t for AC (S6) material is attributed to the excellent adsorption characteristic of porous carbon materials (Figure 7c inset) [29,38,39].
AC/TiO 2 composites with mass mixing ratios of (4:1), (3:2), and (2:3) have higher MB removal performance than either TiO 2 or AC, which is attributed to the synergistic effect of the high adsorption capability of AC and the high photocatalytic activity of TiO 2 .As illustrated in Figure 7d, under UV-VIS irradiation, electron/hole (e − /h + ) pairs are generated, which lead subsequently to oxidation and reduction reactions in the treated solution to generate highly active free radicals, primarily • OH and • O 2 − (Figure 7d) [29,38,39], which in turn degrade MB dye.It is well-known that the recombination rate of e − /h + in TiO 2 is fast.Meanwhile, in TiO 2 /AC composites, the photogenerated carriers can be transferred between TiO 2 and AC, allowing an increase in e − /h + separation, reducing the e − /h + recombination rate, and consequently enhancing the photocatalytic activity.The AC (S6) removes MB with k of 18.7 × 10 −3 min −1 (58.3% of k for TiO 2 ), suggesting the AC has high adsorption capacity with many sufficient active adsorption sites and a large surface area [29,38,39].The lower k value of S5 than S1 indicates that a composite with too little TiO 2 and too much AC contents (TiO 2 /AC = 1:4 for the present case) will not give an enhancement of the photocatalytic activity.An advantage of a combination of a good photocatalyst with a good adsorption material (i.e., TiO 2 /AC) is that TiO 2 degrades MB to create renewable active adsorption sites on the AC to adsorb more MB molecules, as evidenced by the higher adsorption efficiency of pollutants near TiO 2 positions on the composite surfaces [29,40].Also, TiO 2 plays a role in the photocatalytic regeneration of AC, which allows for minimizing operational costs and product waste.The optimal TiO 2 /AC (S2) obtained a high MB removal efficiency of 96.6% after 60 min of treatment at the initial MB concentration of 10 mg/L and a composite dosage of 333.3 mg/L.This efficiency is comparable to that for the TiO 2 /AC (i.e., 86.5%, 97.1%, and 99.4%, depending on the type of AC) under similar experimental conditions (i.e., reaction time of 60 min, UV light source, catalyst dosage of 400 mg/L, and MB initial concentration of 20 mg/L) [29].

Conclusions
In this study, TiO 2 /AC composites with various mass mixing ratios were prepared through a facile process.The composites exhibited tight decoration of TiO 2 nanoparticles on micron-/submicron AC particles.TiO 2 had crystal phases of dominant anatase and minor rutile, and the crystallite size of TiO 2 was ~21 nm.Meanwhile, AC presented the XRD peaks of graphite and carbon, and it had a crystallite size of ~49 nm.TiO 2 /AC composites are reasonably composed of the main elements (O, Ti, C).The EDS mapping confirmed the uniform distribution of elements, indicating the formation of uniform TiO 2 /AC composites.Among the TiO 2 /AC composites prepared at different mixing ratios of (4:1), (3:2), (2:3), (1:4), bare TiO 2 , and bare AC, the TiO 2 /AC = 4:1 possessed the highest photocatalytic activity in degradation of MB under UV-Vis irradiation, which yielded a high MB removal efficiency of 96.6% after 60 min treatment.The enhanced photocatalytic activity of TiO 2 /AC composites is attributed to the synergistic effect of the high adsorption capability of AC and the high photocatalytic activity of TiO 2 .In addition, TiO 2 /AC composites allow charge transfer for enhanced e − /h + separation to reduce their recombination rate and enhance their photocatalytic activity.The TiO 2 /AC composite, with an optimal weight ratio of 4:1, is suggested for treating industrial or household wastewater containing organic pollutants.

Micromachines 2024 , 12 Figure 1 .
Figure 1.TiO2/AC preparation process: (a) Crushing TiO2-AC powder using a mortar and pestle, (b) Mixing the powder in 95% methanol under ultrasonic shaking, (c) Filtering the powder using a paper filter, (d) Annealing the powder in an oven at 160 °C.

Figure 1 .
Figure 1.TiO 2 /AC preparation process: (a) Crushing TiO 2 -AC powder using a mortar and pestle, (b) Mixing the powder in 95% methanol under ultrasonic shaking, (c) Filtering the powder using a paper filter, (d) Annealing the powder in an oven at 160 • C.

Figure 2 .
Figure 2. (a) X-ray diffraction patterns of TiO2, TiO2/AC composites, and AC.The symbol C (hkl) is the spectral peak of carbon (IMCSD #0013020); the symbols G (hkl) and G3D (hkl) are the peaks of graphite (IMCSD #0000049) and graphite 3D structures (IMCSD #0013980), respectively; and A (hkl) and R (hkl) indicate the patterns of TiO2 anatase and TiO2 rutile phases, respectively.(b) The crystallite size of the materials; the inset is the zoom-in view of high-intensity characteristic peaks for TiO2 and AC.

Figure 2 .
Figure 2. (a) X-ray diffraction patterns of TiO 2 , TiO 2 /AC composites, and AC.The symbol C (hkl) is the spectral peak of carbon (IMCSD #0013020); the symbols G (hkl) and G 3D (hkl) are the peaks of graphite (IMCSD #0000049) and graphite 3D structures (IMCSD #0013980), respectively; and A (hkl) and R (hkl) indicate the patterns of TiO 2 anatase and TiO 2 rutile phases, respectively.(b) The crystallite size of the materials; the inset is the zoom-in view of high-intensity characteristic peaks for TiO 2 and AC.

Figure 4 .
Figure 4.An EDS spectrum of a TiO2/AC prepared by dropping a DI water solution containing S4 powder on a clean Si substrate; and an EDS mapping of the S4 powder sample.

Figure 4 .
Figure 4.An EDS spectrum of a TiO2/AC prepared by dropping a DI water solution containing S4 powder on a clean Si substrate; and an EDS mapping of the S4 powder sample.

Figure 4 .
Figure 4.An EDS spectrum of a TiO 2 /AC prepared by dropping a DI water solution containing S4 powder on a clean Si substrate; and an EDS mapping of the S4 powder sample.

Figure 7 .Figure 7 .
Figure 7. (a) Typical photodegradation of methylene blue (MB) using AC/TiO2 materials (e.g., S2 sample case).(b) Photocatalytic degradation of MB by AC/TiO2 materials prepared at various mass ratios under the UV-Vis irradiation of Xenon lamp (120 mW/cm 2 ).(c) The pseudo-first-order kinetics constant k of photolysis (P) and photocatalytic reaction using AC/TiO2 (S1-S6); Inset in (c) shows a Figure 7. (a) Typical photodegradation of methylene blue (MB) using AC/TiO 2 materials (e.g., S2 sample case).(b) Photocatalytic degradation of MB by AC/TiO 2 materials prepared at various mass ratios under the UV-Vis irradiation of Xenon lamp (120 mW/cm 2 ).(c) The pseudo-first-order kinetics constant k of photolysis (P) and photocatalytic reaction using AC/TiO 2 (S1-S6); Inset in (c) shows a schematic of porous AC/TiO 2 in MB solution.(d) A schematic diagram for the photocatalytic degradation mechanism of MB using AC/TiO 2 composite.