Heterogeneous semiconductor photocatalysis have received great attention due to the effectiveness and economy in environmental protection^[1]. TiO₂ is one of the most suitable photocatalysts because of its high activity, low price, safety and excellent stability^[2,3,4]. TiO₂ nanoparticles are easy to agglomerate, which will cause a remarkable deterioration of photocatalytic property. To overcome these problems, it is vital to find a promising support material to uniformly load TiO₂^[5,6,7]. Many studies have been reported porous materials can be used as suitable candidates for supporting TiO₂ to obtain high active catalysts. For example, Hiromi, et al^[8] showed that TiO₂-loaded MacroMeso-SiO₂ exhibited unique adsorption properties and enhanced photocatalytic degradation properties for organic dye. Particularly, the combination of TiO₂ nanoparticles with a number of carbonaceous materials is considered to be an effective method for enhancing photocatalytic efficiency. Fan, et al^[9] demonstrated that uniform dispersion of TiO₂ on graphene was critical for improving photocatalytic efficiency of the photocatalyst.

Bamboo charcoal (BC) will be an ideal substrate due to its porous structure, high surface area and good mechanical and chemical stability. In addition, BC has high electrical conductivity, which can better transport photoinduced charges in TiO₂ under light^[10]. Moreover, BC is one of the fastest growing plants on the earth and it's very cheap^[11]. However, the poor water-dispersibility of pristine BC will hinder the uniform loading of TiO₂ nanoparticles. Therefore, it is of significant value to improve the dispersibility of BC in water. In recent years, many researches have been devoted to the surface functionalization of carbon materials^[12,13,14]. A very significant method is that using wet oxidation process to modify BC. This way not only increases oxygen-containing functional groups on BC but also improves its water-dispersibility. In this article, the synthesis and characterization of SMBC/TiO₂ nanocomposites using SMBC as supporting materials by a simple and green method was reported. Adsorption and photocatalytic activities of pure TiO₂, BC/TiO₂ in comparison to SMBC/TiO₂ nanocomposites were also investigated. The characteristic micro-nano hierarchical structures of SMBC/TiO₂ nanocomposites can improve the light utilization efficiency of TiO₂ and promote the effective contact between TiO₂ nanoparticles and photocatalytic reactants. Thus the SMBC/TiO₂ nanocomposites prepared here would exhibit a higher adsorption and photocatalytic activities.

Titanium dioxide (TiO₂, anatase), absolute ethanol (≥99.7%), ammonium persulfate (APS, ≥98%), nitric acid (HNO₃, 65%-68%), sulphuric acid (H₂SO₄, 95%-98%) and methylene blue were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Bamboo charcoal (BC, laboratory prepared from 3-5 years of moso bamboo sheets) were used directly.

30 mL of 1 mol/L APS solution (prepared in 2 mol/L H₂SO₄ and 1 mol/L HNO₃) was added to a round bottom flask containing 0.5 g of BC. The mixture was stirred and refluxed at 60 ℃ for 12 h. SMBC were obtained by collecting the precipitation after centrifugation of resultant reaction mixture, then washed thoroughly with water and dried at 60 ℃ under vacuum overnight.

TiO₂ (0.3 g), SMBC (0.5 g) and deionized water (50 mL) were added into a 100 mL round bottom flask. And the mixture was stirred for 10 h at room temperature. The resultant mixture was centrifuged and the precipitation was collected, then dried in vacuum at 60 ℃ for 24 h to obtain SMBC/TiO₂ nanocomposites.

Fourier transform infrared spectroscopy (FT-IR) analysis of the samples was taken on a Spectrum One FT-IR spectrometer (Perkin-Bhaskar-Elmer Co, USA). The morphology of the samples was determined by field emission scanning electron microscopy (FESEM, JSM7100F, Japan). Crystallinity study of the samples were performed on an X-ray diffraction (XRD, D/MAX-IIIC, Japan), taken from 5° to 80° with Cu-Kα (λ = 0.154 nm) radiation to the sample at the scanning rate of 10°/min. N₂ adsorption- desorption isotherms were measured at -196 ℃ with JW-BK112 analyzer.

