In situ fabrication of porous biochar reinforced W18O49 nanocomposite for methylene blue photodegradation

In this paper, a novel cow dung based activated carbon (CDAC) was successfully modified by W18O49 nanowires as a photocatalyst using KOH activation and a hydrothermal method. The activity of photocatalytic degradation of methylene blue (MB) under full-spectrum light illumination shows great improvement, and the degradation rate of MB could reach 98% after 240 min (67% for W18O49), with a final degradation rate of 98%. The porous structure with specific surface area of CDAC (∼479 m2 g−1) increases the adsorption of W18O49 reactants and also raises the concentration of reactants in the photocatalytic region. The high electrical conductivity and good electron storage capacity of CDAC allow the electrons excited in the conduction band (CB) of W18O49 to migrate smoothly into CDAC, which are the keys to enhancing the photocatalytic activity. Moreover, the photocatalytic mechanism was proposed. The results show that the CDAC/W18O49 nanowire composite can be used as an efficient photocatalyst for removal of MB dye from wastewater and indicate remarkable future potential in dye wastewater treatment technologies.


Introduction
With the rapid development of industrial processes, the problem of environmental pollution is receiving more and more attention. For a long time, many kinds of organic dyes have been widely used in various industries, such as tanneries, and paper and textile production, 1,2 and the wastewater produced during their production is the main source of dye pollution in water bodies. [3][4][5][6] Even low concentrations of dyes can cause great harm to humans, for example, methylene blue (MB) can cause serious skin problems, chromosome breakage, mutagenesis and human respiratory toxicity. [6][7][8] Therefore, the challenge of how to remove organic dyes from wastewater has attracted widespread interest in the community.
To date, several techniques have been developed to address organic dyes in wastewater, such as solvent extraction, chemical oxidation, photocatalytic degradation, biodegradation, and adsorption. [9][10][11][12][13][14][15] Among them, photocatalysis is a green, effective, environmentally friendly and highly promising advanced oxidation process for wastewater treatment, which uses semiconductor materials and light to remove organic pollutants.
Tungsten oxide (WO 3Àx, 0 # x # 1), a semiconductor material has the advantages of narrow band gap (range of 2.4 to 3.0 eV) favorable for visible light absorption, deep valence band for oxidation reaction, high carrier mobility and good stability. [16][17][18][19] This makes WO 3Àx a favorable candidate for solar-driven chemical reactions. Among them, non-stoichiometric W 18 O 49 structure is rich in W 5+ defects and oxygen vacancies, which can be used as reaction sites to facilitate the adsorption and activation of oxygen molecules, hence it has a signicant important for the development of efficient photocatalysts for the removal of organic dyes from water. 20,21 Unfortunately, the efficiency of narrow bandgap photocatalysts is relatively low due to the fast-compounding efficiency of photogenerated charge carriers, which inhibits the migration of these charges to reach the semiconductor surface to participate in redox reactions. In addition, their photostability is easily compromised because the oxidation and/or reduction potentials tend to lie within the band levels that induce photooxidation and/or reduction. 22 For W 18 O 49 , it is easily deactive during photocatalysis due to the formation of WO 3 by photooxidation of holes accumulated in the valence band. 21 Therefore, the photocatalytic activity of most single-component photocatalysts is still far from satisfactory. 23 In recent years, carbon-based-tungsten oxide composites have been studied more frequently to effectively improve the light-driven performance of W 18 O 49 by taking advantage of the large specic surface area, exible structure, excellent charge carrier mobility, and good electrical and thermal conductivity of carbon-based materials. 24,25 Yang et al. reported a composite of highly ordered mesoporous WO 3 nanocrystals grown on RGO, which was used as visible-light-driven photocatalyst for oxygen production. Under visible light irradiation, the amount of oxygen evolution from the optimized photocatalyst containing ca. 6 wt% RGO reached 437.3 mmol g À1 , which was 5.1 times as high as that from m-WO 3 . 26 Li et al. synthesized a lightweight 3-D porous aerogel using one-dimensional tungsten oxide nanowires and two-dimensional reduced graphene oxide sheets, and investigated the photocatalytic activity of the aerogel under visible light irradiation by degrading six different organic dyes. 18 Deng et al. reported a rationally designed novel layered W 18 O 49 /g-C 3 N 4 composite with enhanced photocatalytic activity by controlling the ow of dual-channel charge carrier separation and transfer processes, and the prepared composite exhibited enhanced photocatalytic performance under both full-spectrum light and near-infrared (NIR) light irradiation due to an effective strategy combining morphological structure and energy band structure modulation. Under optimal conditions, the degradation rate of W 18 O 49 /g-C 3 N 4 composites to MB was 0.0677 min À1 , which was 3 times and 5 times of g-C 3 N 4 (0.0276 min À1 ) and W 18 16 Biochar is a cheap and green carbon-based material obtained by pyrolysis of biomass feedstock at high temperatures and under anaerobic conditions. 27 Biochar has a high surface area and porous structure, structural defect sites and various surface functional groups, which provide excellent electrical conductivity and electron storage capacity in photocatalytic processes. 28 Electrons that leap under light can be transferred to biochar, contributing to a lower electron-hole complexation rate in the photocatalytic process, which improves the oxidative removal of target compounds, while the raw material is renewable and easily available. 29,30 Cow dung is a common livestock waste product, which is mainly derived from undigested cellulose-based feed residues and without proper treatment will cause environmental problems such as deterioration of air quality, public hazards (e.g., infectious pathogens and asphyxiation poisoning), greenhouse gas emissions, and water pollution. 31 Currently, cattle manure is used by some as cooking fuel, disinfectant cleaner, construction material, insulation material, waterproong material for walls and oors of rural houses, and for electricity generation. There is also the use of cattle manure as a raw material for the preparation of biochar, which can be an effective solution for a large amount of livestock waste. 32,33 Thus, cattle manure can be used as a cheap and abundant source of carbon material.
In this paper, high specic surface area porous structure cow dung active carbon (CDAC) was prepared for the rst time by using cow dung as carbon raw material and active by KOH, and W 18 O 49 was loaded on CDAC by hydrothermal method to synthesize CDAC/W 18 O 49 composites, which beneted from the large specic surface area structure of CDAC and the interface between amorphous carbon CDAC and W 18 O 49 could extend the current The photocatalytic degradation of MB by CDAC/W 18 O 49 composites was enhanced by the large surface area structure of CDAC and the interface between amorphous carbon CDAC and W 18 O 49 , which could extend the current carrier lifetime and accelerate the charge transfer. In addition, a schematic diagram of the reaction mechanism was constructed, and a possible photocatalytic mechanism was proposed.

