Synthesis and Characterization of Bi2O3 NPS and Photocatalytic Application with Methylene Blue

Nanoparticles of bismuth oxide were successfully synthesized by hydrothermal process which included Bismuth (III) nitrate (Bi(NO 3 ) 3 .6H 2 O), sodium hydroxide(NaOH) and Nitric acid(HNO3) without further purification along with ultra-pure water. To investigate the structural, optical, and photocatalytic activity of two samples (1:5) and (1:6) respectively with two different NaOH precipitating agent molar ratios. The prepared nanoparticles were characterized by X-ray diffraction (XRD), UV- Vis’s spectrometer, scanning electron microscope (SEM), Energy-dispersive X-ray spectroscopy (EDAX), Fourier transform infrared spectroscopy (FTIR) and X-ray Photo electron Spectroscopy (XPS). Based on the obtained results the hydrothermally synthesized Bi2O3 NPs exhibit good efficiency to photocatalytic degradation of Methylene blue under the irradiation of LED white light.

metal doping or material hybridization) in order to improve the photoactivity and energy conservation.
The complex degradation behaviour of natural organic matter (NOM) was investigated using photocatalytic oxidation systems with a novel catalyst based on a hybrid composite of zinc-bismuth oxides and g-C3N4 (ZBO-CN). by Hai Bang Truong et al. Under low-intensity visible light irradiation, the photooxidation procedure effectively removed NOM, with removal rates of 53-74 % and 65-88 % respectively, based on dissolved organic carbon (DOC) and UV absorption coefficient (UV254) at 1.5 g/L of catalyst [2].
Variety of synthesis techniques have already been developed to produce Bi2O3 in powder and thin film form, its various properties strongly depend on its structure including the crystal size, orientation, morphology and density. The controlled synthesis of monodisperse Bi2O3 nanoparticles remains a challenge. Chemical synthesis is a straight forward synthesis route and the temperature is the key parameter which effectively hydrolyses to starting solution for the well-defined nano scaled Bi2O3. The photocatalytic degradation of dyes in aqueous solution is shown by the synthesis of Bi2O3 NPs using a high yield approach Among all the synthesis routes, hydrothermal synthesis should be further improved, to meets the requirement of environmental production [6].
Methylene blue (MB) is a water-soluble dye which widely used in pharmaceuticals, food industries and textile printing. The safe removal of MB dye is the prime aim of our present study. This study concentrates to examine the effect of pH value and the reaction of photocatalytic performance by using Bi2O3 NPs was reported here. They removal of dye to decompose by the photocatalytic process explains the separation of photogenerated charge carriers for enhancing the photocatalytic activity. However, micron grain sized Bi2O3 which act as a semiconductor so its surface area is very low and photogenerated charge carriers cannot be transferred for the fast charge carrier's recombination.
In this work, we report the synthesis of photocatalytic activity of Bi2O3-NPs structure studied by hydrothermal process. X-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDXS), Field Emission scanning electronmicroscope (FE-SEM), transmission electron microscope (TEM), UV-VIS-NIR spectrophotometer and Fourier transfer infrared spectroscopy (FTIR) was used to confirm and exhibits the different physical properties of Bi2O3-NPs.The photocatalytic response of as-synthesized Bi2O3-NPs as promising photocatalyst was checked using the degradation of Methylene blue (MB) under irradiation of LED white light also examined the effect of crystallite size of the catalyst on (MB) dye concentration prepared by hydrothermal synthesis using organic pollutants such as phenol, 4chloro phenol, and 4-nitro phenol in water [1].
During the reaction, the pH of the mixture increases gradually and attained approximately above 10, precipitation process started to form white precipitate. After 30 min continuous stirring, white precipitate obtained was transferred into autoclave with Teflon lining and kept at 160º C for 12 hours for hydrothermal treatment. The autoclave was cooled to room temperature naturally. The yellow precipitate was obtained by centrifugation (6 min with 7500 rpm) and washed several times with ultra-pure water and dried at 80º C for 12 hours. Finally, the products were calcined at 350º C for 3 hours for further characterization [8].

