Specially designed B4C/SnO2 nanocomposite for photocatalysis: traditional ceramic with unique properties

Boron carbide: A traditional ceramic material shows unique properties when explored in nano-range. Specially designed boron based nanocomposite has been synthesized by reflux method. The addition of SnO2 in base matrix increase the defect states in boron carbide and shows unique catalytic properties. The calculated texture coefficient and Nelson Riley factor shows that the synthesized nanocomposite have very high defect states. Also this composite is explored for the first time for catalysis degradation of industrial used dyes. The industrial pollutants such as Novacron red and methylene blue dye degradation analysis reveal that the composite is an efficient catalyst. Degradation study shows that 1 g/L catalyst concentration of B4C/SnO2 degrade Novacron red Huntsman dye upto 97.38% approximately in 20 minutes under sunlight irradiation time. This water insoluble catalyst can be recovered and reused.


Introduction
Dyes and pigments are extensively used in textile industries, food, cosmetics, paper, leather and plastic industries for coloring the products. The estimated production of different dyes is about approximately 0.7 to 7 million tons per year [1][2][3]. The extent of pollution caused by discharge of dyes into the environment is unknown. The use of dyes is main source of environmental pollution and also become a big threat to aquatic life. The removal of dye from waste water is crucial need of time for better sustainability as most of the dyes and their secondary products are carcinogenic or mutagenic and noxious in nature [4 -5]. The various methods are to be used by researchers to decolourize different dyeing effluents. The industrial waste water treatment involves processes such as physical (adsorption) [6][7], chemical (ozonation) [8], biological [9][10][11] reverse osmosis and as well as photocatalysis [12]. The high cost physical as well as chemical methods have failed to treat waste water as this economically unrealistic and also results in unavoidable secondary pollution. The photocatalysis technology is economically viable method used for degradation of dyes at ambient conditions. Carbides have attracted attention of researchers in recent years due to its extraordinary properties such as high mechanical strength, high melting point and their chemical inertness owing to their potential applications in thermionic electron sources. Nanostructured carbides have been used in various fields such as biomaterials, light weight/high strength materials, high temperature resistant materials, semiconducting devices [13][14]. Various synthesis methods have been used in synthesis of boron carbide nanostructures such as carbothermal method from reduction of boron oxide (B 2 O 3 ) over 1000 °C, thermal decomposition method, gaseous reaction between boron trichloride (BCl 3 ) and a methane hydrogen mixture in presence of radio frequency argon plasma, reduction of BCl 3 by CH 4 at 1500 °C with laser [15][16]. A wide range of high temperature synthesis methods can be used for preparation of boron carbide nanostructures directly from boron and carbon [17]. But these methods are economically not viable due to the use of expensive precursors. As metal oxides has attracted considerable attraction in research field due to their physical and chemical properties. Among them, tin oxide (SnO 2 ) has a wide band gap (3.6 eV) and high thermal stability so SnO 2 is used as a potential candidate as a photocatalyst [18][19]. The barriers in photocatalytic efficiency, in case of SnO 2 nanoparticles are their aggregation as size decreases and electron hole recombination process. B 4 C/SnO 2 composite has been synthesized using wet chemical synthesis method in order to obtain the improved photocatalytic efficiency for waste water treatment. The transition metal oxides, phosphides, sulfides replaced the high cost noble metals based electrocatalysts as well as photo-catalysts in the past years due to their low cost and high activity [20]. Though, corrosion and passivation under acidic conditions cause main hurdle for most of these materials. Besides, there is need of developing a stable and catalytically active material for photocatalysis process in order to reduce water pollution. The transition metal oxides usually showed their failures in field of active site engineering. In recent years, the catalysts that contain non-metallic nature and earth abundance such as carbon are employed as alternative catalyst materials for water purification process. Also, B 4 C is a semiconductor with band gap of about 1.5 eV [21].
In this work, we emphasized on the synthesis of an efficient catalyst B 4 C/SnO 2 composite for removal of industrial pollutants with the purpose of water purification process. The B 4 C/SnO 2 composite has been synthesized using reflux method. The composite of SnO 2 with B 4 C as base matrix can show remarkable photocatalytic properties. Also the catalyst can be recovered and reused. Present study deals with synthesis and photocatalytic properties analysis of B 4 C/SnO 2 nanocomposite. Industrial pollutants methylene blue (MB) and Novacron red Huntsman (NRH) dyes were used as target materials. Their degradation analysis is studied in details and degradation mechanism is also proposed for this study.

Materials and methods
Boric acid (H 3 BO 3 , 99.9%), activated magnesium (Mg, 98%) and acetone (used as carbon source), SnCl 2 , Hydrochloric acid were purchased from Sigma Aldrich. All the chemicals were used as received without any further purification.