Materials
Cow dung was obtained from Guangxi Buffalo Research Institute (Nanning, China). Tungsten hexachloride (WCl 6 ), cyclohexanol and methylene blue (MB) were purchased from Shanghai Macklin Biochemical Co. Potassium hydroxide (KOH), disodium EDTA-2Na, anhydrous ethanol, tert-butanol (t-BuOH), hydrochloric acid (HCl) and 1,4-benzoquinone (BQ) were purchased from Guandong Guanghua Sci-Tech Co. Deionized (Dl) water was obtained from an ultrapure water production facility and used throughout the experiments. All chemicals and reagents were used as received without further purication.

Preparation of the cow dung active carbon
CDAC was synthesized by chemical activation using KOH as the activator and pre-carbonized cow dung charcoal as the precursor. First, the sun-dried cow dung was pre-carbonized in the tube furnace at 450 C for 2 h under an argon gas ow rate of 50 ml min À1 . Then, the charcoal was mixed with KOH in a 1 : 2 ratio. The mixture was transferred to an alumina crucible and pyrolyzed at a rate of 10 C min À1 in a constant stream of argon at 800 C, and then held for 2 h before natural cooling. The obtained product was washed with 1.0 M hydrochloric acid to remove residual KOH, followed by DI water until the ltrate became neutral. Finally, the fabricated CDAC was collected aer drying at 80 C overnight. 32

Preparation of the CDAC/W 18 O 49 composite
Firstly, 40 mg of CDAC obtained was added to 70 ml of cyclohexanol solution and stirred at 40 C for 1 h to obtain a homogeneous mixture. Then, 83.3 mg of WCl 6 was dissolved in 70 ml of the mixed solution and kept stirring for 15 min. Then mixture was sealed in an autoclave with a Teon liner and heated at 200 C for 6 h. Finally, the black-blue precipitate was collected by centrifugation, washed several times with ethanol and deionized water, and dried in vacuum at 60 C for 10 h. For comparison, pure W 18 O 49 was also synthesized in the same way, without the addition of CDAC.

Characterization of materials
The crystal structure of the catalysts was characterized with Cu Ka1 radiation (l ¼ 1.54056Å) using an X-ray diffractometer (XRD, Rigaku D/MAX 2500 V, Rigaku Corporation) at an accelerating voltage of 45 kV, scan rate (2q): 0.05 s À1 , recording a 2q range of 10 to 80 . Scanning electron microscopy (SEM, Sigma 300, Carl Zeiss) was used to study the microstructure and morphology of the prepared photocatalysts. The FT-IR of the synthesized photocatalysts was collected by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) (F20 S-TWIN electron microscope, Tecnai G2, FEI Co.) at an accelerating voltage of 200 kV. FT-IR spectra of the synthesized photocatalysts were collected by Fourier transform infrared spectrometer (IRAffinity-1, Shimadzu, Japan). A surface area analyzer (TriStar II 3020, Micromeritics) was used to analyze the specic surface area and pore volume of the synthesized photocatalysts. To obtain Raman spectra, a microscopic Raman spectrometer (HORIBA Jobin Yvon, Lab RAM HR Evolution) with Raman shis measured between 100 cm À1 and 2000 cm À1 was used. X-ray photoelectron spectroscopy (XPS) of the prepared samples was performed on an XPS spectrometer with Al Ka source (XPS, ESCALAB 250XI, Thermo Fisher), exploring the valence states of W and C elements in the prepared samples. The UV-vis-NIR diffuse reectance spectra (UV-vis-NIR DRS) of the prepared samples were performed by an UV-vis-NIR spectrophotometer equipped with an integrating sphere (PerkinElmer LAMBDA 365 UV/Vis Spectrophotometer) using BaSO 4 as a reference.

