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

Abstract

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., E_1g⁽¹⁾, B_1g⁽¹⁾, E_g⁽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.

1. 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 (TiO₂) 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, TiO₂ 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, TiO₂ exhibits rapid recombination of photoexcited carriers (electrons and holes) [9], prompting considerable interest in heterocatalyst and composite approaches.

TiO₂ 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 practical-scale application. However, TiO₂ 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 TiO₂ films developed on rigid substrates by the anodizing method (e.g., TiO₂ nanotubes [15], TiO₂ nanowires on TiO₂ nanotube arrays [16,17,18]) have been studied for photocatalyst, solar energy conversion, and other applications [15]. In addition, nanoparticular TiO₂ 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/cm²) [20]. Also, Ag and Ag/TiO₂ 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 TiO₂ nanomaterials faces a big challenge in scaling up to the practical scale of water treatment. Another approach is that TiO₂ 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, TiO₂ has been impregnated on several porous supports such as silica, alumina, zeolite, and carbon materials [e.g., activated carbon (AC)] [22,23,24]. TiO₂/AC has been found to possess higher photocatalytic activity than bare TiO_2, as it combines the good photocatalytic activity of TiO₂ with the high surface area of AC [23,24,25,26,27,28,29,30]. In addition, TiO₂ nanoparticles loaded in the AC immobilization can resolve the limitation of using TiO₂ 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 TiO₂/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 TiO₂ from tetrabutylorthotitanate and following heat treatment [25]. Noticeably, a complicated preparation process of TiO₂/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 TiO₂ P25/AC using the two commercial raw materials by a facile method by mixing TiO₂/AC mechanically at different mass ratios. Differing from the previous studies, the compositional weight portions of TiO₂/AC were in some specific range, such as TiO₂/(1–15 wt.%)-AC [30], TiO₂/(5–75 wt.%)-AC prepared by the sol-gel method [23], while the weight ratios of TiO₂/AC in this study were TiO₂/AC = 5:0, TiO₂/AC = 4:1, TiO₂/AC = 3:2, TiO₂/AC = 2:3, TiO₂/AC = 1:4, and TiO₂/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 TiO₂/AC material into their household water treatment plant. Moreover, this study has two reference samples (i.e., TiO₂, AC) to obtain insight into the role of either TiO₂ 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 TiO₂/AC on the morphological, structural, compositional, and photocatalytic properties of the composite.

2. Materials and Methods

2.1. Materials

Commercial Degussa TiO₂ 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, TiO₂ 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 TiO₂ nanoparticles (P25) at different weight ratios, while the total weight of the material (TiO₂ + AC) was fixed at 0.5 g. Six types of samples were prepared, including S1 (TiO₂), S2 (TiO₂/AC = 4:1), S3 (TiO₂/AC = 3:2), S4 (TiO₂/AC = 2:3), S5 (AC/TiO₂ = 1:4), and S6 (AC). Specifically, for example, we mixed 0.3 g TiO₂ with 0.2 g AC to prepare the S3 sample. To obtain a uniform mixture and surface functionalize for the TiO₂/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.

2.2. Photocatalytic Activity in Degradation MB of TiO₂/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/cm²). 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.

2.3. 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 = 1.5406 Å) 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 TiO₂/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 TiO₂ and selected TiO₂/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 TiO₂/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.

3. Results and Discussion

Figure 2a presents the XRD pattern of TiO₂ P25 nanoparticles (S1), TiO₂/AC composites prepared at various mass ratios (S2–S5), and AC (S6). Generally, TiO₂ had a dominant anatase phase characterized by diffraction peaks of A(101) at 25.3°, A(004) at 37.9°, A(200) at 48.2°, and A(204) at 62.8°, and a minor rutile phase with the observed peaks of R(110) at 27.5° and R(211) at 54.1°.