The adsorption capacity and photocatalytic activities of synthesized samples were quantitatively studied by using a typical photocatalytic model reaction: 12 mg of SMBC/TiO₂ composite photocatalysts or pristine BC/TiO₂ composite photocatalysts were homogeneously dispersed into the 100 mL of 15 mg/L methylene blue (MB) aqueous solutions. For comparison purposes, pure TiO₂ nanoparticles were also homogeneously dispersed into 100 mL of 15 mg/L MB aqueous solutions at mass ratio maintaining a similar amount of TiO₂. The prepared dispersion were irradiated in air with a lamp that simulated solar irradiation (LanPu-XQ 350 W adjustable xenon lamp). The distance of light source to the experimental dispersion was setted to 10 cm and optical power of reactor was found to be 50 W/m². Before illumination, the dispersion were stirred in dark for 30 min at room temperature, enabling adsorption- desorption equilibrium. Then 5 mL of the solution were taken from the reactor and then separated by centrifuging (12000 r/min, 10 min) to separate the catalyst. At beginning, 5 mL solution was sampled every 10 min during one hour of illumination. MB degradation was determined using a UV-Vis spectrophotometer (UV-3600, Shimadzu) at 664 nm wavelength.

The application of BC is seriously limited due to its poor water-dispersibility. Herein, we use a wet oxidation process to modify pristine BC for improving its water- dispersibility, after modification, a large amount of carboxyl groups are generated on the surface, which intensify interactions of SMBC with TiO₂ nanoparticles and would do favor to TiO₂ loading. Then commercial TiO₂ with a size of 10 nm are mixed with micron sized SMBC in deionized water to obtain SMBC/TiO₂ nanocomposites with micro-nano hierarchical structures. The characteristic micro-nano hierarchical structures can improve the light utilization efficiency of TiO₂ and promote the effective contact between TiO₂ nanoparticles and photocatalytic reactants^[4]. The detailed preparation process of SMBC/TiO₂ nanocomposites is illustrated in Scheme 1. Fig. 1(a) shows FT-IR spectra of BC and SMBC, the weak peaks at 2924 and 1450 cm^-1 can be assigned to the stretching vibrations and bending absorption bands of -CH₂ groups of pristine BC, and the peaks at 3434 cm^-1 are attributed to the stretching vibrations of active -OH or -COOH groups. After modification, all the characteristic peaks of -CH₂ groups turn weaker, revealing that most of -CH₂ groups were oxidized. And the characteristic peaks of C=O and C-O stretching vibrations of aliphatic carboxyl groups are observed at 1725 cm^-1 and 1209 cm^-1, respectively. All these results indicate that a large amount of carboxyl groups generated on the surface of SMBC after modification.

XRD patterns of pure TiO₂ and SMBC/TiO₂ nanocomposites are shown in Fig. 1(b). The major peaks of pure TiO₂ at about 25°, 38°, 48°, 54°, 55°, 63° are corresponded to (101), (004), (200), (105), (211) and (204) crystal planes of anatase TiO₂^[15]. After loading on the surface of SMBC, all the peaks appeared at the same position and no other peak was observed, indicating that the loading of TiO₂ doesn’t change the crystal structure.

The morphologies of the samples were characterized by FESEM and the images are presented in Fig. 2(a-d). Fig. 2(a-b) exhibit the images of BC/TiO₂ nanocomposites and reveal that most of TiO₂ nanoparticles are agglomerated. When using SMBC as supporting materials, it is clearly observed that TiO₂ nanoparticles are even covered on the surface of SMBC (Fig. 2(c-d)). This is attributed to the chemical adsorption between SMBC and TiO₂. The above phenomenon is in good consistence with the FT-IR results, in which the pristine BC modified by APS will have good water-dispersibilities and then be uniformly covered with TiO₂ nanoparticles.

N₂ adsorption-desorption isotherms of BC/TiO₂ and SMBC/TiO₂ nanocomposites are shown in Fig. 3. The curves show a classical type-IV characteristic, suggesting the presence of mesopores. Then the textural properties of nanocomposites were analyzed by Brunauer-Emmett- Teller (BET) and Barrett-Joyner-Halenda (BJH) methods. Table 1 shows the specific surface area (S_BET), pore size, and pore volume (V_t) of BC/TiO₂ and SMBC/TiO₂ composites. Due to the aggregation of TiO₂ nanoparticles and poor water-dispersibilities of BC, S_BET of BC/TiO₂ nanocomposites was only 71 m²/g. By using SMBC as supporting materials, TiO₂ nanoparticles can be loaded uniformly on the surface and SMBC/TiO₂ nanocomposites display much higher S_BET and pore volume. S_BET is 155 m²/g, nearly 2.2 times as large as BC/TiO₂. The higher S_BET and V_t would offer more surface active sites and do favor to improve the adsorption and photocatalytic degradation performance.