Characterization
The crystalline properties of the synthesized Bi2O3 -NPs were studied by XRD using a Bruker (D5005) X-ray diffractometer equipped with graphite monochromatized CuKα radiation (λ =1.54056Å). An accelerating voltage of 40 kV and emission current of 30 mA were adopted for the measurements. In addition to XRD, FTIR spectroscopy measurements were also performed to confirm the structure of the Bi2O3 -NPs. The chemical composition of Bi2O3 -NPs were studied using SEM. The SEM measurements were performed by a Hitachi S-4800 high resolution (HR) field emission scanning electron microscope. The FE-SEM equipment was also furnished with an EDAX spectrometer that was used for elemental analysis. Absorption spectra of the samples in the diffused reflectance spectrum (DRS) mode were recorded in the wavelength range of 200-1000 nm using a spectrophotometer (Jasco V 670), with BaSO4 as a reference. From the adsorption edge, the band gap values were calculated by extrapolation.

Photocatalytic Activity
Photocatalytic activity of synthesized Bi2O3 -NPs were examined by the rate of degradation of MB under the effect LED white light irradiation. All photo catalytic reaction were performed out in photo catalytic reactor system, which consists of a cylindrical borosilicate glass reactor vessel with volume of 250 mL, a cooling water jacket, and a LED white light, Institute of Electric Light Source, Beijing) positioned axially at the center as a visible light. The reaction temperature was kept at 25° C by circulating the cooling water. A special glass frit as an air diffuser was fixed at the reactor to uniformly disperse air into the solution. For each run the reaction suspension was freshly prepared by adding 0.250 g of catalyst into 250 mL of initial concentration of 5 mg/L of MB. After the degradation reaction, filtration was done for all samples using syringe and syringe filter 0.45 µm to remove any precipitated particles. The filtrate was analysed by an UV-Vis spectrometer (UC-2450-SHIMADZU). The maximum characteristic absorption wavelength of MB was positioned [9].

XRD analysis
The phase crystallinity and purity of hydrothermally synthesized Bi2O3 -NPs sample were investigated using XRD analysis. Figure 1 exhibits the X-ray diffraction pattern of Bi2O3 materials exhibited reflection peaks at 31.923 of glancing angle. All reflection peaks can be well indexed with a pure tetragonal phase of crystalline Bi2O3, which are in good agreement with the fiber structure of tetragonal phase (JCPDS card No: 65-4028). The broad reflection peaks suggested that the materials are nano crystalline structure. The crystalline of Bi2O3 NPs is nearly 42 nm, which shows that the product consists of needle shaped nano crystals. The additional reflection peaks indicates that when OH ions are used for the preparation of nano crystalline structure with additional phase of Bi2O3 is strongly influenced by the variation of pH solution caused by particular molar ratio of NO3and hydroxyl ions. In our case the high concentration of reducing agent NaOH which is used to raise the pH value of given solution between 8 to 10 to reach the optimum condition [10]. The temperature of the hydrothermal synthesis increasing concentration of aqueous solution and Bi 3+ ions species into pure obtained phase suggesting that this 160º C optimum temperature of the solvent are used for stabilizing the agent. The materials exhibited crystalline structure of tetragonal with the lattice constants 3.850 Aº and 12.250 Aº which is good agreement with JCPDS No 65-4028.
Calculated structural parameters were tabulated in Table 1 [11] Average crystallite size was determined from predominant XRD peak using Scherrer Eq. (1) where k=0.9 the numerical shape factor which is a constant ,D is crystallite size, λ is wavelength of incident radiation, β is the FWHM in radians, and θ is Bragg angle taken in radians Dislocation density (δ) was calculated from crystallite size using Eq.(2) The lattice constants a and c are calculated for tetragonal Structures Where d is interplanar spacing, h, k, l = miller indices