Synthesis of B 4 C/SnO 2 catalyst
B 4 C nanoparticles were successfully synthesized using solvothermal method [33][34]. The B 4 C/SnO 2 photocatalyst was prepared by reflux method. The freshly prepared aqueous solutions of SnCl 2 , B 4 C and HCl were added to magnetically stirred round bottom flask respectively and refluxed at 100 °C for 5 hours.
The obtained product was cooled to room temperature naturally. The as prepared sample was collected and washed with distilled water so that neutral pH is obtained. The washed precipitates were collected and dried in vacuum at 80 °C for 6 hour.

Characterization
The dried powder of the B 4 C/SnO 2 composite was characterized by powder X-ray diffraction (XRD). The XRD pattern with diffraction intensity versus 2θ was recorded in a Rigaku instrument with Cu-Kα radiation (λ=1.5418 Å). Transmission Electron Microscope (TEM) was carried out on TECNAI G2 20 FEI at 200 keV in order to study the morphology of synthesized material. Optical absorption spectrum was studied using UV-visible Shimadzu UV-2600 spectrophotometer.

Photocatalysis Experiment
The photocatalytic activities of B 4 C/SnO 2 (1 g/L) were evaluated by degradation of aqueous solutions of methylene blue (MB) dye and a textile dye Novacron red Huntsman (NRH) (1 mg/L). All experiments were carried out at room temperature. The aqueous solutions were magnetically stirred for 30 minutes in dark to get the adsorption desorption equilibrium followed by sunlight irradiation. The maximum absorption wavelength of MB at 664 nm was observed. Typically, 20 mg of photocatalyst (1g/L) was added into 20 mL of 1 mg/L MB and NRH aqueous solution. Analytical samples were taken from reaction systems after specified time period and centrifuged to separate photocatalysts before analysis. The concentration of B 4 C/SnO 2 photocatalyst was varied from 0 g/L to 1 g/L. The changes in absorptional intensity in spectra with different catalyst dosage were studied using UV-visible spectrophotometer.

XRD analysis
XRD pattern of the synthesized B 4 C/SnO 2 composite is shown in figure 1 (a). The collected pattern was compared with B 4 C and SnO 2 JCPDS cards i.e. 35-0798 and 41-1445 respectively. The different diffraction peaks arise from B 4 C and SnO 2 respectively. The pattern confirms the formation of B 4 C and SnO 2 phase in the synthesized sample. The broadening of diffraction peaks were used for the determination of crystallite size. The crystallite size was calculated using Debye Scherer formula [22]. The calculated average crystallite size of the synthesized material is equal to ̴ 26 nm.
Further, the XRD pattern was used to determine the texture coefficient for the synthesized composite. The texture coefficient provides the information about the preferred growth orientation of the material. Higher the value of texture coefficient deviated from unit value; more will the growth. For the calculatio n of texture coefficient, standard intensities related to the diffraction planes were taken from standard JCPDS cards (35-0798 and 41-1445). Texture coefficient [23][24] is calculated using the following relationship: where TC(hkl), I(hkl) and I 0 (hkl) are texture coefficient of the plane specified by miller indices, specimen and standard intensities (taken from JCPDS cards) respectively for a given diffraction peak. The value of n represents the number of different peaks. The texture coefficient analysis reveals that the synthesized material is more grown along (211) with a texture coefficient value 4.5131. The growth of the synthesized composite along (211) can be ascribed to the presence of defects in the synthesized sample. Further, the presence of defect states has also been studied from XRD results. The variation of Δd/d with Nelson -Riley factor [25][26] determined from XRD pattern shown in figure 1 (b) . Therefore, it can be concluded that the SnO 2 incorporation leads to an increase in the defect states/stacking fault density. The effect of these higher defects states/stacking faults in the dye degradation have been discussed at the end of this section.

Mechanism followed
The presence of structural defects and distortion in B 4 C/SnO 2 influence its structure. These inherent structural defects results in B 4 C/SnO 2 with high efficiency in sunlight harvesting and makes it a good catalyst for industrial pollutants. The existence of defects causes the downshift in conduction band and available the new mid gap states that enable the boron carbide as visible light harvesting material [28].
The defects have also shown their impact on carrier relaxation dynamics, results in charge separation by trapping electron and holes [29][30][31]. As Nelson riley plot as well as texture coefficient indicates the presence of structural defects in B 4 C/SnO 2 . SnO 2 can absorb UV wavelength light and results in electron hole separation. SnO 2 makes electron availability to conduction band of boron carbide. These electrons and holes help to produce the • OH radical and results in degradation of dyes.

Photocatalysis analysis
As the synthesized B 4 C/SnO 2 composite, employed as a photocatalyst for the degradation of MB and NRH in water under sunlight irradiation. The effect of concentration loadings of B 4 C/SnO 2 catalyst on degradation of used dyes (details of MB and NRH are shown in table 1) was studied. The degradation efficiency of a dye was calculated from change in the concentration of MB and NRH dye using following formula: where C 0 and C t were concentrations of dye before and after that reaction.       observed [32].The half-life of NRH dye was estimated as 11.81 minutes (figure 6c).

Conclusion
The B