Fig. 4(a) shows MB removal efficiency of pure TiO₂, BC/TiO₂ and SMBC/TiO₂ nanocomposites under the same condition. Before photocatalysis, all samples are allowed to stand for 30 min to ensure saturated adsorption. Clearly, TiO₂ has almost no adsorption capacity and SMBC/TiO₂ shows a higher MB adsorption efficiency than BC/TiO₂ in dark. It is generally known that the adsorption activity of materials can be greatly influenced by the structural features and surface chemistry^[5]. As for BC/TiO₂, the introduction of BC can greatly increase the adsorption capacity of MB. Furthermore, SMBC with many oxygen-containing functional groups on the surface can improve chemical adsorption. The specific values for maximum adsorption capacities (Q) of all samples are presented in Table 2. As shown in Fig. 4(a), MB are rapidly removed under light irradiation with SMBC/TiO₂ nanocomposites. Specifically speaking, more than 97% of MB with SMBC/TiO₂ has been removed within 1.5 h. Nevertheless, only about 56% of MB could be removed with BC/TiO₂ and 38% removed with pure TiO₂.

As is known to us, when the initial concentration is very low (30 mg/L for MB), the kinetic linear curves of photocatalytic degradation of MB is fitted by Langmuir- Hinshelwood first-order rate law^[2], and the reaction rate constant can be calculated by the following equation:

$\ln ({{C}_{0}}/{{C}_{t}})=kt$ (1)

Where k is the rate constant, C₀ is initial concentration of MB, C_t is MB concentration at time t. The rate constants can be obtained from the slope of linear fitting. As shown in Fig. 4(b), it can be seen that the MB photocatalytic degradation efficiencies of SMBC/TiO₂ are clearly superior to BC/TiO₂ and TiO₂. The reaction rate constant of SMBC/TiO₂ is almost 7 times and 6 times as high as BC/TiO₂ and TiO₂, respectively. The values of reaction rate constant for all samples are listed in Table 2. That BC/TiO₂ shows a little lower photocatalytic degradation rate than TiO₂ may be attributed to the shading effect of BC.

In addition, time-dependent UV-Vis spectra of MB aqueous solution in the presence of all photocatalysts are shown in Fig. 5. Because of the adsorption effect of BC, a sharp drop of absorption intensity were shown at the first 30 min for both BC/TiO₂ and SMBC/TiO₂ (Fig. 5(b) and (c)), while pure TiO₂ basically remained unchanged (Fig. 5(a)). Under UV irradiation, the absorption intensity of MB in the presence of pure TiO₂ or BC/TiO₂ decreased slowly due to the aggregation of TiO₂, while the intensity for SMBC/TiO₂ decreased obviously, indicating that the homogeneous dispersion of TiO₂ on the surface of SMBC enhanced its photocatalytic activity. This superior performance of SMBC/TiO₂ could also be confirmed according to the inset photographs by comparing the color difference of the remaining MB solutions after 90 min irradiation under UV.

In summary, a kind of novel surface modified bamboo charcoal/TiO₂ (SMBC/TiO₂) nanocomposites was prepared by a simple and green wet oxide method. The prepared SMBC/TiO₂ nanocomposites exhibit a higher adsorption and photocatalytic activities as compared to pure TiO₂ and unmodified BC/TiO₂ composites. The saturated adsorption capacity of SMBC/TiO₂ nanocomposites was approximately 1.6 times, 12.1 times as great as BC/TiO₂ and pure TiO₂, respectively. And the rate constant for MB photocatalytic degradation of SMBC/TiO₂ was almost 7 times, 6 times as high as BC/TiO₂ and pure TiO₂, respectively. The synergetic effect of adsorption and catalysis gives SMBC/TiO₂ nanocomposites much higher photocatalytic activity for degradation of MB under UV irradiation. Thus SMBC/TiO₂ composites prepared in this work would be potentially applied in waste water treatment.