XPS analysis
The XPS spectra of Bi2O3 NPs were obtained and shown in figure 2(a). There are two asymmetrical peaks observed at 292 eV and 198 eV correspond to Bi 4f orbitals band its electro chemical reduction of new peaks was consistent with Bi 4f spectra for the Bi metal.
This peak at 534 eV correspond to Bi ions and its signal voltage is assigned to Bi 3+ ion which are in good agreement with reported XPS analysis of Bi2O3 NPs. The figure 2(b) exhibited the two peaks at 173 eV and 168 eV can be assigned to Bi 4f5/2 and Bi 4f3/2, respectively.
The surface analysis of Bi2O3 NPs and its peaks were resulted there is no shoulder of Bi 4f7/2 peaks related to either bivalent or tetravalent states, instead it gives Bi 4f5/2 of Bi2O3 gives a report as the surface of the prepared particle were consist of pure Bi2O3 and they were not any BiO and Bi2O5 phases [12]. The spectrum for the O 1s field is shown in Figure 2(c). A low-intensity signal is seen, indicating there is not any oxygen in this sample. It's possible that the presence of oxygen in the samples is due to adsorption from the surrounding moisture.The valence band offset can be described by the formula,

EDAX and SEM analysis
The

UV DRS analysis
The

FTIR analysis
The obtained Bi2O3 NPs was studied by infrared spectroscopy FTIR. Figure 6 shows the obtained infrared spectra. The phase composition and the vibrational frequency were formed by using FTIR spectroscopy Bi(NO3)3 which can be transformed into single phase Bi2O3 and its OH group was released by the oxidation reactions of reagent dissolved in the preparation. The distilled water OH group was observed for the influence of composition of reaction products containing large amounts of hydroxyl group with high specific surface area 25 m 2 g -1 were treated at the pH condition of 6. Suppose the pH is greater than 7 the IR region showed no OH group in the products the morphology of the product of fiber like structure inferred, the particles were prepared under acidic condition [17].
The crystalline product containing bismuth and oxygen ions having fibre shaped particles resulted a possible reaction mechanism by hydration and were followed by hydrolysis. The formation of Bi2O3 and the condensation of the OH group is strongly influenced by the pH of the reaction medium.The accelerated Bi 3+ ion causes non conventional volume of reaction by the applied temperature inside the chamber [18].

Photocatalytic performance
The Noticeably, 1:6 ratio displays low efficiency for MB degradation due to its low valence band location, although it has been considered an excellent visible-light-driven photocatalyst [19].
The suggested photocatalytic reaction mechanism for the degradation of MB dye over Bi2O3 NPs is depicted in figure 9. The volume of photocatalyst suspended in 100 mL of MB solution was determined using the atypical behavior test [20]. After allowing the suspension to achieve adsorption equilibrium in the dark for 2 hours, the photocatalytic reaction was UV-Vis spectrophotometer at 663 nm [21].
As photodegradation of organic pollutant over Bi2O3 nanoparticles was dominated as the hole oxidation process, the photogenerated holes over Bi2O3 did not exhibit enough over potential for the oxidation of MB, thus leading to the low photocatalytic efficiency.
Importantly, it is found that the entire two ratios of the samples exhibit remarkably enhanced photocatalytic activities for MB degradation as compared to these as prepared samples.
Among them, Bi2O3 nanoparticles with 1:5 molar ratio shows the highest photocatalytic activity and could approximately 80% degrade MB within 3 h. It is believed that Bi2O3 (1:5) ratio lattice varies the location of the valence band somehow; therefore, the photogenerated holes possess higher oxidation power for MB degradation [22].  [23]. To explain this last result, the specific area was estimated for the sample (1-5) it was 25 m 2 /g, while for (1-6) it was 23 m 2 /g. As seen in figure 10, the volume of organic dye adsorbed increases as the actual surface area increases, roughly proportional to the differences in the area values.

In the dark Conditions
Since the solution was exposed to visible light after the dark time, the reaction began to take place. Figure 10 shows that Bi2O3 NP is slightly effective for MB decolourization, the / reduction being about 10 %, a value similar to that of photolysis reaction (14 % [24].

Conclusion
In the present work the Bi2O3 NPs was successfully synthesized by the efficient hydrothermal method with two different molar ratio of precipitating agent of NaOH which

Availability of data and material:
The data that support the findings of this study are available in https://doi.org     Schematic illustration of the possible photocatalytic reaction mechanism over Bi2O3 NPs Figure 10 Photo degradation of MB dye in the presence of Bi2O3 NPs of (1:5) and (1:6)