Photoelectrochemical measurement
A typical three-electrode measurement system based on the CHI 660D workstation was applied to measure the photoelectrochemical properties of the as-prepared samples. Pt electrode and an Ag/AgCl electrode in saturated KCl solution were used as counter electrode and reference electrode, respectively. The photocurrent density was measured in a 1 M sulfuric acid electrolyte solution under visible light provided by a 300 W Xe arc lamp (l > 365 nm). Electrochemical impedance spectroscopy (EIS) was measured based on the photoelectrochemical test system described above.

Photocatalytic activity measurement
The photocatalytic activity of the prepared photocatalysts was investigated by degrading MB under light irradiation. A 300 W xenon lamp (PLS/SXE 300C, Perfectlight Co., Ltd, Beijing) was employed as the light source, and it was directly used as a fullspectrum light source without any lter. Before the photocatalytic experiment started, 10 mg of the prepared photocatalyst was added into 100 mL of 35 mg L À1 MB aqueous solution and stirred for 60 min to reach the adsorption equilibrium under protection from light. During the photocatalytic process, the quantitative aqueous solution was collected at certain time intervals, and the photocatalyst was removed by centrifugation and analyzed.

Results and discussions
3.1. Structure and morphology characteristics  020)  The surface morphology of the catalysts was investigated by SEM and TEM. Fig. 2a shows SEM image of pristine CDAC shows a rough surface lled with porous morphology. Aer passing the hydrothermal reaction, the surfaces of CDAC are modied with linear W 18 O 49 (as in Fig. 2b). Further observation by higher magnication SEM images (Fig. 2c and d) shows that the W 18 O 49 nanowires on CDAC have diameters of 20-60 nm and lengths of 0.3-0.8 mm. Fig. 2e represents transmission electron microscopy (TEM) images of individual CDAC/W 18 O 49 composites, which can be seen to be modied with uniformly sized. Fig. 2f shows elemental mapping image of the CDAC/W 18 O 49 composite shows that the C elements are mainly distributed in the middle region, while the O and W elements are mainly distributed on the outer surface of the C elements. HRTEM image (Fig. 2g)

Specic surface area measurement
The pore structures and SSA of the materials CDAC, W 18 Fig. 3. According to the classication criteria of the International Society for Pristine and Applied Chemistry, the curves can be classied as a combination of Class I and IV curves. 34 At relative pressures P/P 0 < 0.1, the CDAC and CDAC/W 18 O 49 curves show a signicant increase in N 2 adsorption, indicating the presence of a large number of micropores in the material, and at relative pressures in the range of 0.41 < P/P 0 < 0.95, there is a signicant hysteresis loop in both curves, indicating the presence of mesopores in the material, but the adsorption and desorption curves of W 18 O 49 are not signicant. When the relative pressure was in the range of P/P 0 > 0.95, the adsorption and desorption curves were close to vertical, suggesting the presence of macroporosity in all three materials. In addition, the SSA of CDAC is 479.1034 m 2 g À1 , which is larger than that of CDAC/W 18 O 49 , but the relative pore volume is smaller than that of CDAC/W 18 O 49 , as shown in Table 1, which may be caused by the larger number of mesopores in CDAC and the larger number of macropores in CDAC/W 18 O 49 , which is consistent with the adsorption-desorption curve. The larger pore volume is benecial to promote the photocatalytic effect of CDAC/W 18 O 49 on MB.   band) and sp 2 (G-band), respectively. 35 The Raman spectra of W 18 Fig. 4d. There are ve peaks in the spectrum of CDAC, the peak at 460 cm À1 is attributed to Si-O-Si bending vibration, the peak at 1085 cm À1 is attributed to C-C stretching vibration, the peaks at 1212 and 3428 cm À1 are attributed to the -OH stretching mode, and the peak at 1383 cm À1 is attributed to the medium C-O bond axial deformation vibration. -OH stretching mode, and the peak at 1383 cm À1 are an axial deformation vibration in the C-O bond. X-ray photoelectron spectra (XPS) spectra of the prepared samples are also provided to further investigate the interactions between CDAC and W 18 O 49 . The XPS spectra in Fig. 5a show that the prepared CDAC/W 18 O 49 composites are mainly composed of C, W and O elements. Fig. 5b-d show the high-resolution spectra of C 1s, W 4f and O 1s. As shown in the highresolution C 1s spectrum of CDAC in Fig. 5b, the peak at the binding energy of 284.80 eV is mainly attributed to C-C of surface amorphous carbon, while the peaks at 286.45 and 289. 15 (Fig. 4a), the main spectrum is divided into two pairs of peaks, which represent two different oxidation states of element W, namely W 6+ and W 5+ . In the W 4f high-resolution XPS spectrum of pure W 18 O 49 (Fig. 5c), the main spectrum is divided into two pairs of peaks, which represent two different oxidation states of element W, namely W 6+ and W 5+ . In the W 4f high-resolution XPS spectrum of pure W 18 O 49 (Fig. 5c) (Fig. 5d)