Meanwhile, the AC exhibited the diffraction peaks of graphite [e.g., G(002) at 26.6°, G(020) at 45.5°], carbon C(100) at 20.9°, and 3D carbon structures with peaks G_3D(002) at 30.0°. The intensity of TiO₂ and AC peaks varied reasonably with the evolution of TiO₂/AC content (see Figure 2a). The inset of Figure 2b provides a zoom-in view of the TiO₂(101) and G(200) peaks. The intensity of the G(200) peak increases monotonically with the rising AC content from S2 (TiO₂/AC = 4:1) to S6 (TiO₂/AC = 0:5). Reasonably, S1 (TiO₂/AC = 5:0) does not exhibit the G(200) peak, whereas S6 lacks the TiO₂(101) peak (Figure 2b inset). The dominant anatase phase over the rutile one for TiO₂ Degussa P25 in this study is consistent with the results reported in ref. [32]. The XRD results are reasonable with the reported compositions of TiO₂ P25, comprising 77.1% anatase, 15.9% rutile, and 7.0% amorphous TiO₂ [33].

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 43.4–109.0 nm 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 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 gain insight into the crystalline structure of TiO₂/AC composites, a typical Raman spectrum of TiO₂/AC was examined. In Figure 5a, the spectrum exhibited characteristic peaks of the TiO₂ 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 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].

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/cm²). 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), 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).

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 TiO₂/AC (S2, S3, S4) exhibited an enhancement in the photocatalyst activity over either the pristine TiO₂ (S1) or AC (S6). Among the investigated TiO₂/AC composites with various mass mixing ratios (S2–S5), S2 possessed the highest photocatalyst activity, suggesting that TiO₂/AC = 4:1 is the optimal mass mixing ratio between TiO₂ P25 and micron- and sub-micron AC. Further increased AC and decreased TiO₂ contents to TiO₂/AC = 3:2 and TiO₂/AC = 2:3 also exhibited higher photocatalytic activities than that for TiO₂. Meanwhile, the sufficient low TiO₂ and high AC contents for the TiO₂/AC = 1:4 (S5) lead to a decrease in activity as compared to pristine TiO₂ (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/TiO₂ composites with mass mixing ratios of (4:1), (3:2), and (2:3) have higher MB removal performance than either TiO₂ or AC, which is attributed to the synergistic effect of the high adsorption capability of AC and the high photocatalytic activity of TiO₂. 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 ^•O₂⁻ (Figure 7d) [29,38,39], which in turn degrade MB dye. It is well-known that the recombination rate of e⁻/h⁺ in TiO₂ is fast. Meanwhile, in TiO₂/AC composites, the photogenerated carriers can be transferred between TiO₂ 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 TiO₂), 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 TiO₂ and too much AC contents (TiO₂/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., TiO₂/AC) is that TiO₂ 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 TiO₂ positions on the composite surfaces [29,40]. Also, TiO₂ plays a role in the photocatalytic regeneration of AC, which allows for minimizing operational costs and product waste. The optimal TiO₂/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 TiO₂/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].

4. Conclusions

In this study, TiO₂/AC composites with various mass mixing ratios were prepared through a facile process. The composites exhibited tight decoration of TiO₂ nanoparticles on micron-/submicron AC particles. TiO₂ had crystal phases of dominant anatase and minor rutile, and the crystallite size of TiO₂ was ~21 nm. Meanwhile, AC presented the XRD peaks of graphite and carbon, and it had a crystallite size of ~49 nm. TiO₂/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 TiO₂/AC composites. Among the TiO₂/AC composites prepared at different mixing ratios of (4:1), (3:2), (2:3), (1:4), bare TiO₂, and bare AC, the TiO₂/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 TiO₂/AC composites is attributed to the synergistic effect of the high adsorption capability of AC and the high photocatalytic activity of TiO₂. In addition, TiO₂/AC composites allow charge transfer for enhanced e⁻/h⁺ separation to reduce their recombination rate and enhance their photocatalytic activity. The TiO₂/AC composite, with an optimal weight ratio of 4:1, is suggested for treating industrial or household wastewater containing organic pollutants.