Surface chemical composition and group analysis
where a, h, v, E g and A are the absorption coefficient, Planck's constant optical frequency, band gap energy and constant,  respectively. Based on the results shown Fig. 6b and  composites may be attributed to the good electrical conductivity of CDAC as an electron absorber and transport network, the increased specic surface area of the composites with CDAC as a substrate, which allows more exposed active sites, and the lower band gap of W 18 O 49 nanowires and the defect structure caused by numerous oxygen vacancies. In addition, the kinetics of photocatalytic degradation of MB under full-spectrum light irradiation was investigated, and the results showed that the variation of MB concentration versus reaction time on CDAC/W 18 O 49 composites followed a pseudo-rst-order kinetic diagram with the equationln(C/C 0 ) ¼ kt, where t, C 0 and C are the reaction time, initial methyl concentration (mg L À1 ), and methyl at time t, respectively concentration (mg L À1 ). k represents the apparent pseudo primary rate constant (min À1 ). The pseudo primary rate constants of pristine W 18  For practical applications, the reusability and stability of photocatalysts are very important, therefore, to evaluate the photostability of CDAC/W 18 O 49 composites, cycling reactions were performed. Aer each catalytic run, the photocatalyst was separated from the solution, washed with ethanol and vacuum dried to ensure the purity of the recovered catalyst. As shown in Fig. 9, aer ve cycling experiments, the photocatalytic activity of CDAC/ W 18 O 49 did not show a signicant degradation process, indicating that the prepared CDAC/W 18 O 49 composites have high stability.

Photocatalytic reaction mechanism
In order to understand the photocatalytic mechanism of this composite, controlled experiments were performed to capture radicals, with EDTA-2Na, t-BuOH and BQ as hole (h + ), hydroxyl radical (cOH) and superoxide radical (cO 2 À ) trapping agents, respectively. As shown in the Fig. 10, when 1 mM 1,4-benzoquinone (BQ) was added to the reaction system, the photocatalytic degradation efficiency of MB under full-spectrum light irradiation (l > 365 nm) was somewhat suppressed, indicating that superoxide radicals (cO 2 À ) play some roles in the photocatalytic degradation process. The photocatalytic activity of CDAC/W 18 O 49 composites was signicantly reduced when 1 mM tert-butanol (t-BuOH) was added, which indicated that hydroxyl radicals (-OH) played a major role in the photocatalytic degradation process. In contrast, the degradation rate of MB was signicantly increased by adding 1 mM disodium ethylenediaminetetraacetate (EDTA-2Na). The reason why EDTA-2Na can improve the degradation rate is that it can trap the hole (h + ), so that more cO 2 À and cOH reactive substances in the system can participate in the reaction, thus improving the degradation rate of MB.
In summary, a possible mechanism for the photocatalytic degradation of methylene blue (MB) by CDAC/W 18 O 49 composite is proposed. As shown in Fig. 11 under full-spectrum light irradiation, W 18 O 49 can absorb enough energy to generate excited electrons and form electron-hole pairs. The excited electrons in the conduction band (CB) of W 18 O 49 can migrate smoothly into the CDAC due to the high conductivity and good electron storage capacity of CDAC, as evidenced by the photocurrent response and EIS. Thus, the combination of photogenerated carriers can be effectively suppressed and the absorption of visible light can be increased. The photo-induced electrons can react with oxygen and water adsorbed on the surface to generate cO 2 À and cOH, which make great contribution on degradation of MB. 43 The porous structure and large specic surface area of CDAC can adsorb and enrich MB, and increase the concentration of MB in the composite, thus increasing the substrate concentration in the photocatalytic reaction region. 44 Thus, the synergistic effect of effective charge   separation, increased specic surface area, more light absorption and higher local MB concentration improves the photocatalytic activity of CDAC/W 18 O 49 composites.

Conflicts of interest
There are no conicts to declare.